Use FoldHash

2024-08-16 18:24:38 -04:00
209 changed files with 2079 additions and 5001 deletions
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -6,8 +6,6 @@ exclude: |
    crates/red_knot_workspace/resources/.*|
    crates/ruff_linter/resources/.*|
    crates/ruff_linter/src/rules/.*/snapshots/.*|
-    crates/ruff_notebook/resources/.*|
-    crates/ruff_server/resources/.*|
    crates/ruff/resources/.*|
    crates/ruff_python_formatter/resources/.*|
    crates/ruff_python_formatter/tests/snapshots/.*|
@@ -17,7 +15,7 @@ exclude: |

 repos:
  - repo: https://github.com/abravalheri/validate-pyproject
-    rev: v0.19
+    rev: v0.18
    hooks:
      - id: validate-pyproject

@@ -59,7 +57,7 @@ repos:
        pass_filenames: false # This makes it a lot faster

  - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.6.1
+    rev: v0.5.7
    hooks:
      - id: ruff-format
      - id: ruff
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,39 +1,5 @@
 # Changelog

-## 0.6.2
-
-### Preview features
-
- \[`flake8-simplify`\] Extend `open-file-with-context-handler` to work with other standard-library IO modules (`SIM115`) ([#12959](https://github.com/astral-sh/ruff/pull/12959))
- \[`ruff`\] Avoid `unused-async` for functions with FastAPI route decorator (`RUF029`) ([#12938](https://github.com/astral-sh/ruff/pull/12938))
- \[`ruff`\] Ignore `fstring-missing-syntax` (`RUF027`) for `fastAPI` paths ([#12939](https://github.com/astral-sh/ruff/pull/12939))
- \[`ruff`\] Implement check for Decimal called with a float literal (RUF032) ([#12909](https://github.com/astral-sh/ruff/pull/12909))
-
-### Rule changes
-
- \[`flake8-bugbear`\] Update diagnostic message when expression is at the end of function (`B015`) ([#12944](https://github.com/astral-sh/ruff/pull/12944))
- \[`flake8-pyi`\] Skip type annotations in `string-or-bytes-too-long` (`PYI053`) ([#13002](https://github.com/astral-sh/ruff/pull/13002))
- \[`flake8-type-checking`\] Always recognise relative imports as first-party ([#12994](https://github.com/astral-sh/ruff/pull/12994))
- \[`flake8-unused-arguments`\] Ignore unused arguments on stub functions (`ARG001`) ([#12966](https://github.com/astral-sh/ruff/pull/12966))
- \[`pylint`\] Ignore augmented assignment for `self-cls-assignment` (`PLW0642`) ([#12957](https://github.com/astral-sh/ruff/pull/12957))
-
-### Server
-
- Show full context in error log messages ([#13029](https://github.com/astral-sh/ruff/pull/13029))
-
-### Bug fixes
-
- \[`pep8-naming`\] Don't flag `from` imports following conventional import names (`N817`) ([#12946](https://github.com/astral-sh/ruff/pull/12946))
- \[`pylint`\] - Allow `__new__` methods to have `cls` as their first argument even if decorated with `@staticmethod` for `bad-staticmethod-argument` (`PLW0211`) ([#12958](https://github.com/astral-sh/ruff/pull/12958))
-
-### Documentation
-
- Add `hyperfine` installation instructions; update `hyperfine` code samples ([#13034](https://github.com/astral-sh/ruff/pull/13034))
- Expand note to use Ruff with other language server in Kate ([#12806](https://github.com/astral-sh/ruff/pull/12806))
- Update example for `PT001` as per the new default behavior ([#13019](https://github.com/astral-sh/ruff/pull/13019))
- \[`perflint`\] Improve docs for `try-except-in-loop` (`PERF203`) ([#12947](https://github.com/astral-sh/ruff/pull/12947))
- \[`pydocstyle`\] Add reference to `lint.pydocstyle.ignore-decorators` setting to rule docs ([#12996](https://github.com/astral-sh/ruff/pull/12996))
-
 ## 0.6.1

 This is a hotfix release to address an issue with `ruff-pre-commit`. In v0.6,
@@ -95,7 +61,7 @@ The following rules have been stabilized and are no longer in preview:
 - [`invalid-bytes-return-type`](https://docs.astral.sh/ruff/rules/invalid-bytes-return-type/) (`PLE0308`)
 - [`invalid-hash-return-type`](https://docs.astral.sh/ruff/rules/invalid-hash-return-type/) (`PLE0309`)
 - [`invalid-index-return-type`](https://docs.astral.sh/ruff/rules/invalid-index-return-type/) (`PLE0305`)
- [`invalid-length-return-type`](https://docs.astral.sh/ruff/rules/invalid-length-return-type/) (`PLEE303`)
+- [`invalid-length-return-type`](https://docs.astral.sh/ruff/rules/invalid-length-return-type/) (`E303`)
 - [`self-or-cls-assignment`](https://docs.astral.sh/ruff/rules/self-or-cls-assignment/) (`PLW0642`)
 - [`byte-string-usage`](https://docs.astral.sh/ruff/rules/byte-string-usage/) (`PYI057`)
 - [`duplicate-literal-member`](https://docs.astral.sh/ruff/rules/duplicate-literal-member/) (`PYI062`)
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -2,6 +2,35 @@

 Welcome! We're happy to have you here. Thank you in advance for your contribution to Ruff.

+- [The Basics](#the-basics)
+    - [Prerequisites](#prerequisites)
+    - [Development](#development)
+    - [Project Structure](#project-structure)
+    - [Example: Adding a new lint rule](#example-adding-a-new-lint-rule)
+        - [Rule naming convention](#rule-naming-convention)
+        - [Rule testing: fixtures and snapshots](#rule-testing-fixtures-and-snapshots)
+    - [Example: Adding a new configuration option](#example-adding-a-new-configuration-option)
+- [MkDocs](#mkdocs)
+- [Release Process](#release-process)
+    - [Creating a new release](#creating-a-new-release)
+- [Ecosystem CI](#ecosystem-ci)
+- [Benchmarking and Profiling](#benchmarking-and-profiling)
+    - [CPython Benchmark](#cpython-benchmark)
+    - [Microbenchmarks](#microbenchmarks)
+        - [Benchmark-driven Development](#benchmark-driven-development)
+        - [PR Summary](#pr-summary)
+        - [Tips](#tips)
+    - [Profiling Projects](#profiling-projects)
+        - [Linux](#linux)
+        - [Mac](#mac)
+- [`cargo dev`](#cargo-dev)
+- [Subsystems](#subsystems)
+    - [Compilation Pipeline](#compilation-pipeline)
+    - [Import Categorization](#import-categorization)
+        - [Project root](#project-root)
+        - [Package root](#package-root)
+        - [Import categorization](#import-categorization-1)
+
 ## The Basics

 Ruff welcomes contributions in the form of pull requests.
@@ -304,34 +333,22 @@ even patch releases may contain [non-backwards-compatible changes](https://semve
 ### Creating a new release

 1. Install `uv`: `curl -LsSf https://astral.sh/uv/install.sh | sh`
-
 1. Run `./scripts/release.sh`; this command will:
-
    - Generate a temporary virtual environment with `rooster`
    - Generate a changelog entry in `CHANGELOG.md`
    - Update versions in `pyproject.toml` and `Cargo.toml`
    - Update references to versions in the `README.md` and documentation
    - Display contributors for the release
-
 1. The changelog should then be editorialized for consistency
-
    - Often labels will be missing from pull requests they will need to be manually organized into the proper section
    - Changes should be edited to be user-facing descriptions, avoiding internal details
-
 1. Highlight any breaking changes in `BREAKING_CHANGES.md`
-
 1. Run `cargo check`. This should update the lock file with new versions.
-
 1. Create a pull request with the changelog and version updates
-
 1. Merge the PR
-
 1. Run the [release workflow](https://github.com/astral-sh/ruff/actions/workflows/release.yml) with:
-
    - The new version number (without starting `v`)
-
 1. The release workflow will do the following:
-
    1. Build all the assets. If this fails (even though we tested in step 4), we haven't tagged or
        uploaded anything, you can restart after pushing a fix. If you just need to rerun the build,
        make sure you're [re-running all the failed
@@ -342,25 +359,14 @@ even patch releases may contain [non-backwards-compatible changes](https://semve
    1. Attach artifacts to draft GitHub release
    1. Trigger downstream repositories. This can fail non-catastrophically, as we can run any
        downstream jobs manually if needed.
-
 1. Verify the GitHub release:
-
    1. The Changelog should match the content of `CHANGELOG.md`
    1. Append the contributors from the `scripts/release.sh` script
-
 1. If needed, [update the schemastore](https://github.com/astral-sh/ruff/blob/main/scripts/update_schemastore.py).
-
    1. One can determine if an update is needed when
        `git diff old-version-tag new-version-tag -- ruff.schema.json` returns a non-empty diff.
    1. Once run successfully, you should follow the link in the output to create a PR.
-
-1. If needed, update the [`ruff-lsp`](https://github.com/astral-sh/ruff-lsp) and
-    [`ruff-vscode`](https://github.com/astral-sh/ruff-vscode) repositories and follow
-    the release instructions in those repositories. `ruff-lsp` should always be updated
-    before `ruff-vscode`.
-
-    This step is generally not required for a patch release, but should always be done
-    for a minor release.
+1. If needed, update the `ruff-lsp` and `ruff-vscode` repositories.

 ## Ecosystem CI

@@ -383,7 +389,7 @@ We have several ways of benchmarking and profiling Ruff:
 - Microbenchmarks which run the linter or the formatter on individual files. These run on pull requests.
 - Profiling the linter on either the microbenchmarks or entire projects

-> **Note**
+> \[!NOTE\]
 > When running benchmarks, ensure that your CPU is otherwise idle (e.g., close any background
 > applications, like web browsers). You may also want to switch your CPU to a "performance"
 > mode, if it exists, especially when benchmarking short-lived processes.
@@ -397,18 +403,12 @@ which makes it a good target for benchmarking.
 git clone --branch 3.10 https://github.com/python/cpython.git crates/ruff_linter/resources/test/cpython
 ```

-Install `hyperfine`:
-
-```shell
-cargo install hyperfine
-```
-
 To benchmark the release build:

 ```shell
 cargo build --release && hyperfine --warmup 10 \
-  "./target/release/ruff check ./crates/ruff_linter/resources/test/cpython/ --no-cache -e" \
-  "./target/release/ruff check ./crates/ruff_linter/resources/test/cpython/ -e"
+  "./target/release/ruff ./crates/ruff_linter/resources/test/cpython/ --no-cache -e" \
+  "./target/release/ruff ./crates/ruff_linter/resources/test/cpython/ -e"

 Benchmark 1: ./target/release/ruff ./crates/ruff_linter/resources/test/cpython/ --no-cache
  Time (mean ± σ):     293.8 ms ±   3.2 ms    [User: 2384.6 ms, System: 90.3 ms]
@@ -427,7 +427,7 @@ To benchmark against the ecosystem's existing tools:

 ```shell
 hyperfine --ignore-failure --warmup 5 \
-  "./target/release/ruff check ./crates/ruff_linter/resources/test/cpython/ --no-cache" \
+  "./target/release/ruff ./crates/ruff_linter/resources/test/cpython/ --no-cache" \
  "pyflakes crates/ruff_linter/resources/test/cpython" \
  "autoflake --recursive --expand-star-imports --remove-all-unused-imports --remove-unused-variables --remove-duplicate-keys resources/test/cpython" \
  "pycodestyle crates/ruff_linter/resources/test/cpython" \
@@ -473,7 +473,7 @@ To benchmark a subset of rules, e.g. `LineTooLong` and `DocLineTooLong`:

 ```shell
 cargo build --release && hyperfine --warmup 10 \
-  "./target/release/ruff check ./crates/ruff_linter/resources/test/cpython/ --no-cache -e --select W505,E501"
+  "./target/release/ruff ./crates/ruff_linter/resources/test/cpython/ --no-cache -e --select W505,E501"
 ```

 You can run `poetry install` from `./scripts/benchmarks` to create a working environment for the
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -228,9 +228,9 @@ dependencies = [

 [[package]]
 name = "camino"
-version = "1.1.9"
+version = "1.1.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8b96ec4966b5813e2c0507c1f86115c8c5abaadc3980879c3424042a02fd1ad3"
+checksum = "e0ec6b951b160caa93cc0c7b209e5a3bff7aae9062213451ac99493cd844c239"

 [[package]]
 name = "cast"
@@ -270,12 +270,6 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fd16c4719339c4530435d38e511904438d07cce7950afa3718a84ac36c10e89e"

-[[package]]
-name = "cfg_aliases"
-version = "0.2.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "613afe47fcd5fac7ccf1db93babcb082c5994d996f20b8b159f2ad1658eb5724"
-
 [[package]]
 name = "chic"
 version = "1.2.2"
@@ -326,9 +320,9 @@ dependencies = [

 [[package]]
 name = "clap"
-version = "4.5.16"
+version = "4.5.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ed6719fffa43d0d87e5fd8caeab59be1554fb028cd30edc88fc4369b17971019"
+checksum = "11d8838454fda655dafd3accb2b6e2bea645b9e4078abe84a22ceb947235c5cc"
 dependencies = [
 "clap_builder",
 "clap_derive",
@@ -401,7 +395,7 @@ version = "3.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2f8c93eb5f77c9050c7750e14f13ef1033a40a0aac70c6371535b6763a01438c"
 dependencies = [
- "nix 0.28.0",
+ "nix",
 "terminfo",
 "thiserror",
 "which",
@@ -618,12 +612,12 @@ checksum = "7a81dae078cea95a014a339291cec439d2f232ebe854a9d672b796c6afafa9b7"

 [[package]]
 name = "ctrlc"
-version = "3.4.5"
+version = "3.4.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "90eeab0aa92f3f9b4e87f258c72b139c207d251f9cbc1080a0086b86a8870dd3"
+checksum = "672465ae37dc1bc6380a6547a8883d5dd397b0f1faaad4f265726cc7042a5345"
 dependencies = [
- "nix 0.29.0",
- "windows-sys 0.59.0",
+ "nix",
+ "windows-sys 0.52.0",
 ]

 [[package]]
@@ -852,6 +846,12 @@ version = "1.0.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"

+[[package]]
+name = "foldhash"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4deb59dd6330afa472c000b86c0c9ada26274836eb59563506c3e34e4bb9a819"
+
 [[package]]
 name = "form_urlencoded"
 version = "1.2.1"
@@ -1053,9 +1053,9 @@ dependencies = [

 [[package]]
 name = "indexmap"
-version = "2.4.0"
+version = "2.3.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "93ead53efc7ea8ed3cfb0c79fc8023fbb782a5432b52830b6518941cebe6505c"
+checksum = "de3fc2e30ba82dd1b3911c8de1ffc143c74a914a14e99514d7637e3099df5ea0"
 dependencies = [
 "equivalent",
 "hashbrown",
@@ -1221,9 +1221,9 @@ checksum = "8b23360e99b8717f20aaa4598f5a6541efbe30630039fbc7706cf954a87947ae"

 [[package]]
 name = "js-sys"
-version = "0.3.70"
+version = "0.3.69"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1868808506b929d7b0cfa8f75951347aa71bb21144b7791bae35d9bccfcfe37a"
+checksum = "29c15563dc2726973df627357ce0c9ddddbea194836909d655df6a75d2cf296d"
 dependencies = [
 "wasm-bindgen",
 ]
@@ -1256,9 +1256,9 @@ checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"

 [[package]]
 name = "libc"
-version = "0.2.157"
+version = "0.2.155"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "374af5f94e54fa97cf75e945cce8a6b201e88a1a07e688b47dfd2a59c66dbd86"
+checksum = "97b3888a4aecf77e811145cadf6eef5901f4782c53886191b2f693f24761847c"

 [[package]]
 name = "libcst"
@@ -1394,16 +1394,6 @@ dependencies = [
 "libmimalloc-sys",
 ]

-[[package]]
-name = "minicov"
-version = "0.3.5"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5c71e683cd655513b99affab7d317deb690528255a0d5f717f1024093c12b169"
-dependencies = [
- "cc",
- "walkdir",
-]
-
 [[package]]
 name = "minimal-lexical"
 version = "0.2.1"
@@ -1454,19 +1444,7 @@ checksum = "ab2156c4fce2f8df6c499cc1c763e4394b7482525bf2a9701c9d79d215f519e4"
 dependencies = [
 "bitflags 2.6.0",
 "cfg-if",
- "cfg_aliases 0.1.1",
- "libc",
-]
-
-[[package]]
-name = "nix"
-version = "0.29.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "71e2746dc3a24dd78b3cfcb7be93368c6de9963d30f43a6a73998a9cf4b17b46"
-dependencies = [
- "bitflags 2.6.0",
- "cfg-if",
- "cfg_aliases 0.2.1",
+ "cfg_aliases",
 "libc",
 ]

@@ -1553,9 +1531,9 @@ checksum = "04744f49eae99ab78e0d5c0b603ab218f515ea8cfe5a456d7629ad883a3b6e7d"

 [[package]]
 name = "ordermap"
-version = "0.5.2"
+version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "61d7d835be600a7ac71b24e39c92fe6fad9e818b3c71bfc379e3ba65e327d77f"
+checksum = "8c81974681ab4f0cc9fe49cad56f821d1cc67a08cd2caa9b5d58b0adaa5dd36d"
 dependencies = [
 "indexmap",
 ]
@@ -1932,10 +1910,7 @@ dependencies = [
 "ruff_text_size",
 "rustc-hash 2.0.0",
 "salsa",
- "smallvec",
- "static_assertions",
 "tempfile",
- "thiserror",
 "tracing",
 "walkdir",
 "zip",
@@ -1947,17 +1922,19 @@ version = "0.0.0"
 dependencies = [
 "anyhow",
 "crossbeam",
+ "foldhash",
 "jod-thread",
 "libc",
 "lsp-server",
 "lsp-types",
+ "red_knot_python_semantic",
 "red_knot_workspace",
 "ruff_db",
+ "ruff_linter",
 "ruff_notebook",
 "ruff_python_ast",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "serde",
 "serde_json",
 "shellexpand",
@@ -1987,14 +1964,15 @@ version = "0.0.0"
 dependencies = [
 "anyhow",
 "crossbeam",
+ "foldhash",
 "notify",
 "red_knot_python_semantic",
 "ruff_cache",
 "ruff_db",
 "ruff_python_ast",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "salsa",
+ "thiserror",
 "tracing",
 ]

@@ -2088,7 +2066,7 @@ dependencies = [

 [[package]]
 name = "ruff"
-version = "0.6.2"
+version = "0.6.1"
 dependencies = [
 "anyhow",
 "argfile",
@@ -2101,6 +2079,7 @@ dependencies = [
 "clearscreen",
 "colored",
 "filetime",
+ "foldhash",
 "ignore",
 "insta",
 "insta-cmd",
@@ -2123,7 +2102,6 @@ dependencies = [
 "ruff_source_file",
 "ruff_text_size",
 "ruff_workspace",
- "rustc-hash 2.0.0",
 "serde",
 "serde_json",
 "shellexpand",
@@ -2182,6 +2160,7 @@ dependencies = [
 "countme",
 "dashmap 6.0.1",
 "filetime",
+ "foldhash",
 "ignore",
 "insta",
 "matchit",
@@ -2193,7 +2172,6 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "salsa",
 "tempfile",
 "thiserror",
@@ -2259,10 +2237,10 @@ name = "ruff_formatter"
 version = "0.0.0"
 dependencies = [
 "drop_bomb",
+ "foldhash",
 "ruff_cache",
 "ruff_macros",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "schemars",
 "serde",
 "static_assertions",
@@ -2280,7 +2258,7 @@ dependencies = [

 [[package]]
 name = "ruff_linter"
-version = "0.6.2"
+version = "0.6.1"
 dependencies = [
 "aho-corasick",
 "annotate-snippets 0.9.2",
@@ -2290,6 +2268,7 @@ dependencies = [
 "clap",
 "colored",
 "fern",
+ "foldhash",
 "glob",
 "globset",
 "imperative",
@@ -2322,7 +2301,6 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "schemars",
 "serde",
 "serde_json",
@@ -2377,6 +2355,7 @@ dependencies = [
 "aho-corasick",
 "bitflags 2.6.0",
 "compact_str",
+ "foldhash",
 "is-macro",
 "itertools 0.13.0",
 "once_cell",
@@ -2385,7 +2364,6 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "schemars",
 "serde",
 ]
@@ -2420,6 +2398,7 @@ dependencies = [
 "anyhow",
 "clap",
 "countme",
+ "foldhash",
 "insta",
 "itertools 0.13.0",
 "memchr",
@@ -2433,7 +2412,6 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "schemars",
 "serde",
 "serde_json",
@@ -2474,13 +2452,13 @@ dependencies = [
 "bitflags 2.6.0",
 "bstr",
 "compact_str",
+ "foldhash",
 "insta",
 "memchr",
 "ruff_python_ast",
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "static_assertions",
 "unicode-ident",
 "unicode-normalization",
@@ -2503,6 +2481,7 @@ name = "ruff_python_semantic"
 version = "0.0.0"
 dependencies = [
 "bitflags 2.6.0",
+ "foldhash",
 "is-macro",
 "ruff_cache",
 "ruff_index",
@@ -2512,7 +2491,6 @@ dependencies = [
 "ruff_python_stdlib",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.0.0",
 "schemars",
 "serde",
 ]
@@ -2551,6 +2529,7 @@ version = "0.2.2"
 dependencies = [
 "anyhow",
 "crossbeam",
+ "foldhash",
 "ignore",
 "insta",
 "jod-thread",
@@ -2570,7 +2549,6 @@ dependencies = [
 "ruff_source_file",
 "ruff_text_size",
 "ruff_workspace",
- "rustc-hash 2.0.0",
 "serde",
 "serde_json",
 "shellexpand",
@@ -2600,7 +2578,7 @@ dependencies = [

 [[package]]
 name = "ruff_wasm"
-version = "0.6.2"
+version = "0.6.1"
 dependencies = [
 "console_error_panic_hook",
 "console_log",
@@ -2630,6 +2608,7 @@ dependencies = [
 "anyhow",
 "colored",
 "etcetera",
+ "foldhash",
 "glob",
 "globset",
 "ignore",
@@ -2649,7 +2628,6 @@ dependencies = [
 "ruff_python_formatter",
 "ruff_python_semantic",
 "ruff_source_file",
- "rustc-hash 2.0.0",
 "schemars",
 "serde",
 "shellexpand",
@@ -2740,7 +2718,7 @@ checksum = "e86697c916019a8588c99b5fac3cead74ec0b4b819707a682fd4d23fa0ce1ba1"
 [[package]]
 name = "salsa"
 version = "0.18.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=f608ff8b24f07706492027199f51132244034f29#f608ff8b24f07706492027199f51132244034f29"
+source = "git+https://github.com/MichaReiser/salsa.git?tag=red-knot-0.0.1#ece083e15b79f155f9e4368ec1318cec9a08d88b"
 dependencies = [
 "append-only-vec",
 "arc-swap",
@@ -2760,12 +2738,12 @@ dependencies = [
 [[package]]
 name = "salsa-macro-rules"
 version = "0.1.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=f608ff8b24f07706492027199f51132244034f29#f608ff8b24f07706492027199f51132244034f29"
+source = "git+https://github.com/MichaReiser/salsa.git?tag=red-knot-0.0.1#ece083e15b79f155f9e4368ec1318cec9a08d88b"

 [[package]]
 name = "salsa-macros"
 version = "0.18.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=f608ff8b24f07706492027199f51132244034f29#f608ff8b24f07706492027199f51132244034f29"
+source = "git+https://github.com/MichaReiser/salsa.git?tag=red-knot-0.0.1#ece083e15b79f155f9e4368ec1318cec9a08d88b"
 dependencies = [
 "heck",
 "proc-macro2",
@@ -2827,9 +2805,9 @@ checksum = "1c107b6f4780854c8b126e228ea8869f4d7b71260f962fefb57b996b8959ba6b"

 [[package]]
 name = "serde"
-version = "1.0.208"
+version = "1.0.206"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cff085d2cb684faa248efb494c39b68e522822ac0de72ccf08109abde717cfb2"
+checksum = "5b3e4cd94123dd520a128bcd11e34d9e9e423e7e3e50425cb1b4b1e3549d0284"
 dependencies = [
 "serde_derive",
 ]
@@ -2847,9 +2825,9 @@ dependencies = [

 [[package]]
 name = "serde_derive"
-version = "1.0.208"
+version = "1.0.206"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "24008e81ff7613ed8e5ba0cfaf24e2c2f1e5b8a0495711e44fcd4882fca62bcf"
+checksum = "fabfb6138d2383ea8208cf98ccf69cdfb1aff4088460681d84189aa259762f97"
 dependencies = [
 "proc-macro2",
 "quote",
@@ -2869,9 +2847,9 @@ dependencies = [

 [[package]]
 name = "serde_json"
-version = "1.0.125"
+version = "1.0.124"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "83c8e735a073ccf5be70aa8066aa984eaf2fa000db6c8d0100ae605b366d31ed"
+checksum = "66ad62847a56b3dba58cc891acd13884b9c61138d330c0d7b6181713d4fce38d"
 dependencies = [
 "itoa",
 "memchr",
@@ -3030,9 +3008,9 @@ checksum = "81cdd64d312baedb58e21336b31bc043b77e01cc99033ce76ef539f78e965ebc"

 [[package]]
 name = "syn"
-version = "2.0.75"
+version = "2.0.74"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f6af063034fc1935ede7be0122941bafa9bacb949334d090b77ca98b5817c7d9"
+checksum = "1fceb41e3d546d0bd83421d3409b1460cc7444cd389341a4c880fe7a042cb3d7"
 dependencies = [
 "proc-macro2",
 "quote",
@@ -3554,20 +3532,19 @@ checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"

 [[package]]
 name = "wasm-bindgen"
-version = "0.2.93"
+version = "0.2.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a82edfc16a6c469f5f44dc7b571814045d60404b55a0ee849f9bcfa2e63dd9b5"
+checksum = "4be2531df63900aeb2bca0daaaddec08491ee64ceecbee5076636a3b026795a8"
 dependencies = [
 "cfg-if",
- "once_cell",
 "wasm-bindgen-macro",
 ]

 [[package]]
 name = "wasm-bindgen-backend"
-version = "0.2.93"
+version = "0.2.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9de396da306523044d3302746f1208fa71d7532227f15e347e2d93e4145dd77b"
+checksum = "614d787b966d3989fa7bb98a654e369c762374fd3213d212cfc0251257e747da"
 dependencies = [
 "bumpalo",
 "log",
@@ -3580,9 +3557,9 @@ dependencies = [

 [[package]]
 name = "wasm-bindgen-futures"
-version = "0.4.43"
+version = "0.4.42"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "61e9300f63a621e96ed275155c108eb6f843b6a26d053f122ab69724559dc8ed"
+checksum = "76bc14366121efc8dbb487ab05bcc9d346b3b5ec0eaa76e46594cabbe51762c0"
 dependencies = [
 "cfg-if",
 "js-sys",
@@ -3592,9 +3569,9 @@ dependencies = [

 [[package]]
 name = "wasm-bindgen-macro"
-version = "0.2.93"
+version = "0.2.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "585c4c91a46b072c92e908d99cb1dcdf95c5218eeb6f3bf1efa991ee7a68cccf"
+checksum = "a1f8823de937b71b9460c0c34e25f3da88250760bec0ebac694b49997550d726"
 dependencies = [
 "quote",
 "wasm-bindgen-macro-support",
@@ -3602,9 +3579,9 @@ dependencies = [

 [[package]]
 name = "wasm-bindgen-macro-support"
-version = "0.2.93"
+version = "0.2.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "afc340c74d9005395cf9dd098506f7f44e38f2b4a21c6aaacf9a105ea5e1e836"
+checksum = "e94f17b526d0a461a191c78ea52bbce64071ed5c04c9ffe424dcb38f74171bb7"
 dependencies = [
 "proc-macro2",
 "quote",
@@ -3615,19 +3592,18 @@ dependencies = [

 [[package]]
 name = "wasm-bindgen-shared"
-version = "0.2.93"
+version = "0.2.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c62a0a307cb4a311d3a07867860911ca130c3494e8c2719593806c08bc5d0484"
+checksum = "af190c94f2773fdb3729c55b007a722abb5384da03bc0986df4c289bf5567e96"

 [[package]]
 name = "wasm-bindgen-test"
-version = "0.3.43"
+version = "0.3.42"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "68497a05fb21143a08a7d24fc81763384a3072ee43c44e86aad1744d6adef9d9"
+checksum = "d9bf62a58e0780af3e852044583deee40983e5886da43a271dd772379987667b"
 dependencies = [
 "console_error_panic_hook",
 "js-sys",
- "minicov",
 "scoped-tls",
 "wasm-bindgen",
 "wasm-bindgen-futures",
@@ -3636,9 +3612,9 @@ dependencies = [

 [[package]]
 name = "wasm-bindgen-test-macro"
-version = "0.3.43"
+version = "0.3.42"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4b8220be1fa9e4c889b30fd207d4906657e7e90b12e0e6b0c8b8d8709f5de021"
+checksum = "b7f89739351a2e03cb94beb799d47fb2cac01759b40ec441f7de39b00cbf7ef0"
 dependencies = [
 "proc-macro2",
 "quote",
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -105,10 +105,11 @@ pyproject-toml = { version = "0.9.0" }
 quick-junit = { version = "0.4.0" }
 quote = { version = "1.0.23" }
 rand = { version = "0.8.5" }
+rustc-hash = { version = "2.0.0" }
 rayon = { version = "1.10.0" }
 regex = { version = "1.10.2" }
-rustc-hash = { version = "2.0.0" }
-salsa = { git = "https://github.com/salsa-rs/salsa.git", rev = "f608ff8b24f07706492027199f51132244034f29" }
+foldhash = { version = "0.1.0" }
+salsa = { git = "https://github.com/MichaReiser/salsa.git", tag = "red-knot-0.0.1" }
 schemars = { version = "0.8.16" }
 seahash = { version = "4.1.0" }
 serde = { version = "1.0.197", features = ["derive"] }
--- a/README.md
+++ b/README.md
@@ -136,8 +136,8 @@ curl -LsSf https://astral.sh/ruff/install.sh | sh
 powershell -c "irm https://astral.sh/ruff/install.ps1 | iex"

 # For a specific version.
-curl -LsSf https://astral.sh/ruff/0.6.2/install.sh | sh
-powershell -c "irm https://astral.sh/ruff/0.6.2/install.ps1 | iex"
+curl -LsSf https://astral.sh/ruff/0.6.1/install.sh | sh
+powershell -c "irm https://astral.sh/ruff/0.6.1/install.ps1 | iex"
 ```

 You can also install Ruff via [Homebrew](https://formulae.brew.sh/formula/ruff), [Conda](https://anaconda.org/conda-forge/ruff),
@@ -170,7 +170,7 @@ Ruff can also be used as a [pre-commit](https://pre-commit.com/) hook via [`ruff
 ```yaml
 - repo: https://github.com/astral-sh/ruff-pre-commit
  # Ruff version.
-  rev: v0.6.2
+  rev: v0.6.1
  hooks:
    # Run the linter.
    - id: ruff
--- a/crates/red_knot/src/logging.rs
+++ b/crates/red_knot/src/logging.rs
@@ -5,8 +5,8 @@ use colored::Colorize;
 use std::fmt;
 use std::fs::File;
 use std::io::BufWriter;
+use tracing::log::LevelFilter;
 use tracing::{Event, Subscriber};
-use tracing_subscriber::filter::LevelFilter;
 use tracing_subscriber::fmt::format::Writer;
 use tracing_subscriber::fmt::{FmtContext, FormatEvent, FormatFields};
 use tracing_subscriber::registry::LookupSpan;
@@ -60,10 +60,10 @@ pub(crate) enum VerbosityLevel {
 impl VerbosityLevel {
    const fn level_filter(self) -> LevelFilter {
        match self {
-            VerbosityLevel::Default => LevelFilter::WARN,
-            VerbosityLevel::Verbose => LevelFilter::INFO,
-            VerbosityLevel::ExtraVerbose => LevelFilter::DEBUG,
-            VerbosityLevel::Trace => LevelFilter::TRACE,
+            VerbosityLevel::Default => LevelFilter::Warn,
+            VerbosityLevel::Verbose => LevelFilter::Info,
+            VerbosityLevel::ExtraVerbose => LevelFilter::Debug,
+            VerbosityLevel::Trace => LevelFilter::Trace,
        }
    }

@@ -88,7 +88,7 @@ pub(crate) fn setup_tracing(level: VerbosityLevel) -> anyhow::Result<TracingGuar
        match level {
            VerbosityLevel::Default => {
                // Show warning traces
-                EnvFilter::default().add_directive(LevelFilter::WARN.into())
+                EnvFilter::default().add_directive(tracing::level_filters::LevelFilter::WARN.into())
            }
            level => {
                let level_filter = level.level_filter();
--- a/crates/red_knot/src/main.rs
+++ b/crates/red_knot/src/main.rs
@@ -7,12 +7,12 @@ use colored::Colorize;
 use crossbeam::channel as crossbeam_channel;
 use salsa::plumbing::ZalsaDatabase;

-use red_knot_python_semantic::SitePackages;
+use red_knot_python_semantic::{ProgramSettings, SearchPathSettings};
 use red_knot_server::run_server;
 use red_knot_workspace::db::RootDatabase;
+use red_knot_workspace::site_packages::VirtualEnvironment;
 use red_knot_workspace::watch;
 use red_knot_workspace::watch::WorkspaceWatcher;
-use red_knot_workspace::workspace::settings::Configuration;
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_db::system::{OsSystem, System, SystemPath, SystemPathBuf};
 use target_version::TargetVersion;
@@ -65,14 +65,15 @@ to resolve type information for the project's third-party dependencies.",
        value_name = "PATH",
        help = "Additional path to use as a module-resolution source (can be passed multiple times)"
    )]
-    extra_search_path: Option<Vec<SystemPathBuf>>,
+    extra_search_path: Vec<SystemPathBuf>,

    #[arg(
        long,
        help = "Python version to assume when resolving types",
-        value_name = "VERSION"
-    )]
-    target_version: Option<TargetVersion>,
+        default_value_t = TargetVersion::default(),
+        value_name="VERSION")
+    ]
+    target_version: TargetVersion,

    #[clap(flatten)]
    verbosity: Verbosity,
@@ -85,36 +86,6 @@ to resolve type information for the project's third-party dependencies.",
    watch: bool,
 }

-impl Args {
-    fn to_configuration(&self, cli_cwd: &SystemPath) -> Configuration {
-        let mut configuration = Configuration::default();
-
-        if let Some(target_version) = self.target_version {
-            configuration.target_version = Some(target_version.into());
-        }
-
-        if let Some(venv_path) = &self.venv_path {
-            configuration.search_paths.site_packages = Some(SitePackages::Derived {
-                venv_path: SystemPath::absolute(venv_path, cli_cwd),
-            });
-        }
-
-        if let Some(custom_typeshed_dir) = &self.custom_typeshed_dir {
-            configuration.search_paths.custom_typeshed =
-                Some(SystemPath::absolute(custom_typeshed_dir, cli_cwd));
-        }
-
-        if let Some(extra_search_paths) = &self.extra_search_path {
-            configuration.search_paths.extra_paths = extra_search_paths
-                .iter()
-                .map(|path| Some(SystemPath::absolute(path, cli_cwd)))
-                .collect();
-        }
-
-        configuration
-    }
-}
-
 #[derive(Debug, clap::Subcommand)]
 pub enum Command {
    /// Start the language server
@@ -144,13 +115,22 @@ pub fn main() -> ExitStatus {
 }

 fn run() -> anyhow::Result<ExitStatus> {
-    let args = Args::parse_from(std::env::args().collect::<Vec<_>>());
+    let Args {
+        command,
+        current_directory,
+        custom_typeshed_dir,
+        extra_search_path: extra_paths,
+        venv_path,
+        target_version,
+        verbosity,
+        watch,
+    } = Args::parse_from(std::env::args().collect::<Vec<_>>());

-    if matches!(args.command, Some(Command::Server)) {
+    if matches!(command, Some(Command::Server)) {
        return run_server().map(|()| ExitStatus::Success);
    }

-    let verbosity = args.verbosity.level();
+    let verbosity = verbosity.level();
    countme::enable(verbosity.is_trace());
    let _guard = setup_tracing(verbosity)?;

@@ -166,12 +146,10 @@ fn run() -> anyhow::Result<ExitStatus> {
            })?
    };

-    let cwd = args
-        .current_directory
-        .as_ref()
+    let cwd = current_directory
        .map(|cwd| {
            if cwd.as_std_path().is_dir() {
-                Ok(SystemPath::absolute(cwd, &cli_base_path))
+                Ok(SystemPath::absolute(&cwd, &cli_base_path))
            } else {
                Err(anyhow!(
                    "Provided current-directory path '{cwd}' is not a directory."
@@ -182,18 +160,33 @@ fn run() -> anyhow::Result<ExitStatus> {
        .unwrap_or_else(|| cli_base_path.clone());

    let system = OsSystem::new(cwd.clone());
-    let cli_configuration = args.to_configuration(&cwd);
-    let workspace_metadata = WorkspaceMetadata::from_path(
-        system.current_directory(),
-        &system,
-        Some(cli_configuration.clone()),
-    )?;
+    let workspace_metadata = WorkspaceMetadata::from_path(system.current_directory(), &system)?;
+
+    // TODO: Verify the remaining search path settings eagerly.
+    let site_packages = venv_path
+        .map(|path| {
+            VirtualEnvironment::new(path, &OsSystem::new(cli_base_path))
+                .and_then(|venv| venv.site_packages_directories(&system))
+        })
+        .transpose()?
+        .unwrap_or_default();
+
+    // TODO: Respect the settings from the workspace metadata. when resolving the program settings.
+    let program_settings = ProgramSettings {
+        target_version: target_version.into(),
+        search_paths: SearchPathSettings {
+            extra_paths,
+            src_root: workspace_metadata.root().to_path_buf(),
+            custom_typeshed: custom_typeshed_dir,
+            site_packages,
+        },
+    };

    // TODO: Use the `program_settings` to compute the key for the database's persistent
    //   cache and load the cache if it exists.
-    let mut db = RootDatabase::new(workspace_metadata, system)?;
+    let mut db = RootDatabase::new(workspace_metadata, program_settings, system)?;

-    let (main_loop, main_loop_cancellation_token) = MainLoop::new(cli_configuration);
+    let (main_loop, main_loop_cancellation_token) = MainLoop::new();

    // Listen to Ctrl+C and abort the watch mode.
    let main_loop_cancellation_token = Mutex::new(Some(main_loop_cancellation_token));
@@ -205,7 +198,7 @@ fn run() -> anyhow::Result<ExitStatus> {
        }
    })?;

-    let exit_status = if args.watch {
+    let exit_status = if watch {
        main_loop.watch(&mut db)?
    } else {
        main_loop.run(&mut db)
@@ -245,12 +238,10 @@ struct MainLoop {

    /// The file system watcher, if running in watch mode.
    watcher: Option<WorkspaceWatcher>,
-
-    cli_configuration: Configuration,
 }

 impl MainLoop {
-    fn new(cli_configuration: Configuration) -> (Self, MainLoopCancellationToken) {
+    fn new() -> (Self, MainLoopCancellationToken) {
        let (sender, receiver) = crossbeam_channel::bounded(10);

        (
@@ -258,7 +249,6 @@ impl MainLoop {
                sender: sender.clone(),
                receiver,
                watcher: None,
-                cli_configuration,
            },
            MainLoopCancellationToken { sender },
        )
@@ -341,7 +331,7 @@ impl MainLoop {
                MainLoopMessage::ApplyChanges(changes) => {
                    revision += 1;
                    // Automatically cancels any pending queries and waits for them to complete.
-                    db.apply_changes(changes, Some(&self.cli_configuration));
+                    db.apply_changes(changes);
                    if let Some(watcher) = self.watcher.as_mut() {
                        watcher.update(db);
                    }
--- a/crates/red_knot/tests/file_watching.rs
+++ b/crates/red_knot/tests/file_watching.rs
@@ -5,11 +5,12 @@ use std::time::Duration;

 use anyhow::{anyhow, Context};

-use red_knot_python_semantic::{resolve_module, ModuleName, Program, PythonVersion, SitePackages};
+use red_knot_python_semantic::{
+    resolve_module, ModuleName, Program, ProgramSettings, PythonVersion, SearchPathSettings,
+};
 use red_knot_workspace::db::RootDatabase;
 use red_knot_workspace::watch;
 use red_knot_workspace::watch::{directory_watcher, WorkspaceWatcher};
-use red_knot_workspace::workspace::settings::{Configuration, SearchPathConfiguration};
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_db::files::{system_path_to_file, File, FileError};
 use ruff_db::source::source_text;
@@ -24,7 +25,7 @@ struct TestCase {
    /// We need to hold on to it in the test case or the temp files get deleted.
    _temp_dir: tempfile::TempDir,
    root_dir: SystemPathBuf,
-    configuration: Configuration,
+    search_path_settings: SearchPathSettings,
 }

 impl TestCase {
@@ -40,6 +41,10 @@ impl TestCase {
        &self.db
    }

+    fn db_mut(&mut self) -> &mut RootDatabase {
+        &mut self.db
+    }
+
    fn stop_watch(&mut self) -> Vec<watch::ChangeEvent> {
        self.try_stop_watch(Duration::from_secs(10))
            .expect("Expected watch changes but observed none.")
@@ -100,20 +105,16 @@ impl TestCase {
        Some(all_events)
    }

-    fn apply_changes(&mut self, changes: Vec<watch::ChangeEvent>) {
-        self.db.apply_changes(changes, Some(&self.configuration));
-    }
-
    fn update_search_path_settings(
        &mut self,
-        configuration: SearchPathConfiguration,
+        f: impl FnOnce(&SearchPathSettings) -> SearchPathSettings,
    ) -> anyhow::Result<()> {
        let program = Program::get(self.db());

-        self.configuration.search_paths = configuration.clone();
-        let new_settings = configuration.into_settings(self.db.workspace().root(&self.db));
+        let new_settings = f(&self.search_path_settings);

-        program.update_search_paths(&mut self.db, &new_settings)?;
+        program.update_search_paths(&mut self.db, new_settings.clone())?;
+        self.search_path_settings = new_settings;

        if let Some(watcher) = &mut self.watcher {
            watcher.update(&self.db);
@@ -126,6 +127,7 @@ impl TestCase {
    fn collect_package_files(&self, path: &SystemPath) -> Vec<File> {
        let package = self.db().workspace().package(self.db(), path).unwrap();
        let files = package.files(self.db());
+        let files = files.read();
        let mut collected: Vec<_> = files.into_iter().collect();
        collected.sort_unstable_by_key(|file| file.path(self.db()).as_system_path().unwrap());
        collected
@@ -178,14 +180,17 @@ fn setup<F>(setup_files: F) -> anyhow::Result<TestCase>
 where
    F: SetupFiles,
 {
-    setup_with_search_paths(setup_files, |_root, _workspace_path| {
-        SearchPathConfiguration::default()
+    setup_with_search_paths(setup_files, |_root, workspace_path| SearchPathSettings {
+        extra_paths: vec![],
+        src_root: workspace_path.to_path_buf(),
+        custom_typeshed: None,
+        site_packages: vec![],
    })
 }

 fn setup_with_search_paths<F>(
    setup_files: F,
-    create_search_paths: impl FnOnce(&SystemPath, &SystemPath) -> SearchPathConfiguration,
+    create_search_paths: impl FnOnce(&SystemPath, &SystemPath) -> SearchPathSettings,
 ) -> anyhow::Result<TestCase>
 where
    F: SetupFiles,
@@ -217,34 +222,25 @@ where

    let system = OsSystem::new(&workspace_path);

-    let search_paths = create_search_paths(&root_path, &workspace_path);
+    let workspace = WorkspaceMetadata::from_path(&workspace_path, &system)?;
+    let search_path_settings = create_search_paths(&root_path, workspace.root());

-    for path in search_paths
+    for path in search_path_settings
        .extra_paths
        .iter()
-        .flatten()
-        .chain(search_paths.custom_typeshed.iter())
-        .chain(search_paths.site_packages.iter().flat_map(|site_packages| {
-            if let SitePackages::Known(path) = site_packages {
-                path.as_slice()
-            } else {
-                &[]
-            }
-        }))
+        .chain(search_path_settings.site_packages.iter())
+        .chain(search_path_settings.custom_typeshed.iter())
    {
        std::fs::create_dir_all(path.as_std_path())
            .with_context(|| format!("Failed to create search path '{path}'"))?;
    }

-    let configuration = Configuration {
-        target_version: Some(PythonVersion::PY312),
-        search_paths,
+    let settings = ProgramSettings {
+        target_version: PythonVersion::default(),
+        search_paths: search_path_settings.clone(),
    };

-    let workspace =
-        WorkspaceMetadata::from_path(&workspace_path, &system, Some(configuration.clone()))?;
-
-    let db = RootDatabase::new(workspace, system)?;
+    let db = RootDatabase::new(workspace, settings, system)?;

    let (sender, receiver) = crossbeam::channel::unbounded();
    let watcher = directory_watcher(move |events| sender.send(events).unwrap())
@@ -259,7 +255,7 @@ where
        watcher: Some(watcher),
        _temp_dir: temp_dir,
        root_dir: root_path,
-        configuration,
+        search_path_settings,
    };

    // Sometimes the file watcher reports changes for events that happened before the watcher was started.
@@ -312,7 +308,7 @@ fn new_file() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    let foo = case.system_file(&foo_path).expect("foo.py to exist.");

@@ -335,7 +331,7 @@ fn new_ignored_file() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert!(case.system_file(&foo_path).is_ok());
    assert_eq!(&case.collect_package_files(&bar_path), &[bar_file]);
@@ -359,7 +355,7 @@ fn changed_file() -> anyhow::Result<()> {

    assert!(!changes.is_empty());

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert_eq!(source_text(case.db(), foo).as_str(), "print('Version 2')");
    assert_eq!(&case.collect_package_files(&foo_path), &[foo]);
@@ -382,7 +378,7 @@ fn deleted_file() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert!(!foo.exists(case.db()));
    assert_eq!(&case.collect_package_files(&foo_path), &[] as &[File]);
@@ -414,7 +410,7 @@ fn move_file_to_trash() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert!(!foo.exists(case.db()));
    assert_eq!(&case.collect_package_files(&foo_path), &[] as &[File]);
@@ -446,7 +442,7 @@ fn move_file_to_workspace() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    let foo_in_workspace = case.system_file(&foo_in_workspace_path)?;

@@ -474,7 +470,7 @@ fn rename_file() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert!(!foo.exists(case.db()));

@@ -515,7 +511,7 @@ fn directory_moved_to_workspace() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    let init_file = case
        .system_file(sub_new_path.join("__init__.py"))
@@ -566,7 +562,7 @@ fn directory_moved_to_trash() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    // `import sub.a` should no longer resolve
    assert!(resolve_module(case.db().upcast(), ModuleName::new_static("sub.a").unwrap()).is_none());
@@ -620,7 +616,7 @@ fn directory_renamed() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    // `import sub.a` should no longer resolve
    assert!(resolve_module(case.db().upcast(), ModuleName::new_static("sub.a").unwrap()).is_none());
@@ -685,7 +681,7 @@ fn directory_deleted() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    // `import sub.a` should no longer resolve
    assert!(resolve_module(case.db().upcast(), ModuleName::new_static("sub.a").unwrap()).is_none());
@@ -699,13 +695,15 @@ fn directory_deleted() -> anyhow::Result<()> {

 #[test]
 fn search_path() -> anyhow::Result<()> {
-    let mut case = setup_with_search_paths(
-        [("bar.py", "import sub.a")],
-        |root_path, _workspace_path| SearchPathConfiguration {
-            site_packages: Some(SitePackages::Known(vec![root_path.join("site_packages")])),
-            ..SearchPathConfiguration::default()
-        },
-    )?;
+    let mut case =
+        setup_with_search_paths([("bar.py", "import sub.a")], |root_path, workspace_path| {
+            SearchPathSettings {
+                extra_paths: vec![],
+                src_root: workspace_path.to_path_buf(),
+                custom_typeshed: None,
+                site_packages: vec![root_path.join("site_packages")],
+            }
+        })?;

    let site_packages = case.root_path().join("site_packages");

@@ -718,7 +716,7 @@ fn search_path() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert!(resolve_module(case.db().upcast(), ModuleName::new_static("a").unwrap()).is_some());
    assert_eq!(
@@ -739,9 +737,9 @@ fn add_search_path() -> anyhow::Result<()> {
    assert!(resolve_module(case.db().upcast(), ModuleName::new_static("a").unwrap()).is_none());

    // Register site-packages as a search path.
-    case.update_search_path_settings(SearchPathConfiguration {
-        site_packages: Some(SitePackages::Known(vec![site_packages.clone()])),
-        ..SearchPathConfiguration::default()
+    case.update_search_path_settings(|settings| SearchPathSettings {
+        site_packages: vec![site_packages.clone()],
+        ..settings.clone()
    })
    .expect("Search path settings to be valid");

@@ -749,7 +747,7 @@ fn add_search_path() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert!(resolve_module(case.db().upcast(), ModuleName::new_static("a").unwrap()).is_some());

@@ -758,19 +756,21 @@ fn add_search_path() -> anyhow::Result<()> {

 #[test]
 fn remove_search_path() -> anyhow::Result<()> {
-    let mut case = setup_with_search_paths(
-        [("bar.py", "import sub.a")],
-        |root_path, _workspace_path| SearchPathConfiguration {
-            site_packages: Some(SitePackages::Known(vec![root_path.join("site_packages")])),
-            ..SearchPathConfiguration::default()
-        },
-    )?;
+    let mut case =
+        setup_with_search_paths([("bar.py", "import sub.a")], |root_path, workspace_path| {
+            SearchPathSettings {
+                extra_paths: vec![],
+                src_root: workspace_path.to_path_buf(),
+                custom_typeshed: None,
+                site_packages: vec![root_path.join("site_packages")],
+            }
+        })?;

    // Remove site packages from the search path settings.
    let site_packages = case.root_path().join("site_packages");
-    case.update_search_path_settings(SearchPathConfiguration {
-        site_packages: None,
-        ..SearchPathConfiguration::default()
+    case.update_search_path_settings(|settings| SearchPathSettings {
+        site_packages: vec![],
+        ..settings.clone()
    })
    .expect("Search path settings to be valid");

@@ -783,48 +783,6 @@ fn remove_search_path() -> anyhow::Result<()> {
    Ok(())
 }

-#[test]
-fn changed_versions_file() -> anyhow::Result<()> {
-    let mut case = setup_with_search_paths(
-        |root_path: &SystemPath, workspace_path: &SystemPath| {
-            std::fs::write(workspace_path.join("bar.py").as_std_path(), "import sub.a")?;
-            std::fs::create_dir_all(root_path.join("typeshed/stdlib").as_std_path())?;
-            std::fs::write(root_path.join("typeshed/stdlib/VERSIONS").as_std_path(), "")?;
-            std::fs::write(
-                root_path.join("typeshed/stdlib/os.pyi").as_std_path(),
-                "# not important",
-            )?;
-
-            Ok(())
-        },
-        |root_path, _workspace_path| SearchPathConfiguration {
-            custom_typeshed: Some(root_path.join("typeshed")),
-            ..SearchPathConfiguration::default()
-        },
-    )?;
-
-    // Unset the custom typeshed directory.
-    assert_eq!(
-        resolve_module(case.db(), ModuleName::new("os").unwrap()),
-        None
-    );
-
-    std::fs::write(
-        case.root_path()
-            .join("typeshed/stdlib/VERSIONS")
-            .as_std_path(),
-        "os: 3.0-",
-    )?;
-
-    let changes = case.stop_watch();
-
-    case.apply_changes(changes);
-
-    assert!(resolve_module(case.db(), ModuleName::new("os").unwrap()).is_some());
-
-    Ok(())
-}
-
 /// Watch a workspace that contains two files where one file is a hardlink to another.
 ///
 /// Setup:
@@ -871,7 +829,7 @@ fn hard_links_in_workspace() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert_eq!(source_text(case.db(), foo).as_str(), "print('Version 2')");

@@ -942,7 +900,7 @@ fn hard_links_to_target_outside_workspace() -> anyhow::Result<()> {

    let changes = case.stop_watch();

-    case.apply_changes(changes);
+    case.db_mut().apply_changes(changes);

    assert_eq!(source_text(case.db(), bar).as_str(), "print('Version 2')");

@@ -981,7 +939,7 @@ mod unix {

        let changes = case.stop_watch();

-        case.apply_changes(changes);
+        case.db_mut().apply_changes(changes);

        assert_eq!(
            foo.permissions(case.db()),
@@ -1066,7 +1024,7 @@ mod unix {

        let changes = case.take_watch_changes();

-        case.apply_changes(changes);
+        case.db_mut().apply_changes(changes);

        assert_eq!(
            source_text(case.db(), baz.file()).as_str(),
@@ -1079,7 +1037,7 @@ mod unix {

        let changes = case.stop_watch();

-        case.apply_changes(changes);
+        case.db_mut().apply_changes(changes);

        assert_eq!(
            source_text(case.db(), baz.file()).as_str(),
@@ -1150,7 +1108,7 @@ mod unix {

        let changes = case.stop_watch();

-        case.apply_changes(changes);
+        case.db_mut().apply_changes(changes);

        // The file watcher is guaranteed to emit one event for the changed file, but it isn't specified
        // if the event is emitted for the "original" or linked path because both paths are watched.
@@ -1219,11 +1177,11 @@ mod unix {

                Ok(())
            },
-            |_root, workspace| SearchPathConfiguration {
-                site_packages: Some(SitePackages::Known(vec![
-                    workspace.join(".venv/lib/python3.12/site-packages")
-                ])),
-                ..SearchPathConfiguration::default()
+            |_root, workspace| SearchPathSettings {
+                extra_paths: vec![],
+                src_root: workspace.to_path_buf(),
+                custom_typeshed: None,
+                site_packages: vec![workspace.join(".venv/lib/python3.12/site-packages")],
            },
        )?;

@@ -1258,7 +1216,7 @@ mod unix {

        let changes = case.stop_watch();

-        case.apply_changes(changes);
+        case.db_mut().apply_changes(changes);

        assert_eq!(
            source_text(case.db(), baz_original_file).as_str(),
--- a/crates/red_knot_python_semantic/Cargo.toml
+++ b/crates/red_knot_python_semantic/Cargo.toml
@@ -26,12 +26,9 @@ countme = { workspace = true }
 once_cell = { workspace = true }
 ordermap = { workspace = true }
 salsa = { workspace = true }
-thiserror = { workspace = true }
 tracing = { workspace = true }
 rustc-hash = { workspace = true }
 hashbrown = { workspace = true }
-smallvec = { workspace = true }
-static_assertions = { workspace = true }

 [build-dependencies]
 path-slash = { workspace = true }
--- a/crates/red_knot_python_semantic/src/db.rs
+++ b/crates/red_knot_python_semantic/src/db.rs
@@ -1,18 +1,15 @@
-use ruff_db::files::File;
 use ruff_db::{Db as SourceDb, Upcast};

 /// Database giving access to semantic information about a Python program.
 #[salsa::db]
-pub trait Db: SourceDb + Upcast<dyn SourceDb> {
-    fn is_file_open(&self, file: File) -> bool;
-}
+pub trait Db: SourceDb + Upcast<dyn SourceDb> {}

 #[cfg(test)]
 pub(crate) mod tests {
    use std::sync::Arc;

    use crate::module_resolver::vendored_typeshed_stubs;
-    use ruff_db::files::{File, Files};
+    use ruff_db::files::Files;
    use ruff_db::system::{DbWithTestSystem, System, TestSystem};
    use ruff_db::vendored::VendoredFileSystem;
    use ruff_db::{Db as SourceDb, Upcast};
@@ -94,11 +91,7 @@ pub(crate) mod tests {
    }

    #[salsa::db]
-    impl Db for TestDb {
-        fn is_file_open(&self, file: File) -> bool {
-            !file.path(self).is_vendored_path()
-        }
-    }
+    impl Db for TestDb {}

    #[salsa::db]
    impl salsa::Database for TestDb {
--- a/crates/red_knot_python_semantic/src/lib.rs
+++ b/crates/red_knot_python_semantic/src/lib.rs
@@ -5,7 +5,7 @@ use rustc_hash::FxHasher;
 pub use db::Db;
 pub use module_name::ModuleName;
 pub use module_resolver::{resolve_module, system_module_search_paths, vendored_typeshed_stubs};
-pub use program::{Program, ProgramSettings, SearchPathSettings, SitePackages};
+pub use program::{Program, ProgramSettings, SearchPathSettings};
 pub use python_version::PythonVersion;
 pub use semantic_model::{HasTy, SemanticModel};

@@ -19,7 +19,6 @@ mod program;
 mod python_version;
 pub mod semantic_index;
 mod semantic_model;
-pub(crate) mod site_packages;
 pub mod types;

 type FxOrderSet<V> = ordermap::set::OrderSet<V, BuildHasherDefault<FxHasher>>;
--- a/crates/red_knot_python_semantic/src/module_resolver/mod.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/mod.rs
@@ -13,6 +13,7 @@ use resolver::SearchPathIterator;
 mod module;
 mod path;
 mod resolver;
+mod state;
 mod typeshed;

 #[cfg(test)]
--- a/crates/red_knot_python_semantic/src/module_resolver/path.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/path.rs
@@ -9,11 +9,11 @@ use ruff_db::files::{system_path_to_file, vendored_path_to_file, File, FileError
 use ruff_db::system::{System, SystemPath, SystemPathBuf};
 use ruff_db::vendored::{VendoredPath, VendoredPathBuf};

-use super::typeshed::{typeshed_versions, TypeshedVersionsParseError, TypeshedVersionsQueryResult};
 use crate::db::Db;
 use crate::module_name::ModuleName;
-use crate::module_resolver::resolver::ResolverContext;
-use crate::site_packages::SitePackagesDiscoveryError;
+
+use super::state::ResolverState;
+use super::typeshed::{TypeshedVersionsParseError, TypeshedVersionsQueryResult};

 /// A path that points to a Python module.
 ///
@@ -60,7 +60,7 @@ impl ModulePath {
    }

    #[must_use]
-    pub(super) fn is_directory(&self, resolver: &ResolverContext) -> bool {
+    pub(crate) fn is_directory(&self, resolver: &ResolverState) -> bool {
        let ModulePath {
            search_path,
            relative_path,
@@ -74,7 +74,7 @@ impl ModulePath {
                    == Err(FileError::IsADirectory)
            }
            SearchPathInner::StandardLibraryCustom(stdlib_root) => {
-                match query_stdlib_version(relative_path, resolver) {
+                match query_stdlib_version(Some(stdlib_root), relative_path, resolver) {
                    TypeshedVersionsQueryResult::DoesNotExist => false,
                    TypeshedVersionsQueryResult::Exists
                    | TypeshedVersionsQueryResult::MaybeExists => {
@@ -84,7 +84,7 @@ impl ModulePath {
                }
            }
            SearchPathInner::StandardLibraryVendored(stdlib_root) => {
-                match query_stdlib_version(relative_path, resolver) {
+                match query_stdlib_version(None, relative_path, resolver) {
                    TypeshedVersionsQueryResult::DoesNotExist => false,
                    TypeshedVersionsQueryResult::Exists
                    | TypeshedVersionsQueryResult::MaybeExists => resolver
@@ -96,7 +96,7 @@ impl ModulePath {
    }

    #[must_use]
-    pub(super) fn is_regular_package(&self, resolver: &ResolverContext) -> bool {
+    pub(crate) fn is_regular_package(&self, resolver: &ResolverState) -> bool {
        let ModulePath {
            search_path,
            relative_path,
@@ -113,7 +113,7 @@ impl ModulePath {
                        .is_ok()
            }
            SearchPathInner::StandardLibraryCustom(search_path) => {
-                match query_stdlib_version(relative_path, resolver) {
+                match query_stdlib_version(Some(search_path), relative_path, resolver) {
                    TypeshedVersionsQueryResult::DoesNotExist => false,
                    TypeshedVersionsQueryResult::Exists
                    | TypeshedVersionsQueryResult::MaybeExists => system_path_to_file(
@@ -124,7 +124,7 @@ impl ModulePath {
                }
            }
            SearchPathInner::StandardLibraryVendored(search_path) => {
-                match query_stdlib_version(relative_path, resolver) {
+                match query_stdlib_version(None, relative_path, resolver) {
                    TypeshedVersionsQueryResult::DoesNotExist => false,
                    TypeshedVersionsQueryResult::Exists
                    | TypeshedVersionsQueryResult::MaybeExists => resolver
@@ -136,7 +136,7 @@ impl ModulePath {
    }

    #[must_use]
-    pub(super) fn to_file(&self, resolver: &ResolverContext) -> Option<File> {
+    pub(crate) fn to_file(&self, resolver: &ResolverState) -> Option<File> {
        let db = resolver.db.upcast();
        let ModulePath {
            search_path,
@@ -150,7 +150,7 @@ impl ModulePath {
                system_path_to_file(db, search_path.join(relative_path)).ok()
            }
            SearchPathInner::StandardLibraryCustom(stdlib_root) => {
-                match query_stdlib_version(relative_path, resolver) {
+                match query_stdlib_version(Some(stdlib_root), relative_path, resolver) {
                    TypeshedVersionsQueryResult::DoesNotExist => None,
                    TypeshedVersionsQueryResult::Exists
                    | TypeshedVersionsQueryResult::MaybeExists => {
@@ -159,7 +159,7 @@ impl ModulePath {
                }
            }
            SearchPathInner::StandardLibraryVendored(stdlib_root) => {
-                match query_stdlib_version(relative_path, resolver) {
+                match query_stdlib_version(None, relative_path, resolver) {
                    TypeshedVersionsQueryResult::DoesNotExist => None,
                    TypeshedVersionsQueryResult::Exists
                    | TypeshedVersionsQueryResult::MaybeExists => {
@@ -273,15 +273,19 @@ fn stdlib_path_to_module_name(relative_path: &Utf8Path) -> Option<ModuleName> {

 #[must_use]
 fn query_stdlib_version(
+    custom_stdlib_root: Option<&SystemPath>,
    relative_path: &Utf8Path,
-    context: &ResolverContext,
+    resolver: &ResolverState,
 ) -> TypeshedVersionsQueryResult {
    let Some(module_name) = stdlib_path_to_module_name(relative_path) else {
        return TypeshedVersionsQueryResult::DoesNotExist;
    };
-    let ResolverContext { db, target_version } = context;
-
-    typeshed_versions(*db).query_module(&module_name, *target_version)
+    let ResolverState {
+        db,
+        typeshed_versions,
+        target_version,
+    } = resolver;
+    typeshed_versions.query_module(*db, &module_name, custom_stdlib_root, *target_version)
 }

 /// Enumeration describing the various ways in which validation of a search path might fail.
@@ -289,7 +293,7 @@ fn query_stdlib_version(
 /// If validation fails for a search path derived from the user settings,
 /// a message must be displayed to the user,
 /// as type checking cannot be done reliably in these circumstances.
-#[derive(Debug)]
+#[derive(Debug, PartialEq, Eq)]
 pub(crate) enum SearchPathValidationError {
    /// The path provided by the user was not a directory
    NotADirectory(SystemPathBuf),
@@ -300,20 +304,18 @@ pub(crate) enum SearchPathValidationError {
    NoStdlibSubdirectory(SystemPathBuf),

    /// The typeshed path provided by the user is a directory,
-    /// but `stdlib/VERSIONS` could not be read.
+    /// but no `stdlib/VERSIONS` file exists.
    /// (This is only relevant for stdlib search paths.)
-    FailedToReadVersionsFile {
-        path: SystemPathBuf,
-        error: std::io::Error,
-    },
+    NoVersionsFile(SystemPathBuf),
+
+    /// `stdlib/VERSIONS` is a directory.
+    /// (This is only relevant for stdlib search paths.)
+    VersionsIsADirectory(SystemPathBuf),

    /// The path provided by the user is a directory,
    /// and a `stdlib/VERSIONS` file exists, but it fails to parse.
    /// (This is only relevant for stdlib search paths.)
    VersionsParseError(TypeshedVersionsParseError),
-
-    /// Failed to discover the site-packages for the configured virtual environment.
-    SitePackagesDiscovery(SitePackagesDiscoveryError),
 }

 impl fmt::Display for SearchPathValidationError {
@@ -323,16 +325,9 @@ impl fmt::Display for SearchPathValidationError {
            Self::NoStdlibSubdirectory(path) => {
                write!(f, "The directory at {path} has no `stdlib/` subdirectory")
            }
-            Self::FailedToReadVersionsFile { path, error } => {
-                write!(
-                    f,
-                    "Failed to read the custom typeshed versions file '{path}': {error}"
-                )
-            }
+            Self::NoVersionsFile(path) => write!(f, "Expected a file at {path}/stdlib/VERSIONS"),
+            Self::VersionsIsADirectory(path) => write!(f, "{path}/stdlib/VERSIONS is a directory."),
            Self::VersionsParseError(underlying_error) => underlying_error.fmt(f),
-            SearchPathValidationError::SitePackagesDiscovery(error) => {
-                write!(f, "Failed to discover the site-packages directory: {error}")
-            }
        }
    }
 }
@@ -347,18 +342,6 @@ impl std::error::Error for SearchPathValidationError {
    }
 }

-impl From<TypeshedVersionsParseError> for SearchPathValidationError {
-    fn from(value: TypeshedVersionsParseError) -> Self {
-        Self::VersionsParseError(value)
-    }
-}
-
-impl From<SitePackagesDiscoveryError> for SearchPathValidationError {
-    fn from(value: SitePackagesDiscoveryError) -> Self {
-        Self::SitePackagesDiscovery(value)
-    }
-}
-
 type SearchPathResult<T> = Result<T, SearchPathValidationError>;

 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
@@ -401,10 +384,11 @@ pub(crate) struct SearchPath(Arc<SearchPathInner>);

 impl SearchPath {
    fn directory_path(system: &dyn System, root: SystemPathBuf) -> SearchPathResult<SystemPathBuf> {
-        if system.is_directory(&root) {
-            Ok(root)
+        let canonicalized = system.canonicalize_path(&root).unwrap_or(root);
+        if system.is_directory(&canonicalized) {
+            Ok(canonicalized)
        } else {
-            Err(SearchPathValidationError::NotADirectory(root))
+            Err(SearchPathValidationError::NotADirectory(canonicalized))
        }
    }

@@ -423,22 +407,32 @@ impl SearchPath {
    }

    /// Create a new standard-library search path pointing to a custom directory on disk
-    pub(crate) fn custom_stdlib(db: &dyn Db, typeshed: &SystemPath) -> SearchPathResult<Self> {
+    pub(crate) fn custom_stdlib(db: &dyn Db, typeshed: SystemPathBuf) -> SearchPathResult<Self> {
        let system = db.system();
-        if !system.is_directory(typeshed) {
+        if !system.is_directory(&typeshed) {
            return Err(SearchPathValidationError::NotADirectory(
                typeshed.to_path_buf(),
            ));
        }
-
        let stdlib =
            Self::directory_path(system, typeshed.join("stdlib")).map_err(|err| match err {
-                SearchPathValidationError::NotADirectory(_) => {
-                    SearchPathValidationError::NoStdlibSubdirectory(typeshed.to_path_buf())
+                SearchPathValidationError::NotADirectory(path) => {
+                    SearchPathValidationError::NoStdlibSubdirectory(path)
                }
                err => err,
            })?;
-
+        let typeshed_versions =
+            system_path_to_file(db.upcast(), stdlib.join("VERSIONS")).map_err(|err| match err {
+                FileError::NotFound => SearchPathValidationError::NoVersionsFile(typeshed),
+                FileError::IsADirectory => {
+                    SearchPathValidationError::VersionsIsADirectory(typeshed)
+                }
+            })?;
+        super::typeshed::parse_typeshed_versions(db, typeshed_versions)
+            .as_ref()
+            .map_err(|validation_error| {
+                SearchPathValidationError::VersionsParseError(validation_error.clone())
+            })?;
        Ok(Self(Arc::new(SearchPathInner::StandardLibraryCustom(
            stdlib,
        ))))
@@ -629,10 +623,10 @@ mod tests {
    use ruff_db::Db;

    use crate::db::tests::TestDb;
-    use crate::module_resolver::testing::{FileSpec, MockedTypeshed, TestCase, TestCaseBuilder};
-    use crate::python_version::PythonVersion;

    use super::*;
+    use crate::module_resolver::testing::{FileSpec, MockedTypeshed, TestCase, TestCaseBuilder};
+    use crate::python_version::PythonVersion;

    impl ModulePath {
        #[must_use]
@@ -644,6 +638,15 @@ mod tests {
    }

    impl SearchPath {
+        #[must_use]
+        pub(crate) fn is_stdlib_search_path(&self) -> bool {
+            matches!(
+                &*self.0,
+                SearchPathInner::StandardLibraryCustom(_)
+                    | SearchPathInner::StandardLibraryVendored(_)
+            )
+        }
+
        fn join(&self, component: &str) -> ModulePath {
            self.to_module_path().join(component)
        }
@@ -658,7 +661,7 @@ mod tests {
            .build();

        assert_eq!(
-            SearchPath::custom_stdlib(&db, stdlib.parent().unwrap())
+            SearchPath::custom_stdlib(&db, stdlib.parent().unwrap().to_path_buf())
                .unwrap()
                .to_module_path()
                .with_py_extension(),
@@ -666,7 +669,7 @@ mod tests {
        );

        assert_eq!(
-            &SearchPath::custom_stdlib(&db, stdlib.parent().unwrap())
+            &SearchPath::custom_stdlib(&db, stdlib.parent().unwrap().to_path_buf())
                .unwrap()
                .join("foo")
                .with_pyi_extension(),
@@ -777,7 +780,7 @@ mod tests {
        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_custom_typeshed(MockedTypeshed::default())
            .build();
-        SearchPath::custom_stdlib(&db, stdlib.parent().unwrap())
+        SearchPath::custom_stdlib(&db, stdlib.parent().unwrap().to_path_buf())
            .unwrap()
            .to_module_path()
            .push("bar.py");
@@ -789,7 +792,7 @@ mod tests {
        let TestCase { db, stdlib, .. } = TestCaseBuilder::new()
            .with_custom_typeshed(MockedTypeshed::default())
            .build();
-        SearchPath::custom_stdlib(&db, stdlib.parent().unwrap())
+        SearchPath::custom_stdlib(&db, stdlib.parent().unwrap().to_path_buf())
            .unwrap()
            .to_module_path()
            .push("bar.rs");
@@ -821,7 +824,7 @@ mod tests {
            .with_custom_typeshed(MockedTypeshed::default())
            .build();

-        let root = SearchPath::custom_stdlib(&db, stdlib.parent().unwrap()).unwrap();
+        let root = SearchPath::custom_stdlib(&db, stdlib.parent().unwrap().to_path_buf()).unwrap();

        // Must have a `.pyi` extension or no extension:
        let bad_absolute_path = SystemPath::new("foo/stdlib/x.py");
@@ -869,7 +872,8 @@ mod tests {
            .with_custom_typeshed(typeshed)
            .with_target_version(target_version)
            .build();
-        let stdlib = SearchPath::custom_stdlib(&db, stdlib.parent().unwrap()).unwrap();
+        let stdlib =
+            SearchPath::custom_stdlib(&db, stdlib.parent().unwrap().to_path_buf()).unwrap();
        (db, stdlib)
    }

@@ -894,7 +898,7 @@ mod tests {
        };

        let (db, stdlib_path) = py38_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY38);
+        let resolver = ResolverState::new(&db, PythonVersion::PY38);

        let asyncio_regular_package = stdlib_path.join("asyncio");
        assert!(asyncio_regular_package.is_directory(&resolver));
@@ -922,7 +926,7 @@ mod tests {
        };

        let (db, stdlib_path) = py38_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY38);
+        let resolver = ResolverState::new(&db, PythonVersion::PY38);

        let xml_namespace_package = stdlib_path.join("xml");
        assert!(xml_namespace_package.is_directory(&resolver));
@@ -944,7 +948,7 @@ mod tests {
        };

        let (db, stdlib_path) = py38_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY38);
+        let resolver = ResolverState::new(&db, PythonVersion::PY38);

        let functools_module = stdlib_path.join("functools.pyi");
        assert!(functools_module.to_file(&resolver).is_some());
@@ -960,7 +964,7 @@ mod tests {
        };

        let (db, stdlib_path) = py38_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY38);
+        let resolver = ResolverState::new(&db, PythonVersion::PY38);

        let collections_regular_package = stdlib_path.join("collections");
        assert_eq!(collections_regular_package.to_file(&resolver), None);
@@ -976,7 +980,7 @@ mod tests {
        };

        let (db, stdlib_path) = py38_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY38);
+        let resolver = ResolverState::new(&db, PythonVersion::PY38);

        let importlib_namespace_package = stdlib_path.join("importlib");
        assert_eq!(importlib_namespace_package.to_file(&resolver), None);
@@ -997,7 +1001,7 @@ mod tests {
        };

        let (db, stdlib_path) = py38_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY38);
+        let resolver = ResolverState::new(&db, PythonVersion::PY38);

        let non_existent = stdlib_path.join("doesnt_even_exist");
        assert_eq!(non_existent.to_file(&resolver), None);
@@ -1025,7 +1029,7 @@ mod tests {
        };

        let (db, stdlib_path) = py39_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY39);
+        let resolver = ResolverState::new(&db, PythonVersion::PY39);

        // Since we've set the target version to Py39,
        // `collections` should now exist as a directory, according to VERSIONS...
@@ -1054,7 +1058,7 @@ mod tests {
        };

        let (db, stdlib_path) = py39_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY39);
+        let resolver = ResolverState::new(&db, PythonVersion::PY39);

        // The `importlib` directory now also exists
        let importlib_namespace_package = stdlib_path.join("importlib");
@@ -1078,7 +1082,7 @@ mod tests {
        };

        let (db, stdlib_path) = py39_typeshed_test_case(TYPESHED);
-        let resolver = ResolverContext::new(&db, PythonVersion::PY39);
+        let resolver = ResolverState::new(&db, PythonVersion::PY39);

        // The `xml` package no longer exists on py39:
        let xml_namespace_package = stdlib_path.join("xml");
--- a/crates/red_knot_python_semantic/src/module_resolver/resolver.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/resolver.rs
@@ -1,19 +1,19 @@
-use rustc_hash::{FxBuildHasher, FxHashSet};
 use std::borrow::Cow;
 use std::iter::FusedIterator;
-use std::ops::Deref;
+
+use rustc_hash::{FxBuildHasher, FxHashSet};

 use ruff_db::files::{File, FilePath, FileRootKind};
-use ruff_db::system::{DirectoryEntry, System, SystemPath, SystemPathBuf};
-use ruff_db::vendored::{VendoredFileSystem, VendoredPath};
+use ruff_db::system::{DirectoryEntry, SystemPath, SystemPathBuf};
+use ruff_db::vendored::VendoredPath;
+
+use crate::db::Db;
+use crate::module_name::ModuleName;
+use crate::{Program, SearchPathSettings};

 use super::module::{Module, ModuleKind};
 use super::path::{ModulePath, SearchPath, SearchPathValidationError};
-use crate::db::Db;
-use crate::module_name::ModuleName;
-use crate::module_resolver::typeshed::{vendored_typeshed_versions, TypeshedVersions};
-use crate::site_packages::VirtualEnvironment;
-use crate::{Program, PythonVersion, SearchPathSettings, SitePackages};
+use super::state::ResolverState;

 /// Resolves a module name to a module.
 pub fn resolve_module(db: &dyn Db, module_name: ModuleName) -> Option<Module> {
@@ -41,7 +41,7 @@ pub(crate) fn resolve_module_query<'db>(

    let module = Module::new(name.clone(), kind, search_path, module_file);

-    tracing::trace!(
+    tracing::debug!(
        "Resolved module '{name}' to '{path}'.",
        path = module_file.path(db)
    );
@@ -122,7 +122,7 @@ pub(crate) fn search_paths(db: &dyn Db) -> SearchPathIterator {
    Program::get(db).search_paths(db).iter(db)
 }

-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, Default)]
 pub(crate) struct SearchPaths {
    /// Search paths that have been statically determined purely from reading Ruff's configuration settings.
    /// These shouldn't ever change unless the config settings themselves change.
@@ -135,8 +135,6 @@ pub(crate) struct SearchPaths {
    /// in terms of module-resolution priority until we've discovered the editable installs
    /// for the first `site-packages` path
    site_packages: Vec<SearchPath>,
-
-    typeshed_versions: ResolvedTypeshedVersions,
 }

 impl SearchPaths {
@@ -148,14 +146,8 @@ impl SearchPaths {
    /// [module resolution order]: https://typing.readthedocs.io/en/latest/spec/distributing.html#import-resolution-ordering
    pub(crate) fn from_settings(
        db: &dyn Db,
-        settings: &SearchPathSettings,
+        settings: SearchPathSettings,
    ) -> Result<Self, SearchPathValidationError> {
-        fn canonicalize(path: &SystemPath, system: &dyn System) -> SystemPathBuf {
-            system
-                .canonicalize_path(path)
-                .unwrap_or_else(|_| path.to_path_buf())
-        }
-
        let SearchPathSettings {
            extra_paths,
            src_root,
@@ -169,65 +161,45 @@ impl SearchPaths {
        let mut static_paths = vec![];

        for path in extra_paths {
-            let path = canonicalize(path, system);
-            files.try_add_root(db.upcast(), &path, FileRootKind::LibrarySearchPath);
-            tracing::debug!("Adding extra search-path '{path}'");
-
-            static_paths.push(SearchPath::extra(system, path)?);
-        }
-
-        tracing::debug!("Adding first-party search path '{src_root}'");
-        static_paths.push(SearchPath::first_party(system, src_root.to_path_buf())?);
-
-        let (typeshed_versions, stdlib_path) = if let Some(custom_typeshed) = custom_typeshed {
-            let custom_typeshed = canonicalize(custom_typeshed, system);
-            tracing::debug!("Adding custom-stdlib search path '{custom_typeshed}'");
+            tracing::debug!("Adding static extra search-path '{path}'");

+            let search_path = SearchPath::extra(system, path)?;
            files.try_add_root(
                db.upcast(),
-                &custom_typeshed,
+                search_path.as_system_path().unwrap(),
                FileRootKind::LibrarySearchPath,
            );
+            static_paths.push(search_path);
+        }

-            let versions_path = custom_typeshed.join("stdlib/VERSIONS");
+        tracing::debug!("Adding static search path '{src_root}'");
+        static_paths.push(SearchPath::first_party(system, src_root)?);

-            let versions_content = system.read_to_string(&versions_path).map_err(|error| {
-                SearchPathValidationError::FailedToReadVersionsFile {
-                    path: versions_path,
-                    error,
-                }
-            })?;
+        static_paths.push(if let Some(custom_typeshed) = custom_typeshed {
+            tracing::debug!("Adding static custom-sdtlib search-path '{custom_typeshed}'");

-            let parsed: TypeshedVersions = versions_content.parse()?;
-
-            let search_path = SearchPath::custom_stdlib(db, &custom_typeshed)?;
-
-            (ResolvedTypeshedVersions::Custom(parsed), search_path)
+            let search_path = SearchPath::custom_stdlib(db, custom_typeshed)?;
+            files.try_add_root(
+                db.upcast(),
+                search_path.as_system_path().unwrap(),
+                FileRootKind::LibrarySearchPath,
+            );
+            search_path
        } else {
-            tracing::debug!("Using vendored stdlib");
-            (
-                ResolvedTypeshedVersions::Vendored(vendored_typeshed_versions()),
-                SearchPath::vendored_stdlib(),
-            )
-        };
-
-        static_paths.push(stdlib_path);
-
-        let site_packages_paths = match site_packages_paths {
-            SitePackages::Derived { venv_path } => VirtualEnvironment::new(venv_path, system)
-                .and_then(|venv| venv.site_packages_directories(system))?,
-            SitePackages::Known(paths) => paths
-                .iter()
-                .map(|path| canonicalize(path, system))
-                .collect(),
-        };
+            SearchPath::vendored_stdlib()
+        });

        let mut site_packages: Vec<_> = Vec::with_capacity(site_packages_paths.len());

        for path in site_packages_paths {
-            tracing::debug!("Adding site-packages search path '{path}'");
-            files.try_add_root(db.upcast(), &path, FileRootKind::LibrarySearchPath);
-            site_packages.push(SearchPath::site_packages(system, path)?);
+            tracing::debug!("Adding site-package path '{path}'");
+            let search_path = SearchPath::site_packages(system, path)?;
+            files.try_add_root(
+                db.upcast(),
+                search_path.as_system_path().unwrap(),
+                FileRootKind::LibrarySearchPath,
+            );
+            site_packages.push(search_path);
        }

        // TODO vendor typeshed's third-party stubs as well as the stdlib and fallback to them as a final step
@@ -252,48 +224,16 @@ impl SearchPaths {
        Ok(SearchPaths {
            static_paths,
            site_packages,
-            typeshed_versions,
        })
    }

-    pub(super) fn iter<'a>(&'a self, db: &'a dyn Db) -> SearchPathIterator<'a> {
+    pub(crate) fn iter<'a>(&'a self, db: &'a dyn Db) -> SearchPathIterator<'a> {
        SearchPathIterator {
            db,
            static_paths: self.static_paths.iter(),
            dynamic_paths: None,
        }
    }
-
-    pub(crate) fn custom_stdlib(&self) -> Option<&SystemPath> {
-        self.static_paths.iter().find_map(|search_path| {
-            if search_path.is_standard_library() {
-                search_path.as_system_path()
-            } else {
-                None
-            }
-        })
-    }
-
-    pub(super) fn typeshed_versions(&self) -> &TypeshedVersions {
-        &self.typeshed_versions
-    }
-}
-
-#[derive(Debug, PartialEq, Eq)]
-enum ResolvedTypeshedVersions {
-    Vendored(&'static TypeshedVersions),
-    Custom(TypeshedVersions),
-}
-
-impl Deref for ResolvedTypeshedVersions {
-    type Target = TypeshedVersions;
-
-    fn deref(&self) -> &Self::Target {
-        match self {
-            ResolvedTypeshedVersions::Vendored(versions) => versions,
-            ResolvedTypeshedVersions::Custom(versions) => versions,
-        }
-    }
 }

 /// Collect all dynamic search paths. For each `site-packages` path:
@@ -311,7 +251,6 @@ pub(crate) fn dynamic_resolution_paths(db: &dyn Db) -> Vec<SearchPath> {
    let SearchPaths {
        static_paths,
        site_packages,
-        typeshed_versions: _,
    } = Program::get(db).search_paths(db);

    let mut dynamic_paths = Vec::new();
@@ -376,16 +315,12 @@ pub(crate) fn dynamic_resolution_paths(db: &dyn Db) -> Vec<SearchPath> {
        let installations = all_pth_files.iter().flat_map(PthFile::items);

        for installation in installations {
-            let installation = system
-                .canonicalize_path(&installation)
-                .unwrap_or(installation);
-
            if existing_paths.insert(Cow::Owned(installation.clone())) {
-                match SearchPath::editable(system, installation.clone()) {
+                match SearchPath::editable(system, installation) {
                    Ok(search_path) => {
                        tracing::debug!(
                            "Adding editable installation to module resolution path {path}",
-                            path = installation
+                            path = search_path.as_system_path().unwrap()
                        );
                        dynamic_paths.push(search_path);
                    }
@@ -547,7 +482,7 @@ struct ModuleNameIngredient<'db> {
 fn resolve_name(db: &dyn Db, name: &ModuleName) -> Option<(SearchPath, File, ModuleKind)> {
    let program = Program::get(db);
    let target_version = program.target_version(db);
-    let resolver_state = ResolverContext::new(db, target_version);
+    let resolver_state = ResolverState::new(db, target_version);
    let is_builtin_module =
        ruff_python_stdlib::sys::is_builtin_module(target_version.minor, name.as_str());

@@ -610,7 +545,7 @@ fn resolve_name(db: &dyn Db, name: &ModuleName) -> Option<(SearchPath, File, Mod
 fn resolve_package<'a, 'db, I>(
    module_search_path: &SearchPath,
    components: I,
-    resolver_state: &ResolverContext<'db>,
+    resolver_state: &ResolverState<'db>,
 ) -> Result<ResolvedPackage, PackageKind>
 where
    I: Iterator<Item = &'a str>,
@@ -692,21 +627,6 @@ impl PackageKind {
    }
 }

-pub(super) struct ResolverContext<'db> {
-    pub(super) db: &'db dyn Db,
-    pub(super) target_version: PythonVersion,
-}
-
-impl<'db> ResolverContext<'db> {
-    pub(super) fn new(db: &'db dyn Db, target_version: PythonVersion) -> Self {
-        Self { db, target_version }
-    }
-
-    pub(super) fn vendored(&self) -> &VendoredFileSystem {
-        self.db.vendored()
-    }
-}
-
 #[cfg(test)]
 mod tests {
    use ruff_db::files::{system_path_to_file, File, FilePath};
@@ -861,7 +781,7 @@ mod tests {
                "Search path for {module_name} was unexpectedly {search_path:?}"
            );
            assert!(
-                search_path.is_standard_library(),
+                search_path.is_stdlib_search_path(),
                "Expected a stdlib search path, but got {search_path:?}"
            );
        }
@@ -957,7 +877,7 @@ mod tests {
                "Search path for {module_name} was unexpectedly {search_path:?}"
            );
            assert!(
-                search_path.is_standard_library(),
+                search_path.is_stdlib_search_path(),
                "Expected a stdlib search path, but got {search_path:?}"
            );
        }
@@ -1274,13 +1194,13 @@ mod tests {

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version: PythonVersion::PY38,
                search_paths: SearchPathSettings {
                    extra_paths: vec![],
                    src_root: src.clone(),
                    custom_typeshed: Some(custom_typeshed.clone()),
-                    site_packages: SitePackages::Known(vec![site_packages]),
+                    site_packages: vec![site_packages],
                },
            },
        )
@@ -1779,16 +1699,13 @@ not_a_directory

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version: PythonVersion::default(),
                search_paths: SearchPathSettings {
                    extra_paths: vec![],
                    src_root: SystemPathBuf::from("/src"),
                    custom_typeshed: None,
-                    site_packages: SitePackages::Known(vec![
-                        venv_site_packages,
-                        system_site_packages,
-                    ]),
+                    site_packages: vec![venv_site_packages, system_site_packages],
                },
            },
        )
--- a/crates/red_knot_python_semantic/src/module_resolver/state.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/state.rs
@@ -0,0 +1,25 @@
+use ruff_db::vendored::VendoredFileSystem;
+
+use super::typeshed::LazyTypeshedVersions;
+use crate::db::Db;
+use crate::python_version::PythonVersion;
+
+pub(crate) struct ResolverState<'db> {
+    pub(crate) db: &'db dyn Db,
+    pub(crate) typeshed_versions: LazyTypeshedVersions<'db>,
+    pub(crate) target_version: PythonVersion,
+}
+
+impl<'db> ResolverState<'db> {
+    pub(crate) fn new(db: &'db dyn Db, target_version: PythonVersion) -> Self {
+        Self {
+            db,
+            typeshed_versions: LazyTypeshedVersions::new(),
+            target_version,
+        }
+    }
+
+    pub(crate) fn vendored(&self) -> &VendoredFileSystem {
+        self.db.vendored()
+    }
+}
--- a/crates/red_knot_python_semantic/src/module_resolver/testing.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/testing.rs
@@ -4,7 +4,7 @@ use ruff_db::vendored::VendoredPathBuf;
 use crate::db::tests::TestDb;
 use crate::program::{Program, SearchPathSettings};
 use crate::python_version::PythonVersion;
-use crate::{ProgramSettings, SitePackages};
+use crate::ProgramSettings;

 /// A test case for the module resolver.
 ///
@@ -179,7 +179,6 @@ impl TestCaseBuilder<UnspecifiedTypeshed> {
            first_party_files,
            site_packages_files,
        } = self;
-
        TestCaseBuilder {
            typeshed_option: typeshed,
            target_version,
@@ -196,7 +195,6 @@ impl TestCaseBuilder<UnspecifiedTypeshed> {
            site_packages,
            target_version,
        } = self.with_custom_typeshed(MockedTypeshed::default()).build();
-
        TestCase {
            db,
            src,
@@ -225,13 +223,13 @@ impl TestCaseBuilder<MockedTypeshed> {

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version,
                search_paths: SearchPathSettings {
                    extra_paths: vec![],
                    src_root: src.clone(),
                    custom_typeshed: Some(typeshed.clone()),
-                    site_packages: SitePackages::Known(vec![site_packages.clone()]),
+                    site_packages: vec![site_packages.clone()],
                },
            },
        )
@@ -281,11 +279,13 @@ impl TestCaseBuilder<VendoredTypeshed> {

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version,
                search_paths: SearchPathSettings {
-                    site_packages: SitePackages::Known(vec![site_packages.clone()]),
-                    ..SearchPathSettings::new(src.clone())
+                    extra_paths: vec![],
+                    src_root: src.clone(),
+                    custom_typeshed: None,
+                    site_packages: vec![site_packages.clone()],
                },
            },
        )
--- a/crates/red_knot_python_semantic/src/module_resolver/typeshed/mod.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/typeshed/mod.rs
@@ -1,6 +1,6 @@
 pub use self::vendored::vendored_typeshed_stubs;
 pub(super) use self::versions::{
-    typeshed_versions, vendored_typeshed_versions, TypeshedVersions, TypeshedVersionsParseError,
+    parse_typeshed_versions, LazyTypeshedVersions, TypeshedVersionsParseError,
    TypeshedVersionsQueryResult,
 };

--- a/crates/red_knot_python_semantic/src/module_resolver/typeshed/versions.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/typeshed/versions.rs
@@ -1,3 +1,4 @@
+use std::cell::OnceCell;
 use std::collections::BTreeMap;
 use std::fmt;
 use std::num::{NonZeroU16, NonZeroUsize};
@@ -5,12 +6,78 @@ use std::ops::{RangeFrom, RangeInclusive};
 use std::str::FromStr;

 use once_cell::sync::Lazy;
+use ruff_db::system::SystemPath;
 use rustc_hash::FxHashMap;

+use ruff_db::files::{system_path_to_file, File};
+
 use super::vendored::vendored_typeshed_stubs;
 use crate::db::Db;
 use crate::module_name::ModuleName;
-use crate::{Program, PythonVersion};
+use crate::python_version::PythonVersion;
+
+#[derive(Debug)]
+pub(crate) struct LazyTypeshedVersions<'db>(OnceCell<&'db TypeshedVersions>);
+
+impl<'db> LazyTypeshedVersions<'db> {
+    #[must_use]
+    pub(crate) fn new() -> Self {
+        Self(OnceCell::new())
+    }
+
+    /// Query whether a module exists at runtime in the stdlib on a certain Python version.
+    ///
+    /// Simply probing whether a file exists in typeshed is insufficient for this question,
+    /// as a module in the stdlib may have been added in Python 3.10, but the typeshed stub
+    /// will still be available (either in a custom typeshed dir or in our vendored copy)
+    /// even if the user specified Python 3.8 as the target version.
+    ///
+    /// For top-level modules and packages, the VERSIONS file can always provide an unambiguous answer
+    /// as to whether the module exists on the specified target version. However, VERSIONS does not
+    /// provide comprehensive information on all submodules, meaning that this method sometimes
+    /// returns [`TypeshedVersionsQueryResult::MaybeExists`].
+    /// See [`TypeshedVersionsQueryResult`] for more details.
+    #[must_use]
+    pub(crate) fn query_module(
+        &self,
+        db: &'db dyn Db,
+        module: &ModuleName,
+        stdlib_root: Option<&SystemPath>,
+        target_version: PythonVersion,
+    ) -> TypeshedVersionsQueryResult {
+        let versions = self.0.get_or_init(|| {
+            let versions_path = if let Some(system_path) = stdlib_root {
+                system_path.join("VERSIONS")
+            } else {
+                return &VENDORED_VERSIONS;
+            };
+            let Ok(versions_file) = system_path_to_file(db.upcast(), &versions_path) else {
+                todo!(
+                    "Still need to figure out how to handle VERSIONS files being deleted \
+                    from custom typeshed directories! Expected a file to exist at {versions_path}"
+                )
+            };
+            // TODO(Alex/Micha): If VERSIONS is invalid,
+            // this should invalidate not just the specific module resolution we're currently attempting,
+            // but all type inference that depends on any standard-library types.
+            // Unwrapping here is not correct...
+            parse_typeshed_versions(db, versions_file).as_ref().unwrap()
+        });
+        versions.query_module(module, target_version)
+    }
+}
+
+#[salsa::tracked(return_ref)]
+pub(crate) fn parse_typeshed_versions(
+    db: &dyn Db,
+    versions_file: File,
+) -> Result<TypeshedVersions, TypeshedVersionsParseError> {
+    // TODO: Handle IO errors
+    let file_content = versions_file
+        .read_to_string(db.upcast())
+        .unwrap_or_default();
+    file_content.parse()
+}

 static VENDORED_VERSIONS: Lazy<TypeshedVersions> = Lazy::new(|| {
    TypeshedVersions::from_str(
@@ -21,14 +88,6 @@ static VENDORED_VERSIONS: Lazy<TypeshedVersions> = Lazy::new(|| {
    .unwrap()
 });

-pub(crate) fn vendored_typeshed_versions() -> &'static TypeshedVersions {
-    &VENDORED_VERSIONS
-}
-
-pub(crate) fn typeshed_versions(db: &dyn Db) -> &TypeshedVersions {
-    Program::get(db).search_paths(db).typeshed_versions()
-}
-
 #[derive(Debug, PartialEq, Eq, Clone)]
 pub(crate) struct TypeshedVersionsParseError {
    line_number: Option<NonZeroU16>,
@@ -115,7 +174,7 @@ impl TypeshedVersions {
    }

    #[must_use]
-    pub(in crate::module_resolver) fn query_module(
+    fn query_module(
        &self,
        module: &ModuleName,
        target_version: PythonVersion,
@@ -145,7 +204,7 @@ impl TypeshedVersions {
    }
 }

-/// Possible answers [`TypeshedVersions::query_module()`] could give to the question:
+/// Possible answers [`LazyTypeshedVersions::query_module()`] could give to the question:
 /// "Does this module exist in the stdlib at runtime on a certain target version?"
 #[derive(Debug, Copy, PartialEq, Eq, Clone, Hash)]
 pub(crate) enum TypeshedVersionsQueryResult {
--- a/crates/red_knot_python_semantic/src/program.rs
+++ b/crates/red_knot_python_semantic/src/program.rs
@@ -3,7 +3,7 @@ use anyhow::Context;
 use salsa::Durability;
 use salsa::Setter;

-use ruff_db::system::{SystemPath, SystemPathBuf};
+use ruff_db::system::SystemPathBuf;

 use crate::module_resolver::SearchPaths;
 use crate::Db;
@@ -12,31 +12,33 @@ use crate::Db;
 pub struct Program {
    pub target_version: PythonVersion,

+    #[default]
    #[return_ref]
    pub(crate) search_paths: SearchPaths,
 }

 impl Program {
-    pub fn from_settings(db: &dyn Db, settings: &ProgramSettings) -> anyhow::Result<Self> {
+    pub fn from_settings(db: &dyn Db, settings: ProgramSettings) -> anyhow::Result<Self> {
        let ProgramSettings {
            target_version,
            search_paths,
        } = settings;

-        tracing::info!("Target version: Python {target_version}");
+        tracing::info!("Target version: {target_version}");

        let search_paths = SearchPaths::from_settings(db, search_paths)
            .with_context(|| "Invalid search path settings")?;

-        Ok(Program::builder(settings.target_version, search_paths)
+        Ok(Program::builder(settings.target_version)
            .durability(Durability::HIGH)
+            .search_paths(search_paths)
            .new(db))
    }

    pub fn update_search_paths(
-        self,
+        &self,
        db: &mut dyn Db,
-        search_path_settings: &SearchPathSettings,
+        search_path_settings: SearchPathSettings,
    ) -> anyhow::Result<()> {
        let search_paths = SearchPaths::from_settings(db, search_path_settings)?;

@@ -47,20 +49,16 @@ impl Program {

        Ok(())
    }
-
-    pub fn custom_stdlib_search_path(self, db: &dyn Db) -> Option<&SystemPath> {
-        self.search_paths(db).custom_stdlib()
-    }
 }

-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq)]
 pub struct ProgramSettings {
    pub target_version: PythonVersion,
    pub search_paths: SearchPathSettings,
 }

 /// Configures the search paths for module resolution.
-#[derive(Eq, PartialEq, Debug, Clone)]
+#[derive(Eq, PartialEq, Debug, Clone, Default)]
 pub struct SearchPathSettings {
    /// List of user-provided paths that should take first priority in the module resolution.
    /// Examples in other type checkers are mypy's MYPYPATH environment variable,
@@ -76,25 +74,5 @@ pub struct SearchPathSettings {
    pub custom_typeshed: Option<SystemPathBuf>,

    /// The path to the user's `site-packages` directory, where third-party packages from ``PyPI`` are installed.
-    pub site_packages: SitePackages,
-}
-
-impl SearchPathSettings {
-    pub fn new(src_root: SystemPathBuf) -> Self {
-        Self {
-            src_root,
-            extra_paths: vec![],
-            custom_typeshed: None,
-            site_packages: SitePackages::Known(vec![]),
-        }
-    }
-}
-
-#[derive(Debug, Clone, Eq, PartialEq)]
-pub enum SitePackages {
-    Derived {
-        venv_path: SystemPathBuf,
-    },
-    /// Resolved site packages paths
-    Known(Vec<SystemPathBuf>),
+    pub site_packages: Vec<SystemPathBuf>,
 }
--- a/crates/red_knot_python_semantic/src/semantic_index.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index.rs
@@ -16,9 +16,10 @@ use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{
    FileScopeId, NodeWithScopeKey, NodeWithScopeRef, Scope, ScopeId, ScopedSymbolId, SymbolTable,
 };
-use crate::semantic_index::use_def::UseDefMap;
 use crate::Db;

+pub(crate) use self::use_def::UseDefMap;
+
 pub mod ast_ids;
 mod builder;
 pub mod definition;
@@ -26,8 +27,6 @@ pub mod expression;
 pub mod symbol;
 mod use_def;

-pub(crate) use self::use_def::{DefinitionWithConstraints, DefinitionWithConstraintsIterator};
-
 type SymbolMap = hashbrown::HashMap<ScopedSymbolId, (), ()>;

 /// Returns the semantic index for `file`.
@@ -154,10 +153,6 @@ impl<'db> SemanticIndex<'db> {
        &self.scopes[id]
    }

-    pub(crate) fn scope_ids(&self) -> impl Iterator<Item = ScopeId> {
-        self.scope_ids_by_scope.iter().copied()
-    }
-
    /// Returns the id of the parent scope.
    pub(crate) fn parent_scope_id(&self, scope_id: FileScopeId) -> Option<FileScopeId> {
        let scope = self.scope(scope_id);
@@ -315,29 +310,12 @@ mod tests {
    use ruff_text_size::{Ranged, TextRange};

    use crate::db::tests::TestDb;
-    use crate::semantic_index::ast_ids::{HasScopedUseId, ScopedUseId};
-    use crate::semantic_index::definition::{Definition, DefinitionKind};
-    use crate::semantic_index::symbol::{
-        FileScopeId, Scope, ScopeKind, ScopedSymbolId, SymbolTable,
-    };
-    use crate::semantic_index::use_def::UseDefMap;
+    use crate::semantic_index::ast_ids::HasScopedUseId;
+    use crate::semantic_index::definition::DefinitionKind;
+    use crate::semantic_index::symbol::{FileScopeId, Scope, ScopeKind, SymbolTable};
    use crate::semantic_index::{global_scope, semantic_index, symbol_table, use_def_map};
    use crate::Db;

-    impl UseDefMap<'_> {
-        fn first_public_definition(&self, symbol: ScopedSymbolId) -> Option<Definition<'_>> {
-            self.public_definitions(symbol)
-                .next()
-                .map(|constrained_definition| constrained_definition.definition)
-        }
-
-        fn first_use_definition(&self, use_id: ScopedUseId) -> Option<Definition<'_>> {
-            self.use_definitions(use_id)
-                .next()
-                .map(|constrained_definition| constrained_definition.definition)
-        }
-    }
-
    struct TestCase {
        db: TestDb,
        file: File,
@@ -396,7 +374,9 @@ mod tests {
        let foo = global_table.symbol_id_by_name("foo").unwrap();

        let use_def = use_def_map(&db, scope);
-        let definition = use_def.first_public_definition(foo).unwrap();
+        let [definition] = use_def.public_definitions(foo) else {
+            panic!("expected one definition");
+        };
        assert!(matches!(definition.node(&db), DefinitionKind::Import(_)));
    }

@@ -431,13 +411,13 @@ mod tests {
        );

        let use_def = use_def_map(&db, scope);
-        let definition = use_def
-            .first_public_definition(
-                global_table
-                    .symbol_id_by_name("foo")
-                    .expect("symbol to exist"),
-            )
-            .unwrap();
+        let [definition] = use_def.public_definitions(
+            global_table
+                .symbol_id_by_name("foo")
+                .expect("symbol to exist"),
+        ) else {
+            panic!("expected one definition");
+        };
        assert!(matches!(
            definition.node(&db),
            DefinitionKind::ImportFrom(_)
@@ -458,34 +438,17 @@ mod tests {
            "a symbol used but not defined in a scope should have only the used flag"
        );
        let use_def = use_def_map(&db, scope);
-        let definition = use_def
-            .first_public_definition(global_table.symbol_id_by_name("x").expect("symbol exists"))
-            .unwrap();
+        let [definition] =
+            use_def.public_definitions(global_table.symbol_id_by_name("x").expect("symbol exists"))
+        else {
+            panic!("expected one definition");
+        };
        assert!(matches!(
            definition.node(&db),
            DefinitionKind::Assignment(_)
        ));
    }

-    #[test]
-    fn augmented_assignment() {
-        let TestCase { db, file } = test_case("x += 1");
-        let scope = global_scope(&db, file);
-        let global_table = symbol_table(&db, scope);
-
-        assert_eq!(names(&global_table), vec!["x"]);
-
-        let use_def = use_def_map(&db, scope);
-        let definition = use_def
-            .first_public_definition(global_table.symbol_id_by_name("x").unwrap())
-            .unwrap();
-
-        assert!(matches!(
-            definition.node(&db),
-            DefinitionKind::AugmentedAssignment(_)
-        ));
-    }
-
    #[test]
    fn class_scope() {
        let TestCase { db, file } = test_case(
@@ -514,9 +477,11 @@ y = 2
        assert_eq!(names(&class_table), vec!["x"]);

        let use_def = index.use_def_map(class_scope_id);
-        let definition = use_def
-            .first_public_definition(class_table.symbol_id_by_name("x").expect("symbol exists"))
-            .unwrap();
+        let [definition] =
+            use_def.public_definitions(class_table.symbol_id_by_name("x").expect("symbol exists"))
+        else {
+            panic!("expected one definition");
+        };
        assert!(matches!(
            definition.node(&db),
            DefinitionKind::Assignment(_)
@@ -550,13 +515,13 @@ y = 2
        assert_eq!(names(&function_table), vec!["x"]);

        let use_def = index.use_def_map(function_scope_id);
-        let definition = use_def
-            .first_public_definition(
-                function_table
-                    .symbol_id_by_name("x")
-                    .expect("symbol exists"),
-            )
-            .unwrap();
+        let [definition] = use_def.public_definitions(
+            function_table
+                .symbol_id_by_name("x")
+                .expect("symbol exists"),
+        ) else {
+            panic!("expected one definition");
+        };
        assert!(matches!(
            definition.node(&db),
            DefinitionKind::Assignment(_)
@@ -592,26 +557,26 @@ def f(a: str, /, b: str, c: int = 1, *args, d: int = 2, **kwargs):

        let use_def = index.use_def_map(function_scope_id);
        for name in ["a", "b", "c", "d"] {
-            let definition = use_def
-                .first_public_definition(
-                    function_table
-                        .symbol_id_by_name(name)
-                        .expect("symbol exists"),
-                )
-                .unwrap();
+            let [definition] = use_def.public_definitions(
+                function_table
+                    .symbol_id_by_name(name)
+                    .expect("symbol exists"),
+            ) else {
+                panic!("Expected parameter definition for {name}");
+            };
            assert!(matches!(
                definition.node(&db),
                DefinitionKind::ParameterWithDefault(_)
            ));
        }
        for name in ["args", "kwargs"] {
-            let definition = use_def
-                .first_public_definition(
-                    function_table
-                        .symbol_id_by_name(name)
-                        .expect("symbol exists"),
-                )
-                .unwrap();
+            let [definition] = use_def.public_definitions(
+                function_table
+                    .symbol_id_by_name(name)
+                    .expect("symbol exists"),
+            ) else {
+                panic!("Expected parameter definition for {name}");
+            };
            assert!(matches!(definition.node(&db), DefinitionKind::Parameter(_)));
        }
    }
@@ -640,22 +605,22 @@ def f(a: str, /, b: str, c: int = 1, *args, d: int = 2, **kwargs):

        let use_def = index.use_def_map(lambda_scope_id);
        for name in ["a", "b", "c", "d"] {
-            let definition = use_def
-                .first_public_definition(
-                    lambda_table.symbol_id_by_name(name).expect("symbol exists"),
-                )
-                .unwrap();
+            let [definition] = use_def
+                .public_definitions(lambda_table.symbol_id_by_name(name).expect("symbol exists"))
+            else {
+                panic!("Expected parameter definition for {name}");
+            };
            assert!(matches!(
                definition.node(&db),
                DefinitionKind::ParameterWithDefault(_)
            ));
        }
        for name in ["args", "kwargs"] {
-            let definition = use_def
-                .first_public_definition(
-                    lambda_table.symbol_id_by_name(name).expect("symbol exists"),
-                )
-                .unwrap();
+            let [definition] = use_def
+                .public_definitions(lambda_table.symbol_id_by_name(name).expect("symbol exists"))
+            else {
+                panic!("Expected parameter definition for {name}");
+            };
            assert!(matches!(definition.node(&db), DefinitionKind::Parameter(_)));
        }
    }
@@ -726,7 +691,9 @@ def f(a: str, /, b: str, c: int = 1, *args, d: int = 2, **kwargs):
        let element_use_id =
            element.scoped_use_id(&db, comprehension_scope_id.to_scope_id(&db, file));

-        let definition = use_def.first_use_definition(element_use_id).unwrap();
+        let [definition] = use_def.use_definitions(element_use_id) else {
+            panic!("expected one definition")
+        };
        let DefinitionKind::Comprehension(comprehension) = definition.node(&db) else {
            panic!("expected generator definition")
        };
@@ -790,56 +757,6 @@ def f(a: str, /, b: str, c: int = 1, *args, d: int = 2, **kwargs):
        assert_eq!(names(&inner_comprehension_symbol_table), vec!["x"]);
    }

-    #[test]
-    fn with_item_definition() {
-        let TestCase { db, file } = test_case(
-            "
-with item1 as x, item2 as y:
-    pass
-",
-        );
-
-        let index = semantic_index(&db, file);
-        let global_table = index.symbol_table(FileScopeId::global());
-
-        assert_eq!(names(&global_table), vec!["item1", "x", "item2", "y"]);
-
-        let use_def = index.use_def_map(FileScopeId::global());
-        for name in ["x", "y"] {
-            let Some(definition) = use_def.first_public_definition(
-                global_table.symbol_id_by_name(name).expect("symbol exists"),
-            ) else {
-                panic!("Expected with item definition for {name}");
-            };
-            assert!(matches!(definition.node(&db), DefinitionKind::WithItem(_)));
-        }
-    }
-
-    #[test]
-    fn with_item_unpacked_definition() {
-        let TestCase { db, file } = test_case(
-            "
-with context() as (x, y):
-    pass
-",
-        );
-
-        let index = semantic_index(&db, file);
-        let global_table = index.symbol_table(FileScopeId::global());
-
-        assert_eq!(names(&global_table), vec!["context", "x", "y"]);
-
-        let use_def = index.use_def_map(FileScopeId::global());
-        for name in ["x", "y"] {
-            let Some(definition) = use_def.first_public_definition(
-                global_table.symbol_id_by_name(name).expect("symbol exists"),
-            ) else {
-                panic!("Expected with item definition for {name}");
-            };
-            assert!(matches!(definition.node(&db), DefinitionKind::WithItem(_)));
-        }
-    }
-
    #[test]
    fn dupes() {
        let TestCase { db, file } = test_case(
@@ -873,13 +790,13 @@ def func():
        assert_eq!(names(&func2_table), vec!["y"]);

        let use_def = index.use_def_map(FileScopeId::global());
-        let definition = use_def
-            .first_public_definition(
-                global_table
-                    .symbol_id_by_name("func")
-                    .expect("symbol exists"),
-            )
-            .unwrap();
+        let [definition] = use_def.public_definitions(
+            global_table
+                .symbol_id_by_name("func")
+                .expect("symbol exists"),
+        ) else {
+            panic!("expected one definition");
+        };
        assert!(matches!(definition.node(&db), DefinitionKind::Function(_)));
    }

@@ -980,7 +897,9 @@ class C[T]:
        };
        let x_use_id = x_use_expr_name.scoped_use_id(&db, scope);
        let use_def = use_def_map(&db, scope);
-        let definition = use_def.first_use_definition(x_use_id).unwrap();
+        let [definition] = use_def.use_definitions(x_use_id) else {
+            panic!("expected one definition");
+        };
        let DefinitionKind::Assignment(assignment) = definition.node(&db) else {
            panic!("should be an assignment definition")
        };
@@ -1071,28 +990,4 @@ def x():
            vec!["bar", "foo", "Test", "<module>"]
        );
    }
-
-    #[test]
-    fn match_stmt_symbols() {
-        let TestCase { db, file } = test_case(
-            "
-match subject:
-    case a: ...
-    case [b, c, *d]: ...
-    case e as f: ...
-    case {'x': g, **h}: ...
-    case Foo(i, z=j): ...
-    case k | l: ...
-    case _: ...
-",
-        );
-
-        let global_table = symbol_table(&db, global_scope(&db, file));
-
-        assert!(global_table.symbol_by_name("Foo").unwrap().is_used());
-        assert_eq!(
-            names(&global_table),
-            vec!["subject", "a", "b", "c", "d", "f", "e", "h", "g", "Foo", "i", "j", "k", "l"]
-        );
-    }
 }
--- a/crates/red_knot_python_semantic/src/semantic_index/builder.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/builder.rs
@@ -7,8 +7,7 @@ use ruff_db::parsed::ParsedModule;
 use ruff_index::IndexVec;
 use ruff_python_ast as ast;
 use ruff_python_ast::name::Name;
-use ruff_python_ast::visitor::{walk_expr, walk_pattern, walk_stmt, Visitor};
-use ruff_python_ast::AnyParameterRef;
+use ruff_python_ast::visitor::{walk_expr, walk_stmt, Visitor};

 use crate::ast_node_ref::AstNodeRef;
 use crate::semantic_index::ast_ids::node_key::ExpressionNodeKey;
@@ -26,8 +25,6 @@ use crate::semantic_index::use_def::{FlowSnapshot, UseDefMapBuilder};
 use crate::semantic_index::SemanticIndex;
 use crate::Db;

-use super::definition::WithItemDefinitionNodeRef;
-
 pub(super) struct SemanticIndexBuilder<'db> {
    // Builder state
    db: &'db dyn Db,
@@ -158,7 +155,7 @@ impl<'db> SemanticIndexBuilder<'db> {
        self.current_use_def_map_mut().restore(state);
    }

-    fn flow_merge(&mut self, state: FlowSnapshot) {
+    fn flow_merge(&mut self, state: &FlowSnapshot) {
        self.current_use_def_map_mut().merge(state);
    }

@@ -198,16 +195,9 @@ impl<'db> SemanticIndexBuilder<'db> {
        definition
    }

-    fn add_constraint(&mut self, constraint_node: &ast::Expr) -> Expression<'db> {
-        let expression = self.add_standalone_expression(constraint_node);
-        self.current_use_def_map_mut().record_constraint(expression);
-
-        expression
-    }
-
    /// Record an expression that needs to be a Salsa ingredient, because we need to infer its type
    /// standalone (type narrowing tests, RHS of an assignment.)
-    fn add_standalone_expression(&mut self, expression_node: &ast::Expr) -> Expression<'db> {
+    fn add_standalone_expression(&mut self, expression_node: &ast::Expr) {
        let expression = Expression::new(
            self.db,
            self.file,
@@ -220,7 +210,6 @@ impl<'db> SemanticIndexBuilder<'db> {
        );
        self.expressions_by_node
            .insert(expression_node.into(), expression);
-        expression
    }

    fn with_type_params(
@@ -312,23 +301,6 @@ impl<'db> SemanticIndexBuilder<'db> {
        }
    }

-    fn declare_parameter(&mut self, parameter: AnyParameterRef) {
-        let symbol =
-            self.add_or_update_symbol(parameter.name().id().clone(), SymbolFlags::IS_DEFINED);
-
-        let definition = self.add_definition(symbol, parameter);
-
-        if let AnyParameterRef::NonVariadic(with_default) = parameter {
-            // Insert a mapping from the parameter to the same definition.
-            // This ensures that calling `HasTy::ty` on the inner parameter returns
-            // a valid type (and doesn't panic)
-            self.definitions_by_node.insert(
-                DefinitionNodeRef::from(AnyParameterRef::Variadic(&with_default.parameter)).key(),
-                definition,
-            );
-        }
-    }
-
    pub(super) fn build(mut self) -> SemanticIndex<'db> {
        let module = self.module;
        self.visit_body(module.suite());
@@ -419,7 +391,11 @@ where

                        // Add symbols and definitions for the parameters to the function scope.
                        for parameter in &*function_def.parameters {
-                            builder.declare_parameter(parameter);
+                            let symbol = builder.add_or_update_symbol(
+                                parameter.name().id().clone(),
+                                SymbolFlags::IS_DEFINED,
+                            );
+                            builder.add_definition(symbol, parameter);
                        }

                        builder.visit_body(&function_def.body);
@@ -497,24 +473,9 @@ where
                self.visit_expr(&node.target);
                self.current_assignment = None;
            }
-            ast::Stmt::AugAssign(
-                aug_assign @ ast::StmtAugAssign {
-                    range: _,
-                    target,
-                    op: _,
-                    value,
-                },
-            ) => {
-                debug_assert!(self.current_assignment.is_none());
-                self.visit_expr(value);
-                self.current_assignment = Some(aug_assign.into());
-                self.visit_expr(target);
-                self.current_assignment = None;
-            }
            ast::Stmt::If(node) => {
                self.visit_expr(&node.test);
                let pre_if = self.flow_snapshot();
-                self.add_constraint(&node.test);
                self.visit_body(&node.body);
                let mut post_clauses: Vec<FlowSnapshot> = vec![];
                for clause in &node.elif_else_clauses {
@@ -527,7 +488,7 @@ where
                    self.visit_elif_else_clause(clause);
                }
                for post_clause_state in post_clauses {
-                    self.flow_merge(post_clause_state);
+                    self.flow_merge(&post_clause_state);
                }
                let has_else = node
                    .elif_else_clauses
@@ -536,7 +497,7 @@ where
                if !has_else {
                    // if there's no else clause, then it's possible we took none of the branches,
                    // and the pre_if state can reach here
-                    self.flow_merge(pre_if);
+                    self.flow_merge(&pre_if);
                }
            }
            ast::Stmt::While(node) => {
@@ -554,27 +515,15 @@ where

                // We may execute the `else` clause without ever executing the body, so merge in
                // the pre-loop state before visiting `else`.
-                self.flow_merge(pre_loop);
+                self.flow_merge(&pre_loop);
                self.visit_body(&node.orelse);

                // Breaking out of a while loop bypasses the `else` clause, so merge in the break
                // states after visiting `else`.
                for break_state in break_states {
-                    self.flow_merge(break_state);
+                    self.flow_merge(&break_state);
                }
            }
-            ast::Stmt::With(ast::StmtWith { items, body, .. }) => {
-                for item in items {
-                    self.visit_expr(&item.context_expr);
-                    if let Some(optional_vars) = item.optional_vars.as_deref() {
-                        self.add_standalone_expression(&item.context_expr);
-                        self.current_assignment = Some(item.into());
-                        self.visit_expr(optional_vars);
-                        self.current_assignment = None;
-                    }
-                }
-                self.visit_body(body);
-            }
            ast::Stmt::Break(_) => {
                self.loop_break_states.push(self.flow_snapshot());
            }
@@ -591,21 +540,12 @@ where

        match expr {
            ast::Expr::Name(name_node @ ast::ExprName { id, ctx, .. }) => {
-                let mut flags = match ctx {
+                let flags = match ctx {
                    ast::ExprContext::Load => SymbolFlags::IS_USED,
                    ast::ExprContext::Store => SymbolFlags::IS_DEFINED,
                    ast::ExprContext::Del => SymbolFlags::IS_DEFINED,
                    ast::ExprContext::Invalid => SymbolFlags::empty(),
                };
-                if matches!(
-                    self.current_assignment,
-                    Some(CurrentAssignment::AugAssign(_))
-                ) && !ctx.is_invalid()
-                {
-                    // For augmented assignment, the target expression is also used, so we should
-                    // record that as a use.
-                    flags |= SymbolFlags::IS_USED;
-                }
                let symbol = self.add_or_update_symbol(id.clone(), flags);
                if flags.contains(SymbolFlags::IS_DEFINED) {
                    match self.current_assignment {
@@ -621,9 +561,6 @@ where
                        Some(CurrentAssignment::AnnAssign(ann_assign)) => {
                            self.add_definition(symbol, ann_assign);
                        }
-                        Some(CurrentAssignment::AugAssign(aug_assign)) => {
-                            self.add_definition(symbol, aug_assign);
-                        }
                        Some(CurrentAssignment::Named(named)) => {
                            // TODO(dhruvmanila): If the current scope is a comprehension, then the
                            // named expression is implicitly nonlocal. This is yet to be
@@ -636,15 +573,6 @@ where
                                ComprehensionDefinitionNodeRef { node, first },
                            );
                        }
-                        Some(CurrentAssignment::WithItem(with_item)) => {
-                            self.add_definition(
-                                symbol,
-                                WithItemDefinitionNodeRef {
-                                    node: with_item,
-                                    target: name_node,
-                                },
-                            );
-                        }
                        None => {}
                    }
                }
@@ -681,7 +609,11 @@ where
                // Add symbols and definitions for the parameters to the lambda scope.
                if let Some(parameters) = &lambda.parameters {
                    for parameter in &**parameters {
-                        self.declare_parameter(parameter);
+                        let symbol = self.add_or_update_symbol(
+                            parameter.name().id().clone(),
+                            SymbolFlags::IS_DEFINED,
+                        );
+                        self.add_definition(symbol, parameter);
                    }
                }

@@ -699,7 +631,7 @@ where
                let post_body = self.flow_snapshot();
                self.flow_restore(pre_if);
                self.visit_expr(orelse);
-                self.flow_merge(post_body);
+                self.flow_merge(&post_body);
            }
            ast::Expr::ListComp(
                list_comprehension @ ast::ExprListComp {
@@ -770,38 +702,17 @@ where
            self.visit_parameter(parameter);
        }
    }
-
-    fn visit_pattern(&mut self, pattern: &'ast ast::Pattern) {
-        if let ast::Pattern::MatchAs(ast::PatternMatchAs {
-            name: Some(name), ..
-        })
-        | ast::Pattern::MatchStar(ast::PatternMatchStar {
-            name: Some(name),
-            range: _,
-        })
-        | ast::Pattern::MatchMapping(ast::PatternMatchMapping {
-            rest: Some(name), ..
-        }) = pattern
-        {
-            // TODO(dhruvmanila): Add definition
-            self.add_or_update_symbol(name.id.clone(), SymbolFlags::IS_DEFINED);
-        }
-
-        walk_pattern(self, pattern);
-    }
 }

 #[derive(Copy, Clone, Debug)]
 enum CurrentAssignment<'a> {
    Assign(&'a ast::StmtAssign),
    AnnAssign(&'a ast::StmtAnnAssign),
-    AugAssign(&'a ast::StmtAugAssign),
    Named(&'a ast::ExprNamed),
    Comprehension {
        node: &'a ast::Comprehension,
        first: bool,
    },
-    WithItem(&'a ast::WithItem),
 }

 impl<'a> From<&'a ast::StmtAssign> for CurrentAssignment<'a> {
@@ -816,20 +727,8 @@ impl<'a> From<&'a ast::StmtAnnAssign> for CurrentAssignment<'a> {
    }
 }

-impl<'a> From<&'a ast::StmtAugAssign> for CurrentAssignment<'a> {
-    fn from(value: &'a ast::StmtAugAssign) -> Self {
-        Self::AugAssign(value)
-    }
-}
-
 impl<'a> From<&'a ast::ExprNamed> for CurrentAssignment<'a> {
    fn from(value: &'a ast::ExprNamed) -> Self {
        Self::Named(value)
    }
 }
-
-impl<'a> From<&'a ast::WithItem> for CurrentAssignment<'a> {
-    fn from(value: &'a ast::WithItem) -> Self {
-        Self::WithItem(value)
-    }
-}
--- a/crates/red_knot_python_semantic/src/semantic_index/definition.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/definition.rs
@@ -44,10 +44,8 @@ pub(crate) enum DefinitionNodeRef<'a> {
    NamedExpression(&'a ast::ExprNamed),
    Assignment(AssignmentDefinitionNodeRef<'a>),
    AnnotatedAssignment(&'a ast::StmtAnnAssign),
-    AugmentedAssignment(&'a ast::StmtAugAssign),
    Comprehension(ComprehensionDefinitionNodeRef<'a>),
    Parameter(ast::AnyParameterRef<'a>),
-    WithItem(WithItemDefinitionNodeRef<'a>),
 }

 impl<'a> From<&'a ast::StmtFunctionDef> for DefinitionNodeRef<'a> {
@@ -74,12 +72,6 @@ impl<'a> From<&'a ast::StmtAnnAssign> for DefinitionNodeRef<'a> {
    }
 }

-impl<'a> From<&'a ast::StmtAugAssign> for DefinitionNodeRef<'a> {
-    fn from(node: &'a ast::StmtAugAssign) -> Self {
-        Self::AugmentedAssignment(node)
-    }
-}
-
 impl<'a> From<&'a ast::Alias> for DefinitionNodeRef<'a> {
    fn from(node_ref: &'a ast::Alias) -> Self {
        Self::Import(node_ref)
@@ -98,12 +90,6 @@ impl<'a> From<AssignmentDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
    }
 }

-impl<'a> From<WithItemDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
-    fn from(node_ref: WithItemDefinitionNodeRef<'a>) -> Self {
-        Self::WithItem(node_ref)
-    }
-}
-
 impl<'a> From<ComprehensionDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
    fn from(node: ComprehensionDefinitionNodeRef<'a>) -> Self {
        Self::Comprehension(node)
@@ -128,12 +114,6 @@ pub(crate) struct AssignmentDefinitionNodeRef<'a> {
    pub(crate) target: &'a ast::ExprName,
 }

-#[derive(Copy, Clone, Debug)]
-pub(crate) struct WithItemDefinitionNodeRef<'a> {
-    pub(crate) node: &'a ast::WithItem,
-    pub(crate) target: &'a ast::ExprName,
-}
-
 #[derive(Copy, Clone, Debug)]
 pub(crate) struct ComprehensionDefinitionNodeRef<'a> {
    pub(crate) node: &'a ast::Comprehension,
@@ -171,9 +151,6 @@ impl DefinitionNodeRef<'_> {
            DefinitionNodeRef::AnnotatedAssignment(assign) => {
                DefinitionKind::AnnotatedAssignment(AstNodeRef::new(parsed, assign))
            }
-            DefinitionNodeRef::AugmentedAssignment(augmented_assignment) => {
-                DefinitionKind::AugmentedAssignment(AstNodeRef::new(parsed, augmented_assignment))
-            }
            DefinitionNodeRef::Comprehension(ComprehensionDefinitionNodeRef { node, first }) => {
                DefinitionKind::Comprehension(ComprehensionDefinitionKind {
                    node: AstNodeRef::new(parsed, node),
@@ -188,12 +165,6 @@ impl DefinitionNodeRef<'_> {
                    DefinitionKind::ParameterWithDefault(AstNodeRef::new(parsed, parameter))
                }
            },
-            DefinitionNodeRef::WithItem(WithItemDefinitionNodeRef { node, target }) => {
-                DefinitionKind::WithItem(WithItemDefinitionKind {
-                    node: AstNodeRef::new(parsed.clone(), node),
-                    target: AstNodeRef::new(parsed, target),
-                })
-            }
        }
    }

@@ -211,13 +182,11 @@ impl DefinitionNodeRef<'_> {
                target,
            }) => target.into(),
            Self::AnnotatedAssignment(node) => node.into(),
-            Self::AugmentedAssignment(node) => node.into(),
            Self::Comprehension(ComprehensionDefinitionNodeRef { node, first: _ }) => node.into(),
            Self::Parameter(node) => match node {
                ast::AnyParameterRef::Variadic(parameter) => parameter.into(),
                ast::AnyParameterRef::NonVariadic(parameter) => parameter.into(),
            },
-            Self::WithItem(WithItemDefinitionNodeRef { node: _, target }) => target.into(),
        }
    }
 }
@@ -231,11 +200,9 @@ pub enum DefinitionKind {
    NamedExpression(AstNodeRef<ast::ExprNamed>),
    Assignment(AssignmentDefinitionKind),
    AnnotatedAssignment(AstNodeRef<ast::StmtAnnAssign>),
-    AugmentedAssignment(AstNodeRef<ast::StmtAugAssign>),
    Comprehension(ComprehensionDefinitionKind),
    Parameter(AstNodeRef<ast::Parameter>),
    ParameterWithDefault(AstNodeRef<ast::ParameterWithDefault>),
-    WithItem(WithItemDefinitionKind),
 }

 #[derive(Clone, Debug)]
@@ -271,6 +238,7 @@ impl ImportFromDefinitionKind {
 }

 #[derive(Clone, Debug)]
+#[allow(dead_code)]
 pub struct AssignmentDefinitionKind {
    assignment: AstNodeRef<ast::StmtAssign>,
    target: AstNodeRef<ast::ExprName>,
@@ -280,26 +248,6 @@ impl AssignmentDefinitionKind {
    pub(crate) fn assignment(&self) -> &ast::StmtAssign {
        self.assignment.node()
    }
-
-    pub(crate) fn target(&self) -> &ast::ExprName {
-        self.target.node()
-    }
-}
-
-#[derive(Clone, Debug)]
-pub struct WithItemDefinitionKind {
-    node: AstNodeRef<ast::WithItem>,
-    target: AstNodeRef<ast::ExprName>,
-}
-
-impl WithItemDefinitionKind {
-    pub(crate) fn node(&self) -> &ast::WithItem {
-        self.node.node()
-    }
-
-    pub(crate) fn target(&self) -> &ast::ExprName {
-        self.target.node()
-    }
 }

 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
@@ -341,12 +289,6 @@ impl From<&ast::StmtAnnAssign> for DefinitionNodeKey {
    }
 }

-impl From<&ast::StmtAugAssign> for DefinitionNodeKey {
-    fn from(node: &ast::StmtAugAssign) -> Self {
-        Self(NodeKey::from_node(node))
-    }
-}
-
 impl From<&ast::Comprehension> for DefinitionNodeKey {
    fn from(node: &ast::Comprehension) -> Self {
        Self(NodeKey::from_node(node))
--- a/crates/red_knot_python_semantic/src/semantic_index/expression.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/expression.rs
@@ -21,7 +21,7 @@ pub(crate) struct Expression<'db> {
    /// The expression node.
    #[no_eq]
    #[return_ref]
-    pub(crate) node_ref: AstNodeRef<ast::Expr>,
+    pub(crate) node: AstNodeRef<ast::Expr>,

    #[no_eq]
    count: countme::Count<Expression<'static>>,
--- a/crates/red_knot_python_semantic/src/semantic_index/use_def.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/use_def.rs
@@ -1,5 +1,4 @@
-//! Build a map from each use of a symbol to the definitions visible from that use, and the
-//! type-narrowing constraints that apply to each definition.
+//! Build a map from each use of a symbol to the definitions visible from that use.
 //!
 //! Let's take this code sample:
 //!
@@ -7,7 +6,7 @@
 //! x = 1
 //! x = 2
 //! y = x
-//! if y is not None:
+//! if flag:
 //!     x = 3
 //! else:
 //!     x = 4
@@ -35,8 +34,8 @@
 //! [`AstIds`](crate::semantic_index::ast_ids::AstIds) we number all uses (that means a `Name` node
 //! with `Load` context) so we have a `ScopedUseId` to efficiently represent each use.
 //!
-//! Another case we need to handle is when a symbol is referenced from a different scope (the most
-//! obvious example of this is an import). We call this "public" use of a symbol. So the other
+//! The other case we need to handle is when a symbol is referenced from a different scope (the
+//! most obvious example of this is an import). We call this "public" use of a symbol. So the other
 //! question we need to be able to answer is, what are the publicly-visible definitions of each
 //! symbol?
 //!
@@ -54,55 +53,42 @@
 //! start.)
 //!
 //! So this means that the publicly-visible definitions of a symbol are the definitions still
-//! visible at the end of the scope; effectively we have an implicit "use" of every symbol at the
-//! end of the scope.
+//! visible at the end of the scope.
 //!
-//! We also need to know, for a given definition of a symbol, what type-narrowing constraints apply
-//! to it. For instance, in this code sample:
-//!
-//! ```python
-//! x = 1 if flag else None
-//! if x is not None:
-//!     y = x
-//! ```
-//!
-//! At the use of `x` in `y = x`, the visible definition of `x` is `1 if flag else None`, which
-//! would infer as the type `Literal[1] | None`. But the constraint `x is not None` dominates this
-//! use, which means we can rule out the possibility that `x` is `None` here, which should give us
-//! the type `Literal[1]` for this use.
-//!
-//! The data structure we build to answer these questions is the `UseDefMap`. It has a
+//! The data structure we build to answer these two questions is the `UseDefMap`. It has a
 //! `definitions_by_use` vector indexed by [`ScopedUseId`] and a `public_definitions` vector
 //! indexed by [`ScopedSymbolId`]. The values in each of these vectors are (in principle) a list of
-//! visible definitions at that use, or at the end of the scope for that symbol, with a list of the
-//! dominating constraints for each of those definitions.
+//! visible definitions at that use, or at the end of the scope for that symbol.
 //!
-//! In order to avoid vectors-of-vectors-of-vectors and all the allocations that would entail, we
-//! don't actually store these "list of visible definitions" as a vector of [`Definition`].
-//! Instead, the values in `definitions_by_use` and `public_definitions` are a [`SymbolState`]
-//! struct which uses bit-sets to track definitions and constraints in terms of
-//! [`ScopedDefinitionId`] and [`ScopedConstraintId`], which are indices into the `all_definitions`
-//! and `all_constraints` indexvecs in the [`UseDefMap`].
+//! In order to avoid vectors-of-vectors and all the allocations that would entail, we don't
+//! actually store these "list of visible definitions" as a vector of [`Definition`] IDs. Instead,
+//! the values in `definitions_by_use` and `public_definitions` are a [`Definitions`] struct that
+//! keeps a [`Range`] into a third vector of [`Definition`] IDs, `all_definitions`. The trick with
+//! this representation is that it requires that the definitions visible at any given use of a
+//! symbol are stored sequentially in `all_definitions`.
 //!
-//! There is another special kind of possible "definition" for a symbol: there might be a path from
-//! the scope entry to a given use in which the symbol is never bound.
+//! There is another special kind of possible "definition" for a symbol: it might be unbound in the
+//! scope. (This isn't equivalent to "zero visible definitions", since we may go through an `if`
+//! that has a definition for the symbol, leaving us with one visible definition, but still also
+//! the "unbound" possibility, since we might not have taken the `if` branch.)
 //!
 //! The simplest way to model "unbound" would be as an actual [`Definition`] itself: the initial
 //! visible [`Definition`] for each symbol in a scope. But actually modeling it this way would
-//! unnecessarily increase the number of [`Definition`] that Salsa must track. Since "unbound" is a
+//! dramatically increase the number of [`Definition`] that Salsa must track. Since "unbound" is a
 //! special definition in that all symbols share it, and it doesn't have any additional per-symbol
-//! state, and constraints are irrelevant to it, we can represent it more efficiently: we use the
-//! `may_be_unbound` boolean on the [`SymbolState`] struct. If this flag is `true`, it means the
-//! symbol/use really has one additional visible "definition", which is the unbound state. If this
-//! flag is `false`, it means we've eliminated the possibility of unbound: every path we've
-//! followed includes a definition for this symbol.
+//! state, we can represent it more efficiently: we use the `may_be_unbound` boolean on the
+//! [`Definitions`] struct. If this flag is `true`, it means the symbol/use really has one
+//! additional visible "definition", which is the unbound state. If this flag is `false`, it means
+//! we've eliminated the possibility of unbound: every path we've followed includes a definition
+//! for this symbol.
 //!
-//! To build a [`UseDefMap`], the [`UseDefMapBuilder`] is notified of each new use, definition, and
-//! constraint as they are encountered by the
+//! To build a [`UseDefMap`], the [`UseDefMapBuilder`] is notified of each new use and definition
+//! as they are encountered by the
 //! [`SemanticIndexBuilder`](crate::semantic_index::builder::SemanticIndexBuilder) AST visit. For
-//! each symbol, the builder tracks the `SymbolState` for that symbol. When we hit a use of a
-//! symbol, it records the current state for that symbol for that use. When we reach the end of the
-//! scope, it records the state for each symbol as the public definitions of that symbol.
+//! each symbol, the builder tracks the currently-visible definitions for that symbol. When we hit
+//! a use of a symbol, it records the currently-visible definitions for that symbol as the visible
+//! definitions for that use. When we reach the end of the scope, it records the currently-visible
+//! definitions for each symbol as the public definitions of that symbol.
 //!
 //! Let's walk through the above example. Initially we record for `x` that it has no visible
 //! definitions, and may be unbound. When we see `x = 1`, we record that as the sole visible
@@ -112,11 +98,10 @@
 //!
 //! Then we hit the `if` branch. We visit the `test` node (`flag` in this case), since that will
 //! happen regardless. Then we take a pre-branch snapshot of the currently visible definitions for
-//! all symbols, which we'll need later. Then we record `flag` as a possible constraint on the
-//! currently visible definition (`x = 2`), and go ahead and visit the `if` body. When we see `x =
-//! 3`, it replaces `x = 2` (constrained by `flag`) as the sole visible definition of `x`. At the
-//! end of the `if` body, we take another snapshot of the currently-visible definitions; we'll call
-//! this the post-if-body snapshot.
+//! all symbols, which we'll need later. Then we go ahead and visit the `if` body. When we see `x =
+//! 3`, it replaces `x = 2` as the sole visible definition of `x`. At the end of the `if` body, we
+//! take another snapshot of the currently-visible definitions; we'll call this the post-if-body
+//! snapshot.
 //!
 //! Now we need to visit the `else` clause. The conditions when entering the `else` clause should
 //! be the pre-if conditions; if we are entering the `else` clause, we know that the `if` test
@@ -140,142 +125,98 @@
 //! (In the future we may have some other questions we want to answer as well, such as "is this
 //! definition used?", which will require tracking a bit more info in our map, e.g. a "used" bit
 //! for each [`Definition`] which is flipped to true when we record that definition for a use.)
-use self::symbol_state::{
-    ConstraintIdIterator, DefinitionIdWithConstraintsIterator, ScopedConstraintId,
-    ScopedDefinitionId, SymbolState,
-};
 use crate::semantic_index::ast_ids::ScopedUseId;
 use crate::semantic_index::definition::Definition;
-use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::ScopedSymbolId;
 use ruff_index::IndexVec;
+use std::ops::Range;

-mod bitset;
-mod symbol_state;
-
-/// Applicable definitions and constraints for every use of a name.
+/// All definitions that can reach a given use of a name.
 #[derive(Debug, PartialEq, Eq)]
 pub(crate) struct UseDefMap<'db> {
-    /// Array of [`Definition`] in this scope.
-    all_definitions: IndexVec<ScopedDefinitionId, Definition<'db>>,
+    // TODO store constraints with definitions for type narrowing
+    /// Definition IDs array for `definitions_by_use` and `public_definitions` to slice into.
+    all_definitions: Vec<Definition<'db>>,

-    /// Array of constraints (as [`Expression`]) in this scope.
-    all_constraints: IndexVec<ScopedConstraintId, Expression<'db>>,
+    /// Definitions that can reach a [`ScopedUseId`].
+    definitions_by_use: IndexVec<ScopedUseId, Definitions>,

-    /// [`SymbolState`] visible at a [`ScopedUseId`].
-    definitions_by_use: IndexVec<ScopedUseId, SymbolState>,
-
-    /// [`SymbolState`] visible at end of scope for each symbol.
-    public_definitions: IndexVec<ScopedSymbolId, SymbolState>,
+    /// Definitions of each symbol visible at end of scope.
+    public_definitions: IndexVec<ScopedSymbolId, Definitions>,
 }

 impl<'db> UseDefMap<'db> {
-    pub(crate) fn use_definitions(
-        &self,
-        use_id: ScopedUseId,
-    ) -> DefinitionWithConstraintsIterator<'_, 'db> {
-        DefinitionWithConstraintsIterator {
-            all_definitions: &self.all_definitions,
-            all_constraints: &self.all_constraints,
-            inner: self.definitions_by_use[use_id].visible_definitions(),
-        }
+    pub(crate) fn use_definitions(&self, use_id: ScopedUseId) -> &[Definition<'db>] {
+        &self.all_definitions[self.definitions_by_use[use_id].definitions_range.clone()]
    }

    pub(crate) fn use_may_be_unbound(&self, use_id: ScopedUseId) -> bool {
-        self.definitions_by_use[use_id].may_be_unbound()
+        self.definitions_by_use[use_id].may_be_unbound
    }

-    pub(crate) fn public_definitions(
-        &self,
-        symbol: ScopedSymbolId,
-    ) -> DefinitionWithConstraintsIterator<'_, 'db> {
-        DefinitionWithConstraintsIterator {
-            all_definitions: &self.all_definitions,
-            all_constraints: &self.all_constraints,
-            inner: self.public_definitions[symbol].visible_definitions(),
-        }
+    pub(crate) fn public_definitions(&self, symbol: ScopedSymbolId) -> &[Definition<'db>] {
+        &self.all_definitions[self.public_definitions[symbol].definitions_range.clone()]
    }

    pub(crate) fn public_may_be_unbound(&self, symbol: ScopedSymbolId) -> bool {
-        self.public_definitions[symbol].may_be_unbound()
+        self.public_definitions[symbol].may_be_unbound
    }
 }

+/// Definitions visible for a symbol at a particular use (or end-of-scope).
+#[derive(Clone, Debug, PartialEq, Eq)]
+struct Definitions {
+    /// [`Range`] in `all_definitions` of the visible definition IDs.
+    definitions_range: Range<usize>,
+    /// Is the symbol possibly unbound at this point?
+    may_be_unbound: bool,
+}
+
+impl Definitions {
+    /// The default state of a symbol is "no definitions, may be unbound", aka definitely-unbound.
+    fn unbound() -> Self {
+        Self {
+            definitions_range: Range::default(),
+            may_be_unbound: true,
+        }
+    }
+}
+
+impl Default for Definitions {
+    fn default() -> Self {
+        Definitions::unbound()
+    }
+}
+
+/// A snapshot of the visible definitions for each symbol at a particular point in control flow.
+#[derive(Clone, Debug)]
+pub(super) struct FlowSnapshot {
+    definitions_by_symbol: IndexVec<ScopedSymbolId, Definitions>,
+}
+
 #[derive(Debug)]
-pub(crate) struct DefinitionWithConstraintsIterator<'map, 'db> {
-    all_definitions: &'map IndexVec<ScopedDefinitionId, Definition<'db>>,
-    all_constraints: &'map IndexVec<ScopedConstraintId, Expression<'db>>,
-    inner: DefinitionIdWithConstraintsIterator<'map>,
-}
-
-impl<'map, 'db> Iterator for DefinitionWithConstraintsIterator<'map, 'db> {
-    type Item = DefinitionWithConstraints<'map, 'db>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.inner
-            .next()
-            .map(|def_id_with_constraints| DefinitionWithConstraints {
-                definition: self.all_definitions[def_id_with_constraints.definition],
-                constraints: ConstraintsIterator {
-                    all_constraints: self.all_constraints,
-                    constraint_ids: def_id_with_constraints.constraint_ids,
-                },
-            })
-    }
-}
-
-impl std::iter::FusedIterator for DefinitionWithConstraintsIterator<'_, '_> {}
-
-pub(crate) struct DefinitionWithConstraints<'map, 'db> {
-    pub(crate) definition: Definition<'db>,
-    pub(crate) constraints: ConstraintsIterator<'map, 'db>,
-}
-
-pub(crate) struct ConstraintsIterator<'map, 'db> {
-    all_constraints: &'map IndexVec<ScopedConstraintId, Expression<'db>>,
-    constraint_ids: ConstraintIdIterator<'map>,
-}
-
-impl<'map, 'db> Iterator for ConstraintsIterator<'map, 'db> {
-    type Item = Expression<'db>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.constraint_ids
-            .next()
-            .map(|constraint_id| self.all_constraints[constraint_id])
-    }
-}
-
-impl std::iter::FusedIterator for ConstraintsIterator<'_, '_> {}
-
-/// A snapshot of the definitions and constraints state at a particular point in control flow.
-#[derive(Clone, Debug)]
-pub(super) struct FlowSnapshot {
-    definitions_by_symbol: IndexVec<ScopedSymbolId, SymbolState>,
-}
-
-#[derive(Debug, Default)]
 pub(super) struct UseDefMapBuilder<'db> {
-    /// Append-only array of [`Definition`]; None is unbound.
-    all_definitions: IndexVec<ScopedDefinitionId, Definition<'db>>,
-
-    /// Append-only array of constraints (as [`Expression`]).
-    all_constraints: IndexVec<ScopedConstraintId, Expression<'db>>,
+    /// Definition IDs array for `definitions_by_use` and `definitions_by_symbol` to slice into.
+    all_definitions: Vec<Definition<'db>>,

    /// Visible definitions at each so-far-recorded use.
-    definitions_by_use: IndexVec<ScopedUseId, SymbolState>,
+    definitions_by_use: IndexVec<ScopedUseId, Definitions>,

    /// Currently visible definitions for each symbol.
-    definitions_by_symbol: IndexVec<ScopedSymbolId, SymbolState>,
+    definitions_by_symbol: IndexVec<ScopedSymbolId, Definitions>,
 }

 impl<'db> UseDefMapBuilder<'db> {
    pub(super) fn new() -> Self {
-        Self::default()
+        Self {
+            all_definitions: Vec::new(),
+            definitions_by_use: IndexVec::new(),
+            definitions_by_symbol: IndexVec::new(),
+        }
    }

    pub(super) fn add_symbol(&mut self, symbol: ScopedSymbolId) {
-        let new_symbol = self.definitions_by_symbol.push(SymbolState::unbound());
+        let new_symbol = self.definitions_by_symbol.push(Definitions::unbound());
        debug_assert_eq!(symbol, new_symbol);
    }

@@ -286,15 +227,13 @@ impl<'db> UseDefMapBuilder<'db> {
    ) {
        // We have a new definition of a symbol; this replaces any previous definitions in this
        // path.
-        let def_id = self.all_definitions.push(definition);
-        self.definitions_by_symbol[symbol] = SymbolState::with(def_id);
-    }
-
-    pub(super) fn record_constraint(&mut self, constraint: Expression<'db>) {
-        let constraint_id = self.all_constraints.push(constraint);
-        for definitions in &mut self.definitions_by_symbol {
-            definitions.add_constraint(constraint_id);
-        }
+        let def_idx = self.all_definitions.len();
+        self.all_definitions.push(definition);
+        self.definitions_by_symbol[symbol] = Definitions {
+            #[allow(clippy::range_plus_one)]
+            definitions_range: def_idx..(def_idx + 1),
+            may_be_unbound: false,
+        };
    }

    pub(super) fn record_use(&mut self, symbol: ScopedSymbolId, use_id: ScopedUseId) {
@@ -326,15 +265,15 @@ impl<'db> UseDefMapBuilder<'db> {

        // If the snapshot we are restoring is missing some symbols we've recorded since, we need
        // to fill them in so the symbol IDs continue to line up. Since they don't exist in the
-        // snapshot, the correct state to fill them in with is "unbound".
+        // snapshot, the correct state to fill them in with is "unbound", the default.
        self.definitions_by_symbol
-            .resize(num_symbols, SymbolState::unbound());
+            .resize(num_symbols, Definitions::unbound());
    }

    /// Merge the given snapshot into the current state, reflecting that we might have taken either
    /// path to get here. The new visible-definitions state for each symbol should include
    /// definitions from both the prior state and the snapshot.
-    pub(super) fn merge(&mut self, snapshot: FlowSnapshot) {
+    pub(super) fn merge(&mut self, snapshot: &FlowSnapshot) {
        // The tricky thing about merging two Ranges pointing into `all_definitions` is that if the
        // two Ranges aren't already adjacent in `all_definitions`, we will have to copy at least
        // one or the other of the ranges to the end of `all_definitions` so as to make them
@@ -348,26 +287,66 @@ impl<'db> UseDefMapBuilder<'db> {
        // greater than the number of known symbols in a previously-taken snapshot.
        debug_assert!(self.definitions_by_symbol.len() >= snapshot.definitions_by_symbol.len());

-        let mut snapshot_definitions_iter = snapshot.definitions_by_symbol.into_iter();
-        for current in &mut self.definitions_by_symbol {
-            if let Some(snapshot) = snapshot_definitions_iter.next() {
-                current.merge(snapshot);
-            } else {
+        for (symbol_id, current) in self.definitions_by_symbol.iter_mut_enumerated() {
+            let Some(snapshot) = snapshot.definitions_by_symbol.get(symbol_id) else {
                // Symbol not present in snapshot, so it's unbound from that path.
-                current.add_unbound();
+                current.may_be_unbound = true;
+                continue;
+            };
+
+            // If the symbol can be unbound in either predecessor, it can be unbound post-merge.
+            current.may_be_unbound |= snapshot.may_be_unbound;
+
+            // Merge the definition ranges.
+            let current = &mut current.definitions_range;
+            let snapshot = &snapshot.definitions_range;
+
+            // We never create reversed ranges.
+            debug_assert!(current.end >= current.start);
+            debug_assert!(snapshot.end >= snapshot.start);
+
+            if current == snapshot {
+                // Ranges already identical, nothing to do.
+            } else if snapshot.is_empty() {
+                // Merging from an empty range; nothing to do.
+            } else if (*current).is_empty() {
+                // Merging to an empty range; just use the incoming range.
+                *current = snapshot.clone();
+            } else if snapshot.end >= current.start && snapshot.start <= current.end {
+                // Ranges are adjacent or overlapping, merge them in-place.
+                *current = current.start.min(snapshot.start)..current.end.max(snapshot.end);
+            } else if current.end == self.all_definitions.len() {
+                // Ranges are not adjacent or overlapping, `current` is at the end of
+                // `all_definitions`, we need to copy `snapshot` to the end so they are adjacent
+                // and can be merged into one range.
+                self.all_definitions.extend_from_within(snapshot.clone());
+                current.end = self.all_definitions.len();
+            } else if snapshot.end == self.all_definitions.len() {
+                // Ranges are not adjacent or overlapping, `snapshot` is at the end of
+                // `all_definitions`, we need to copy `current` to the end so they are adjacent and
+                // can be merged into one range.
+                self.all_definitions.extend_from_within(current.clone());
+                current.start = snapshot.start;
+                current.end = self.all_definitions.len();
+            } else {
+                // Ranges are not adjacent and neither one is at the end of `all_definitions`, we
+                // have to copy both to the end so they are adjacent and we can merge them.
+                let start = self.all_definitions.len();
+                self.all_definitions.extend_from_within(current.clone());
+                self.all_definitions.extend_from_within(snapshot.clone());
+                current.start = start;
+                current.end = self.all_definitions.len();
            }
        }
    }

    pub(super) fn finish(mut self) -> UseDefMap<'db> {
        self.all_definitions.shrink_to_fit();
-        self.all_constraints.shrink_to_fit();
        self.definitions_by_symbol.shrink_to_fit();
        self.definitions_by_use.shrink_to_fit();

        UseDefMap {
            all_definitions: self.all_definitions,
-            all_constraints: self.all_constraints,
            definitions_by_use: self.definitions_by_use,
            public_definitions: self.definitions_by_symbol,
        }
--- a/crates/red_knot_python_semantic/src/semantic_index/use_def/bitset.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/use_def/bitset.rs
@@ -1,228 +0,0 @@
-/// Ordered set of `u32`.
-///
-/// Uses an inline bit-set for small values (up to 64 * B), falls back to heap allocated vector of
-/// blocks for larger values.
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub(super) enum BitSet<const B: usize> {
-    /// Bit-set (in 64-bit blocks) for the first 64 * B entries.
-    Inline([u64; B]),
-
-    /// Overflow beyond 64 * B.
-    Heap(Vec<u64>),
-}
-
-impl<const B: usize> Default for BitSet<B> {
-    fn default() -> Self {
-        // B * 64 must fit in a u32, or else we have unusable bits; this assertion makes the
-        // truncating casts to u32 below safe. This would be better as a const assertion, but
-        // that's not possible on stable with const generic params. (B should never really be
-        // anywhere close to this large.)
-        assert!(B * 64 < (u32::MAX as usize));
-        // This implementation requires usize >= 32 bits.
-        static_assertions::const_assert!(usize::BITS >= 32);
-        Self::Inline([0; B])
-    }
-}
-
-impl<const B: usize> BitSet<B> {
-    /// Create and return a new [`BitSet`] with a single `value` inserted.
-    pub(super) fn with(value: u32) -> Self {
-        let mut bitset = Self::default();
-        bitset.insert(value);
-        bitset
-    }
-
-    /// Convert from Inline to Heap, if needed, and resize the Heap vector, if needed.
-    fn resize(&mut self, value: u32) {
-        let num_blocks_needed = (value / 64) + 1;
-        match self {
-            Self::Inline(blocks) => {
-                let mut vec = blocks.to_vec();
-                vec.resize(num_blocks_needed as usize, 0);
-                *self = Self::Heap(vec);
-            }
-            Self::Heap(vec) => {
-                vec.resize(num_blocks_needed as usize, 0);
-            }
-        }
-    }
-
-    fn blocks_mut(&mut self) -> &mut [u64] {
-        match self {
-            Self::Inline(blocks) => blocks.as_mut_slice(),
-            Self::Heap(blocks) => blocks.as_mut_slice(),
-        }
-    }
-
-    fn blocks(&self) -> &[u64] {
-        match self {
-            Self::Inline(blocks) => blocks.as_slice(),
-            Self::Heap(blocks) => blocks.as_slice(),
-        }
-    }
-
-    /// Insert a value into the [`BitSet`].
-    ///
-    /// Return true if the value was newly inserted, false if already present.
-    pub(super) fn insert(&mut self, value: u32) -> bool {
-        let value_usize = value as usize;
-        let (block, index) = (value_usize / 64, value_usize % 64);
-        if block >= self.blocks().len() {
-            self.resize(value);
-        }
-        let blocks = self.blocks_mut();
-        let missing = blocks[block] & (1 << index) == 0;
-        blocks[block] |= 1 << index;
-        missing
-    }
-
-    /// Intersect in-place with another [`BitSet`].
-    pub(super) fn intersect(&mut self, other: &BitSet<B>) {
-        let my_blocks = self.blocks_mut();
-        let other_blocks = other.blocks();
-        let min_len = my_blocks.len().min(other_blocks.len());
-        for i in 0..min_len {
-            my_blocks[i] &= other_blocks[i];
-        }
-        for block in my_blocks.iter_mut().skip(min_len) {
-            *block = 0;
-        }
-    }
-
-    /// Return an iterator over the values (in ascending order) in this [`BitSet`].
-    pub(super) fn iter(&self) -> BitSetIterator<'_, B> {
-        let blocks = self.blocks();
-        BitSetIterator {
-            blocks,
-            current_block_index: 0,
-            current_block: blocks[0],
-        }
-    }
-}
-
-/// Iterator over values in a [`BitSet`].
-#[derive(Debug)]
-pub(super) struct BitSetIterator<'a, const B: usize> {
-    /// The blocks we are iterating over.
-    blocks: &'a [u64],
-
-    /// The index of the block we are currently iterating through.
-    current_block_index: usize,
-
-    /// The block we are currently iterating through (and zeroing as we go.)
-    current_block: u64,
-}
-
-impl<const B: usize> Iterator for BitSetIterator<'_, B> {
-    type Item = u32;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        while self.current_block == 0 {
-            if self.current_block_index + 1 >= self.blocks.len() {
-                return None;
-            }
-            self.current_block_index += 1;
-            self.current_block = self.blocks[self.current_block_index];
-        }
-        let lowest_bit_set = self.current_block.trailing_zeros();
-        // reset the lowest set bit, without a data dependency on `lowest_bit_set`
-        self.current_block &= self.current_block.wrapping_sub(1);
-        // SAFETY: `lowest_bit_set` cannot be more than 64, `current_block_index` cannot be more
-        // than `B - 1`, and we check above that `B * 64 < u32::MAX`. So both `64 *
-        // current_block_index` and the final value here must fit in u32.
-        #[allow(clippy::cast_possible_truncation)]
-        Some(lowest_bit_set + (64 * self.current_block_index) as u32)
-    }
-}
-
-impl<const B: usize> std::iter::FusedIterator for BitSetIterator<'_, B> {}
-
-#[cfg(test)]
-mod tests {
-    use super::BitSet;
-
-    fn assert_bitset<const B: usize>(bitset: &BitSet<B>, contents: &[u32]) {
-        assert_eq!(bitset.iter().collect::<Vec<_>>(), contents);
-    }
-
-    #[test]
-    fn iter() {
-        let mut b = BitSet::<1>::with(3);
-        b.insert(27);
-        b.insert(6);
-        assert!(matches!(b, BitSet::Inline(_)));
-        assert_bitset(&b, &[3, 6, 27]);
-    }
-
-    #[test]
-    fn iter_overflow() {
-        let mut b = BitSet::<1>::with(140);
-        b.insert(100);
-        b.insert(129);
-        assert!(matches!(b, BitSet::Heap(_)));
-        assert_bitset(&b, &[100, 129, 140]);
-    }
-
-    #[test]
-    fn intersect() {
-        let mut b1 = BitSet::<1>::with(4);
-        let mut b2 = BitSet::<1>::with(4);
-        b1.insert(23);
-        b2.insert(5);
-
-        b1.intersect(&b2);
-        assert_bitset(&b1, &[4]);
-    }
-
-    #[test]
-    fn intersect_mixed_1() {
-        let mut b1 = BitSet::<1>::with(4);
-        let mut b2 = BitSet::<1>::with(4);
-        b1.insert(89);
-        b2.insert(5);
-
-        b1.intersect(&b2);
-        assert_bitset(&b1, &[4]);
-    }
-
-    #[test]
-    fn intersect_mixed_2() {
-        let mut b1 = BitSet::<1>::with(4);
-        let mut b2 = BitSet::<1>::with(4);
-        b1.insert(23);
-        b2.insert(89);
-
-        b1.intersect(&b2);
-        assert_bitset(&b1, &[4]);
-    }
-
-    #[test]
-    fn intersect_heap() {
-        let mut b1 = BitSet::<1>::with(4);
-        let mut b2 = BitSet::<1>::with(4);
-        b1.insert(89);
-        b2.insert(90);
-
-        b1.intersect(&b2);
-        assert_bitset(&b1, &[4]);
-    }
-
-    #[test]
-    fn intersect_heap_2() {
-        let mut b1 = BitSet::<1>::with(89);
-        let mut b2 = BitSet::<1>::with(89);
-        b1.insert(91);
-        b2.insert(90);
-
-        b1.intersect(&b2);
-        assert_bitset(&b1, &[89]);
-    }
-
-    #[test]
-    fn multiple_blocks() {
-        let mut b = BitSet::<2>::with(120);
-        b.insert(45);
-        assert!(matches!(b, BitSet::Inline(_)));
-        assert_bitset(&b, &[45, 120]);
-    }
-}
--- a/crates/red_knot_python_semantic/src/semantic_index/use_def/symbol_state.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/use_def/symbol_state.rs
@@ -1,374 +0,0 @@
-//! Track visible definitions of a symbol, and applicable constraints per definition.
-//!
-//! These data structures operate entirely on scope-local newtype-indices for definitions and
-//! constraints, referring to their location in the `all_definitions` and `all_constraints`
-//! indexvecs in [`super::UseDefMapBuilder`].
-//!
-//! We need to track arbitrary associations between definitions and constraints, not just a single
-//! set of currently dominating constraints (where "dominating" means "control flow must have
-//! passed through it to reach this point"), because we can have dominating constraints that apply
-//! to some definitions but not others, as in this code:
-//!
-//! ```python
-//! x = 1 if flag else None
-//! if x is not None:
-//!     if flag2:
-//!         x = 2 if flag else None
-//!     x
-//! ```
-//!
-//! The `x is not None` constraint dominates the final use of `x`, but it applies only to the first
-//! definition of `x`, not the second, so `None` is a possible value for `x`.
-//!
-//! And we can't just track, for each definition, an index into a list of dominating constraints,
-//! either, because we can have definitions which are still visible, but subject to constraints
-//! that are no longer dominating, as in this code:
-//!
-//! ```python
-//! x = 0
-//! if flag1:
-//!     x = 1 if flag2 else None
-//!     assert x is not None
-//! x
-//! ```
-//!
-//! From the point of view of the final use of `x`, the `x is not None` constraint no longer
-//! dominates, but it does dominate the `x = 1 if flag2 else None` definition, so we have to keep
-//! track of that.
-//!
-//! The data structures used here ([`BitSet`] and [`smallvec::SmallVec`]) optimize for keeping all
-//! data inline (avoiding lots of scattered allocations) in small-to-medium cases, and falling back
-//! to heap allocation to be able to scale to arbitrary numbers of definitions and constraints when
-//! needed.
-use super::bitset::{BitSet, BitSetIterator};
-use ruff_index::newtype_index;
-use smallvec::SmallVec;
-
-/// A newtype-index for a definition in a particular scope.
-#[newtype_index]
-pub(super) struct ScopedDefinitionId;
-
-/// A newtype-index for a constraint expression in a particular scope.
-#[newtype_index]
-pub(super) struct ScopedConstraintId;
-
-/// Can reference this * 64 total definitions inline; more will fall back to the heap.
-const INLINE_DEFINITION_BLOCKS: usize = 3;
-
-/// A [`BitSet`] of [`ScopedDefinitionId`], representing visible definitions of a symbol in a scope.
-type Definitions = BitSet<INLINE_DEFINITION_BLOCKS>;
-type DefinitionsIterator<'a> = BitSetIterator<'a, INLINE_DEFINITION_BLOCKS>;
-
-/// Can reference this * 64 total constraints inline; more will fall back to the heap.
-const INLINE_CONSTRAINT_BLOCKS: usize = 2;
-
-/// Can keep inline this many visible definitions per symbol at a given time; more will go to heap.
-const INLINE_VISIBLE_DEFINITIONS_PER_SYMBOL: usize = 4;
-
-/// One [`BitSet`] of applicable [`ScopedConstraintId`] per visible definition.
-type InlineConstraintArray =
-    [BitSet<INLINE_CONSTRAINT_BLOCKS>; INLINE_VISIBLE_DEFINITIONS_PER_SYMBOL];
-type Constraints = SmallVec<InlineConstraintArray>;
-type ConstraintsIterator<'a> = std::slice::Iter<'a, BitSet<INLINE_CONSTRAINT_BLOCKS>>;
-type ConstraintsIntoIterator = smallvec::IntoIter<InlineConstraintArray>;
-
-/// Visible definitions and narrowing constraints for a single symbol at some point in control flow.
-#[derive(Clone, Debug, PartialEq, Eq)]
-pub(super) struct SymbolState {
-    /// [`BitSet`]: which [`ScopedDefinitionId`] are visible for this symbol?
-    visible_definitions: Definitions,
-
-    /// For each definition, which [`ScopedConstraintId`] apply?
-    ///
-    /// This is a [`smallvec::SmallVec`] which should always have one [`BitSet`] of constraints per
-    /// definition in `visible_definitions`.
-    constraints: Constraints,
-
-    /// Could the symbol be unbound at this point?
-    may_be_unbound: bool,
-}
-
-/// A single [`ScopedDefinitionId`] with an iterator of its applicable [`ScopedConstraintId`].
-#[derive(Debug)]
-pub(super) struct DefinitionIdWithConstraints<'a> {
-    pub(super) definition: ScopedDefinitionId,
-    pub(super) constraint_ids: ConstraintIdIterator<'a>,
-}
-
-impl SymbolState {
-    /// Return a new [`SymbolState`] representing an unbound symbol.
-    pub(super) fn unbound() -> Self {
-        Self {
-            visible_definitions: Definitions::default(),
-            constraints: Constraints::default(),
-            may_be_unbound: true,
-        }
-    }
-
-    /// Return a new [`SymbolState`] representing a symbol with a single visible definition.
-    pub(super) fn with(definition_id: ScopedDefinitionId) -> Self {
-        let mut constraints = Constraints::with_capacity(1);
-        constraints.push(BitSet::default());
-        Self {
-            visible_definitions: Definitions::with(definition_id.into()),
-            constraints,
-            may_be_unbound: false,
-        }
-    }
-
-    /// Add Unbound as a possibility for this symbol.
-    pub(super) fn add_unbound(&mut self) {
-        self.may_be_unbound = true;
-    }
-
-    /// Add given constraint to all currently-visible definitions.
-    pub(super) fn add_constraint(&mut self, constraint_id: ScopedConstraintId) {
-        for bitset in &mut self.constraints {
-            bitset.insert(constraint_id.into());
-        }
-    }
-
-    /// Merge another [`SymbolState`] into this one.
-    pub(super) fn merge(&mut self, b: SymbolState) {
-        let mut a = Self {
-            visible_definitions: Definitions::default(),
-            constraints: Constraints::default(),
-            may_be_unbound: self.may_be_unbound || b.may_be_unbound,
-        };
-        std::mem::swap(&mut a, self);
-        let mut a_defs_iter = a.visible_definitions.iter();
-        let mut b_defs_iter = b.visible_definitions.iter();
-        let mut a_constraints_iter = a.constraints.into_iter();
-        let mut b_constraints_iter = b.constraints.into_iter();
-
-        let mut opt_a_def: Option<u32> = a_defs_iter.next();
-        let mut opt_b_def: Option<u32> = b_defs_iter.next();
-
-        // Iterate through the definitions from `a` and `b`, always processing the lower definition
-        // ID first, and pushing each definition onto the merged `SymbolState` with its
-        // constraints. If a definition is found in both `a` and `b`, push it with the intersection
-        // of the constraints from the two paths; a constraint that applies from only one possible
-        // path is irrelevant.
-
-        // Helper to push `def`, with constraints in `constraints_iter`, onto `self`.
-        let push = |def, constraints_iter: &mut ConstraintsIntoIterator, merged: &mut Self| {
-            merged.visible_definitions.insert(def);
-            // SAFETY: we only ever create SymbolState with either no definitions and no constraint
-            // bitsets (`::unbound`) or one definition and one constraint bitset (`::with`), and
-            // `::merge` always pushes one definition and one constraint bitset together (just
-            // below), so the number of definitions and the number of constraint bitsets can never
-            // get out of sync.
-            let constraints = constraints_iter
-                .next()
-                .expect("definitions and constraints length mismatch");
-            merged.constraints.push(constraints);
-        };
-
-        loop {
-            match (opt_a_def, opt_b_def) {
-                (Some(a_def), Some(b_def)) => match a_def.cmp(&b_def) {
-                    std::cmp::Ordering::Less => {
-                        // Next definition ID is only in `a`, push it to `self` and advance `a`.
-                        push(a_def, &mut a_constraints_iter, self);
-                        opt_a_def = a_defs_iter.next();
-                    }
-                    std::cmp::Ordering::Greater => {
-                        // Next definition ID is only in `b`, push it to `self` and advance `b`.
-                        push(b_def, &mut b_constraints_iter, self);
-                        opt_b_def = b_defs_iter.next();
-                    }
-                    std::cmp::Ordering::Equal => {
-                        // Next definition is in both; push to `self` and intersect constraints.
-                        push(a_def, &mut b_constraints_iter, self);
-                        // SAFETY: we only ever create SymbolState with either no definitions and
-                        // no constraint bitsets (`::unbound`) or one definition and one constraint
-                        // bitset (`::with`), and `::merge` always pushes one definition and one
-                        // constraint bitset together (just below), so the number of definitions
-                        // and the number of constraint bitsets can never get out of sync.
-                        let a_constraints = a_constraints_iter
-                            .next()
-                            .expect("definitions and constraints length mismatch");
-                        // If the same definition is visible through both paths, any constraint
-                        // that applies on only one path is irrelevant to the resulting type from
-                        // unioning the two paths, so we intersect the constraints.
-                        self.constraints
-                            .last_mut()
-                            .unwrap()
-                            .intersect(&a_constraints);
-                        opt_a_def = a_defs_iter.next();
-                        opt_b_def = b_defs_iter.next();
-                    }
-                },
-                (Some(a_def), None) => {
-                    // We've exhausted `b`, just push the def from `a` and move on to the next.
-                    push(a_def, &mut a_constraints_iter, self);
-                    opt_a_def = a_defs_iter.next();
-                }
-                (None, Some(b_def)) => {
-                    // We've exhausted `a`, just push the def from `b` and move on to the next.
-                    push(b_def, &mut b_constraints_iter, self);
-                    opt_b_def = b_defs_iter.next();
-                }
-                (None, None) => break,
-            }
-        }
-    }
-
-    /// Get iterator over visible definitions with constraints.
-    pub(super) fn visible_definitions(&self) -> DefinitionIdWithConstraintsIterator {
-        DefinitionIdWithConstraintsIterator {
-            definitions: self.visible_definitions.iter(),
-            constraints: self.constraints.iter(),
-        }
-    }
-
-    /// Could the symbol be unbound?
-    pub(super) fn may_be_unbound(&self) -> bool {
-        self.may_be_unbound
-    }
-}
-
-/// The default state of a symbol (if we've seen no definitions of it) is unbound.
-impl Default for SymbolState {
-    fn default() -> Self {
-        SymbolState::unbound()
-    }
-}
-
-#[derive(Debug)]
-pub(super) struct DefinitionIdWithConstraintsIterator<'a> {
-    definitions: DefinitionsIterator<'a>,
-    constraints: ConstraintsIterator<'a>,
-}
-
-impl<'a> Iterator for DefinitionIdWithConstraintsIterator<'a> {
-    type Item = DefinitionIdWithConstraints<'a>;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        match (self.definitions.next(), self.constraints.next()) {
-            (None, None) => None,
-            (Some(def), Some(constraints)) => Some(DefinitionIdWithConstraints {
-                definition: ScopedDefinitionId::from_u32(def),
-                constraint_ids: ConstraintIdIterator {
-                    wrapped: constraints.iter(),
-                },
-            }),
-            // SAFETY: see above.
-            _ => unreachable!("definitions and constraints length mismatch"),
-        }
-    }
-}
-
-impl std::iter::FusedIterator for DefinitionIdWithConstraintsIterator<'_> {}
-
-#[derive(Debug)]
-pub(super) struct ConstraintIdIterator<'a> {
-    wrapped: BitSetIterator<'a, INLINE_CONSTRAINT_BLOCKS>,
-}
-
-impl Iterator for ConstraintIdIterator<'_> {
-    type Item = ScopedConstraintId;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.wrapped.next().map(ScopedConstraintId::from_u32)
-    }
-}
-
-impl std::iter::FusedIterator for ConstraintIdIterator<'_> {}
-
-#[cfg(test)]
-mod tests {
-    use super::{ScopedConstraintId, ScopedDefinitionId, SymbolState};
-
-    impl SymbolState {
-        pub(crate) fn assert(&self, may_be_unbound: bool, expected: &[&str]) {
-            assert_eq!(self.may_be_unbound(), may_be_unbound);
-            let actual = self
-                .visible_definitions()
-                .map(|def_id_with_constraints| {
-                    format!(
-                        "{}<{}>",
-                        def_id_with_constraints.definition.as_u32(),
-                        def_id_with_constraints
-                            .constraint_ids
-                            .map(ScopedConstraintId::as_u32)
-                            .map(|idx| idx.to_string())
-                            .collect::<Vec<_>>()
-                            .join(", ")
-                    )
-                })
-                .collect::<Vec<_>>();
-            assert_eq!(actual, expected);
-        }
-    }
-
-    #[test]
-    fn unbound() {
-        let cd = SymbolState::unbound();
-
-        cd.assert(true, &[]);
-    }
-
-    #[test]
-    fn with() {
-        let cd = SymbolState::with(ScopedDefinitionId::from_u32(0));
-
-        cd.assert(false, &["0<>"]);
-    }
-
-    #[test]
-    fn add_unbound() {
-        let mut cd = SymbolState::with(ScopedDefinitionId::from_u32(0));
-        cd.add_unbound();
-
-        cd.assert(true, &["0<>"]);
-    }
-
-    #[test]
-    fn add_constraint() {
-        let mut cd = SymbolState::with(ScopedDefinitionId::from_u32(0));
-        cd.add_constraint(ScopedConstraintId::from_u32(0));
-
-        cd.assert(false, &["0<0>"]);
-    }
-
-    #[test]
-    fn merge() {
-        // merging the same definition with the same constraint keeps the constraint
-        let mut cd0a = SymbolState::with(ScopedDefinitionId::from_u32(0));
-        cd0a.add_constraint(ScopedConstraintId::from_u32(0));
-
-        let mut cd0b = SymbolState::with(ScopedDefinitionId::from_u32(0));
-        cd0b.add_constraint(ScopedConstraintId::from_u32(0));
-
-        cd0a.merge(cd0b);
-        let mut cd0 = cd0a;
-        cd0.assert(false, &["0<0>"]);
-
-        // merging the same definition with differing constraints drops all constraints
-        let mut cd1a = SymbolState::with(ScopedDefinitionId::from_u32(1));
-        cd1a.add_constraint(ScopedConstraintId::from_u32(1));
-
-        let mut cd1b = SymbolState::with(ScopedDefinitionId::from_u32(1));
-        cd1b.add_constraint(ScopedConstraintId::from_u32(2));
-
-        cd1a.merge(cd1b);
-        let cd1 = cd1a;
-        cd1.assert(false, &["1<>"]);
-
-        // merging a constrained definition with unbound keeps both
-        let mut cd2a = SymbolState::with(ScopedDefinitionId::from_u32(2));
-        cd2a.add_constraint(ScopedConstraintId::from_u32(3));
-
-        let cd2b = SymbolState::unbound();
-
-        cd2a.merge(cd2b);
-        let cd2 = cd2a;
-        cd2.assert(true, &["2<3>"]);
-
-        // merging different definitions keeps them each with their existing constraints
-        cd0.merge(cd2);
-        let cd = cd0;
-        cd.assert(true, &["0<0>", "2<3>"]);
-    }
-}
--- a/crates/red_knot_python_semantic/src/semantic_model.rs
+++ b/crates/red_knot_python_semantic/src/semantic_model.rs
@@ -1,7 +1,7 @@
 use ruff_db::files::{File, FilePath};
 use ruff_db::source::line_index;
 use ruff_python_ast as ast;
-use ruff_python_ast::{Expr, ExpressionRef};
+use ruff_python_ast::{Expr, ExpressionRef, StmtClassDef};
 use ruff_source_file::LineIndex;

 use crate::module_name::ModuleName;
@@ -147,24 +147,29 @@ impl HasTy for ast::Expr {
    }
 }

-macro_rules! impl_definition_has_ty {
-    ($ty: ty) => {
-        impl HasTy for $ty {
-            #[inline]
-            fn ty<'db>(&self, model: &SemanticModel<'db>) -> Type<'db> {
-                let index = semantic_index(model.db, model.file);
-                let definition = index.definition(self);
-                definition_ty(model.db, definition)
-            }
-        }
-    };
+impl HasTy for ast::StmtFunctionDef {
+    fn ty<'db>(&self, model: &SemanticModel<'db>) -> Type<'db> {
+        let index = semantic_index(model.db, model.file);
+        let definition = index.definition(self);
+        definition_ty(model.db, definition)
+    }
 }

-impl_definition_has_ty!(ast::StmtFunctionDef);
-impl_definition_has_ty!(ast::StmtClassDef);
-impl_definition_has_ty!(ast::Alias);
-impl_definition_has_ty!(ast::Parameter);
-impl_definition_has_ty!(ast::ParameterWithDefault);
+impl HasTy for StmtClassDef {
+    fn ty<'db>(&self, model: &SemanticModel<'db>) -> Type<'db> {
+        let index = semantic_index(model.db, model.file);
+        let definition = index.definition(self);
+        definition_ty(model.db, definition)
+    }
+}
+
+impl HasTy for ast::Alias {
+    fn ty<'db>(&self, model: &SemanticModel<'db>) -> Type<'db> {
+        let index = semantic_index(model.db, model.file);
+        let definition = index.definition(self);
+        definition_ty(model.db, definition)
+    }
+}

 #[cfg(test)]
 mod tests {
@@ -184,9 +189,14 @@ mod tests {

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version: PythonVersion::default(),
-                search_paths: SearchPathSettings::new(SystemPathBuf::from("/src")),
+                search_paths: SearchPathSettings {
+                    extra_paths: vec![],
+                    src_root: SystemPathBuf::from("/src"),
+                    site_packages: vec![],
+                    custom_typeshed: None,
+                },
            },
        )?;

--- a/crates/red_knot_python_semantic/src/types.rs
+++ b/crates/red_knot_python_semantic/src/types.rs
@@ -4,38 +4,15 @@ use ruff_python_ast::name::Name;
 use crate::builtins::builtins_scope;
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::symbol::{ScopeId, ScopedSymbolId};
-use crate::semantic_index::{
-    global_scope, semantic_index, symbol_table, use_def_map, DefinitionWithConstraints,
-    DefinitionWithConstraintsIterator,
-};
-use crate::types::narrow::narrowing_constraint;
+use crate::semantic_index::{global_scope, symbol_table, use_def_map};
 use crate::{Db, FxOrderSet};

-pub(crate) use self::builder::{IntersectionBuilder, UnionBuilder};
-pub(crate) use self::diagnostic::TypeCheckDiagnostics;
-pub(crate) use self::infer::{
-    infer_definition_types, infer_expression_types, infer_scope_types, TypeInference,
-};
-
 mod builder;
-mod diagnostic;
 mod display;
 mod infer;
-mod narrow;

-pub fn check_types(db: &dyn Db, file: File) -> TypeCheckDiagnostics {
-    let _span = tracing::trace_span!("check_types", file=?file.path(db)).entered();
-
-    let index = semantic_index(db, file);
-    let mut diagnostics = TypeCheckDiagnostics::new();
-
-    for scope_id in index.scope_ids() {
-        let result = infer_scope_types(db, scope_id);
-        diagnostics.extend(result.diagnostics());
-    }
-
-    diagnostics
-}
+pub(crate) use self::builder::UnionBuilder;
+pub(crate) use self::infer::{infer_definition_types, infer_scope_types};

 /// Infer the public type of a symbol (its type as seen from outside its scope).
 pub(crate) fn symbol_ty<'db>(
@@ -105,31 +82,10 @@ pub(crate) fn definition_ty<'db>(db: &'db dyn Db, definition: Definition<'db>) -
 /// provide an `unbound_ty`.
 pub(crate) fn definitions_ty<'db>(
    db: &'db dyn Db,
-    definitions_with_constraints: DefinitionWithConstraintsIterator<'_, 'db>,
+    definitions: &[Definition<'db>],
    unbound_ty: Option<Type<'db>>,
 ) -> Type<'db> {
-    let def_types = definitions_with_constraints.map(
-        |DefinitionWithConstraints {
-             definition,
-             constraints,
-         }| {
-            let mut constraint_tys =
-                constraints.filter_map(|test| narrowing_constraint(db, test, definition));
-            let definition_ty = definition_ty(db, definition);
-            if let Some(first_constraint_ty) = constraint_tys.next() {
-                let mut builder = IntersectionBuilder::new(db);
-                builder = builder
-                    .add_positive(definition_ty)
-                    .add_positive(first_constraint_ty);
-                for constraint_ty in constraint_tys {
-                    builder = builder.add_positive(constraint_ty);
-                }
-                builder.build()
-            } else {
-                definition_ty
-            }
-        },
-    );
+    let def_types = definitions.iter().map(|def| definition_ty(db, *def));
    let mut all_types = unbound_ty.into_iter().chain(def_types);

    let Some(first) = all_types.next() else {
@@ -222,42 +178,20 @@ impl<'db> Type<'db> {
        }
    }

-    /// Resolve a member access of a type.
-    ///
-    /// For example, if `foo` is `Type::Instance(<Bar>)`,
-    /// `foo.member(&db, "baz")` returns the type of `baz` attributes
-    /// as accessed from instances of the `Bar` class.
-    ///
-    /// TODO: use of this method currently requires manually checking
-    /// whether the returned type is `Unknown`/`Unbound`
-    /// (or a union with `Unknown`/`Unbound`) in many places.
-    /// Ideally we'd use a more type-safe pattern, such as returning
-    /// an `Option` or a `Result` from this method, which would force
-    /// us to explicitly consider whether to handle an error or propagate
-    /// it up the call stack.
    #[must_use]
    pub fn member(&self, db: &'db dyn Db, name: &Name) -> Type<'db> {
        match self {
            Type::Any => Type::Any,
-            Type::Never => {
-                // TODO: attribute lookup on Never type
-                Type::Unknown
-            }
+            Type::Never => todo!("attribute lookup on Never type"),
            Type::Unknown => Type::Unknown,
            Type::Unbound => Type::Unbound,
-            Type::None => {
-                // TODO: attribute lookup on None type
-                Type::Unknown
-            }
-            Type::Function(_) => {
-                // TODO: attribute lookup on function type
-                Type::Unknown
-            }
+            Type::None => todo!("attribute lookup on None type"),
+            Type::Function(_) => todo!("attribute lookup on Function type"),
            Type::Module(file) => global_symbol_ty_by_name(db, *file, name),
            Type::Class(class) => class.class_member(db, name),
            Type::Instance(_) => {
                // TODO MRO? get_own_instance_member, get_instance_member
-                Type::Unknown
+                todo!("attribute lookup on Instance type")
            }
            Type::Union(union) => union
                .elements(db)
@@ -269,7 +203,7 @@ impl<'db> Type<'db> {
            Type::Intersection(_) => {
                // TODO perform the get_member on each type in the intersection
                // TODO return the intersection of those results
-                Type::Unknown
+                todo!("attribute lookup on Intersection type")
            }
            Type::IntLiteral(_) => {
                // TODO raise error
@@ -371,110 +305,3 @@ pub struct IntersectionType<'db> {
    /// directly in intersections rather than as a separate type.
    negative: FxOrderSet<Type<'db>>,
 }
-
-#[cfg(test)]
-mod tests {
-    use anyhow::Context;
-
-    use ruff_db::files::system_path_to_file;
-    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
-
-    use crate::db::tests::TestDb;
-    use crate::{Program, ProgramSettings, PythonVersion, SearchPathSettings};
-
-    use super::TypeCheckDiagnostics;
-
-    fn setup_db() -> TestDb {
-        let db = TestDb::new();
-        db.memory_file_system()
-            .create_directory_all("/src")
-            .unwrap();
-
-        Program::from_settings(
-            &db,
-            &ProgramSettings {
-                target_version: PythonVersion::default(),
-                search_paths: SearchPathSettings::new(SystemPathBuf::from("/src")),
-            },
-        )
-        .expect("Valid search path settings");
-
-        db
-    }
-
-    fn assert_diagnostic_messages(diagnostics: &TypeCheckDiagnostics, expected: &[&str]) {
-        let messages: Vec<&str> = diagnostics
-            .iter()
-            .map(|diagnostic| diagnostic.message())
-            .collect();
-        assert_eq!(&messages, expected);
-    }
-
-    #[test]
-    fn unresolved_import_statement() -> anyhow::Result<()> {
-        let mut db = setup_db();
-
-        db.write_file("src/foo.py", "import bar\n")
-            .context("Failed to write foo.py")?;
-
-        let foo = system_path_to_file(&db, "src/foo.py").context("Failed to resolve foo.py")?;
-
-        let diagnostics = super::check_types(&db, foo);
-        assert_diagnostic_messages(&diagnostics, &["Import 'bar' could not be resolved."]);
-
-        Ok(())
-    }
-
-    #[test]
-    fn unresolved_import_from_statement() {
-        let mut db = setup_db();
-
-        db.write_file("src/foo.py", "from bar import baz\n")
-            .unwrap();
-        let foo = system_path_to_file(&db, "src/foo.py").unwrap();
-        let diagnostics = super::check_types(&db, foo);
-        assert_diagnostic_messages(&diagnostics, &["Import 'bar' could not be resolved."]);
-    }
-
-    #[test]
-    fn unresolved_import_from_resolved_module() {
-        let mut db = setup_db();
-
-        db.write_files([("/src/a.py", ""), ("/src/b.py", "from a import thing")])
-            .unwrap();
-
-        let b_file = system_path_to_file(&db, "/src/b.py").unwrap();
-        let b_file_diagnostics = super::check_types(&db, b_file);
-        assert_diagnostic_messages(
-            &b_file_diagnostics,
-            &["Could not resolve import of 'thing' from 'a'"],
-        );
-    }
-
-    #[ignore = "\
-A spurious second 'Unresolved import' diagnostic message is emitted on `b.py`, \
-despite the symbol existing in the symbol table for `a.py`"]
-    #[test]
-    fn resolved_import_of_symbol_from_unresolved_import() {
-        let mut db = setup_db();
-
-        db.write_files([
-            ("/src/a.py", "import foo as foo"),
-            ("/src/b.py", "from a import foo"),
-        ])
-        .unwrap();
-
-        let a_file = system_path_to_file(&db, "/src/a.py").unwrap();
-        let a_file_diagnostics = super::check_types(&db, a_file);
-        assert_diagnostic_messages(
-            &a_file_diagnostics,
-            &["Import 'foo' could not be resolved."],
-        );
-
-        // Importing the unresolved import into a second first-party file should not trigger
-        // an additional "unresolved import" violation
-        let b_file = system_path_to_file(&db, "/src/b.py").unwrap();
-        let b_file_diagnostics = super::check_types(&db, b_file);
-        assert_eq!(&*b_file_diagnostics, &[]);
-    }
-}
--- a/crates/red_knot_python_semantic/src/types/builder.rs
+++ b/crates/red_knot_python_semantic/src/types/builder.rs
@@ -65,6 +65,7 @@ impl<'db> UnionBuilder<'db> {
    }
 }

+#[allow(unused)]
 #[derive(Clone)]
 pub(crate) struct IntersectionBuilder<'db> {
    // Really this builds a union-of-intersections, because we always keep our set-theoretic types
@@ -77,7 +78,8 @@ pub(crate) struct IntersectionBuilder<'db> {
 }

 impl<'db> IntersectionBuilder<'db> {
-    pub(crate) fn new(db: &'db dyn Db) -> Self {
+    #[allow(dead_code)]
+    fn new(db: &'db dyn Db) -> Self {
        Self {
            db,
            intersections: vec![InnerIntersectionBuilder::new()],
@@ -91,7 +93,8 @@ impl<'db> IntersectionBuilder<'db> {
        }
    }

-    pub(crate) fn add_positive(mut self, ty: Type<'db>) -> Self {
+    #[allow(dead_code)]
+    fn add_positive(mut self, ty: Type<'db>) -> Self {
        if let Type::Union(union) = ty {
            // Distribute ourself over this union: for each union element, clone ourself and
            // intersect with that union element, then create a new union-of-intersections with all
@@ -119,7 +122,8 @@ impl<'db> IntersectionBuilder<'db> {
        }
    }

-    pub(crate) fn add_negative(mut self, ty: Type<'db>) -> Self {
+    #[allow(dead_code)]
+    fn add_negative(mut self, ty: Type<'db>) -> Self {
        // See comments above in `add_positive`; this is just the negated version.
        if let Type::Union(union) = ty {
            union
@@ -138,7 +142,8 @@ impl<'db> IntersectionBuilder<'db> {
        }
    }

-    pub(crate) fn build(mut self) -> Type<'db> {
+    #[allow(dead_code)]
+    fn build(mut self) -> Type<'db> {
        // Avoid allocating the UnionBuilder unnecessarily if we have just one intersection:
        if self.intersections.len() == 1 {
            self.intersections.pop().unwrap().build(self.db)
@@ -152,6 +157,7 @@ impl<'db> IntersectionBuilder<'db> {
    }
 }

+#[allow(unused)]
 #[derive(Debug, Clone, Default)]
 struct InnerIntersectionBuilder<'db> {
    positive: FxOrderSet<Type<'db>>,
@@ -217,16 +223,6 @@ impl<'db> InnerIntersectionBuilder<'db> {
            self.positive.retain(Type::is_unbound);
            self.negative.clear();
        }
-
-        // None intersects only with object
-        for pos in &self.positive {
-            if let Type::Instance(_) = pos {
-                // could be `object` type
-            } else {
-                self.negative.remove(&Type::None);
-                break;
-            }
-        }
    }

    fn build(mut self, db: &'db dyn Db) -> Type<'db> {
@@ -457,15 +453,4 @@ mod tests {

        assert_eq!(ty, Type::IntLiteral(1));
    }
-
-    #[test]
-    fn build_intersection_simplify_negative_none() {
-        let db = setup_db();
-        let ty = IntersectionBuilder::new(&db)
-            .add_negative(Type::None)
-            .add_positive(Type::IntLiteral(1))
-            .build();
-
-        assert_eq!(ty, Type::IntLiteral(1));
-    }
 }
--- a/crates/red_knot_python_semantic/src/types/diagnostic.rs
+++ b/crates/red_knot_python_semantic/src/types/diagnostic.rs
@@ -1,111 +0,0 @@
-use ruff_db::files::File;
-use ruff_text_size::{Ranged, TextRange};
-use std::fmt::Formatter;
-use std::ops::Deref;
-use std::sync::Arc;
-
-#[derive(Debug, Eq, PartialEq)]
-pub struct TypeCheckDiagnostic {
-    // TODO: Don't use string keys for rules
-    pub(super) rule: String,
-    pub(super) message: String,
-    pub(super) range: TextRange,
-    pub(super) file: File,
-}
-
-impl TypeCheckDiagnostic {
-    pub fn rule(&self) -> &str {
-        &self.rule
-    }
-
-    pub fn message(&self) -> &str {
-        &self.message
-    }
-
-    pub fn file(&self) -> File {
-        self.file
-    }
-}
-
-impl Ranged for TypeCheckDiagnostic {
-    fn range(&self) -> TextRange {
-        self.range
-    }
-}
-
-/// A collection of type check diagnostics.
-///
-/// The diagnostics are wrapped in an `Arc` because they need to be cloned multiple times
-/// when going from `infer_expression` to `check_file`. We could consider
-/// making [`TypeCheckDiagnostic`] a Salsa struct to have them Arena-allocated (once the Tables refactor is done).
-/// Using Salsa struct does have the downside that it leaks the Salsa dependency into diagnostics and
-/// each Salsa-struct comes with an overhead.
-#[derive(Default, Eq, PartialEq)]
-pub struct TypeCheckDiagnostics {
-    inner: Vec<std::sync::Arc<TypeCheckDiagnostic>>,
-}
-
-impl TypeCheckDiagnostics {
-    pub fn new() -> Self {
-        Self { inner: Vec::new() }
-    }
-
-    pub(super) fn push(&mut self, diagnostic: TypeCheckDiagnostic) {
-        self.inner.push(Arc::new(diagnostic));
-    }
-
-    pub(crate) fn shrink_to_fit(&mut self) {
-        self.inner.shrink_to_fit();
-    }
-}
-
-impl Extend<TypeCheckDiagnostic> for TypeCheckDiagnostics {
-    fn extend<T: IntoIterator<Item = TypeCheckDiagnostic>>(&mut self, iter: T) {
-        self.inner.extend(iter.into_iter().map(std::sync::Arc::new));
-    }
-}
-
-impl Extend<std::sync::Arc<TypeCheckDiagnostic>> for TypeCheckDiagnostics {
-    fn extend<T: IntoIterator<Item = Arc<TypeCheckDiagnostic>>>(&mut self, iter: T) {
-        self.inner.extend(iter);
-    }
-}
-
-impl<'a> Extend<&'a std::sync::Arc<TypeCheckDiagnostic>> for TypeCheckDiagnostics {
-    fn extend<T: IntoIterator<Item = &'a Arc<TypeCheckDiagnostic>>>(&mut self, iter: T) {
-        self.inner
-            .extend(iter.into_iter().map(std::sync::Arc::clone));
-    }
-}
-
-impl std::fmt::Debug for TypeCheckDiagnostics {
-    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
-        self.inner.fmt(f)
-    }
-}
-
-impl Deref for TypeCheckDiagnostics {
-    type Target = [std::sync::Arc<TypeCheckDiagnostic>];
-
-    fn deref(&self) -> &Self::Target {
-        &self.inner
-    }
-}
-
-impl IntoIterator for TypeCheckDiagnostics {
-    type Item = Arc<TypeCheckDiagnostic>;
-    type IntoIter = std::vec::IntoIter<std::sync::Arc<TypeCheckDiagnostic>>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.inner.into_iter()
-    }
-}
-
-impl<'a> IntoIterator for &'a TypeCheckDiagnostics {
-    type Item = &'a Arc<TypeCheckDiagnostic>;
-    type IntoIter = std::slice::Iter<'a, std::sync::Arc<TypeCheckDiagnostic>>;
-
-    fn into_iter(self) -> Self::IntoIter {
-        self.inner.iter()
-    }
-}
--- a/crates/red_knot_python_semantic/src/types/infer.rs
+++ b/crates/red_knot_python_semantic/src/types/infer.rs
@@ -29,8 +29,7 @@ use salsa::plumbing::AsId;
 use ruff_db::files::File;
 use ruff_db::parsed::parsed_module;
 use ruff_python_ast as ast;
-use ruff_python_ast::{AnyNodeRef, ExprContext};
-use ruff_text_size::Ranged;
+use ruff_python_ast::{ExprContext, TypeParams};

 use crate::builtins::builtins_scope;
 use crate::module_name::ModuleName;
@@ -41,7 +40,6 @@ use crate::semantic_index::expression::Expression;
 use crate::semantic_index::semantic_index;
 use crate::semantic_index::symbol::{FileScopeId, NodeWithScopeKind, NodeWithScopeRef, ScopeId};
 use crate::semantic_index::SemanticIndex;
-use crate::types::diagnostic::{TypeCheckDiagnostic, TypeCheckDiagnostics};
 use crate::types::{
    builtins_symbol_ty_by_name, definitions_ty, global_symbol_ty_by_name, ClassType, FunctionType,
    Name, Type, UnionBuilder,
@@ -125,16 +123,13 @@ pub(crate) enum InferenceRegion<'db> {
 }

 /// The inferred types for a single region.
-#[derive(Debug, Eq, PartialEq, Default)]
+#[derive(Debug, Eq, PartialEq, Default, Clone)]
 pub(crate) struct TypeInference<'db> {
    /// The types of every expression in this region.
    expressions: FxHashMap<ScopedExpressionId, Type<'db>>,

    /// The types of every definition in this region.
    definitions: FxHashMap<Definition<'db>, Type<'db>>,
-
-    /// The diagnostics for this region.
-    diagnostics: TypeCheckDiagnostics,
 }

 impl<'db> TypeInference<'db> {
@@ -147,14 +142,9 @@ impl<'db> TypeInference<'db> {
        self.definitions[&definition]
    }

-    pub(crate) fn diagnostics(&self) -> &[std::sync::Arc<TypeCheckDiagnostic>] {
-        &self.diagnostics
-    }
-
    fn shrink_to_fit(&mut self) {
        self.expressions.shrink_to_fit();
        self.definitions.shrink_to_fit();
-        self.diagnostics.shrink_to_fit();
    }
 }

@@ -245,7 +235,6 @@ impl<'db> TypeInferenceBuilder<'db> {
    fn extend(&mut self, inference: &TypeInference<'db>) {
        self.types.definitions.extend(inference.definitions.iter());
        self.types.expressions.extend(inference.expressions.iter());
-        self.types.diagnostics.extend(&inference.diagnostics);
    }

    /// Infers types in the given [`InferenceRegion`].
@@ -305,18 +294,11 @@ impl<'db> TypeInferenceBuilder<'db> {
                );
            }
            DefinitionKind::Assignment(assignment) => {
-                self.infer_assignment_definition(
-                    assignment.target(),
-                    assignment.assignment(),
-                    definition,
-                );
+                self.infer_assignment_definition(assignment.assignment(), definition);
            }
            DefinitionKind::AnnotatedAssignment(annotated_assignment) => {
                self.infer_annotated_assignment_definition(annotated_assignment.node(), definition);
            }
-            DefinitionKind::AugmentedAssignment(augmented_assignment) => {
-                self.infer_augment_assignment_definition(augmented_assignment.node(), definition);
-            }
            DefinitionKind::NamedExpression(named_expression) => {
                self.infer_named_expression_definition(named_expression.node(), definition);
            }
@@ -333,14 +315,11 @@ impl<'db> TypeInferenceBuilder<'db> {
            DefinitionKind::ParameterWithDefault(parameter_with_default) => {
                self.infer_parameter_with_default_definition(parameter_with_default, definition);
            }
-            DefinitionKind::WithItem(with_item) => {
-                self.infer_with_item_definition(with_item.target(), with_item.node(), definition);
-            }
        }
    }

    fn infer_region_expression(&mut self, expression: Expression<'db>) {
-        self.infer_expression(expression.node_ref(self.db));
+        self.infer_expression(expression.node(self.db));
    }

    fn infer_module(&mut self, module: &ast::ModModule) {
@@ -621,42 +600,13 @@ impl<'db> TypeInferenceBuilder<'db> {
        } = with_statement;

        for item in items {
-            match item.optional_vars.as_deref() {
-                Some(ast::Expr::Name(name)) => {
-                    self.infer_definition(name);
-                }
-                _ => {
-                    // TODO infer definitions in unpacking assignment
-                    self.infer_expression(&item.context_expr);
-                }
-            }
+            self.infer_expression(&item.context_expr);
+            self.infer_optional_expression(item.optional_vars.as_deref());
        }

        self.infer_body(body);
    }

-    fn infer_with_item_definition(
-        &mut self,
-        target: &ast::ExprName,
-        with_item: &ast::WithItem,
-        definition: Definition<'db>,
-    ) {
-        let expression = self.index.expression(&with_item.context_expr);
-        let result = infer_expression_types(self.db, expression);
-        self.extend(result);
-
-        // TODO(dhruvmanila): The correct type inference here is the return type of the __enter__
-        // method of the context manager.
-        let context_expr_ty = self
-            .types
-            .expression_ty(with_item.context_expr.scoped_ast_id(self.db, self.scope));
-
-        self.types
-            .expressions
-            .insert(target.scoped_ast_id(self.db, self.scope), context_expr_ty);
-        self.types.definitions.insert(definition, context_expr_ty);
-    }
-
    fn infer_match_statement(&mut self, match_statement: &ast::StmtMatch) {
        let ast::StmtMatch {
            range: _,
@@ -756,7 +706,6 @@ impl<'db> TypeInferenceBuilder<'db> {

    fn infer_assignment_definition(
        &mut self,
-        target: &ast::ExprName,
        assignment: &ast::StmtAssign,
        definition: Definition<'db>,
    ) {
@@ -766,9 +715,6 @@ impl<'db> TypeInferenceBuilder<'db> {
        let value_ty = self
            .types
            .expression_ty(assignment.value.scoped_ast_id(self.db, self.scope));
-        self.types
-            .expressions
-            .insert(target.scoped_ast_id(self.db, self.scope), value_ty);
        self.types.definitions.insert(definition, value_ty);
    }

@@ -809,35 +755,15 @@ impl<'db> TypeInferenceBuilder<'db> {
    }

    fn infer_augmented_assignment_statement(&mut self, assignment: &ast::StmtAugAssign) {
-        if assignment.target.is_name_expr() {
-            self.infer_definition(assignment);
-        } else {
-            // TODO currently we don't consider assignments to non-Names to be Definitions
-            self.infer_augment_assignment(assignment);
-        }
-    }
-
-    fn infer_augment_assignment_definition(
-        &mut self,
-        assignment: &ast::StmtAugAssign,
-        definition: Definition<'db>,
-    ) {
-        let target_ty = self.infer_augment_assignment(assignment);
-        self.types.definitions.insert(definition, target_ty);
-    }
-
-    fn infer_augment_assignment(&mut self, assignment: &ast::StmtAugAssign) -> Type<'db> {
+        // TODO this should be a Definition
        let ast::StmtAugAssign {
            range: _,
            target,
            op: _,
            value,
        } = assignment;
-        self.infer_expression(value);
        self.infer_expression(target);
-
-        // TODO(dhruvmanila): Resolve the target type using the value type and the operator
-        Type::Unknown
+        self.infer_expression(value);
    }

    fn infer_type_alias_statement(&mut self, type_alias_statement: &ast::StmtTypeAlias) {
@@ -898,26 +824,7 @@ impl<'db> TypeInferenceBuilder<'db> {
            asname: _,
        } = alias;

-        let module_ty = ModuleName::new(name)
-            .ok_or(ModuleResolutionError::InvalidSyntax)
-            .and_then(|module_name| self.module_ty_from_name(module_name));
-
-        let module_ty = match module_ty {
-            Ok(ty) => ty,
-            Err(ModuleResolutionError::InvalidSyntax) => {
-                tracing::debug!("Failed to resolve import due to invalid syntax");
-                Type::Unknown
-            }
-            Err(ModuleResolutionError::UnresolvedModule) => {
-                self.add_diagnostic(
-                    AnyNodeRef::Alias(alias),
-                    "unresolved-import",
-                    format_args!("Import '{name}' could not be resolved."),
-                );
-                Type::Unknown
-            }
-        };
-
+        let module_ty = self.module_ty_from_name(ModuleName::new(name));
        self.types.definitions.insert(definition, module_ty);
    }

@@ -965,18 +872,10 @@ impl<'db> TypeInferenceBuilder<'db> {
    /// - `tail` is the relative module name stripped of all leading dots:
    ///   - `from .foo import bar` => `tail == "foo"`
    ///   - `from ..foo.bar import baz` => `tail == "foo.bar"`
-    fn relative_module_name(
-        &self,
-        tail: Option<&str>,
-        level: NonZeroU32,
-    ) -> Result<ModuleName, ModuleResolutionError> {
+    fn relative_module_name(&self, tail: Option<&str>, level: NonZeroU32) -> Option<ModuleName> {
        let Some(module) = file_to_module(self.db, self.file) else {
-            tracing::debug!(
-                "Relative module resolution '{}' failed; could not resolve file '{}' to a module",
-                format_import_from_module(level.get(), tail),
-                self.file.path(self.db)
-            );
-            return Err(ModuleResolutionError::UnresolvedModule);
+            tracing::debug!("Failed to resolve file {:?} to a module", self.file);
+            return None;
        };
        let mut level = level.get();
        if module.kind().is_package() {
@@ -984,19 +883,17 @@ impl<'db> TypeInferenceBuilder<'db> {
        }
        let mut module_name = module.name().to_owned();
        for _ in 0..level {
-            module_name = module_name
-                .parent()
-                .ok_or(ModuleResolutionError::UnresolvedModule)?;
+            module_name = module_name.parent()?;
        }
        if let Some(tail) = tail {
            if let Some(valid_tail) = ModuleName::new(tail) {
                module_name.extend(&valid_tail);
            } else {
-                tracing::debug!("Relative module resolution failed: invalid syntax");
-                return Err(ModuleResolutionError::InvalidSyntax);
+                tracing::debug!("Failed to resolve relative import due to invalid syntax");
+                return None;
            }
        }
-        Ok(module_name)
+        Some(module_name)
    }

    fn infer_import_from_definition(
@@ -1016,27 +913,16 @@ impl<'db> TypeInferenceBuilder<'db> {
        // `follow_nonexistent_import_bare_to_module()`.
        let ast::StmtImportFrom { module, level, .. } = import_from;
        tracing::trace!("Resolving imported object {alias:?} from statement {import_from:?}");
-        let module = module.as_deref();
        let module_name = if let Some(level) = NonZeroU32::new(*level) {
-            tracing::trace!(
-                "Resolving imported object '{}' from module '{}' relative to file '{}'",
-                alias.name,
-                format_import_from_module(level.get(), module),
-                self.file.path(self.db),
-            );
-            self.relative_module_name(module, level)
+            self.relative_module_name(module.as_deref(), level)
        } else {
-            tracing::trace!(
-                "Resolving imported object '{}' from module '{}'",
-                alias.name,
-                format_import_from_module(*level, module),
-            );
-            module
-                .and_then(ModuleName::new)
-                .ok_or(ModuleResolutionError::InvalidSyntax)
+            let module_name = module
+                .as_ref()
+                .expect("Non-relative import should always have a non-None `module`!");
+            ModuleName::new(module_name)
        };

-        let module_ty = module_name.and_then(|module_name| self.module_ty_from_name(module_name));
+        let module_ty = self.module_ty_from_name(module_name);

        let ast::Alias {
            range: _,
@@ -1049,34 +935,11 @@ impl<'db> TypeInferenceBuilder<'db> {
        // the runtime error will occur immediately (rather than when the symbol is *used*,
        // as would be the case for a symbol with type `Unbound`), so it's appropriate to
        // think of the type of the imported symbol as `Unknown` rather than `Unbound`
-        let member_ty = module_ty
-            .unwrap_or(Type::Unbound)
+        let ty = module_ty
            .member(self.db, &Name::new(&name.id))
            .replace_unbound_with(self.db, Type::Unknown);

-        if matches!(module_ty, Err(ModuleResolutionError::UnresolvedModule)) {
-            self.add_diagnostic(
-                AnyNodeRef::StmtImportFrom(import_from),
-                "unresolved-import",
-                format_args!(
-                    "Import '{}{}' could not be resolved.",
-                    ".".repeat(*level as usize),
-                    module.unwrap_or_default()
-                ),
-            );
-        } else if module_ty.is_ok() && member_ty.is_unknown() {
-            self.add_diagnostic(
-                AnyNodeRef::Alias(alias),
-                "unresolved-import",
-                format_args!(
-                    "Could not resolve import of '{name}' from '{}{}'",
-                    ".".repeat(*level as usize),
-                    module.unwrap_or_default()
-                ),
-            );
-        }
-
-        self.types.definitions.insert(definition, member_ty);
+        self.types.definitions.insert(definition, ty);
    }

    fn infer_return_statement(&mut self, ret: &ast::StmtReturn) {
@@ -1090,13 +953,10 @@ impl<'db> TypeInferenceBuilder<'db> {
        }
    }

-    fn module_ty_from_name(
-        &self,
-        module_name: ModuleName,
-    ) -> Result<Type<'db>, ModuleResolutionError> {
-        resolve_module(self.db, module_name)
-            .map(|module| Type::Module(module.file()))
-            .ok_or(ModuleResolutionError::UnresolvedModule)
+    fn module_ty_from_name(&self, module_name: Option<ModuleName>) -> Type<'db> {
+        module_name
+            .and_then(|module_name| resolve_module(self.db, module_name))
+            .map_or(Type::Unknown, |module| Type::Module(module.file()))
    }

    fn infer_decorator(&mut self, decorator: &ast::Decorator) -> Type<'db> {
@@ -1139,9 +999,6 @@ impl<'db> TypeInferenceBuilder<'db> {
            ast::Expr::NumberLiteral(literal) => self.infer_number_literal_expression(literal),
            ast::Expr::BooleanLiteral(literal) => self.infer_boolean_literal_expression(literal),
            ast::Expr::StringLiteral(literal) => self.infer_string_literal_expression(literal),
-            ast::Expr::BytesLiteral(bytes_literal) => {
-                self.infer_bytes_literal_expression(bytes_literal)
-            }
            ast::Expr::FString(fstring) => self.infer_fstring_expression(fstring),
            ast::Expr::EllipsisLiteral(literal) => self.infer_ellipsis_literal_expression(literal),
            ast::Expr::Tuple(tuple) => self.infer_tuple_expression(tuple),
@@ -1168,7 +1025,8 @@ impl<'db> TypeInferenceBuilder<'db> {
            ast::Expr::Yield(yield_expression) => self.infer_yield_expression(yield_expression),
            ast::Expr::YieldFrom(yield_from) => self.infer_yield_from_expression(yield_from),
            ast::Expr::Await(await_expression) => self.infer_await_expression(await_expression),
-            ast::Expr::IpyEscapeCommand(_) => todo!("Implement Ipy escape command support"),
+
+            _ => todo!("expression type resolution for {:?}", expression),
        };

        let expr_id = expression.scoped_ast_id(self.db, self.scope);
@@ -1205,12 +1063,6 @@ impl<'db> TypeInferenceBuilder<'db> {
        Type::Unknown
    }

-    #[allow(clippy::unused_self)]
-    fn infer_bytes_literal_expression(&mut self, _literal: &ast::ExprBytesLiteral) -> Type<'db> {
-        // TODO
-        Type::Unknown
-    }
-
    fn infer_fstring_expression(&mut self, fstring: &ast::ExprFString) -> Type<'db> {
        let ast::ExprFString { range: _, value } = fstring;

@@ -1226,7 +1078,21 @@ impl<'db> TypeInferenceBuilder<'db> {
                        flags: _,
                    } = fstring;
                    for element in elements {
-                        self.infer_fstring_element(element);
+                        match element {
+                            ast::FStringElement::Literal(_) => {
+                                // TODO string literal type
+                            }
+                            ast::FStringElement::Expression(expr_element) => {
+                                let ast::FStringExpressionElement {
+                                    range: _,
+                                    expression,
+                                    debug_text: _,
+                                    conversion: _,
+                                    format_spec: _,
+                                } = expr_element;
+                                self.infer_expression(expression);
+                            }
+                        }
                    }
                }
            }
@@ -1236,30 +1102,6 @@ impl<'db> TypeInferenceBuilder<'db> {
        Type::Unknown
    }

-    fn infer_fstring_element(&mut self, element: &ast::FStringElement) {
-        match element {
-            ast::FStringElement::Literal(_) => {
-                // TODO string literal type
-            }
-            ast::FStringElement::Expression(expr_element) => {
-                let ast::FStringExpressionElement {
-                    range: _,
-                    expression,
-                    debug_text: _,
-                    conversion: _,
-                    format_spec,
-                } = expr_element;
-                self.infer_expression(expression);
-
-                if let Some(format_spec) = format_spec {
-                    for spec_element in &format_spec.elements {
-                        self.infer_fstring_element(spec_element);
-                    }
-                }
-            }
-        }
-    }
-
    #[allow(clippy::unused_self)]
    fn infer_ellipsis_literal_expression(
        &mut self,
@@ -1788,7 +1630,7 @@ impl<'db> TypeInferenceBuilder<'db> {
        Type::Unknown
    }

-    fn infer_type_parameters(&mut self, type_parameters: &ast::TypeParams) {
+    fn infer_type_parameters(&mut self, type_parameters: &TypeParams) {
        let ast::TypeParams {
            range: _,
            type_params,
@@ -1825,28 +1667,6 @@ impl<'db> TypeInferenceBuilder<'db> {
        }
    }

-    /// Adds a new diagnostic.
-    ///
-    /// The diagnostic does not get added if the rule isn't enabled for this file.
-    fn add_diagnostic(&mut self, node: AnyNodeRef, rule: &str, message: std::fmt::Arguments) {
-        if !self.db.is_file_open(self.file) {
-            return;
-        }
-
-        // TODO: Don't emit the diagnostic if:
-        // * The enclosing node contains any syntax errors
-        // * The rule is disabled for this file. We probably want to introduce a new query that
-        //   returns a rule selector for a given file that respects the package's settings,
-        //   any global pragma comments in the file, and any per-file-ignores.
-
-        self.types.diagnostics.push(TypeCheckDiagnostic {
-            file: self.file,
-            rule: rule.to_string(),
-            message: message.to_string(),
-            range: node.range(),
-        });
-    }
-
    pub(super) fn finish(mut self) -> TypeInference<'db> {
        self.infer_region();
        self.types.shrink_to_fit();
@@ -1854,24 +1674,9 @@ impl<'db> TypeInferenceBuilder<'db> {
    }
 }

-fn format_import_from_module(level: u32, module: Option<&str>) -> String {
-    format!(
-        "{}{}",
-        ".".repeat(level as usize),
-        module.unwrap_or_default()
-    )
-}
-
-#[derive(Debug, Copy, Clone, PartialEq, Eq)]
-enum ModuleResolutionError {
-    InvalidSyntax,
-    UnresolvedModule,
-}
-
 #[cfg(test)]
 mod tests {
    use anyhow::Context;
-
    use ruff_db::files::{system_path_to_file, File};
    use ruff_db::parsed::parsed_module;
    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
@@ -1898,9 +1703,14 @@ mod tests {

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version: PythonVersion::default(),
-                search_paths: SearchPathSettings::new(src_root),
+                search_paths: SearchPathSettings {
+                    extra_paths: Vec::new(),
+                    src_root,
+                    site_packages: vec![],
+                    custom_typeshed: None,
+                },
            },
        )
        .expect("Valid search path settings");
@@ -1920,11 +1730,13 @@ mod tests {

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version: PythonVersion::default(),
                search_paths: SearchPathSettings {
+                    extra_paths: Vec::new(),
+                    src_root,
+                    site_packages: vec![],
                    custom_typeshed: Some(SystemPathBuf::from(typeshed)),
-                    ..SearchPathSettings::new(src_root)
                },
            },
        )
@@ -2114,16 +1926,6 @@ mod tests {
        Ok(())
    }

-    #[test]
-    fn from_import_with_no_module_name() -> anyhow::Result<()> {
-        // This test checks that invalid syntax in a `StmtImportFrom` node
-        // leads to the type being inferred as `Unknown`
-        let mut db = setup_db();
-        db.write_file("src/foo.py", "from import bar")?;
-        assert_public_ty(&db, "src/foo.py", "bar", "Unknown");
-        Ok(())
-    }
-
    #[test]
    fn resolve_base_class_by_name() -> anyhow::Result<()> {
        let mut db = setup_db();
@@ -2785,26 +2587,6 @@ mod tests {
        Ok(())
    }

-    #[test]
-    fn narrow_not_none() -> anyhow::Result<()> {
-        let mut db = setup_db();
-
-        db.write_dedented(
-            "/src/a.py",
-            "
-            x = None if flag else 1
-            y = 0
-            if x is not None:
-                y = x
-            ",
-        )?;
-
-        assert_public_ty(&db, "/src/a.py", "x", "Literal[1] | None");
-        assert_public_ty(&db, "/src/a.py", "y", "Literal[0, 1]");
-
-        Ok(())
-    }
-
    #[test]
    fn while_loop() -> anyhow::Result<()> {
        let mut db = setup_db();
@@ -2902,11 +2684,10 @@ mod tests {

    fn first_public_def<'db>(db: &'db TestDb, file: File, name: &str) -> Definition<'db> {
        let scope = global_scope(db, file);
-        use_def_map(db, scope)
+        *use_def_map(db, scope)
            .public_definitions(symbol_table(db, scope).symbol_id_by_name(name).unwrap())
-            .next()
+            .first()
            .unwrap()
-            .definition
    }

    #[test]
--- a/crates/red_knot_python_semantic/src/types/narrow.rs
+++ b/crates/red_knot_python_semantic/src/types/narrow.rs
@@ -1,115 +0,0 @@
-use crate::semantic_index::ast_ids::HasScopedAstId;
-use crate::semantic_index::definition::Definition;
-use crate::semantic_index::expression::Expression;
-use crate::semantic_index::symbol::{ScopeId, ScopedSymbolId, SymbolTable};
-use crate::semantic_index::symbol_table;
-use crate::types::{infer_expression_types, IntersectionBuilder, Type, TypeInference};
-use crate::Db;
-use ruff_python_ast as ast;
-use rustc_hash::FxHashMap;
-use std::sync::Arc;
-
-/// Return the type constraint that `test` (if true) would place on `definition`, if any.
-///
-/// For example, if we have this code:
-///
-/// ```python
-/// y = 1 if flag else None
-/// x = 1 if flag else None
-/// if x is not None:
-///     ...
-/// ```
-///
-/// The `test` expression `x is not None` places the constraint "not None" on the definition of
-/// `x`, so in that case we'd return `Some(Type::Intersection(negative=[Type::None]))`.
-///
-/// But if we called this with the same `test` expression, but the `definition` of `y`, no
-/// constraint is applied to that definition, so we'd just return `None`.
-pub(crate) fn narrowing_constraint<'db>(
-    db: &'db dyn Db,
-    test: Expression<'db>,
-    definition: Definition<'db>,
-) -> Option<Type<'db>> {
-    all_narrowing_constraints(db, test)
-        .get(&definition.symbol(db))
-        .copied()
-}
-
-#[salsa::tracked(return_ref)]
-fn all_narrowing_constraints<'db>(
-    db: &'db dyn Db,
-    test: Expression<'db>,
-) -> NarrowingConstraints<'db> {
-    NarrowingConstraintsBuilder::new(db, test).finish()
-}
-
-type NarrowingConstraints<'db> = FxHashMap<ScopedSymbolId, Type<'db>>;
-
-struct NarrowingConstraintsBuilder<'db> {
-    db: &'db dyn Db,
-    expression: Expression<'db>,
-    constraints: NarrowingConstraints<'db>,
-}
-
-impl<'db> NarrowingConstraintsBuilder<'db> {
-    fn new(db: &'db dyn Db, expression: Expression<'db>) -> Self {
-        Self {
-            db,
-            expression,
-            constraints: NarrowingConstraints::default(),
-        }
-    }
-
-    fn finish(mut self) -> NarrowingConstraints<'db> {
-        if let ast::Expr::Compare(expr_compare) = self.expression.node_ref(self.db).node() {
-            self.add_expr_compare(expr_compare);
-        }
-        // TODO other test expression kinds
-
-        self.constraints.shrink_to_fit();
-        self.constraints
-    }
-
-    fn symbols(&self) -> Arc<SymbolTable> {
-        symbol_table(self.db, self.scope())
-    }
-
-    fn scope(&self) -> ScopeId<'db> {
-        self.expression.scope(self.db)
-    }
-
-    fn inference(&self) -> &'db TypeInference<'db> {
-        infer_expression_types(self.db, self.expression)
-    }
-
-    fn add_expr_compare(&mut self, expr_compare: &ast::ExprCompare) {
-        let ast::ExprCompare {
-            range: _,
-            left,
-            ops,
-            comparators,
-        } = expr_compare;
-
-        if let ast::Expr::Name(ast::ExprName {
-            range: _,
-            id,
-            ctx: _,
-        }) = left.as_ref()
-        {
-            // SAFETY: we should always have a symbol for every Name node.
-            let symbol = self.symbols().symbol_id_by_name(id).unwrap();
-            let scope = self.scope();
-            let inference = self.inference();
-            for (op, comparator) in std::iter::zip(&**ops, &**comparators) {
-                let comp_ty = inference.expression_ty(comparator.scoped_ast_id(self.db, scope));
-                if matches!(op, ast::CmpOp::IsNot) {
-                    let ty = IntersectionBuilder::new(self.db)
-                        .add_negative(comp_ty)
-                        .build();
-                    self.constraints.insert(symbol, ty);
-                };
-                // TODO other comparison types
-            }
-        }
-    }
-}
--- a/crates/red_knot_server/Cargo.toml
+++ b/crates/red_knot_server/Cargo.toml
@@ -11,8 +11,10 @@ repository = { workspace = true }
 license = { workspace = true }

 [dependencies]
+red_knot_python_semantic = { workspace = true }
 red_knot_workspace = { workspace = true }
 ruff_db = { workspace = true }
+ruff_linter = { workspace = true }
 ruff_notebook = { workspace = true }
 ruff_python_ast = { workspace = true }
 ruff_source_file = { workspace = true }
@@ -23,7 +25,7 @@ crossbeam = { workspace = true }
 jod-thread = { workspace = true }
 lsp-server = { workspace = true }
 lsp-types = { workspace = true }
-rustc-hash = { workspace = true }
+foldhash = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 shellexpand = { workspace = true }
--- a/crates/red_knot_server/src/edit/notebook.rs
+++ b/crates/red_knot_server/src/edit/notebook.rs
@@ -1,7 +1,7 @@
 use anyhow::Ok;
+use foldhash::{HashMap, HashMapExt};
 use lsp_types::NotebookCellKind;
 use ruff_notebook::CellMetadata;
-use rustc_hash::{FxBuildHasher, FxHashMap};

 use crate::{PositionEncoding, TextDocument};

@@ -17,7 +17,7 @@ pub struct NotebookDocument {
    metadata: ruff_notebook::RawNotebookMetadata,
    version: DocumentVersion,
    // Used to quickly find the index of a cell for a given URL.
-    cell_index: FxHashMap<lsp_types::Url, CellId>,
+    cell_index: HashMap<lsp_types::Url, CellId>,
 }

 /// A single cell within a notebook, which has text contents represented as a `TextDocument`.
@@ -35,7 +35,7 @@ impl NotebookDocument {
        metadata: serde_json::Map<String, serde_json::Value>,
        cell_documents: Vec<lsp_types::TextDocumentItem>,
    ) -> crate::Result<Self> {
-        let mut cell_contents: FxHashMap<_, _> = cell_documents
+        let mut cell_contents: HashMap<_, _> = cell_documents
            .into_iter()
            .map(|document| (document.uri, document.text))
            .collect();
@@ -122,7 +122,7 @@ impl NotebookDocument {
                // Instead, it only provides that (a) these cell URIs were removed, and (b) these
                // cell URIs were added.
                // https://github.com/astral-sh/ruff/issues/12573
-                let mut deleted_cells = FxHashMap::default();
+                let mut deleted_cells = HashMap::default();

                // First, delete the cells and remove them from the index.
                if delete > 0 {
@@ -216,8 +216,8 @@ impl NotebookDocument {
        self.cells.get_mut(*self.cell_index.get(uri)?)
    }

-    fn make_cell_index(cells: &[NotebookCell]) -> FxHashMap<lsp_types::Url, CellId> {
-        let mut index = FxHashMap::with_capacity_and_hasher(cells.len(), FxBuildHasher);
+    fn make_cell_index(cells: &[NotebookCell]) -> HashMap<lsp_types::Url, CellId> {
+        let mut index = HashMap::with_capacity(cells.len());
        for (i, cell) in cells.iter().enumerate() {
            index.insert(cell.url.clone(), i);
        }
--- a/crates/red_knot_server/src/server.rs
+++ b/crates/red_knot_server/src/server.rs
@@ -3,10 +3,11 @@
 use std::num::NonZeroUsize;
 use std::panic::PanicInfo;

-use lsp_server::Message;
+use lsp_server as lsp;
+use lsp_types as types;
 use lsp_types::{
-    ClientCapabilities, DiagnosticOptions, DiagnosticServerCapabilities, MessageType,
-    ServerCapabilities, TextDocumentSyncCapability, TextDocumentSyncOptions, Url,
+    ClientCapabilities, DiagnosticOptions, NotebookCellSelector, NotebookDocumentSyncOptions,
+    NotebookSelector, TextDocumentSyncCapability, TextDocumentSyncOptions,
 };

 use self::connection::{Connection, ConnectionInitializer};
@@ -73,7 +74,7 @@ impl Server {
            init_params.client_info.as_ref(),
        );

-        let mut workspace_for_url = |url: Url| {
+        let mut workspace_for_url = |url: lsp_types::Url| {
            let Some(workspace_settings) = workspace_settings.as_mut() else {
                return (url, ClientSettings::default());
            };
@@ -92,7 +93,7 @@ impl Server {
            }).collect())
            .or_else(|| {
                tracing::warn!("No workspace(s) were provided during initialization. Using the current working directory as a default workspace...");
-                let uri = Url::from_file_path(std::env::current_dir().ok()?).ok()?;
+                let uri = types::Url::from_file_path(std::env::current_dir().ok()?).ok()?;
                Some(vec![workspace_for_url(uri)])
            })
            .ok_or_else(|| {
@@ -148,7 +149,7 @@ impl Server {
            try_show_message(
                "The Ruff language server exited with a panic. See the logs for more details."
                    .to_string(),
-                MessageType::ERROR,
+                lsp_types::MessageType::ERROR,
            )
            .ok();
        }));
@@ -181,9 +182,9 @@ impl Server {
                break;
            }
            let task = match msg {
-                Message::Request(req) => api::request(req),
-                Message::Notification(notification) => api::notification(notification),
-                Message::Response(response) => scheduler.response(response),
+                lsp::Message::Request(req) => api::request(req),
+                lsp::Message::Notification(notification) => api::notification(notification),
+                lsp::Message::Response(response) => scheduler.response(response),
            };
            scheduler.dispatch(task);
        }
@@ -205,12 +206,24 @@ impl Server {
            .unwrap_or_default()
    }

-    fn server_capabilities(position_encoding: PositionEncoding) -> ServerCapabilities {
-        ServerCapabilities {
+    fn server_capabilities(position_encoding: PositionEncoding) -> types::ServerCapabilities {
+        types::ServerCapabilities {
            position_encoding: Some(position_encoding.into()),
-            diagnostic_provider: Some(DiagnosticServerCapabilities::Options(DiagnosticOptions {
-                identifier: Some(crate::DIAGNOSTIC_NAME.into()),
-                ..Default::default()
+            diagnostic_provider: Some(types::DiagnosticServerCapabilities::Options(
+                DiagnosticOptions {
+                    identifier: Some(crate::DIAGNOSTIC_NAME.into()),
+                    ..Default::default()
+                },
+            )),
+            notebook_document_sync: Some(types::OneOf::Left(NotebookDocumentSyncOptions {
+                save: Some(false),
+                notebook_selector: [NotebookSelector::ByCells {
+                    notebook: None,
+                    cells: vec![NotebookCellSelector {
+                        language: "python".to_string(),
+                    }],
+                }]
+                .to_vec(),
            })),
            text_document_sync: Some(TextDocumentSyncCapability::Options(
                TextDocumentSyncOptions {
--- a/crates/red_knot_server/src/server/client.rs
+++ b/crates/red_knot_server/src/server/client.rs
@@ -1,7 +1,7 @@
 use std::any::TypeId;

+use foldhash::HashMap;
 use lsp_server::{Notification, RequestId};
-use rustc_hash::FxHashMap;
 use serde_json::Value;

 use super::{schedule::Task, ClientSender};
@@ -23,7 +23,7 @@ pub(crate) struct Responder(ClientSender);
 pub(crate) struct Requester<'s> {
    sender: ClientSender,
    next_request_id: i32,
-    response_handlers: FxHashMap<lsp_server::RequestId, ResponseBuilder<'s>>,
+    response_handlers: HashMap<lsp_server::RequestId, ResponseBuilder<'s>>,
 }

 impl<'s> Client<'s> {
@@ -34,7 +34,7 @@ impl<'s> Client<'s> {
            requester: Requester {
                sender,
                next_request_id: 1,
-                response_handlers: FxHashMap::default(),
+                response_handlers: HashMap::default(),
            },
        }
    }
--- a/crates/red_knot_server/src/session.rs
+++ b/crates/red_knot_server/src/session.rs
@@ -8,6 +8,7 @@ use std::sync::Arc;
 use anyhow::anyhow;
 use lsp_types::{ClientCapabilities, Url};

+use red_knot_python_semantic::{ProgramSettings, PythonVersion, SearchPathSettings};
 use red_knot_workspace::db::RootDatabase;
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_db::files::{system_path_to_file, File};
@@ -66,10 +67,19 @@ impl Session {
                .ok_or_else(|| anyhow!("Workspace path is not a valid UTF-8 path: {:?}", path))?;
            let system = LSPSystem::new(index.clone());

+            let metadata = WorkspaceMetadata::from_path(system_path, &system)?;
            // TODO(dhruvmanila): Get the values from the client settings
-            let metadata = WorkspaceMetadata::from_path(system_path, &system, None)?;
+            let program_settings = ProgramSettings {
+                target_version: PythonVersion::default(),
+                search_paths: SearchPathSettings {
+                    extra_paths: vec![],
+                    src_root: system_path.to_path_buf(),
+                    site_packages: vec![],
+                    custom_typeshed: None,
+                },
+            };
            // TODO(micha): Handle the case where the program settings are incorrect more gracefully.
-            workspaces.insert(path, RootDatabase::new(metadata, system)?);
+            workspaces.insert(path, RootDatabase::new(metadata, program_settings, system)?);
        }

        Ok(Self {
--- a/crates/red_knot_server/src/session/capabilities.rs
+++ b/crates/red_knot_server/src/session/capabilities.rs
@@ -1,4 +1,5 @@
 use lsp_types::ClientCapabilities;
+use ruff_linter::display_settings;

 #[derive(Debug, Clone, PartialEq, Eq, Default)]
 #[allow(clippy::struct_excessive_bools)]
@@ -65,3 +66,20 @@ impl ResolvedClientCapabilities {
        }
    }
 }
+
+impl std::fmt::Display for ResolvedClientCapabilities {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        display_settings! {
+            formatter = f,
+            namespace = "capabilities",
+            fields = [
+                self.code_action_deferred_edit_resolution,
+                self.apply_edit,
+                self.document_changes,
+                self.workspace_refresh,
+                self.pull_diagnostics,
+            ]
+        };
+        Ok(())
+    }
+}
--- a/crates/red_knot_server/src/session/index.rs
+++ b/crates/red_knot_server/src/session/index.rs
@@ -2,8 +2,8 @@ use std::borrow::Cow;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;

+use foldhash::HashMap;
 use lsp_types::Url;
-use rustc_hash::FxHashMap;

 use crate::{
    edit::{DocumentKey, DocumentVersion, NotebookDocument},
@@ -16,10 +16,10 @@ use super::ClientSettings;
 #[derive(Default, Debug)]
 pub(crate) struct Index {
    /// Maps all document file URLs to the associated document controller
-    documents: FxHashMap<Url, DocumentController>,
+    documents: HashMap<Url, DocumentController>,

    /// Maps opaque cell URLs to a notebook URL (document)
-    notebook_cells: FxHashMap<Url, Url>,
+    notebook_cells: HashMap<Url, Url>,

    /// Global settings provided by the client.
    global_settings: ClientSettings,
@@ -28,8 +28,8 @@ pub(crate) struct Index {
 impl Index {
    pub(super) fn new(global_settings: ClientSettings) -> Self {
        Self {
-            documents: FxHashMap::default(),
-            notebook_cells: FxHashMap::default(),
+            documents: HashMap::default(),
+            notebook_cells: HashMap::default(),
            global_settings,
        }
    }
@@ -278,6 +278,18 @@ impl DocumentQuery {
        }
    }

+    /// Generate a source kind used by the linter.
+    pub(crate) fn make_source_kind(&self) -> ruff_linter::source_kind::SourceKind {
+        match self {
+            Self::Text { document, .. } => {
+                ruff_linter::source_kind::SourceKind::Python(document.contents().to_string())
+            }
+            Self::Notebook { notebook, .. } => {
+                ruff_linter::source_kind::SourceKind::IpyNotebook(notebook.make_ruff_notebook())
+            }
+        }
+    }
+
    /// Attempts to access the underlying notebook document that this query is selecting.
    pub fn as_notebook(&self) -> Option<&NotebookDocument> {
        match self {
--- a/crates/red_knot_server/src/session/settings.rs
+++ b/crates/red_knot_server/src/session/settings.rs
@@ -1,11 +1,11 @@
 use std::path::PathBuf;

+use foldhash::HashMap;
 use lsp_types::Url;
-use rustc_hash::FxHashMap;
 use serde::Deserialize;

 /// Maps a workspace URI to its associated client settings. Used during server initialization.
-pub(crate) type WorkspaceSettingsMap = FxHashMap<Url, ClientSettings>;
+pub(crate) type WorkspaceSettingsMap = HashMap<Url, ClientSettings>;

 /// This is a direct representation of the settings schema sent by the client.
 #[derive(Debug, Deserialize, Default)]
--- a/crates/red_knot_wasm/src/lib.rs
+++ b/crates/red_knot_wasm/src/lib.rs
@@ -3,8 +3,8 @@ use std::any::Any;
 use js_sys::Error;
 use wasm_bindgen::prelude::*;

+use red_knot_python_semantic::{ProgramSettings, SearchPathSettings};
 use red_knot_workspace::db::RootDatabase;
-use red_knot_workspace::workspace::settings::Configuration;
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_db::files::{system_path_to_file, File};
 use ruff_db::system::walk_directory::WalkDirectoryBuilder;
@@ -41,17 +41,16 @@ impl Workspace {
    #[wasm_bindgen(constructor)]
    pub fn new(root: &str, settings: &Settings) -> Result<Workspace, Error> {
        let system = WasmSystem::new(SystemPath::new(root));
-        let workspace = WorkspaceMetadata::from_path(
-            SystemPath::new(root),
-            &system,
-            Some(Configuration {
-                target_version: Some(settings.target_version.into()),
-                ..Configuration::default()
-            }),
-        )
-        .map_err(into_error)?;
+        let workspace =
+            WorkspaceMetadata::from_path(SystemPath::new(root), &system).map_err(into_error)?;

-        let db = RootDatabase::new(workspace, system.clone()).map_err(into_error)?;
+        let program_settings = ProgramSettings {
+            target_version: settings.target_version.into(),
+            search_paths: SearchPathSettings::default(),
+        };
+
+        let db =
+            RootDatabase::new(workspace, program_settings, system.clone()).map_err(into_error)?;

        Ok(Self { db, system })
    }
@@ -110,7 +109,7 @@ impl Workspace {
    pub fn check_file(&self, file_id: &FileHandle) -> Result<Vec<String>, Error> {
        let result = self.db.check_file(file_id.file).map_err(into_error)?;

-        Ok(result.clone())
+        Ok(result.to_vec())
    }

    /// Checks all open files
--- a/crates/red_knot_wasm/tests/api.rs
+++ b/crates/red_knot_wasm/tests/api.rs
@@ -17,8 +17,5 @@ fn check() {

    let result = workspace.check_file(&test).expect("Check to succeed");

-    assert_eq!(
-        result,
-        vec!["/test.py:1:8: Import 'random22' could not be resolved.",]
-    );
+    assert_eq!(result, vec!["/test.py:1:8: Unresolved import 'random22'"]);
 }
--- a/crates/red_knot_workspace/Cargo.toml
+++ b/crates/red_knot_workspace/Cargo.toml
@@ -22,12 +22,12 @@ ruff_text_size = { workspace = true }
 anyhow = { workspace = true }
 crossbeam = { workspace = true }
 notify = { workspace = true }
-rustc-hash = { workspace = true }
+foldhash = { workspace = true }
 salsa = { workspace = true }
+thiserror = { workspace = true }
 tracing = { workspace = true }

 [dev-dependencies]
-ruff_db = { workspace = true, features = ["testing"]}

 [lints]
 workspace = true
--- a/crates/red_knot_workspace/src/db.rs
+++ b/crates/red_knot_workspace/src/db.rs
@@ -1,15 +1,17 @@
 use std::panic::RefUnwindSafe;
 use std::sync::Arc;

-use salsa::plumbing::ZalsaDatabase;
-use salsa::{Cancelled, Event};
-
-use red_knot_python_semantic::{vendored_typeshed_stubs, Db as SemanticDb, Program};
+use red_knot_python_semantic::{
+    vendored_typeshed_stubs, Db as SemanticDb, Program, ProgramSettings,
+};
 use ruff_db::files::{File, Files};
 use ruff_db::system::System;
 use ruff_db::vendored::VendoredFileSystem;
 use ruff_db::{Db as SourceDb, Upcast};
+use salsa::plumbing::ZalsaDatabase;
+use salsa::{Cancelled, Event};

+use crate::lint::Diagnostics;
 use crate::workspace::{check_file, Workspace, WorkspaceMetadata};

 mod changes;
@@ -26,7 +28,11 @@ pub struct RootDatabase {
 }

 impl RootDatabase {
-    pub fn new<S>(workspace: WorkspaceMetadata, system: S) -> anyhow::Result<Self>
+    pub fn new<S>(
+        workspace: WorkspaceMetadata,
+        settings: ProgramSettings,
+        system: S,
+    ) -> anyhow::Result<Self>
    where
        S: System + 'static + Send + Sync + RefUnwindSafe,
    {
@@ -37,11 +43,11 @@ impl RootDatabase {
            system: Arc::new(system),
        };

+        let workspace = Workspace::from_metadata(&db, workspace);
        // Initialize the `Program` singleton
-        Program::from_settings(&db, workspace.settings().program())?;
-
-        db.workspace = Some(Workspace::from_metadata(&db, workspace));
+        Program::from_settings(&db, settings)?;

+        db.workspace = Some(workspace);
        Ok(db)
    }

@@ -55,7 +61,7 @@ impl RootDatabase {
        self.with_db(|db| db.workspace().check(db))
    }

-    pub fn check_file(&self, file: File) -> Result<Vec<String>, Cancelled> {
+    pub fn check_file(&self, file: File) -> Result<Diagnostics, Cancelled> {
        self.with_db(|db| check_file(db, file))
    }

@@ -109,15 +115,7 @@ impl Upcast<dyn SourceDb> for RootDatabase {
 }

 #[salsa::db]
-impl SemanticDb for RootDatabase {
-    fn is_file_open(&self, file: File) -> bool {
-        let Some(workspace) = &self.workspace else {
-            return false;
-        };
-
-        workspace.is_file_open(self, file)
-    }
-}
+impl SemanticDb for RootDatabase {}

 #[salsa::db]
 impl SourceDb for RootDatabase {
@@ -155,9 +153,8 @@ impl Db for RootDatabase {}

 #[cfg(test)]
 pub(crate) mod tests {
-    use std::sync::Arc;
-
    use salsa::Event;
+    use std::sync::Arc;

    use red_knot_python_semantic::{vendored_typeshed_stubs, Db as SemanticDb};
    use ruff_db::files::Files;
@@ -245,12 +242,7 @@ pub(crate) mod tests {
    }

    #[salsa::db]
-    impl red_knot_python_semantic::Db for TestDb {
-        fn is_file_open(&self, file: ruff_db::files::File) -> bool {
-            !file.path(self).is_vendored_path()
-        }
-    }
-
+    impl red_knot_python_semantic::Db for TestDb {}
    #[salsa::db]
    impl Db for TestDb {}

--- a/crates/red_knot_workspace/src/db/changes.rs
+++ b/crates/red_knot_workspace/src/db/changes.rs
@@ -1,41 +1,30 @@
-use red_knot_python_semantic::Program;
+use foldhash::HashSet;
+
 use ruff_db::files::{system_path_to_file, File, Files};
 use ruff_db::system::walk_directory::WalkState;
 use ruff_db::system::SystemPath;
 use ruff_db::Db;
-use rustc_hash::FxHashSet;

 use crate::db::RootDatabase;
 use crate::watch;
 use crate::watch::{CreatedKind, DeletedKind};
-use crate::workspace::settings::Configuration;
 use crate::workspace::WorkspaceMetadata;

 impl RootDatabase {
-    #[tracing::instrument(level = "debug", skip(self, changes, base_configuration))]
-    pub fn apply_changes(
-        &mut self,
-        changes: Vec<watch::ChangeEvent>,
-        base_configuration: Option<&Configuration>,
-    ) {
+    #[tracing::instrument(level = "debug", skip(self, changes))]
+    pub fn apply_changes(&mut self, changes: Vec<watch::ChangeEvent>) {
        let workspace = self.workspace();
        let workspace_path = workspace.root(self).to_path_buf();
-        let program = Program::get(self);
-        let custom_stdlib_versions_path = program
-            .custom_stdlib_search_path(self)
-            .map(|path| path.join("VERSIONS"));

        let mut workspace_change = false;
-        // Changes to a custom stdlib path's VERSIONS
-        let mut custom_stdlib_change = false;
        // Packages that need reloading
-        let mut changed_packages = FxHashSet::default();
+        let mut changed_packages = HashSet::default();
        // Paths that were added
-        let mut added_paths = FxHashSet::default();
+        let mut added_paths = HashSet::default();

        // Deduplicate the `sync` calls. Many file watchers emit multiple events for the same path.
-        let mut synced_files = FxHashSet::default();
-        let mut synced_recursively = FxHashSet::default();
+        let mut synced_files = HashSet::default();
+        let mut synced_recursively = HashSet::default();

        let mut sync_path = |db: &mut RootDatabase, path: &SystemPath| {
            if synced_files.insert(path.to_path_buf()) {
@@ -65,10 +54,6 @@ impl RootDatabase {

                    continue;
                }
-
-                if Some(path) == custom_stdlib_versions_path.as_deref() {
-                    custom_stdlib_change = true;
-                }
            }

            match change {
@@ -115,13 +100,7 @@ impl RootDatabase {
                    } else {
                        sync_recursively(self, &path);

-                        if custom_stdlib_versions_path
-                            .as_ref()
-                            .is_some_and(|versions_path| versions_path.starts_with(&path))
-                        {
-                            custom_stdlib_change = true;
-                        }
-
+                        // TODO: Remove after converting `package.files()` to a salsa query.
                        if let Some(package) = workspace.package(self, &path) {
                            changed_packages.insert(package);
                        } else {
@@ -139,13 +118,9 @@ impl RootDatabase {
        }

        if workspace_change {
-            match WorkspaceMetadata::from_path(
-                &workspace_path,
-                self.system(),
-                base_configuration.cloned(),
-            ) {
+            match WorkspaceMetadata::from_path(&workspace_path, self.system()) {
                Ok(metadata) => {
-                    tracing::debug!("Reloading workspace after structural change.");
+                    tracing::debug!("Reload workspace after structural change.");
                    // TODO: Handle changes in the program settings.
                    workspace.reload(self, metadata);
                }
@@ -155,11 +130,6 @@ impl RootDatabase {
            }

            return;
-        } else if custom_stdlib_change {
-            let search_paths = workspace.search_path_settings(self).clone();
-            if let Err(error) = program.update_search_paths(self, &search_paths) {
-                tracing::error!("Failed to set the new search paths: {error}");
-            }
        }

        let mut added_paths = added_paths.into_iter().filter(|path| {
--- a/crates/red_knot_workspace/src/lib.rs
+++ b/crates/red_knot_workspace/src/lib.rs
@@ -1,4 +1,5 @@
 pub mod db;
 pub mod lint;
+pub mod site_packages;
 pub mod watch;
 pub mod workspace;
--- a/crates/red_knot_workspace/src/lint.rs
+++ b/crates/red_knot_workspace/src/lint.rs
@@ -1,4 +1,5 @@
 use std::cell::RefCell;
+use std::ops::Deref;
 use std::time::Duration;

 use tracing::debug_span;
@@ -21,7 +22,7 @@ use crate::db::Db;
 pub(crate) fn unwind_if_cancelled(db: &dyn Db) {}

 #[salsa::tracked(return_ref)]
-pub(crate) fn lint_syntax(db: &dyn Db, file_id: File) -> Vec<String> {
+pub(crate) fn lint_syntax(db: &dyn Db, file_id: File) -> Diagnostics {
    #[allow(clippy::print_stdout)]
    if std::env::var("RED_KNOT_SLOW_LINT").is_ok() {
        for i in 0..10 {
@@ -63,7 +64,7 @@ pub(crate) fn lint_syntax(db: &dyn Db, file_id: File) -> Vec<String> {
        }));
    }

-    diagnostics
+    Diagnostics::from(diagnostics)
 }

 fn lint_lines(source: &str, diagnostics: &mut Vec<String>) {
@@ -85,7 +86,7 @@ fn lint_lines(source: &str, diagnostics: &mut Vec<String>) {

 #[allow(unreachable_pub)]
 #[salsa::tracked(return_ref)]
-pub fn lint_semantic(db: &dyn Db, file_id: File) -> Vec<String> {
+pub fn lint_semantic(db: &dyn Db, file_id: File) -> Diagnostics {
    let _span = debug_span!("lint_semantic", file=%file_id.path(db)).entered();

    let source = source_text(db.upcast(), file_id);
@@ -93,7 +94,7 @@ pub fn lint_semantic(db: &dyn Db, file_id: File) -> Vec<String> {
    let semantic = SemanticModel::new(db.upcast(), file_id);

    if !parsed.is_valid() {
-        return vec![];
+        return Diagnostics::Empty;
    }

    let context = SemanticLintContext {
@@ -105,7 +106,7 @@ pub fn lint_semantic(db: &dyn Db, file_id: File) -> Vec<String> {

    SemanticVisitor { context: &context }.visit_body(parsed.suite());

-    context.diagnostics.take()
+    Diagnostics::from(context.diagnostics.take())
 }

 fn format_diagnostic(context: &SemanticLintContext, message: &str, start: TextSize) -> String {
@@ -115,13 +116,48 @@ fn format_diagnostic(context: &SemanticLintContext, message: &str, start: TextSi
        .source_location(start, context.source_text());
    format!(
        "{}:{}:{}: {}",
-        context.semantic.file_path(),
+        context.semantic.file_path().as_str(),
        source_location.row,
        source_location.column,
        message,
    )
 }

+fn lint_unresolved_imports(context: &SemanticLintContext, import: AnyImportRef) {
+    // TODO: this treats any symbol with `Type::Unknown` as an unresolved import,
+    // which isn't really correct: if it exists but has `Type::Unknown` in the
+    // module we're importing it from, we shouldn't really emit a diagnostic here,
+    // but currently do.
+    match import {
+        AnyImportRef::Import(import) => {
+            for alias in &import.names {
+                let ty = alias.ty(&context.semantic);
+
+                if ty.is_unknown() {
+                    context.push_diagnostic(format_diagnostic(
+                        context,
+                        &format!("Unresolved import '{}'", &alias.name),
+                        alias.start(),
+                    ));
+                }
+            }
+        }
+        AnyImportRef::ImportFrom(import) => {
+            for alias in &import.names {
+                let ty = alias.ty(&context.semantic);
+
+                if ty.is_unknown() {
+                    context.push_diagnostic(format_diagnostic(
+                        context,
+                        &format!("Unresolved import '{}'", &alias.name),
+                        alias.start(),
+                    ));
+                }
+            }
+        }
+    }
+}
+
 fn lint_maybe_undefined(context: &SemanticLintContext, name: &ast::ExprName) {
    if !matches!(name.ctx, ast::ExprContext::Load) {
        return;
@@ -244,8 +280,17 @@ struct SemanticVisitor<'a> {

 impl Visitor<'_> for SemanticVisitor<'_> {
    fn visit_stmt(&mut self, stmt: &ast::Stmt) {
-        if let ast::Stmt::ClassDef(class) = stmt {
-            lint_bad_override(self.context, class);
+        match stmt {
+            ast::Stmt::ClassDef(class) => {
+                lint_bad_override(self.context, class);
+            }
+            ast::Stmt::Import(import) => {
+                lint_unresolved_imports(self.context, AnyImportRef::Import(import));
+            }
+            ast::Stmt::ImportFrom(import) => {
+                lint_unresolved_imports(self.context, AnyImportRef::ImportFrom(import));
+            }
+            _ => {}
        }

        walk_stmt(self, stmt);
@@ -263,6 +308,53 @@ impl Visitor<'_> for SemanticVisitor<'_> {
    }
 }

+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Diagnostics {
+    Empty,
+    List(Vec<String>),
+}
+
+impl Diagnostics {
+    pub fn as_slice(&self) -> &[String] {
+        match self {
+            Diagnostics::Empty => &[],
+            Diagnostics::List(list) => list.as_slice(),
+        }
+    }
+}
+
+impl Deref for Diagnostics {
+    type Target = [String];
+    fn deref(&self) -> &Self::Target {
+        self.as_slice()
+    }
+}
+
+impl From<Vec<String>> for Diagnostics {
+    fn from(value: Vec<String>) -> Self {
+        if value.is_empty() {
+            Diagnostics::Empty
+        } else {
+            Diagnostics::List(value)
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+enum AnyImportRef<'a> {
+    Import(&'a ast::StmtImport),
+    ImportFrom(&'a ast::StmtImportFrom),
+}
+
+impl Ranged for AnyImportRef<'_> {
+    fn range(&self) -> ruff_text_size::TextRange {
+        match self {
+            AnyImportRef::Import(import) => import.range(),
+            AnyImportRef::ImportFrom(import) => import.range(),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
    use red_knot_python_semantic::{Program, ProgramSettings, PythonVersion, SearchPathSettings};
@@ -271,7 +363,7 @@ mod tests {

    use crate::db::tests::TestDb;

-    use super::lint_semantic;
+    use super::{lint_semantic, Diagnostics};

    fn setup_db() -> TestDb {
        setup_db_with_root(SystemPathBuf::from("/src"))
@@ -286,9 +378,14 @@ mod tests {

        Program::from_settings(
            &db,
-            &ProgramSettings {
+            ProgramSettings {
                target_version: PythonVersion::default(),
-                search_paths: SearchPathSettings::new(src_root),
+                search_paths: SearchPathSettings {
+                    extra_paths: Vec::new(),
+                    src_root,
+                    site_packages: vec![],
+                    custom_typeshed: None,
+                },
            },
        )
        .expect("Valid program settings");
@@ -312,9 +409,9 @@ mod tests {
        .unwrap();

        let file = system_path_to_file(&db, "/src/a.py").expect("file to exist");
-        let messages = lint_semantic(&db, file);
-
-        assert_ne!(messages, &[] as &[String], "expected some diagnostics");
+        let Diagnostics::List(messages) = lint_semantic(&db, file) else {
+            panic!("expected some diagnostics");
+        };

        assert_eq!(
            *messages,
--- a/crates/red_knot_python_semantic/src/site_packages.rs
+++ b/crates/red_knot_python_semantic/src/site_packages.rs
@@ -13,10 +13,9 @@ use std::io;
 use std::num::NonZeroUsize;
 use std::ops::Deref;

+use red_knot_python_semantic::PythonVersion;
 use ruff_db::system::{System, SystemPath, SystemPathBuf};

-use crate::PythonVersion;
-
 type SitePackagesDiscoveryResult<T> = Result<T, SitePackagesDiscoveryError>;

 /// Abstraction for a Python virtual environment.
@@ -25,7 +24,7 @@ type SitePackagesDiscoveryResult<T> = Result<T, SitePackagesDiscoveryError>;
 /// The format of this file is not defined anywhere, and exactly which keys are present
 /// depends on the tool that was used to create the virtual environment.
 #[derive(Debug)]
-pub(crate) struct VirtualEnvironment {
+pub struct VirtualEnvironment {
    venv_path: SysPrefixPath,
    base_executable_home_path: PythonHomePath,
    include_system_site_packages: bool,
@@ -42,7 +41,7 @@ pub(crate) struct VirtualEnvironment {
 }

 impl VirtualEnvironment {
-    pub(crate) fn new(
+    pub fn new(
        path: impl AsRef<SystemPath>,
        system: &dyn System,
    ) -> SitePackagesDiscoveryResult<Self> {
@@ -56,7 +55,7 @@ impl VirtualEnvironment {

        let venv_path = SysPrefixPath::new(path, system)?;
        let pyvenv_cfg_path = venv_path.join("pyvenv.cfg");
-        tracing::debug!("Attempting to parse virtual environment metadata at '{pyvenv_cfg_path}'");
+        tracing::debug!("Attempting to parse virtual environment metadata at {pyvenv_cfg_path}");

        let pyvenv_cfg = system
            .read_to_string(&pyvenv_cfg_path)
@@ -158,7 +157,7 @@ impl VirtualEnvironment {
    /// Return a list of `site-packages` directories that are available from this virtual environment
    ///
    /// See the documentation for `site_packages_dir_from_sys_prefix` for more details.
-    pub(crate) fn site_packages_directories(
+    pub fn site_packages_directories(
        &self,
        system: &dyn System,
    ) -> SitePackagesDiscoveryResult<Vec<SystemPathBuf>> {
@@ -192,7 +191,7 @@ impl VirtualEnvironment {
            } else {
                tracing::warn!(
                    "Failed to resolve `sys.prefix` of the system Python installation \
-from the `home` value in the `pyvenv.cfg` file at '{}'. \
+from the `home` value in the `pyvenv.cfg` file at {}. \
 System site-packages will not be used for module resolution.",
                    venv_path.join("pyvenv.cfg")
                );
@@ -205,7 +204,7 @@ System site-packages will not be used for module resolution.",
 }

 #[derive(Debug, thiserror::Error)]
-pub(crate) enum SitePackagesDiscoveryError {
+pub enum SitePackagesDiscoveryError {
    #[error("Invalid --venv-path argument: {0} could not be canonicalized")]
    VenvDirCanonicalizationError(SystemPathBuf, #[source] io::Error),
    #[error("Invalid --venv-path argument: {0} does not point to a directory on disk")]
@@ -222,7 +221,7 @@ pub(crate) enum SitePackagesDiscoveryError {

 /// The various ways in which parsing a `pyvenv.cfg` file could fail
 #[derive(Debug)]
-pub(crate) enum PyvenvCfgParseErrorKind {
+pub enum PyvenvCfgParseErrorKind {
    TooManyEquals { line_number: NonZeroUsize },
    MalformedKeyValuePair { line_number: NonZeroUsize },
    NoHomeKey,
@@ -371,7 +370,7 @@ fn site_packages_directory_from_sys_prefix(
 ///
 /// [`sys.prefix`]: https://docs.python.org/3/library/sys.html#sys.prefix
 #[derive(Debug, PartialEq, Eq, Clone)]
-pub(crate) struct SysPrefixPath(SystemPathBuf);
+pub struct SysPrefixPath(SystemPathBuf);

 impl SysPrefixPath {
    fn new(
@@ -426,7 +425,7 @@ impl Deref for SysPrefixPath {

 impl fmt::Display for SysPrefixPath {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "`sys.prefix` path '{}'", self.0)
+        write!(f, "`sys.prefix` path {}", self.0)
    }
 }

@@ -483,7 +482,7 @@ impl Deref for PythonHomePath {

 impl fmt::Display for PythonHomePath {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "`home` location '{}'", self.0)
+        write!(f, "`home` location {}", self.0)
    }
 }

--- a/crates/red_knot_workspace/src/watch/watcher.rs
+++ b/crates/red_knot_workspace/src/watch/watcher.rs
@@ -109,7 +109,7 @@ struct WatcherInner {
 impl Watcher {
    /// Sets up file watching for `path`.
    pub fn watch(&mut self, path: &SystemPath) -> notify::Result<()> {
-        tracing::debug!("Watching path: '{path}'.");
+        tracing::debug!("Watching path: {path}.");

        self.inner_mut()
            .watcher
@@ -118,7 +118,7 @@ impl Watcher {

    /// Stops file watching for `path`.
    pub fn unwatch(&mut self, path: &SystemPath) -> notify::Result<()> {
-        tracing::debug!("Unwatching path: '{path}'.");
+        tracing::debug!("Unwatching path: {path}.");

        self.inner_mut().watcher.unwatch(path.as_std_path())
    }
@@ -351,7 +351,7 @@ impl Debouncer {
            }

            EventKind::Any => {
-                tracing::debug!("Skipping any FS event for '{path}'.");
+                tracing::debug!("Skip any FS event for {path}.");
                return;
            }
        };
--- a/crates/red_knot_workspace/src/workspace.rs
+++ b/crates/red_knot_workspace/src/workspace.rs
@@ -1,28 +1,24 @@
 use std::{collections::BTreeMap, sync::Arc};

-use rustc_hash::{FxBuildHasher, FxHashSet};
+use foldhash::{HashMapExt, HashSet, HashSetExt};
 use salsa::{Durability, Setter as _};

 pub use metadata::{PackageMetadata, WorkspaceMetadata};
-use red_knot_python_semantic::types::check_types;
-use red_knot_python_semantic::SearchPathSettings;
-use ruff_db::source::{line_index, source_text, SourceDiagnostic};
+use ruff_db::source::{source_text, SourceDiagnostic};
 use ruff_db::{
    files::{system_path_to_file, File},
    system::{walk_directory::WalkState, SystemPath, SystemPathBuf},
 };
 use ruff_python_ast::{name::Name, PySourceType};
-use ruff_text_size::Ranged;

-use crate::workspace::files::{Index, Indexed, PackageFiles};
+use crate::workspace::files::{Index, IndexedFiles, PackageFiles};
 use crate::{
    db::Db,
-    lint::{lint_semantic, lint_syntax},
+    lint::{lint_semantic, lint_syntax, Diagnostics},
 };

 mod files;
 mod metadata;
-pub mod settings;

 /// The project workspace as a Salsa ingredient.
 ///
@@ -78,15 +74,11 @@ pub struct Workspace {
    /// open files rather than all files in the workspace.
    #[return_ref]
    #[default]
-    open_fileset: Option<Arc<FxHashSet<File>>>,
+    open_fileset: Option<Arc<HashSet<File>>>,

    /// The (first-party) packages in this workspace.
    #[return_ref]
    package_tree: BTreeMap<SystemPathBuf, Package>,
-
-    /// The unresolved search path configuration.
-    #[return_ref]
-    pub search_path_settings: SearchPathSettings,
 }

 /// A first-party package in a workspace.
@@ -100,8 +92,8 @@ pub struct Package {
    root_buf: SystemPathBuf,

    /// The files that are part of this package.
-    #[default]
    #[return_ref]
+    #[default]
    file_set: PackageFiles,
    // TODO: Add the loaded settings.
 }
@@ -115,14 +107,10 @@ impl Workspace {
            packages.insert(package.root.clone(), Package::from_metadata(db, package));
        }

-        Workspace::builder(
-            metadata.root,
-            packages,
-            metadata.settings.program.search_paths,
-        )
-        .durability(Durability::MEDIUM)
-        .open_fileset_durability(Durability::LOW)
-        .new(db)
+        Workspace::builder(metadata.root, packages)
+            .durability(Durability::MEDIUM)
+            .open_fileset_durability(Durability::LOW)
+            .new(db)
    }

    pub fn root(self, db: &dyn Db) -> &SystemPath {
@@ -153,12 +141,9 @@ impl Workspace {
            new_packages.insert(path, package);
        }

-        if &metadata.settings.program.search_paths != self.search_path_settings(db) {
-            self.set_search_path_settings(db)
-                .to(metadata.settings.program.search_paths);
-        }
-
-        self.set_package_tree(db).to(new_packages);
+        self.set_package_tree(db)
+            .with_durability(Durability::MEDIUM)
+            .to(new_packages);
    }

    pub fn update_package(self, db: &mut dyn Db, metadata: PackageMetadata) -> anyhow::Result<()> {
@@ -212,7 +197,7 @@ impl Workspace {
    ///
    /// This changes the behavior of `check` to only check the open files rather than all files in the workspace.
    pub fn open_file(self, db: &mut dyn Db, file: File) {
-        tracing::debug!("Opening file '{}'", file.path(db));
+        tracing::debug!("Opening file {}", file.path(db));

        let mut open_files = self.take_open_files(db);
        open_files.insert(file);
@@ -221,7 +206,7 @@ impl Workspace {

    /// Closes a file in the workspace.
    pub fn close_file(self, db: &mut dyn Db, file: File) -> bool {
-        tracing::debug!("Closing file '{}'", file.path(db));
+        tracing::debug!("Closing file {}", file.path(db));

        let mut open_files = self.take_open_files(db);
        let removed = open_files.remove(&file);
@@ -234,7 +219,7 @@ impl Workspace {
    }

    /// Returns the open files in the workspace or `None` if the entire workspace should be checked.
-    pub fn open_files(self, db: &dyn Db) -> Option<&FxHashSet<File>> {
+    pub fn open_files(self, db: &dyn Db) -> Option<&HashSet<File>> {
        self.open_fileset(db).as_deref()
    }

@@ -242,7 +227,7 @@ impl Workspace {
    ///
    /// This changes the behavior of `check` to only check the open files rather than all files in the workspace.
    #[tracing::instrument(level = "debug", skip(self, db))]
-    pub fn set_open_files(self, db: &mut dyn Db, open_files: FxHashSet<File>) {
+    pub fn set_open_files(self, db: &mut dyn Db, open_files: HashSet<File>) {
        tracing::debug!("Set open workspace files (count: {})", open_files.len());

        self.set_open_fileset(db).to(Some(Arc::new(open_files)));
@@ -251,7 +236,7 @@ impl Workspace {
    /// This takes the open files from the workspace and returns them.
    ///
    /// This changes the behavior of `check` to check all files in the workspace instead of just the open files.
-    pub fn take_open_files(self, db: &mut dyn Db) -> FxHashSet<File> {
+    pub fn take_open_files(self, db: &mut dyn Db) -> HashSet<File> {
        tracing::debug!("Take open workspace files");

        // Salsa will cancel any pending queries and remove its own reference to `open_files`
@@ -261,24 +246,7 @@ impl Workspace {
        if let Some(open_files) = open_files {
            Arc::try_unwrap(open_files).unwrap()
        } else {
-            FxHashSet::default()
-        }
-    }
-
-    /// Returns `true` if the file is open in the workspace.
-    ///
-    /// A file is considered open when:
-    /// * explicitly set as an open file using [`open_file`](Self::open_file)
-    /// * It has a [`SystemPath`] and belongs to a package's `src` files
-    /// * It has a [`SystemVirtualPath`](ruff_db::system::SystemVirtualPath)
-    pub fn is_file_open(self, db: &dyn Db, file: File) -> bool {
-        if let Some(open_files) = self.open_files(db) {
-            open_files.contains(&file)
-        } else if let Some(system_path) = file.path(db).as_system_path() {
-            self.package(db, system_path)
-                .map_or(false, |package| package.contains_file(db, file))
-        } else {
-            file.path(db).is_system_virtual_path()
+            HashSet::default()
        }
    }
 }
@@ -291,13 +259,13 @@ impl Package {

    /// Returns `true` if `file` is a first-party file part of this package.
    pub fn contains_file(self, db: &dyn Db, file: File) -> bool {
-        self.files(db).contains(&file)
+        self.files(db).read().contains(&file)
    }

    #[tracing::instrument(level = "debug", skip(db))]
    pub fn remove_file(self, db: &mut dyn Db, file: File) {
        tracing::debug!(
-            "Removing file '{}' from package '{}'",
+            "Remove file {} from package {}",
            file.path(db),
            self.name(db)
        );
@@ -310,11 +278,7 @@ impl Package {
    }

    pub fn add_file(self, db: &mut dyn Db, file: File) {
-        tracing::debug!(
-            "Adding file '{}' to package '{}'",
-            file.path(db),
-            self.name(db)
-        );
+        tracing::debug!("Add file {} to package {}", file.path(db), self.name(db));

        let Some(mut index) = PackageFiles::indexed_mut(db, self) else {
            return;
@@ -325,10 +289,10 @@ impl Package {

    #[tracing::instrument(level = "debug", skip(db))]
    pub(crate) fn check(self, db: &dyn Db) -> Vec<String> {
-        tracing::debug!("Checking package '{}'", self.root(db));
+        tracing::debug!("Checking package {}", self.root(db));

        let mut result = Vec::new();
-        for file in &self.files(db) {
+        for file in &self.files(db).read() {
            let diagnostics = check_file(db, file);
            result.extend_from_slice(&diagnostics);
        }
@@ -337,16 +301,15 @@ impl Package {
    }

    /// Returns the files belonging to this package.
-    pub fn files(self, db: &dyn Db) -> Indexed<'_> {
+    #[salsa::tracked]
+    pub fn files(self, db: &dyn Db) -> IndexedFiles {
+        let _entered = tracing::debug_span!("files").entered();
        let files = self.file_set(db);

        let indexed = match files.get() {
            Index::Lazy(vacant) => {
-                let _entered =
-                    tracing::debug_span!("index_package_files", package = %self.name(db)).entered();
-
+                tracing::debug!("Indexing files for package {}", self.name(db));
                let files = discover_package_files(db, self.root(db));
-                tracing::info!("Found {} files in package '{}'", files.len(), self.name(db));
                vacant.set(files)
            }
            Index::Indexed(indexed) => indexed,
@@ -367,12 +330,14 @@ impl Package {
        assert_eq!(root, metadata.root());

        if self.name(db) != metadata.name() {
-            self.set_name(db).to(metadata.name);
+            self.set_name(db)
+                .with_durability(Durability::MEDIUM)
+                .to(metadata.name);
        }
    }

    pub fn reload_files(self, db: &mut dyn Db) {
-        tracing::debug!("Reloading files for package '{}'", self.name(db));
+        tracing::debug!("Reload files for package {}", self.name(db));

        if !self.file_set(db).is_lazy() {
            // Force a re-index of the files in the next revision.
@@ -382,10 +347,10 @@ impl Package {
 }

 #[salsa::tracked]
-pub(super) fn check_file(db: &dyn Db, file: File) -> Vec<String> {
+pub(super) fn check_file(db: &dyn Db, file: File) -> Diagnostics {
    let path = file.path(db);
    let _span = tracing::debug_span!("check_file", file=%path).entered();
-    tracing::debug!("Checking file '{path}'");
+    tracing::debug!("Checking file {path}");

    let mut diagnostics = Vec::new();

@@ -398,28 +363,16 @@ pub(super) fn check_file(db: &dyn Db, file: File) -> Vec<String> {
    );

    // Abort checking if there are IO errors.
-    let source = source_text(db.upcast(), file);
-
-    if source.has_read_error() {
-        return diagnostics;
-    }
-
-    for diagnostic in check_types(db.upcast(), file) {
-        let index = line_index(db.upcast(), diagnostic.file());
-        let location = index.source_location(diagnostic.start(), source.as_str());
-        diagnostics.push(format!(
-            "{path}:{location}: {message}",
-            path = file.path(db),
-            message = diagnostic.message()
-        ));
+    if source_text(db.upcast(), file).has_read_error() {
+        return Diagnostics::from(diagnostics);
    }

    diagnostics.extend_from_slice(lint_syntax(db, file));
    diagnostics.extend_from_slice(lint_semantic(db, file));
-    diagnostics
+    Diagnostics::from(diagnostics)
 }

-fn discover_package_files(db: &dyn Db, path: &SystemPath) -> FxHashSet<File> {
+fn discover_package_files(db: &dyn Db, path: &SystemPath) -> HashSet<File> {
    let paths = std::sync::Mutex::new(Vec::new());

    db.system().walk_directory(path).run(|| {
@@ -449,7 +402,7 @@ fn discover_package_files(db: &dyn Db, path: &SystemPath) -> FxHashSet<File> {
    });

    let paths = paths.into_inner().unwrap();
-    let mut files = FxHashSet::with_capacity_and_hasher(paths.len(), FxBuildHasher);
+    let mut files = HashSet::with_capacity(paths.len());

    for path in paths {
        // If this returns `None`, then the file was deleted between the `walk_directory` call and now.
@@ -470,7 +423,7 @@ mod tests {
    use ruff_db::testing::assert_function_query_was_not_run;

    use crate::db::tests::TestDb;
-    use crate::lint::lint_syntax;
+    use crate::lint::{lint_syntax, Diagnostics};
    use crate::workspace::check_file;

    #[test]
@@ -488,7 +441,9 @@ mod tests {
        assert_eq!(source_text(&db, file).as_str(), "");
        assert_eq!(
            check_file(&db, file),
-            vec!["Failed to read file: No such file or directory".to_string()]
+            Diagnostics::List(vec![
+                "Failed to read file: No such file or directory".to_string()
+            ])
        );

        let events = db.take_salsa_events();
@@ -499,7 +454,7 @@ mod tests {
        db.write_file(path, "").unwrap();

        assert_eq!(source_text(&db, file).as_str(), "");
-        assert_eq!(check_file(&db, file), vec![] as Vec<String>);
+        assert_eq!(check_file(&db, file), Diagnostics::Empty);

        Ok(())
    }
--- a/crates/red_knot_workspace/src/workspace/files.rs
+++ b/crates/red_knot_workspace/src/workspace/files.rs
@@ -1,8 +1,8 @@
-use std::marker::PhantomData;
+use std::iter::FusedIterator;
 use std::ops::Deref;
 use std::sync::Arc;

-use rustc_hash::FxHashSet;
+use foldhash::HashSet;
 use salsa::Setter;

 use ruff_db::files::File;
@@ -10,9 +10,6 @@ use ruff_db::files::File;
 use crate::db::Db;
 use crate::workspace::Package;

-/// Cheap cloneable hash set of files.
-type FileSet = Arc<FxHashSet<File>>;
-
 /// The indexed files of a package.
 ///
 /// The indexing happens lazily, but the files are then cached for subsequent reads.
@@ -21,7 +18,7 @@ type FileSet = Arc<FxHashSet<File>>;
 /// The implementation uses internal mutability to transition between the lazy and indexed state
 /// without triggering a new salsa revision. This is safe because the initial indexing happens on first access,
 /// so no query can be depending on the contents of the indexed files before that. All subsequent mutations to
-/// the indexed files must go through `IndexedMut`, which uses the Salsa setter `package.set_file_set` to
+/// the indexed files must go through `IndexedFilesMut`, which uses the Salsa setter `package.set_file_set` to
 /// ensure that Salsa always knows when the set of indexed files have changed.
 #[derive(Debug)]
 pub struct PackageFiles {
@@ -35,67 +32,46 @@ impl PackageFiles {
        }
    }

-    fn indexed(files: FileSet) -> Self {
+    fn indexed(indexed_files: IndexedFiles) -> Self {
        Self {
-            state: std::sync::Mutex::new(State::Indexed(files)),
+            state: std::sync::Mutex::new(State::Indexed(indexed_files)),
        }
    }

-    pub(super) fn get(&self) -> Index {
+    pub fn get(&self) -> Index {
        let state = self.state.lock().unwrap();

        match &*state {
            State::Lazy => Index::Lazy(LazyFiles { files: state }),
-            State::Indexed(files) => Index::Indexed(Indexed {
-                files: Arc::clone(files),
-                _lifetime: PhantomData,
-            }),
+            State::Indexed(files) => Index::Indexed(files.clone()),
        }
    }

-    pub(super) fn is_lazy(&self) -> bool {
+    pub fn is_lazy(&self) -> bool {
        matches!(*self.state.lock().unwrap(), State::Lazy)
    }

    /// Returns a mutable view on the index that allows cheap in-place mutations.
    ///
    /// The changes are automatically written back to the database once the view is dropped.
-    pub(super) fn indexed_mut(db: &mut dyn Db, package: Package) -> Option<IndexedMut> {
+    pub fn indexed_mut(db: &mut dyn Db, package: Package) -> Option<IndexedFilesMut> {
        // Calling `zalsa_mut` cancels all pending salsa queries. This ensures that there are no pending
        // reads to the file set.
        // TODO: Use a non-internal API instead https://salsa.zulipchat.com/#narrow/stream/333573-salsa-3.2E0/topic/Expose.20an.20API.20to.20cancel.20other.20queries
        let _ = db.as_dyn_database_mut().zalsa_mut();

-        // Replace the state with lazy. The `IndexedMut` guard restores the state
-        // to `State::Indexed`  or sets a new `PackageFiles` when it gets dropped to ensure the state
-        // is restored to how it has been before replacing the value.
-        //
-        // It isn't necessary to hold on to the lock after this point:
-        // * The above call to `zalsa_mut` guarantees that there's exactly **one** DB reference.
-        // * `Indexed` has a `'db` lifetime, and this method requires a `&mut db`.
-        //   This means that there can't be any pending reference to `Indexed` because Rust
-        //   doesn't allow borrowing `db` as mutable (to call this method) and immutable (`Indexed<'db>`) at the same time.
-        //   There can't be any other `Indexed<'db>` references created by clones of this DB because
-        //   all clones must have been dropped at this point and the `Indexed`
-        //   can't outlive the database (constrained by the `db` lifetime).
-        let state = {
-            let files = package.file_set(db);
-            let mut locked = files.state.lock().unwrap();
-            std::mem::replace(&mut *locked, State::Lazy)
-        };
+        let files = package.file_set(db);

-        let indexed = match state {
-            // If it's already lazy, just return. We also don't need to restore anything because the
-            // replace above was a no-op.
+        let indexed = match &*files.state.lock().unwrap() {
            State::Lazy => return None,
-            State::Indexed(indexed) => indexed,
+            State::Indexed(indexed) => indexed.clone(),
        };

-        Some(IndexedMut {
+        Some(IndexedFilesMut {
            db: Some(db),
            package,
-            files: indexed,
-            did_change: false,
+            new_revision: indexed.revision,
+            indexed,
        })
    }
 }
@@ -112,93 +88,152 @@ enum State {
    Lazy,

    /// The files are indexed. Stores the known files of a package.
-    Indexed(FileSet),
+    Indexed(IndexedFiles),
 }

-pub(super) enum Index<'db> {
+pub enum Index<'a> {
    /// The index has not yet been computed. Allows inserting the files.
-    Lazy(LazyFiles<'db>),
+    Lazy(LazyFiles<'a>),

-    Indexed(Indexed<'db>),
+    Indexed(IndexedFiles),
 }

 /// Package files that have not been indexed yet.
-pub(super) struct LazyFiles<'db> {
-    files: std::sync::MutexGuard<'db, State>,
+pub struct LazyFiles<'a> {
+    files: std::sync::MutexGuard<'a, State>,
 }

-impl<'db> LazyFiles<'db> {
+impl<'a> LazyFiles<'a> {
    /// Sets the indexed files of a package to `files`.
-    pub(super) fn set(mut self, files: FxHashSet<File>) -> Indexed<'db> {
-        let files = Indexed {
-            files: Arc::new(files),
-            _lifetime: PhantomData,
-        };
-        *self.files = State::Indexed(Arc::clone(&files.files));
+    pub fn set(mut self, files: HashSet<File>) -> IndexedFiles {
+        let files = IndexedFiles::new(files);
+        *self.files = State::Indexed(files.clone());
        files
    }
 }

 /// The indexed files of a package.
 ///
-/// Note: This type is intentionally non-cloneable. Making it cloneable requires
-/// revisiting the locking behavior in [`PackageFiles::indexed_mut`].
-#[derive(Debug, PartialEq, Eq)]
-pub struct Indexed<'db> {
-    files: FileSet,
-    // Preserve the lifetime of `PackageFiles`.
-    _lifetime: PhantomData<&'db ()>,
+/// # Salsa integration
+/// The type is cheap clonable and allows for in-place mutation of the files. The in-place mutation requires
+/// extra care because the type is used as the result of Salsa queries and Salsa relies on a type's equality
+/// to determine if the output has changed. This is accomplished by using a `revision` that gets incremented
+/// whenever the files are changed. The revision ensures that salsa's comparison of the
+/// previous [`IndexedFiles`] with the next [`IndexedFiles`] returns false even though they both
+/// point to the same underlying hash set.
+///
+/// # Equality
+/// Two [`IndexedFiles`] are only equal if they have the same revision and point to the **same** (identity) hash set.
+#[derive(Debug, Clone)]
+pub struct IndexedFiles {
+    revision: u64,
+    files: Arc<std::sync::Mutex<HashSet<File>>>,
 }

-impl Deref for Indexed<'_> {
-    type Target = FxHashSet<File>;
+impl IndexedFiles {
+    fn new(files: HashSet<File>) -> Self {
+        Self {
+            files: Arc::new(std::sync::Mutex::new(files)),
+            revision: 0,
+        }
+    }
+
+    /// Locks the file index for reading.
+    pub fn read(&self) -> IndexedFilesGuard {
+        IndexedFilesGuard {
+            guard: self.files.lock().unwrap(),
+        }
+    }
+}
+
+impl PartialEq for IndexedFiles {
+    fn eq(&self, other: &Self) -> bool {
+        self.revision == other.revision && Arc::ptr_eq(&self.files, &other.files)
+    }
+}
+
+impl Eq for IndexedFiles {}
+
+pub struct IndexedFilesGuard<'a> {
+    guard: std::sync::MutexGuard<'a, HashSet<File>>,
+}
+
+impl Deref for IndexedFilesGuard<'_> {
+    type Target = HashSet<File>;

    fn deref(&self) -> &Self::Target {
-        &self.files
+        &self.guard
    }
 }

-impl<'a> IntoIterator for &'a Indexed<'_> {
+impl<'a> IntoIterator for &'a IndexedFilesGuard<'a> {
    type Item = File;
-    type IntoIter = std::iter::Copied<std::collections::hash_set::Iter<'a, File>>;
+    type IntoIter = IndexedFilesIter<'a>;

    fn into_iter(self) -> Self::IntoIter {
-        self.files.iter().copied()
+        IndexedFilesIter {
+            inner: self.guard.iter(),
+        }
    }
 }

+/// Iterator over the indexed files.
+///
+/// # Locks
+/// Holding on to the iterator locks the file index for reading.
+pub struct IndexedFilesIter<'a> {
+    inner: std::collections::hash_set::Iter<'a, File>,
+}
+
+impl<'a> Iterator for IndexedFilesIter<'a> {
+    type Item = File;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.inner.next().copied()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.inner.size_hint()
+    }
+}
+
+impl FusedIterator for IndexedFilesIter<'_> {}
+
+impl ExactSizeIterator for IndexedFilesIter<'_> {}
+
 /// A Mutable view of a package's indexed files.
 ///
 /// Allows in-place mutation of the files without deep cloning the hash set.
 /// The changes are written back when the mutable view is dropped or by calling [`Self::set`] manually.
-pub(super) struct IndexedMut<'db> {
+pub struct IndexedFilesMut<'db> {
    db: Option<&'db mut dyn Db>,
    package: Package,
-    files: FileSet,
-    did_change: bool,
+    indexed: IndexedFiles,
+    new_revision: u64,
 }

-impl IndexedMut<'_> {
-    pub(super) fn insert(&mut self, file: File) -> bool {
-        if self.files_mut().insert(file) {
-            self.did_change = true;
+impl IndexedFilesMut<'_> {
+    pub fn insert(&mut self, file: File) -> bool {
+        if self.indexed.files.lock().unwrap().insert(file) {
+            self.new_revision += 1;
            true
        } else {
            false
        }
    }

-    pub(super) fn remove(&mut self, file: File) -> bool {
-        if self.files_mut().remove(&file) {
-            self.did_change = true;
+    pub fn remove(&mut self, file: File) -> bool {
+        if self.indexed.files.lock().unwrap().remove(&file) {
+            self.new_revision += 1;
            true
        } else {
            false
        }
    }

-    fn files_mut(&mut self) -> &mut FxHashSet<File> {
-        Arc::get_mut(&mut self.files).expect("All references to `FilesSet` to have been dropped")
+    /// Writes the changes back to the database.
+    pub fn set(mut self) {
+        self.set_impl();
    }

    fn set_impl(&mut self) {
@@ -206,70 +241,19 @@ impl IndexedMut<'_> {
            return;
        };

-        let files = Arc::clone(&self.files);
-
-        if self.did_change {
-            // If there are changes, set the new file_set to trigger a salsa revision change.
+        if self.indexed.revision != self.new_revision {
            self.package
                .set_file_set(db)
-                .to(PackageFiles::indexed(files));
-        } else {
-            // The `indexed_mut` replaced the `state` with Lazy. Restore it back to the indexed state.
-            *self.package.file_set(db).state.lock().unwrap() = State::Indexed(files);
+                .to(PackageFiles::indexed(IndexedFiles {
+                    revision: self.new_revision,
+                    files: self.indexed.files.clone(),
+                }));
        }
    }
 }

-impl Drop for IndexedMut<'_> {
+impl Drop for IndexedFilesMut<'_> {
    fn drop(&mut self) {
        self.set_impl();
    }
 }
-
-#[cfg(test)]
-mod tests {
-    use rustc_hash::FxHashSet;
-
-    use ruff_db::files::system_path_to_file;
-    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
-    use ruff_python_ast::name::Name;
-
-    use crate::db::tests::TestDb;
-    use crate::workspace::files::Index;
-    use crate::workspace::Package;
-
-    #[test]
-    fn re_entrance() -> anyhow::Result<()> {
-        let mut db = TestDb::new();
-
-        db.write_file("test.py", "")?;
-
-        let package = Package::new(&db, Name::new("test"), SystemPathBuf::from("/test"));
-
-        let file = system_path_to_file(&db, "test.py").unwrap();
-
-        let files = match package.file_set(&db).get() {
-            Index::Lazy(lazy) => lazy.set(FxHashSet::from_iter([file])),
-            Index::Indexed(files) => files,
-        };
-
-        // Calling files a second time should not dead-lock.
-        // This can e.g. happen when `check_file` iterates over all files and
-        // `is_file_open` queries the open files.
-        let files_2 = package.file_set(&db).get();
-
-        match files_2 {
-            Index::Lazy(_) => {
-                panic!("Expected indexed files, got lazy files");
-            }
-            Index::Indexed(files_2) => {
-                assert_eq!(
-                    files_2.iter().collect::<Vec<_>>(),
-                    files.iter().collect::<Vec<_>>()
-                );
-            }
-        }
-
-        Ok(())
-    }
-}
--- a/crates/red_knot_workspace/src/workspace/metadata.rs
+++ b/crates/red_knot_workspace/src/workspace/metadata.rs
@@ -1,4 +1,3 @@
-use crate::workspace::settings::{Configuration, WorkspaceSettings};
 use ruff_db::system::{System, SystemPath, SystemPathBuf};
 use ruff_python_ast::name::Name;

@@ -8,8 +7,6 @@ pub struct WorkspaceMetadata {

    /// The (first-party) packages in this workspace.
    pub(super) packages: Vec<PackageMetadata>,
-
-    pub(super) settings: WorkspaceSettings,
 }

 /// A first-party package in a workspace.
@@ -24,11 +21,7 @@ pub struct PackageMetadata {

 impl WorkspaceMetadata {
    /// Discovers the closest workspace at `path` and returns its metadata.
-    pub fn from_path(
-        path: &SystemPath,
-        system: &dyn System,
-        base_configuration: Option<Configuration>,
-    ) -> anyhow::Result<WorkspaceMetadata> {
+    pub fn from_path(path: &SystemPath, system: &dyn System) -> anyhow::Result<WorkspaceMetadata> {
        assert!(
            system.is_directory(path),
            "Workspace root path must be a directory"
@@ -45,20 +38,9 @@ impl WorkspaceMetadata {
            root: root.clone(),
        };

-        // TODO: Load the configuration from disk.
-        let mut configuration = Configuration::default();
-
-        if let Some(base_configuration) = base_configuration {
-            configuration.extend(base_configuration);
-        }
-
-        // TODO: Respect the package configurations when resolving settings (e.g. for the target version).
-        let settings = configuration.into_workspace_settings(&root);
-
        let workspace = WorkspaceMetadata {
            root,
            packages: vec![package],
-            settings,
        };

        Ok(workspace)
@@ -71,10 +53,6 @@ impl WorkspaceMetadata {
    pub fn packages(&self) -> &[PackageMetadata] {
        &self.packages
    }
-
-    pub fn settings(&self) -> &WorkspaceSettings {
-        &self.settings
-    }
 }

 impl PackageMetadata {
--- a/crates/red_knot_workspace/src/workspace/settings.rs
+++ b/crates/red_knot_workspace/src/workspace/settings.rs
@@ -1,89 +0,0 @@
-use red_knot_python_semantic::{ProgramSettings, PythonVersion, SearchPathSettings, SitePackages};
-use ruff_db::system::{SystemPath, SystemPathBuf};
-
-/// The resolved configurations.
-///
-/// The main difference to [`Configuration`] is that default values are filled in.
-#[derive(Debug, Clone)]
-pub struct WorkspaceSettings {
-    pub(super) program: ProgramSettings,
-}
-
-impl WorkspaceSettings {
-    pub fn program(&self) -> &ProgramSettings {
-        &self.program
-    }
-}
-
-/// The configuration for the workspace or a package.
-#[derive(Debug, Default, Clone)]
-pub struct Configuration {
-    pub target_version: Option<PythonVersion>,
-    pub search_paths: SearchPathConfiguration,
-}
-
-impl Configuration {
-    /// Extends this configuration by using the values from `with` for all values that are absent in `self`.
-    pub fn extend(&mut self, with: Configuration) {
-        self.target_version = self.target_version.or(with.target_version);
-        self.search_paths.extend(with.search_paths);
-    }
-
-    pub fn into_workspace_settings(self, workspace_root: &SystemPath) -> WorkspaceSettings {
-        WorkspaceSettings {
-            program: ProgramSettings {
-                target_version: self.target_version.unwrap_or_default(),
-                search_paths: self.search_paths.into_settings(workspace_root),
-            },
-        }
-    }
-}
-
-#[derive(Debug, Default, Clone, Eq, PartialEq)]
-pub struct SearchPathConfiguration {
-    /// List of user-provided paths that should take first priority in the module resolution.
-    /// Examples in other type checkers are mypy's MYPYPATH environment variable,
-    /// or pyright's stubPath configuration setting.
-    pub extra_paths: Option<Vec<SystemPathBuf>>,
-
-    /// The root of the workspace, used for finding first-party modules.
-    pub src_root: Option<SystemPathBuf>,
-
-    /// Optional path to a "custom typeshed" directory on disk for us to use for standard-library types.
-    /// If this is not provided, we will fallback to our vendored typeshed stubs for the stdlib,
-    /// bundled as a zip file in the binary
-    pub custom_typeshed: Option<SystemPathBuf>,
-
-    /// The path to the user's `site-packages` directory, where third-party packages from ``PyPI`` are installed.
-    pub site_packages: Option<SitePackages>,
-}
-
-impl SearchPathConfiguration {
-    pub fn into_settings(self, workspace_root: &SystemPath) -> SearchPathSettings {
-        let site_packages = self.site_packages.unwrap_or(SitePackages::Known(vec![]));
-
-        SearchPathSettings {
-            extra_paths: self.extra_paths.unwrap_or_default(),
-            src_root: self
-                .src_root
-                .unwrap_or_else(|| workspace_root.to_path_buf()),
-            custom_typeshed: self.custom_typeshed,
-            site_packages,
-        }
-    }
-
-    pub fn extend(&mut self, with: SearchPathConfiguration) {
-        if let Some(extra_paths) = with.extra_paths {
-            self.extra_paths.get_or_insert(extra_paths);
-        }
-        if let Some(src_root) = with.src_root {
-            self.src_root.get_or_insert(src_root);
-        }
-        if let Some(custom_typeshed) = with.custom_typeshed {
-            self.custom_typeshed.get_or_insert(custom_typeshed);
-        }
-        if let Some(site_packages) = with.site_packages {
-            self.site_packages.get_or_insert(site_packages);
-        }
-    }
-}
--- a/crates/red_knot_workspace/tests/check.rs
+++ b/crates/red_knot_workspace/tests/check.rs
@@ -1,20 +1,26 @@
+use red_knot_python_semantic::{ProgramSettings, PythonVersion, SearchPathSettings};
+use red_knot_workspace::db::RootDatabase;
+use red_knot_workspace::lint::lint_semantic;
+use red_knot_workspace::workspace::WorkspaceMetadata;
+use ruff_db::files::system_path_to_file;
+use ruff_db::system::{OsSystem, SystemPathBuf};
 use std::fs;
 use std::path::PathBuf;

-use red_knot_python_semantic::{HasTy, SemanticModel};
-use red_knot_workspace::db::RootDatabase;
-use red_knot_workspace::workspace::WorkspaceMetadata;
-use ruff_db::files::{system_path_to_file, File};
-use ruff_db::parsed::parsed_module;
-use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf};
-use ruff_python_ast::visitor::source_order;
-use ruff_python_ast::visitor::source_order::SourceOrderVisitor;
-use ruff_python_ast::{Alias, Expr, Parameter, ParameterWithDefault, Stmt};
-
-fn setup_db(workspace_root: &SystemPath) -> anyhow::Result<RootDatabase> {
-    let system = OsSystem::new(workspace_root);
-    let workspace = WorkspaceMetadata::from_path(workspace_root, &system, None)?;
-    RootDatabase::new(workspace, system)
+fn setup_db(workspace_root: SystemPathBuf) -> anyhow::Result<RootDatabase> {
+    let system = OsSystem::new(&workspace_root);
+    let workspace = WorkspaceMetadata::from_path(&workspace_root, &system)?;
+    let search_paths = SearchPathSettings {
+        extra_paths: vec![],
+        src_root: workspace_root,
+        custom_typeshed: None,
+        site_packages: vec![],
+    };
+    let settings = ProgramSettings {
+        target_version: PythonVersion::default(),
+        search_paths,
+    };
+    RootDatabase::new(workspace, settings, system)
 }

 /// Test that all snippets in testcorpus can be checked without panic
@@ -24,99 +30,15 @@ fn corpus_no_panic() -> anyhow::Result<()> {
    let corpus = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("resources/test/corpus");
    let system_corpus =
        SystemPathBuf::from_path_buf(corpus.clone()).expect("corpus path to be UTF8");
-    let db = setup_db(&system_corpus)?;
+    let db = setup_db(system_corpus.clone())?;

    for path in fs::read_dir(&corpus).expect("corpus to be a directory") {
        let path = path.expect("path to not be an error").path();
        println!("checking {path:?}");
        let path = SystemPathBuf::from_path_buf(path.clone()).expect("path to be UTF-8");
-        // this test is only asserting that we can pull every expression type without a panic
-        // (and some non-expressions that clearly define a single type)
+        // this test is only asserting that we can run the lint without a panic
        let file = system_path_to_file(&db, path).expect("file to exist");
-
-        pull_types(&db, file);
+        lint_semantic(&db, file);
    }
    Ok(())
 }
-
-fn pull_types(db: &RootDatabase, file: File) {
-    let mut visitor = PullTypesVisitor::new(db, file);
-
-    let ast = parsed_module(db, file);
-
-    visitor.visit_body(ast.suite());
-}
-
-struct PullTypesVisitor<'db> {
-    model: SemanticModel<'db>,
-}
-
-impl<'db> PullTypesVisitor<'db> {
-    fn new(db: &'db RootDatabase, file: File) -> Self {
-        Self {
-            model: SemanticModel::new(db, file),
-        }
-    }
-}
-
-impl SourceOrderVisitor<'_> for PullTypesVisitor<'_> {
-    fn visit_stmt(&mut self, stmt: &Stmt) {
-        match stmt {
-            Stmt::FunctionDef(function) => {
-                let _ty = function.ty(&self.model);
-            }
-            Stmt::ClassDef(class) => {
-                let _ty = class.ty(&self.model);
-            }
-            Stmt::AnnAssign(_)
-            | Stmt::Return(_)
-            | Stmt::Delete(_)
-            | Stmt::Assign(_)
-            | Stmt::AugAssign(_)
-            | Stmt::TypeAlias(_)
-            | Stmt::For(_)
-            | Stmt::While(_)
-            | Stmt::If(_)
-            | Stmt::With(_)
-            | Stmt::Match(_)
-            | Stmt::Raise(_)
-            | Stmt::Try(_)
-            | Stmt::Assert(_)
-            | Stmt::Import(_)
-            | Stmt::ImportFrom(_)
-            | Stmt::Global(_)
-            | Stmt::Nonlocal(_)
-            | Stmt::Expr(_)
-            | Stmt::Pass(_)
-            | Stmt::Break(_)
-            | Stmt::Continue(_)
-            | Stmt::IpyEscapeCommand(_) => {}
-        }
-
-        source_order::walk_stmt(self, stmt);
-    }
-
-    fn visit_expr(&mut self, expr: &Expr) {
-        let _ty = expr.ty(&self.model);
-
-        source_order::walk_expr(self, expr);
-    }
-
-    fn visit_parameter(&mut self, parameter: &Parameter) {
-        let _ty = parameter.ty(&self.model);
-
-        source_order::walk_parameter(self, parameter);
-    }
-
-    fn visit_parameter_with_default(&mut self, parameter_with_default: &ParameterWithDefault) {
-        let _ty = parameter_with_default.ty(&self.model);
-
-        source_order::walk_parameter_with_default(self, parameter_with_default);
-    }
-
-    fn visit_alias(&mut self, alias: &Alias) {
-        let _ty = alias.ty(&self.model);
-
-        source_order::walk_alias(self, alias);
-    }
-}
--- a/crates/ruff/Cargo.toml
+++ b/crates/ruff/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "ruff"
-version = "0.6.2"
+version = "0.6.1"
 publish = true
 authors = { workspace = true }
 edition = { workspace = true }
@@ -44,7 +44,7 @@ notify = { workspace = true }
 path-absolutize = { workspace = true, features = ["once_cell_cache"] }
 rayon = { workspace = true }
 regex = { workspace = true }
-rustc-hash = { workspace = true }
+foldhash = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
 shellexpand = { workspace = true }
--- a/crates/ruff/src/args.rs
+++ b/crates/ruff/src/args.rs
@@ -9,9 +9,9 @@ use anyhow::{anyhow, bail};
 use clap::builder::{TypedValueParser, ValueParserFactory};
 use clap::{command, Parser};
 use colored::Colorize;
+use foldhash::HashMap;
 use path_absolutize::path_dedot;
 use regex::Regex;
-use rustc_hash::FxHashMap;
 use toml;

 use ruff_linter::line_width::LineLength;
@@ -1278,7 +1278,7 @@ impl ConfigurationTransformer for ExplicitConfigOverrides {

 /// Convert a list of `PatternPrefixPair` structs to `PerFileIgnore`.
 pub fn collect_per_file_ignores(pairs: Vec<PatternPrefixPair>) -> Vec<PerFileIgnore> {
-    let mut per_file_ignores: FxHashMap<String, Vec<RuleSelector>> = FxHashMap::default();
+    let mut per_file_ignores: HashMap<String, Vec<RuleSelector>> = HashMap::default();
    for pair in pairs {
        per_file_ignores
            .entry(pair.pattern)
--- a/crates/ruff/src/cache.rs
+++ b/crates/ruff/src/cache.rs
@@ -9,11 +9,11 @@ use std::time::{Duration, SystemTime};

 use anyhow::{Context, Result};
 use filetime::FileTime;
+use foldhash::HashMap;
 use itertools::Itertools;
 use log::{debug, error};
 use rayon::iter::ParallelIterator;
 use rayon::iter::{IntoParallelIterator, ParallelBridge};
-use rustc_hash::FxHashMap;
 use serde::{Deserialize, Serialize};
 use tempfile::NamedTempFile;

@@ -140,7 +140,7 @@ impl Cache {
    fn empty(path: PathBuf, package_root: PathBuf) -> Self {
        let package = PackageCache {
            package_root,
-            files: FxHashMap::default(),
+            files: HashMap::default(),
        };
        Cache::new(path, package)
    }
@@ -318,7 +318,7 @@ struct PackageCache {
    /// single file "packages", e.g. scripts.
    package_root: PathBuf,
    /// Mapping of source file path to it's cached data.
-    files: FxHashMap<RelativePathBuf, FileCache>,
+    files: HashMap<RelativePathBuf, FileCache>,
 }

 /// On disk representation of the cache per source file.
@@ -357,9 +357,9 @@ impl FileCache {
                    .collect()
            };
            let notebook_indexes = if let Some(notebook_index) = lint.notebook_index.as_ref() {
-                FxHashMap::from_iter([(path.to_string_lossy().to_string(), notebook_index.clone())])
+                HashMap::from_iter([(path.to_string_lossy().to_string(), notebook_index.clone())])
            } else {
-                FxHashMap::default()
+                HashMap::default()
            };
            Diagnostics::new(messages, notebook_indexes)
        })
@@ -493,11 +493,11 @@ where
    }
 }

-pub(crate) struct PackageCacheMap<'a>(FxHashMap<&'a Path, Cache>);
+pub(crate) struct PackageCacheMap<'a>(HashMap<&'a Path, Cache>);

 impl<'a> PackageCacheMap<'a> {
    pub(crate) fn init(
-        package_roots: &FxHashMap<&'a Path, Option<&'a Path>>,
+        package_roots: &HashMap<&'a Path, Option<&'a Path>>,
        resolver: &Resolver,
    ) -> Self {
        fn init_cache(path: &Path) {
--- a/crates/ruff/src/commands/check.rs
+++ b/crates/ruff/src/commands/check.rs
@@ -5,11 +5,11 @@ use std::time::Instant;

 use anyhow::Result;
 use colored::Colorize;
+use foldhash::HashMap;
 use ignore::Error;
 use log::{debug, error, warn};
 #[cfg(not(target_family = "wasm"))]
 use rayon::prelude::*;
-use rustc_hash::FxHashMap;

 use ruff_diagnostics::Diagnostic;
 use ruff_linter::message::Message;
@@ -133,7 +133,7 @@ pub(crate) fn check(
                            dummy,
                            TextSize::default(),
                        )],
-                        FxHashMap::default(),
+                        HashMap::default(),
                    )
                } else {
                    warn!(
@@ -221,7 +221,7 @@ mod test {
    use std::os::unix::fs::OpenOptionsExt;

    use anyhow::Result;
-    use rustc_hash::FxHashMap;
+    use foldhash::HashMap;
    use tempfile::TempDir;

    use ruff_linter::message::{Emitter, EmitterContext, TextEmitter};
@@ -284,7 +284,7 @@ mod test {
            .emit(
                &mut output,
                &diagnostics.messages,
-                &EmitterContext::new(&FxHashMap::default()),
+                &EmitterContext::new(&HashMap::default()),
            )
            .unwrap();

--- a/crates/ruff/src/commands/format.rs
+++ b/crates/ruff/src/commands/format.rs
@@ -7,11 +7,11 @@ use std::time::Instant;

 use anyhow::Result;
 use colored::Colorize;
+use foldhash::HashSet;
 use itertools::Itertools;
 use log::{error, warn};
 use rayon::iter::Either::{Left, Right};
 use rayon::iter::{IntoParallelRefIterator, ParallelIterator};
-use rustc_hash::FxHashSet;
 use thiserror::Error;
 use tracing::debug;

@@ -782,7 +782,7 @@ impl Display for FormatCommandError {

 pub(super) fn warn_incompatible_formatter_settings(resolver: &Resolver) {
    // First, collect all rules that are incompatible regardless of the linter-specific settings.
-    let mut incompatible_rules = FxHashSet::default();
+    let mut incompatible_rules = HashSet::default();
    for setting in resolver.settings() {
        for rule in [
            // The formatter might collapse implicit string concatenation on a single line.
--- a/crates/ruff/src/diagnostics.rs
+++ b/crates/ruff/src/diagnostics.rs
@@ -9,8 +9,8 @@ use std::path::Path;

 use anyhow::{Context, Result};
 use colored::Colorize;
+use foldhash::HashMap;
 use log::{debug, warn};
-use rustc_hash::FxHashMap;

 use ruff_diagnostics::Diagnostic;
 use ruff_linter::codes::Rule;
@@ -33,13 +33,13 @@ use crate::cache::{Cache, FileCacheKey, LintCacheData};
 pub(crate) struct Diagnostics {
    pub(crate) messages: Vec<Message>,
    pub(crate) fixed: FixMap,
-    pub(crate) notebook_indexes: FxHashMap<String, NotebookIndex>,
+    pub(crate) notebook_indexes: HashMap<String, NotebookIndex>,
 }

 impl Diagnostics {
    pub(crate) fn new(
        messages: Vec<Message>,
-        notebook_indexes: FxHashMap<String, NotebookIndex>,
+        notebook_indexes: HashMap<String, NotebookIndex>,
    ) -> Self {
        Self {
            messages,
@@ -72,7 +72,7 @@ impl Diagnostics {
                            source_file,
                            TextSize::default(),
                        )],
-                        FxHashMap::default(),
+                        HashMap::default(),
                    )
                } else {
                    match path {
@@ -106,7 +106,7 @@ impl Diagnostics {
                        range: TextRange::default(),
                        file: dummy,
                    })],
-                    FxHashMap::default(),
+                    HashMap::default(),
                )
            }
        }
@@ -132,7 +132,7 @@ impl AddAssign for Diagnostics {

 /// A collection of fixes indexed by file path.
 #[derive(Debug, Default, PartialEq)]
-pub(crate) struct FixMap(FxHashMap<String, FixTable>);
+pub(crate) struct FixMap(HashMap<String, FixTable>);

 impl FixMap {
    /// Returns `true` if there are no fixes in the map.
@@ -314,7 +314,7 @@ pub(crate) fn lint_path(
                ParseSource::None,
            );
            let transformed = source_kind;
-            let fixed = FxHashMap::default();
+            let fixed = HashMap::default();
            (result, transformed, fixed)
        }
    } else {
@@ -328,7 +328,7 @@ pub(crate) fn lint_path(
            ParseSource::None,
        );
        let transformed = source_kind;
-        let fixed = FxHashMap::default();
+        let fixed = HashMap::default();
        (result, transformed, fixed)
    };

@@ -357,9 +357,9 @@ pub(crate) fn lint_path(
    }

    let notebook_indexes = if let SourceKind::IpyNotebook(notebook) = transformed {
-        FxHashMap::from_iter([(path.to_string_lossy().to_string(), notebook.into_index())])
+        HashMap::from_iter([(path.to_string_lossy().to_string(), notebook.into_index())])
    } else {
-        FxHashMap::default()
+        HashMap::default()
    };

    Ok(Diagnostics {
@@ -456,7 +456,7 @@ pub(crate) fn lint_stdin(
                }

                let transformed = source_kind;
-                let fixed = FxHashMap::default();
+                let fixed = HashMap::default();
                (result, transformed, fixed)
            }
        } else {
@@ -470,17 +470,17 @@ pub(crate) fn lint_stdin(
                ParseSource::None,
            );
            let transformed = source_kind;
-            let fixed = FxHashMap::default();
+            let fixed = HashMap::default();
            (result, transformed, fixed)
        };

    let notebook_indexes = if let SourceKind::IpyNotebook(notebook) = transformed {
-        FxHashMap::from_iter([(
+        HashMap::from_iter([(
            path.map_or_else(|| "-".into(), |path| path.to_string_lossy().to_string()),
            notebook.into_index(),
        )])
    } else {
-        FxHashMap::default()
+        HashMap::default()
    };

    Ok(Diagnostics {
--- a/crates/ruff/tests/lint.rs
+++ b/crates/ruff/tests/lint.rs
@@ -1434,7 +1434,7 @@ def unused(x):

    insta::assert_snapshot!(test_code, @r###"

-    def unused(x):  # noqa: ANN001, ANN201, D103
+    def unused(x):  # noqa: ANN001, ANN201, ARG001, D103
        pass
    "###);

--- a/crates/ruff_benchmark/benches/red_knot.rs
+++ b/crates/ruff_benchmark/benches/red_knot.rs
@@ -1,9 +1,7 @@
 #![allow(clippy::disallowed_names)]

-use red_knot_python_semantic::PythonVersion;
+use red_knot_python_semantic::{ProgramSettings, PythonVersion, SearchPathSettings};
 use red_knot_workspace::db::RootDatabase;
-use red_knot_workspace::watch::{ChangeEvent, ChangedKind};
-use red_knot_workspace::workspace::settings::Configuration;
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_benchmark::criterion::{criterion_group, criterion_main, BatchSize, Criterion};
 use ruff_benchmark::TestFile;
@@ -14,53 +12,13 @@ use ruff_db::system::{MemoryFileSystem, SystemPath, TestSystem};
 struct Case {
    db: RootDatabase,
    fs: MemoryFileSystem,
+    parser: File,
    re: File,
    re_path: &'static SystemPath,
 }

 const TOMLLIB_312_URL: &str = "https://raw.githubusercontent.com/python/cpython/8e8a4baf652f6e1cee7acde9d78c4b6154539748/Lib/tomllib";

-// This first "unresolved import" is because we don't understand `*` imports yet.
-// The following "unresolved import" violations are because we can't distinguish currently from
-// "Symbol exists in the module but its type is unknown" and
-// "Symbol does not exist in the module"
-static EXPECTED_DIAGNOSTICS: &[&str] = &[
-    "/src/tomllib/_parser.py:7:29: Could not resolve import of 'Iterable' from 'collections.abc'",
-    "/src/tomllib/_parser.py:10:20: Could not resolve import of 'Any' from 'typing'",
-    "/src/tomllib/_parser.py:13:5: Could not resolve import of 'RE_DATETIME' from '._re'",
-    "/src/tomllib/_parser.py:14:5: Could not resolve import of 'RE_LOCALTIME' from '._re'",
-    "/src/tomllib/_parser.py:15:5: Could not resolve import of 'RE_NUMBER' from '._re'",
-    "/src/tomllib/_parser.py:20:21: Could not resolve import of 'Key' from '._types'",
-    "/src/tomllib/_parser.py:20:26: Could not resolve import of 'ParseFloat' from '._types'",
-    "Line 69 is too long (89 characters)",
-    "Use double quotes for strings",
-    "Use double quotes for strings",
-    "Use double quotes for strings",
-    "Use double quotes for strings",
-    "Use double quotes for strings",
-    "Use double quotes for strings",
-    "Use double quotes for strings",
-    "/src/tomllib/_parser.py:153:22: Name 'key' used when not defined.",
-    "/src/tomllib/_parser.py:153:27: Name 'flag' used when not defined.",
-    "/src/tomllib/_parser.py:159:16: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:161:25: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:168:16: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:169:22: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:170:25: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:180:16: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:182:31: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:206:16: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:207:22: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:208:25: Name 'k' used when not defined.",
-    "/src/tomllib/_parser.py:330:32: Name 'header' used when not defined.",
-    "/src/tomllib/_parser.py:330:41: Name 'key' used when not defined.",
-    "/src/tomllib/_parser.py:333:26: Name 'cont_key' used when not defined.",
-    "/src/tomllib/_parser.py:334:71: Name 'cont_key' used when not defined.",
-    "/src/tomllib/_parser.py:337:31: Name 'cont_key' used when not defined.",
-    "/src/tomllib/_parser.py:628:75: Name 'e' used when not defined.",
-    "/src/tomllib/_parser.py:686:23: Name 'parse_float' used when not defined.",
-];
-
 fn get_test_file(name: &str) -> TestFile {
    let path = format!("tomllib/{name}");
    let url = format!("{TOMLLIB_312_URL}/{name}");
@@ -70,34 +28,31 @@ fn get_test_file(name: &str) -> TestFile {
 fn setup_case() -> Case {
    let system = TestSystem::default();
    let fs = system.memory_file_system().clone();
+    let init_path = SystemPath::new("/src/tomllib/__init__.py");
    let parser_path = SystemPath::new("/src/tomllib/_parser.py");
    let re_path = SystemPath::new("/src/tomllib/_re.py");
+    let types_path = SystemPath::new("/src/tomllib/_types.py");
    fs.write_files([
-        (
-            SystemPath::new("/src/tomllib/__init__.py"),
-            get_test_file("__init__.py").code(),
-        ),
+        (init_path, get_test_file("__init__.py").code()),
        (parser_path, get_test_file("_parser.py").code()),
        (re_path, get_test_file("_re.py").code()),
-        (
-            SystemPath::new("/src/tomllib/_types.py"),
-            get_test_file("_types.py").code(),
-        ),
+        (types_path, get_test_file("_types.py").code()),
    ])
    .unwrap();

    let src_root = SystemPath::new("/src");
-    let metadata = WorkspaceMetadata::from_path(
-        src_root,
-        &system,
-        Some(Configuration {
-            target_version: Some(PythonVersion::PY312),
-            ..Configuration::default()
-        }),
-    )
-    .unwrap();
+    let metadata = WorkspaceMetadata::from_path(src_root, &system).unwrap();
+    let settings = ProgramSettings {
+        target_version: PythonVersion::PY312,
+        search_paths: SearchPathSettings {
+            extra_paths: vec![],
+            src_root: src_root.to_path_buf(),
+            site_packages: vec![],
+            custom_typeshed: None,
+        },
+    };

-    let mut db = RootDatabase::new(metadata, system).unwrap();
+    let mut db = RootDatabase::new(metadata, settings, system).unwrap();
    let parser = system_path_to_file(&db, parser_path).unwrap();

    db.workspace().open_file(&mut db, parser);
@@ -107,6 +62,7 @@ fn setup_case() -> Case {
    Case {
        db,
        fs,
+        parser,
        re,
        re_path,
    }
@@ -116,8 +72,8 @@ fn benchmark_incremental(criterion: &mut Criterion) {
    criterion.bench_function("red_knot_check_file[incremental]", |b| {
        b.iter_batched_ref(
            || {
-                let case = setup_case();
-                case.db.check().unwrap();
+                let mut case = setup_case();
+                case.db.check_file(case.parser).unwrap();

                case.fs
                    .write_file(
@@ -126,22 +82,14 @@ fn benchmark_incremental(criterion: &mut Criterion) {
                    )
                    .unwrap();

+                case.re.sync(&mut case.db);
                case
            },
            |case| {
-                let Case { db, .. } = case;
+                let Case { db, parser, .. } = case;
+                let result = db.check_file(*parser).unwrap();

-                db.apply_changes(
-                    vec![ChangeEvent::Changed {
-                        path: case.re_path.to_path_buf(),
-                        kind: ChangedKind::FileContent,
-                    }],
-                    None,
-                );
-
-                let result = db.check().unwrap();
-
-                assert_eq!(result, EXPECTED_DIAGNOSTICS);
+                assert_eq!(result.len(), 34);
            },
            BatchSize::SmallInput,
        );
@@ -153,10 +101,10 @@ fn benchmark_cold(criterion: &mut Criterion) {
        b.iter_batched_ref(
            setup_case,
            |case| {
-                let Case { db, .. } = case;
-                let result = db.check().unwrap();
+                let Case { db, parser, .. } = case;
+                let result = db.check_file(*parser).unwrap();

-                assert_eq!(result, EXPECTED_DIAGNOSTICS);
+                assert_eq!(result.len(), 34);
            },
            BatchSize::SmallInput,
        );
--- a/crates/ruff_db/Cargo.toml
+++ b/crates/ruff_db/Cargo.toml
@@ -31,7 +31,7 @@ thiserror = { workspace = true }
 tracing = { workspace = true }
 tracing-subscriber = { workspace = true, optional = true }
 tracing-tree = { workspace = true, optional = true }
-rustc-hash = { workspace = true }
+foldhash = { workspace = true }

 [target.'cfg(not(target_arch="wasm32"))'.dependencies]
 zip = { workspace = true, features = ["zstd"] }
--- a/crates/ruff_db/src/files.rs
+++ b/crates/ruff_db/src/files.rs
@@ -6,6 +6,7 @@ use dashmap::mapref::entry::Entry;
 use salsa::{Durability, Setter};

 pub use file_root::{FileRoot, FileRootKind};
+use foldhash::{HashMapExt, HashSetExt};
 pub use path::FilePath;
 use ruff_notebook::{Notebook, NotebookError};

@@ -85,7 +86,7 @@ impl Files {
            .system_by_path
            .entry(absolute.clone())
            .or_insert_with(|| {
-                tracing::trace!("Adding file '{path}'");
+                tracing::trace!("Adding file {path}");

                let metadata = db.system().path_metadata(path);
                let durability = self
@@ -131,7 +132,7 @@ impl Files {
                    Err(_) => return Err(FileError::NotFound),
                };

-                tracing::trace!("Adding vendored file '{}'", path);
+                tracing::trace!("Adding vendored file {}", path);
                let file = File::builder(FilePath::Vendored(path.to_path_buf()))
                    .permissions(Some(0o444))
                    .revision(metadata.revision())
@@ -158,7 +159,7 @@ impl Files {
            Entry::Vacant(entry) => {
                let metadata = db.system().virtual_path_metadata(path).ok()?;

-                tracing::trace!("Adding virtual file '{}'", path);
+                tracing::trace!("Adding virtual file {}", path);

                let file = File::builder(FilePath::SystemVirtual(path.to_path_buf()))
                    .revision(metadata.revision())
@@ -211,7 +212,7 @@ impl Files {
    /// That's why [`File::sync_path`] and [`File::sync_path`] is preferred if it is known that the path is a file.
    pub fn sync_recursively(db: &mut dyn Db, path: &SystemPath) {
        let path = SystemPath::absolute(path, db.system().current_directory());
-        tracing::debug!("Syncing all files in '{path}'");
+        tracing::debug!("Syncing all files in {path}");

        let inner = Arc::clone(&db.files().inner);
        for entry in inner.system_by_path.iter_mut() {
@@ -224,7 +225,9 @@ impl Files {

        for root in roots.all() {
            if root.path(db).starts_with(&path) {
-                root.set_revision(db).to(FileRevision::now());
+                root.set_revision(db)
+                    .with_durability(Durability::HIGH)
+                    .to(FileRevision::now());
            }
        }
    }
@@ -247,7 +250,9 @@ impl Files {
        let roots = inner.roots.read().unwrap();

        for root in roots.all() {
-            root.set_revision(db).to(FileRevision::now());
+            root.set_revision(db)
+                .with_durability(Durability::HIGH)
+                .to(FileRevision::now());
        }
    }

@@ -377,17 +382,23 @@ impl File {
            return;
        };
        let metadata = db.system().path_metadata(path);
-        Self::sync_impl(db, metadata, file);
+        let durability = db.files().root(db, path).map(|root| root.durability(db));
+        Self::sync_impl(db, metadata, file, durability);
    }

    fn sync_system_virtual_path(db: &mut dyn Db, path: &SystemVirtualPath, file: File) {
        let metadata = db.system().virtual_path_metadata(path);
-        Self::sync_impl(db, metadata, file);
+        Self::sync_impl(db, metadata, file, None);
    }

    /// Private method providing the implementation for [`Self::sync_system_path`] and
    /// [`Self::sync_system_virtual_path`].
-    fn sync_impl(db: &mut dyn Db, metadata: crate::system::Result<Metadata>, file: File) {
+    fn sync_impl(
+        db: &mut dyn Db,
+        metadata: crate::system::Result<Metadata>,
+        file: File,
+        durability: Option<Durability>,
+    ) {
        let (status, revision, permission) = match metadata {
            Ok(metadata) if metadata.file_type().is_file() => (
                FileStatus::Exists,
@@ -400,19 +411,25 @@ impl File {
            _ => (FileStatus::NotFound, FileRevision::zero(), None),
        };

+        let durability = durability.unwrap_or_default();
+
        if file.status(db) != status {
-            tracing::debug!("Updating the status of '{}'", file.path(db),);
-            file.set_status(db).to(status);
+            tracing::debug!("Updating the status of {}", file.path(db),);
+            file.set_status(db).with_durability(durability).to(status);
        }

        if file.revision(db) != revision {
-            tracing::debug!("Updating the revision of '{}'", file.path(db));
-            file.set_revision(db).to(revision);
+            tracing::debug!("Updating the revision of {}", file.path(db));
+            file.set_revision(db)
+                .with_durability(durability)
+                .to(revision);
        }

        if file.permissions(db) != permission {
-            tracing::debug!("Updating the permissions of '{}'", file.path(db),);
-            file.set_permissions(db).to(permission);
+            tracing::debug!("Updating the permissions of {}", file.path(db),);
+            file.set_permissions(db)
+                .with_durability(durability)
+                .to(permission);
        }
    }

--- a/crates/ruff_db/src/lib.rs
+++ b/crates/ruff_db/src/lib.rs
@@ -1,6 +1,4 @@
-use std::hash::BuildHasherDefault;
-
-use rustc_hash::FxHasher;
+use foldhash::fast::RandomState;

 use crate::files::Files;
 use crate::system::System;
@@ -15,8 +13,8 @@ pub mod system;
 pub mod testing;
 pub mod vendored;

-pub type FxDashMap<K, V> = dashmap::DashMap<K, V, BuildHasherDefault<FxHasher>>;
-pub type FxDashSet<K> = dashmap::DashSet<K, BuildHasherDefault<FxHasher>>;
+pub type FxDashMap<K, V> = dashmap::DashMap<K, V, RandomState>;
+pub type FxDashSet<K> = dashmap::DashSet<K, RandomState>;

 /// Most basic database that gives access to files, the host system, source code, and parsed AST.
 #[salsa::db]
--- a/crates/ruff_db/src/source.rs
+++ b/crates/ruff_db/src/source.rs
@@ -22,7 +22,7 @@ pub fn source_text(db: &dyn Db, file: File) -> SourceText {
    let kind = if is_notebook(file.path(db)) {
        file.read_to_notebook(db)
            .unwrap_or_else(|error| {
-                tracing::debug!("Failed to read notebook '{path}': {error}");
+                tracing::debug!("Failed to read notebook {path}: {error}");

                has_read_error = true;
                SourceDiagnostic(Arc::new(SourceTextError::FailedToReadNotebook(error)))
@@ -33,7 +33,7 @@ pub fn source_text(db: &dyn Db, file: File) -> SourceText {
    } else {
        file.read_to_string(db)
            .unwrap_or_else(|error| {
-                tracing::debug!("Failed to read file '{path}': {error}");
+                tracing::debug!("Failed to read file {path}: {error}");

                has_read_error = true;
                SourceDiagnostic(Arc::new(SourceTextError::FailedToReadFile(error))).accumulate(db);
--- a/crates/ruff_db/src/system/memory_fs.rs
+++ b/crates/ruff_db/src/system/memory_fs.rs
@@ -4,7 +4,7 @@ use std::sync::{Arc, RwLock, RwLockWriteGuard};

 use camino::{Utf8Path, Utf8PathBuf};
 use filetime::FileTime;
-use rustc_hash::FxHashMap;
+use foldhash::{HashMap, HashMapExt, HashSetExt};

 use ruff_notebook::{Notebook, NotebookError};

@@ -54,7 +54,7 @@ impl MemoryFileSystem {
        let fs = Self {
            inner: Arc::new(MemoryFileSystemInner {
                by_path: RwLock::new(BTreeMap::default()),
-                virtual_files: RwLock::new(FxHashMap::default()),
+                virtual_files: RwLock::new(HashMap::default()),
                cwd: cwd.clone(),
            }),
        };
@@ -385,7 +385,7 @@ impl std::fmt::Debug for MemoryFileSystem {

 struct MemoryFileSystemInner {
    by_path: RwLock<BTreeMap<Utf8PathBuf, Entry>>,
-    virtual_files: RwLock<FxHashMap<SystemVirtualPathBuf, File>>,
+    virtual_files: RwLock<HashMap<SystemVirtualPathBuf, File>>,
    cwd: SystemPathBuf,
 }

--- a/crates/ruff_db/src/testing.rs
+++ b/crates/ruff_db/src/testing.rs
@@ -31,20 +31,10 @@ pub fn assert_const_function_query_was_not_run<Db, Q, QDb, R>(
    Db: salsa::Database,
    Q: Fn(QDb) -> R,
 {
-    // Salsa now interns singleton ingredients. But we know that it is a singleton, so we can just search for
-    // any event of that ingredient.
-    let query_name = query_name(&query);
-
-    let event = events.iter().find(|event| {
-        if let salsa::EventKind::WillExecute { database_key } = event.kind {
-            db.ingredient_debug_name(database_key.ingredient_index()) == query_name
-        } else {
-            false
-        }
-    });
+    let (query_name, will_execute_event) = find_will_execute_event(db, query, (), events);

    db.attach(|_| {
-        if let Some(will_execute_event) = event {
+        if let Some(will_execute_event) = will_execute_event {
            panic!(
                "Expected query {query_name}() not to have run but it did: {will_execute_event:?}"
            );
--- a/crates/ruff_db/src/vendored.rs
+++ b/crates/ruff_db/src/vendored.rs
@@ -97,16 +97,7 @@ impl VendoredFileSystem {
        fn read_to_string(fs: &VendoredFileSystem, path: &VendoredPath) -> Result<String> {
            let mut archive = fs.lock_archive();
            let mut zip_file = archive.lookup_path(&NormalizedVendoredPath::from(path))?;
-
-            // Pre-allocate the buffer with the size specified in the ZIP file metadata
-            // because `read_to_string` passes `None` as the size hint.
-            // But let's not trust the zip file metadata (even though it's vendored)
-            // and limit it to a reasonable size.
-            let mut buffer = String::with_capacity(
-                usize::try_from(zip_file.size())
-                    .unwrap_or(usize::MAX)
-                    .min(10_000_000),
-            );
+            let mut buffer = String::new();
            zip_file.read_to_string(&mut buffer)?;
            Ok(buffer)
        }
--- a/crates/ruff_formatter/Cargo.toml
+++ b/crates/ruff_formatter/Cargo.toml
@@ -16,7 +16,7 @@ ruff_macros = { workspace = true }
 ruff_text_size = { workspace = true }

 drop_bomb = { workspace = true }
-rustc-hash = { workspace = true }
+foldhash = { workspace = true }
 schemars = { workspace = true, optional = true }
 serde = { workspace = true, optional = true }
 static_assertions = { workspace = true }
--- a/crates/ruff_formatter/src/buffer.rs
+++ b/crates/ruff_formatter/src/buffer.rs
@@ -2,7 +2,7 @@ use super::{write, Arguments, FormatElement};
 use crate::format_element::Interned;
 use crate::prelude::LineMode;
 use crate::{FormatResult, FormatState};
-use rustc_hash::FxHashMap;
+use foldhash::HashMap;
 use std::any::{Any, TypeId};
 use std::fmt::Debug;
 use std::ops::{Deref, DerefMut};
@@ -349,7 +349,7 @@ pub struct RemoveSoftLinesBuffer<'a, Context> {
    ///
    /// It's fine to not snapshot the cache. The worst that can happen is that it holds on interned elements
    /// that are now unused. But there's little harm in that and the cache is cleaned when dropping the buffer.
-    interned_cache: FxHashMap<Interned, Interned>,
+    interned_cache: HashMap<Interned, Interned>,
 }

 impl<'a, Context> RemoveSoftLinesBuffer<'a, Context> {
@@ -357,7 +357,7 @@ impl<'a, Context> RemoveSoftLinesBuffer<'a, Context> {
    pub fn new(inner: &'a mut dyn Buffer<Context = Context>) -> Self {
        Self {
            inner,
-            interned_cache: FxHashMap::default(),
+            interned_cache: HashMap::default(),
        }
    }

@@ -370,7 +370,7 @@ impl<'a, Context> RemoveSoftLinesBuffer<'a, Context> {
 // Extracted to function to avoid monomorphization
 fn clean_interned(
    interned: &Interned,
-    interned_cache: &mut FxHashMap<Interned, Interned>,
+    interned_cache: &mut HashMap<Interned, Interned>,
 ) -> Interned {
    if let Some(cleaned) = interned_cache.get(interned) {
        cleaned.clone()
--- a/crates/ruff_formatter/src/format_element/document.rs
+++ b/crates/ruff_formatter/src/format_element/document.rs
@@ -1,7 +1,6 @@
-use std::collections::HashMap;
 use std::ops::Deref;

-use rustc_hash::FxHashMap;
+use foldhash::HashMap;

 use crate::format_element::tag::{Condition, DedentMode};
 use crate::prelude::tag::GroupMode;
@@ -57,7 +56,7 @@ impl Document {
        fn propagate_expands<'a>(
            elements: &'a [FormatElement],
            enclosing: &mut Vec<Enclosing<'a>>,
-            checked_interned: &mut FxHashMap<&'a Interned, bool>,
+            checked_interned: &mut HashMap<&'a Interned, bool>,
        ) -> bool {
            let mut expands = false;
            for element in elements {
@@ -147,7 +146,7 @@ impl Document {
        } else {
            self.len().ilog2() as usize
        });
-        let mut interned = FxHashMap::default();
+        let mut interned = HashMap::default();
        propagate_expands(self, &mut enclosing, &mut interned);
    }

@@ -210,7 +209,7 @@ impl<'a> IrFormatContext<'a> {
    fn new(source_code: SourceCode<'a>) -> Self {
        Self {
            source_code,
-            printed_interned_elements: HashMap::new(),
+            printed_interned_elements: HashMap::default(),
        }
    }
 }
--- a/crates/ruff_linter/Cargo.toml
+++ b/crates/ruff_linter/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "ruff_linter"
-version = "0.6.2"
+version = "0.6.1"
 publish = false
 authors = { workspace = true }
 edition = { workspace = true }
@@ -56,7 +56,7 @@ pep440_rs = { workspace = true, features = ["serde"] }
 pyproject-toml = { workspace = true }
 quick-junit = { workspace = true }
 regex = { workspace = true }
-rustc-hash = { workspace = true }
+foldhash = { workspace = true }
 schemars = { workspace = true, optional = true }
 serde = { workspace = true }
 serde_json = { workspace = true }
--- a/crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B015.py
+++ b/crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B015.py
@@ -17,11 +17,6 @@ def test():
    1 in (1, 2)


-def test2():
-    1 in (1, 2)
-    return
-
-
 data = [x for x in [1, 2, 3] if x in (1, 2)]


--- a/crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI053.pyi
+++ b/crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI053.pyi
@@ -66,6 +66,3 @@ def not_warnings_dot_deprecated(
 def not_a_deprecated_function() -> None: ...

 fbaz: str = f"51 character {foo} stringgggggggggggggggggggggggggg"  # Error: PYI053
-
-# see https://github.com/astral-sh/ruff/issues/12995
-def foo(bar: typing.Literal["a", "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"]):...
--- a/crates/ruff_linter/resources/test/fixtures/flake8_simplify/SIM115.py
+++ b/crates/ruff_linter/resources/test/fixtures/flake8_simplify/SIM115.py
@@ -47,6 +47,7 @@ with contextlib.ExitStack() as exit_stack:
 open("filename", "w").close()
 pathlib.Path("filename").open("w").close()

+
 # OK (custom context manager)
 class MyFile:
    def __init__(self, filename: str):
@@ -57,189 +58,3 @@ class MyFile:

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.file.close()
-
-
-import tempfile
-import tarfile
-from tarfile import TarFile
-import zipfile
-import io
-import codecs
-import bz2
-import gzip
-import dbm
-import dbm.gnu
-import dbm.ndbm
-import dbm.dumb
-import lzma
-import shelve
-import tokenize
-import wave
-import fileinput
-
-f = tempfile.NamedTemporaryFile()
-f = tempfile.TemporaryFile()
-f = tempfile.SpooledTemporaryFile()
-f = tarfile.open("foo.tar")
-f = TarFile("foo.tar").open()
-f = tarfile.TarFile("foo.tar").open()
-f = tarfile.TarFile().open()
-f = zipfile.ZipFile("foo.zip").open("foo.txt")
-f = io.open("foo.txt")
-f = io.open_code("foo.txt")
-f = codecs.open("foo.txt")
-f = bz2.open("foo.txt")
-f = gzip.open("foo.txt")
-f = dbm.open("foo.db")
-f = dbm.gnu.open("foo.db")
-f = dbm.ndbm.open("foo.db")
-f = dbm.dumb.open("foo.db")
-f = lzma.open("foo.xz")
-f = lzma.LZMAFile("foo.xz")
-f = shelve.open("foo.db")
-f = tokenize.open("foo.py")
-f = wave.open("foo.wav")
-f = tarfile.TarFile.taropen("foo.tar")
-f = fileinput.input("foo.txt")
-f = fileinput.FileInput("foo.txt")
-
-with contextlib.suppress(Exception):
-    # The following line is for example's sake.
-    # For some f's above, this would raise an error (since it'd be f.readline() etc.)
-    data = f.read()
-
-f.close()
-
-# OK
-with tempfile.TemporaryFile() as f:
-    data = f.read()
-
-# OK
-with tarfile.open("foo.tar") as f:
-    data = f.add("foo.txt")
-
-# OK
-with tarfile.TarFile("foo.tar") as f:
-    data = f.add("foo.txt")
-
-# OK
-with tarfile.TarFile("foo.tar").open() as f:
-    data = f.add("foo.txt")
-
-# OK
-with zipfile.ZipFile("foo.zip") as f:
-    data = f.read("foo.txt")
-
-# OK
-with zipfile.ZipFile("foo.zip").open("foo.txt") as f:
-    data = f.read()
-
-# OK
-with zipfile.ZipFile("foo.zip") as zf:
-    with zf.open("foo.txt") as f:
-        data = f.read()
-
-# OK
-with io.open("foo.txt") as f:
-    data = f.read()
-
-# OK
-with io.open_code("foo.txt") as f:
-    data = f.read()
-
-# OK
-with codecs.open("foo.txt") as f:
-    data = f.read()
-
-# OK
-with bz2.open("foo.txt") as f:
-    data = f.read()
-
-# OK
-with gzip.open("foo.txt") as f:
-    data = f.read()
-
-# OK
-with dbm.open("foo.db") as f:
-    data = f.get("foo")
-
-# OK
-with dbm.gnu.open("foo.db") as f:
-    data = f.get("foo")
-
-# OK
-with dbm.ndbm.open("foo.db") as f:
-    data = f.get("foo")
-
-# OK
-with dbm.dumb.open("foo.db") as f:
-    data = f.get("foo")
-
-# OK
-with lzma.open("foo.xz") as f:
-    data = f.read()
-
-# OK
-with lzma.LZMAFile("foo.xz") as f:
-    data = f.read()
-
-# OK
-with shelve.open("foo.db") as f:
-    data = f["foo"]
-
-# OK
-with tokenize.open("foo.py") as f:
-    data = f.read()
-
-# OK
-with wave.open("foo.wav") as f:
-    data = f.readframes(1024)
-
-# OK
-with tarfile.TarFile.taropen("foo.tar") as f:
-    data = f.add("foo.txt")
-
-# OK
-with fileinput.input("foo.txt") as f:
-    data = f.readline()
-
-# OK
-with fileinput.FileInput("foo.txt") as f:
-    data = f.readline()
-
-# OK (quick one-liner to clear file contents)
-tempfile.NamedTemporaryFile().close()
-tempfile.TemporaryFile().close()
-tempfile.SpooledTemporaryFile().close()
-tarfile.open("foo.tar").close()
-tarfile.TarFile("foo.tar").close()
-tarfile.TarFile("foo.tar").open().close()
-tarfile.TarFile.open("foo.tar").close()
-zipfile.ZipFile("foo.zip").close()
-zipfile.ZipFile("foo.zip").open("foo.txt").close()
-io.open("foo.txt").close()
-io.open_code("foo.txt").close()
-codecs.open("foo.txt").close()
-bz2.open("foo.txt").close()
-gzip.open("foo.txt").close()
-dbm.open("foo.db").close()
-dbm.gnu.open("foo.db").close()
-dbm.ndbm.open("foo.db").close()
-dbm.dumb.open("foo.db").close()
-lzma.open("foo.xz").close()
-lzma.LZMAFile("foo.xz").close()
-shelve.open("foo.db").close()
-tokenize.open("foo.py").close()
-wave.open("foo.wav").close()
-tarfile.TarFile.taropen("foo.tar").close()
-fileinput.input("foo.txt").close()
-fileinput.FileInput("foo.txt").close()
-
-def aliased():
-    from shelve import open as open_shelf
-    x = open_shelf("foo.dbm")
-    x.close()
-
-    from tarfile import TarFile as TF
-    f = TF("foo").open()
-    f.close()
--- a/crates/ruff_linter/resources/test/fixtures/pep8_naming/N817.py
+++ b/crates/ruff_linter/resources/test/fixtures/pep8_naming/N817.py
@@ -4,7 +4,3 @@ from mod import CamelCase as CC

 # OK depending on configured import convention
 import xml.etree.ElementTree as ET
-from xml.etree import ElementTree as ET
-
-# Always an error (relative import)
-from ..xml.eltree import ElementTree as ET
--- a/crates/ruff_linter/resources/test/fixtures/pylint/bad_staticmethod_argument.py
+++ b/crates/ruff_linter/resources/test/fixtures/pylint/bad_staticmethod_argument.py
@@ -42,9 +42,3 @@ class Foo:
    @classmethod
    def graze(cls, x, y, z):
        pass
-
-
-class Foo:
-    @staticmethod
-    def __new__(cls, x, y, z):  # OK, see https://docs.python.org/3/reference/datamodel.html#basic-customization
-        pass
--- a/crates/ruff_linter/resources/test/fixtures/pylint/self_or_cls_assignment.py
+++ b/crates/ruff_linter/resources/test/fixtures/pylint/self_or_cls_assignment.py
@@ -3,7 +3,7 @@ class Fruit:
    def list_fruits(cls) -> None:
        cls = "apple"  # PLW0642
        cls: Fruit = "apple"  # PLW0642
-        cls += "orange"  # OK, augmented assignments are ignored
+        cls += "orange"  # PLW0642
        *cls = "banana"  # PLW0642
        cls, blah = "apple", "orange"  # PLW0642
        blah, (cls, blah2) = "apple", ("orange", "banana")  # PLW0642
@@ -16,7 +16,7 @@ class Fruit:
    def print_color(self) -> None:
        self = "red"  # PLW0642
        self: Self = "red"  # PLW0642
-        self += "blue"  # OK, augmented assignments are ignored
+        self += "blue"  # PLW0642
        *self = "blue"  # PLW0642
        self, blah = "red", "blue"  # PLW0642
        blah, (self, blah2) = "apple", ("orange", "banana")  # PLW0642
--- a/crates/ruff_linter/resources/test/fixtures/ruff/RUF027_1.py
+++ b/crates/ruff_linter/resources/test/fixtures/ruff/RUF027_1.py
@@ -59,12 +59,3 @@ def negative_cases():
    # See https://docs.python.org/3/howto/logging-cookbook.html#formatting-styles
    import logging
    logging.info("yet {another} non-f-string")
-
-    # See https://fastapi.tiangolo.com/tutorial/path-params/
-    from fastapi import FastAPI
-    app = FastAPI()
-    item_id = 42
-
-    @app.get("/items/{item_id}")
-    async def read_item(item_id):
-        return {"item_id": item_id}
--- a/crates/ruff_linter/resources/test/fixtures/ruff/RUF029.py
+++ b/crates/ruff_linter/resources/test/fixtures/ruff/RUF029.py
@@ -78,13 +78,3 @@ async def test():
 async def test() -> str:
    vals = [str(val) for val in await async_func(1)]
    return ",".join(vals)
-
-
-from fastapi import FastAPI
-
-app = FastAPI()
-
-
-@app.post("/count")
-async def fastapi_route(): # Ok: FastApi routes can be async without actually using await
-    return 1
--- a/crates/ruff_linter/resources/test/fixtures/ruff/RUF032.py
+++ b/crates/ruff_linter/resources/test/fixtures/ruff/RUF032.py
@@ -1,120 +0,0 @@
-import decimal
-
-# Tests with fully qualified import
-decimal.Decimal(0)
-
-decimal.Decimal(0.0)  # Should error
-
-decimal.Decimal("0.0")
-
-decimal.Decimal(10)
-
-decimal.Decimal(10.0)  # Should error
-
-decimal.Decimal("10.0")
-
-decimal.Decimal(-10)
-
-decimal.Decimal(-10.0)  # Should error
-
-decimal.Decimal("-10.0")
-
-a = 10.0
-
-decimal.Decimal(a)
-
-
-# Tests with relative import
-from decimal import Decimal
-
-
-val = Decimal(0)
-
-val = Decimal(0.0)  # Should error
-
-val = Decimal("0.0")
-
-val = Decimal(10)
-
-val = Decimal(10.0)  # Should error
-
-val = Decimal("10.0")
-
-val = Decimal(-10)
-
-val = Decimal(-10.0)  # Should error
-
-val = Decimal("-10.0")
-
-a = 10.0
-
-val = Decimal(a)
-
-
-# Tests with shadowed name
-class Decimal():
-    value: float | int | str
-
-    def __init__(self, value: float | int | str) -> None:
-        self.value = value
-
-
-val = Decimal(0.0)
-
-val = Decimal("0.0")
-
-val = Decimal(10.0)
-
-val = Decimal("10.0")
-
-val = Decimal(-10.0)
-
-val = Decimal("-10.0")
-
-a = 10.0
-
-val = Decimal(a)
-
-
-# Retest with fully qualified import
-
-val = decimal.Decimal(0.0)  # Should error
-
-val = decimal.Decimal("0.0")
-
-val = decimal.Decimal(10.0)  # Should error
-
-val = decimal.Decimal("10.0")
-
-val = decimal.Decimal(-10.0)  # Should error
-
-val = decimal.Decimal("-10.0")
-
-a = 10.0
-
-val = decimal.Decimal(a)
-
-
-class decimal():
-    class Decimal():
-        value: float | int | str
-
-        def __init__(self, value: float | int | str) -> None:
-            self.value = value
-
-
-val = decimal.Decimal(0.0)
-
-val = decimal.Decimal("0.0")
-
-val = decimal.Decimal(10.0)
-
-val = decimal.Decimal("10.0")
-
-val = decimal.Decimal(-10.0)
-
-val = decimal.Decimal("-10.0")
-
-a = 10.0
-
-val = decimal.Decimal(a)
--- a/crates/ruff_linter/src/checkers/ast/analyze/expression.rs
+++ b/crates/ruff_linter/src/checkers/ast/analyze/expression.rs
@@ -883,7 +883,7 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                flake8_simplify::rules::use_capital_environment_variables(checker, expr);
            }
            if checker.enabled(Rule::OpenFileWithContextHandler) {
-                flake8_simplify::rules::open_file_with_context_handler(checker, call);
+                flake8_simplify::rules::open_file_with_context_handler(checker, func);
            }
            if checker.enabled(Rule::DictGetWithNoneDefault) {
                flake8_simplify::rules::dict_get_with_none_default(checker, expr);
@@ -1011,9 +1011,6 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
            if checker.enabled(Rule::UnnecessaryIterableAllocationForFirstElement) {
                ruff::rules::unnecessary_iterable_allocation_for_first_element(checker, expr);
            }
-            if checker.enabled(Rule::DecimalFromFloatLiteral) {
-                ruff::rules::decimal_from_float_literal_syntax(checker, call);
-            }
            if checker.enabled(Rule::IntOnSlicedStr) {
                refurb::rules::int_on_sliced_str(checker, call);
            }
--- a/crates/ruff_linter/src/checkers/ast/analyze/statement.rs
+++ b/crates/ruff_linter/src/checkers/ast/analyze/statement.rs
@@ -1112,6 +1112,9 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
            }
        }
        Stmt::AugAssign(aug_assign @ ast::StmtAugAssign { target, .. }) => {
+            if checker.enabled(Rule::SelfOrClsAssignment) {
+                pylint::rules::self_or_cls_assignment(checker, target);
+            }
            if checker.enabled(Rule::GlobalStatement) {
                if let Expr::Name(ast::ExprName { id, .. }) = target.as_ref() {
                    pylint::rules::global_statement(checker, id);
--- a/crates/ruff_linter/src/checkers/noqa.rs
+++ b/crates/ruff_linter/src/checkers/noqa.rs
@@ -2,8 +2,8 @@

 use std::path::Path;

+use foldhash::HashSet;
 use itertools::Itertools;
-use rustc_hash::FxHashSet;

 use ruff_diagnostics::{Diagnostic, Edit, Fix};
 use ruff_python_trivia::CommentRanges;
@@ -132,7 +132,7 @@ pub(crate) fn check_noqa(
                    let mut unknown_codes = vec![];
                    let mut unmatched_codes = vec![];
                    let mut valid_codes = vec![];
-                    let mut seen_codes = FxHashSet::default();
+                    let mut seen_codes = HashSet::default();
                    let mut self_ignore = false;
                    for original_code in directive.iter().map(Code::as_str) {
                        let code = get_redirect_target(original_code).unwrap_or(original_code);
--- a/crates/ruff_linter/src/codes.rs
+++ b/crates/ruff_linter/src/codes.rs
@@ -959,7 +959,6 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
        (Ruff, "029") => (RuleGroup::Preview, rules::ruff::rules::UnusedAsync),
        (Ruff, "030") => (RuleGroup::Preview, rules::ruff::rules::AssertWithPrintMessage),
        (Ruff, "031") => (RuleGroup::Preview, rules::ruff::rules::IncorrectlyParenthesizedTupleInSubscript),
-        (Ruff, "032") => (RuleGroup::Preview, rules::ruff::rules::DecimalFromFloatLiteral),
        (Ruff, "100") => (RuleGroup::Stable, rules::ruff::rules::UnusedNOQA),
        (Ruff, "101") => (RuleGroup::Stable, rules::ruff::rules::RedirectedNOQA),

--- a/crates/ruff_linter/src/fix/mod.rs
+++ b/crates/ruff_linter/src/fix/mod.rs
@@ -1,8 +1,8 @@
 use itertools::Itertools;
 use std::collections::BTreeSet;

+use foldhash::{HashMap, HashMapExt, HashSet};
 use ruff_text_size::{Ranged, TextLen, TextRange, TextSize};
-use rustc_hash::{FxHashMap, FxHashSet};

 use ruff_diagnostics::{Diagnostic, Edit, Fix, IsolationLevel, SourceMap};
 use ruff_source_file::Locator;
@@ -57,8 +57,8 @@ fn apply_fixes<'a>(
    let mut output = String::with_capacity(locator.len());
    let mut last_pos: Option<TextSize> = None;
    let mut applied: BTreeSet<&Edit> = BTreeSet::default();
-    let mut isolated: FxHashSet<u32> = FxHashSet::default();
-    let mut fixed = FxHashMap::default();
+    let mut isolated: HashSet<u32> = HashSet::default();
+    let mut fixed = HashMap::default();
    let mut source_map = SourceMap::default();

    for (rule, fix) in diagnostics
--- a/crates/ruff_linter/src/linter.rs
+++ b/crates/ruff_linter/src/linter.rs
@@ -4,8 +4,8 @@ use std::path::Path;

 use anyhow::{anyhow, Result};
 use colored::Colorize;
+use foldhash::HashMap;
 use itertools::Itertools;
-use rustc_hash::FxHashMap;

 use ruff_diagnostics::Diagnostic;
 use ruff_notebook::Notebook;
@@ -43,7 +43,7 @@ pub struct LinterResult {
    pub has_syntax_error: bool,
 }

-pub type FixTable = FxHashMap<Rule, usize>;
+pub type FixTable = HashMap<Rule, usize>;

 pub struct FixerResult<'a> {
    /// The result returned by the linter, after applying any fixes.
@@ -476,7 +476,7 @@ pub fn lint_fix<'a>(
    let mut transformed = Cow::Borrowed(source_kind);

    // Track the number of fixed errors across iterations.
-    let mut fixed = FxHashMap::default();
+    let mut fixed = HashMap::default();

    // As an escape hatch, bail after 100 iterations.
    let mut iterations = 0;
--- a/crates/ruff_linter/src/logging.rs
+++ b/crates/ruff_linter/src/logging.rs
@@ -5,10 +5,10 @@ use std::sync::Mutex;
 use anyhow::Result;
 use colored::Colorize;
 use fern;
+use foldhash::HashSet;
 use log::Level;
 use once_cell::sync::Lazy;
 use ruff_python_parser::{ParseError, ParseErrorType};
-use rustc_hash::FxHashSet;

 use ruff_source_file::{LineIndex, OneIndexed, SourceCode, SourceLocation};

@@ -35,7 +35,7 @@ macro_rules! warn_user_once_by_id {
    };
 }

-pub static MESSAGES: Lazy<Mutex<FxHashSet<String>>> = Lazy::new(Mutex::default);
+pub static MESSAGES: Lazy<Mutex<HashSet<String>>> = Lazy::new(Mutex::default);

 /// Warn a user once, if warnings are enabled, with uniqueness determined by the content of the
 /// message.
--- a/crates/ruff_linter/src/message/mod.rs
+++ b/crates/ruff_linter/src/message/mod.rs
@@ -3,7 +3,7 @@ use std::collections::BTreeMap;
 use std::io::Write;
 use std::ops::Deref;

-use rustc_hash::FxHashMap;
+use foldhash::HashMap;

 pub use azure::AzureEmitter;
 pub use github::GithubEmitter;
@@ -285,11 +285,11 @@ pub trait Emitter {

 /// Context passed to [`Emitter`].
 pub struct EmitterContext<'a> {
-    notebook_indexes: &'a FxHashMap<String, NotebookIndex>,
+    notebook_indexes: &'a HashMap<String, NotebookIndex>,
 }

 impl<'a> EmitterContext<'a> {
-    pub fn new(notebook_indexes: &'a FxHashMap<String, NotebookIndex>) -> Self {
+    pub fn new(notebook_indexes: &'a HashMap<String, NotebookIndex>) -> Self {
        Self { notebook_indexes }
    }

@@ -305,7 +305,7 @@ impl<'a> EmitterContext<'a> {

 #[cfg(test)]
 mod tests {
-    use rustc_hash::FxHashMap;
+    use foldhash::HashMap;

    use ruff_diagnostics::{Diagnostic, DiagnosticKind, Edit, Fix};
    use ruff_notebook::NotebookIndex;
@@ -399,7 +399,7 @@ def fibonacci(n):
        ]
    }

-    pub(super) fn create_notebook_messages() -> (Vec<Message>, FxHashMap<String, NotebookIndex>) {
+    pub(super) fn create_notebook_messages() -> (Vec<Message>, HashMap<String, NotebookIndex>) {
        let notebook = r"# cell 1
 import os
 # cell 2
@@ -453,7 +453,7 @@ def foo():

        let notebook_source = SourceFileBuilder::new("notebook.ipynb", notebook).finish();

-        let mut notebook_indexes = FxHashMap::default();
+        let mut notebook_indexes = HashMap::default();
        notebook_indexes.insert(
            "notebook.ipynb".to_string(),
            NotebookIndex::new(
@@ -510,7 +510,7 @@ def foo():
        emitter: &mut dyn Emitter,
        messages: &[Message],
    ) -> String {
-        let notebook_indexes = FxHashMap::default();
+        let notebook_indexes = HashMap::default();
        let context = EmitterContext::new(&notebook_indexes);
        let mut output: Vec<u8> = Vec::new();
        emitter.emit(&mut output, messages, &context).unwrap();
@@ -521,7 +521,7 @@ def foo():
    pub(super) fn capture_emitter_notebook_output(
        emitter: &mut dyn Emitter,
        messages: &[Message],
-        notebook_indexes: &FxHashMap<String, NotebookIndex>,
+        notebook_indexes: &HashMap<String, NotebookIndex>,
    ) -> String {
        let context = EmitterContext::new(notebook_indexes);
        let mut output: Vec<u8> = Vec::new();
--- a/crates/ruff_linter/src/rules/fastapi/rules/mod.rs
+++ b/crates/ruff_linter/src/rules/fastapi/rules/mod.rs
@@ -6,15 +6,12 @@ mod fastapi_non_annotated_dependency;
 mod fastapi_redundant_response_model;
 mod fastapi_unused_path_parameter;

-use ruff_python_ast as ast;
+use ruff_python_ast::{Decorator, ExprCall, StmtFunctionDef};
 use ruff_python_semantic::analyze::typing::resolve_assignment;
 use ruff_python_semantic::SemanticModel;

 /// Returns `true` if the function is a FastAPI route.
-pub(crate) fn is_fastapi_route(
-    function_def: &ast::StmtFunctionDef,
-    semantic: &SemanticModel,
-) -> bool {
+pub(crate) fn is_fastapi_route(function_def: &StmtFunctionDef, semantic: &SemanticModel) -> bool {
    return function_def
        .decorator_list
        .iter()
@@ -23,29 +20,27 @@ pub(crate) fn is_fastapi_route(

 /// Returns `true` if the decorator is indicative of a FastAPI route.
 pub(crate) fn is_fastapi_route_decorator<'a>(
-    decorator: &'a ast::Decorator,
+    decorator: &'a Decorator,
    semantic: &'a SemanticModel,
-) -> Option<&'a ast::ExprCall> {
+) -> Option<&'a ExprCall> {
    let call = decorator.expression.as_call_expr()?;
-    is_fastapi_route_call(call, semantic).then_some(call)
-}
-
-pub(crate) fn is_fastapi_route_call(call_expr: &ast::ExprCall, semantic: &SemanticModel) -> bool {
-    let ast::Expr::Attribute(ast::ExprAttribute { attr, value, .. }) = &*call_expr.func else {
-        return false;
-    };
+    let decorator_method = call.func.as_attribute_expr()?;
+    let method_name = &decorator_method.attr;

    if !matches!(
-        attr.as_str(),
+        method_name.as_str(),
        "get" | "post" | "put" | "delete" | "patch" | "options" | "head" | "trace"
    ) {
-        return false;
+        return None;
    }

-    resolve_assignment(value, semantic).is_some_and(|qualified_name| {
-        matches!(
-            qualified_name.segments(),
-            ["fastapi", "FastAPI" | "APIRouter"]
-        )
-    })
+    let qualified_name = resolve_assignment(&decorator_method.value, semantic)?;
+    if matches!(
+        qualified_name.segments(),
+        ["fastapi", "FastAPI" | "APIRouter"]
+    ) {
+        Some(call)
+    } else {
+        None
+    }
 }
--- a/crates/ruff_linter/src/rules/flake8_annotations/helpers.rs
+++ b/crates/ruff_linter/src/rules/flake8_annotations/helpers.rs
@@ -1,5 +1,5 @@
+use foldhash::HashSet;
 use itertools::Itertools;
-use rustc_hash::FxHashSet;

 use ruff_diagnostics::Edit;
 use ruff_python_ast::helpers::{
@@ -109,7 +109,7 @@ pub(crate) fn auto_return_type(function: &ast::StmtFunctionDef) -> Option<AutoPy
 pub(crate) enum AutoPythonType {
    Never,
    Atom(PythonType),
-    Union(FxHashSet<PythonType>),
+    Union(HashSet<PythonType>),
 }

 impl AutoPythonType {
--- a/crates/ruff_linter/src/rules/flake8_bugbear/rules/duplicate_exceptions.rs
+++ b/crates/ruff_linter/src/rules/flake8_bugbear/rules/duplicate_exceptions.rs
@@ -1,5 +1,5 @@
+use foldhash::{HashMap, HashMapExt, HashSet};
 use itertools::Itertools;
-use rustc_hash::{FxHashMap, FxHashSet};

 use ruff_diagnostics::{AlwaysFixableViolation, Violation};
 use ruff_diagnostics::{Diagnostic, Edit, Fix};
@@ -118,9 +118,9 @@ fn duplicate_handler_exceptions<'a>(
    checker: &mut Checker,
    expr: &'a Expr,
    elts: &'a [Expr],
-) -> FxHashMap<UnqualifiedName<'a>, &'a Expr> {
-    let mut seen: FxHashMap<UnqualifiedName, &Expr> = FxHashMap::default();
-    let mut duplicates: FxHashSet<UnqualifiedName> = FxHashSet::default();
+) -> HashMap<UnqualifiedName<'a>, &'a Expr> {
+    let mut seen: HashMap<UnqualifiedName, &Expr> = HashMap::default();
+    let mut duplicates: HashSet<UnqualifiedName> = HashSet::default();
    let mut unique_elts: Vec<&Expr> = Vec::default();
    for type_ in elts {
        if let Some(name) = UnqualifiedName::from_expr(type_) {
@@ -171,8 +171,8 @@ fn duplicate_handler_exceptions<'a>(

 /// B025
 pub(crate) fn duplicate_exceptions(checker: &mut Checker, handlers: &[ExceptHandler]) {
-    let mut seen: FxHashSet<UnqualifiedName> = FxHashSet::default();
-    let mut duplicates: FxHashMap<UnqualifiedName, Vec<&Expr>> = FxHashMap::default();
+    let mut seen: HashSet<UnqualifiedName> = HashSet::default();
+    let mut duplicates: HashMap<UnqualifiedName, Vec<&Expr>> = HashMap::default();
    for handler in handlers {
        let ExceptHandler::ExceptHandler(ast::ExceptHandlerExceptHandler {
            type_: Some(type_),
--- a/crates/ruff_linter/src/rules/flake8_bugbear/rules/duplicate_value.rs
+++ b/crates/ruff_linter/src/rules/flake8_bugbear/rules/duplicate_value.rs
@@ -1,5 +1,5 @@
 use anyhow::{Context, Result};
-use rustc_hash::FxHashSet;
+use foldhash::HashSet;

 use ruff_diagnostics::{Diagnostic, Edit, Fix, FixAvailability, Violation};
 use ruff_macros::{derive_message_formats, violation};
@@ -48,7 +48,7 @@ impl Violation for DuplicateValue {

 /// B033
 pub(crate) fn duplicate_value(checker: &mut Checker, set: &ast::ExprSet) {
-    let mut seen_values: FxHashSet<ComparableExpr> = FxHashSet::default();
+    let mut seen_values: HashSet<ComparableExpr> = HashSet::default();
    for (index, value) in set.iter().enumerate() {
        if value.is_literal_expr() {
            let comparable_value = ComparableExpr::from(value);
--- a/crates/ruff_linter/src/rules/flake8_bugbear/rules/loop_variable_overrides_iterator.rs
+++ b/crates/ruff_linter/src/rules/flake8_bugbear/rules/loop_variable_overrides_iterator.rs
@@ -1,5 +1,5 @@
+use foldhash::HashMap;
 use ruff_python_ast::{self as ast, Expr, ParameterWithDefault};
-use rustc_hash::FxHashMap;

 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
@@ -76,7 +76,7 @@ pub(crate) fn loop_variable_overrides_iterator(checker: &mut Checker, target: &E

 #[derive(Default)]
 struct NameFinder<'a> {
-    names: FxHashMap<&'a str, &'a Expr>,
+    names: HashMap<&'a str, &'a Expr>,
 }

 impl<'a> Visitor<'a> for NameFinder<'a> {
--- a/crates/ruff_linter/src/rules/flake8_bugbear/rules/static_key_dict_comprehension.rs
+++ b/crates/ruff_linter/src/rules/flake8_bugbear/rules/static_key_dict_comprehension.rs
@@ -1,4 +1,4 @@
-use rustc_hash::FxHashMap;
+use foldhash::HashMap;

 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
@@ -70,7 +70,7 @@ pub(crate) fn static_key_dict_comprehension(checker: &mut Checker, dict_comp: &a

 /// Returns `true` if the given expression is a constant in the context of the dictionary
 /// comprehension.
-fn is_constant(key: &Expr, names: &FxHashMap<&str, &ast::ExprName>) -> bool {
+fn is_constant(key: &Expr, names: &HashMap<&str, &ast::ExprName>) -> bool {
    match key {
        Expr::Tuple(tuple) => tuple.iter().all(|elem| is_constant(elem, names)),
        Expr::Name(ast::ExprName { id, .. }) => !names.contains_key(id.as_str()),
--- a/crates/ruff_linter/src/rules/flake8_bugbear/rules/useless_comparison.rs
+++ b/crates/ruff_linter/src/rules/flake8_bugbear/rules/useless_comparison.rs
@@ -1,7 +1,6 @@
 use ruff_diagnostics::{Diagnostic, Violation};
 use ruff_macros::{derive_message_formats, violation};
-use ruff_python_ast::{Expr, Stmt};
-use ruff_python_semantic::ScopeKind;
+use ruff_python_ast::Expr;
 use ruff_text_size::Ranged;

 use crate::checkers::ast::Checker;
@@ -34,34 +33,24 @@ use super::super::helpers::at_last_top_level_expression_in_cell;
 /// ## References
 /// - [Python documentation: `assert` statement](https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement)
 #[violation]
-pub struct UselessComparison {
-    at: ComparisonLocationAt,
-}
+pub struct UselessComparison;

 impl Violation for UselessComparison {
    #[derive_message_formats]
    fn message(&self) -> String {
-        match self.at {
-            ComparisonLocationAt::MiddleBody => format!(
-                "Pointless comparison. Did you mean to assign a value? \
-                Otherwise, prepend `assert` or remove it."
-            ),
-            ComparisonLocationAt::EndOfFunction => format!(
-                "Pointless comparison at end of function scope. Did you mean \
-                to return the expression result?"
-            ),
-        }
+        format!(
+            "Pointless comparison. Did you mean to assign a value? \
+             Otherwise, prepend `assert` or remove it."
+        )
    }
 }

 /// B015
 pub(crate) fn useless_comparison(checker: &mut Checker, expr: &Expr) {
    if expr.is_compare_expr() {
-        let semantic = checker.semantic();
-
        if checker.source_type.is_ipynb()
            && at_last_top_level_expression_in_cell(
-                semantic,
+                checker.semantic(),
                checker.locator(),
                checker.cell_offsets(),
            )
@@ -69,34 +58,8 @@ pub(crate) fn useless_comparison(checker: &mut Checker, expr: &Expr) {
            return;
        }

-        if let ScopeKind::Function(func_def) = semantic.current_scope().kind {
-            if func_def
-                .body
-                .last()
-                .and_then(Stmt::as_expr_stmt)
-                .is_some_and(|last_stmt| &*last_stmt.value == expr)
-            {
-                checker.diagnostics.push(Diagnostic::new(
-                    UselessComparison {
-                        at: ComparisonLocationAt::EndOfFunction,
-                    },
-                    expr.range(),
-                ));
-                return;
-            }
-        }
-
-        checker.diagnostics.push(Diagnostic::new(
-            UselessComparison {
-                at: ComparisonLocationAt::MiddleBody,
-            },
-            expr.range(),
-        ));
+        checker
+            .diagnostics
+            .push(Diagnostic::new(UselessComparison, expr.range()));
    }
 }
-
-#[derive(Debug, PartialEq, Eq, Copy, Clone)]
-enum ComparisonLocationAt {
-    MiddleBody,
-    EndOfFunction,
-}
--- a/crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linterrulesflake8_bugbeartestsB015_B015.py.snap
+++ b/crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linterrulesflake8_bugbeartestsB015_B015.py.snap
@@ -1,6 +1,5 @@
 ---
 source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
-assertion_line: 74
 ---
 B015.py:3:1: B015 Pointless comparison. Did you mean to assign a value? Otherwise, prepend `assert` or remove it.
  |
@@ -20,7 +19,7 @@ B015.py:7:1: B015 Pointless comparison. Did you mean to assign a value? Otherwis
  | ^^^^^^^^^^^ B015
  |

-B015.py:17:5: B015 Pointless comparison at end of function scope. Did you mean to return the expression result?
+B015.py:17:5: B015 Pointless comparison. Did you mean to assign a value? Otherwise, prepend `assert` or remove it.
   |
 15 |     assert 1 in (1, 2)
 16 | 
@@ -28,17 +27,11 @@ B015.py:17:5: B015 Pointless comparison at end of function scope. Did you mean t
   |     ^^^^^^^^^^^ B015
   |

-B015.py:21:5: B015 Pointless comparison. Did you mean to assign a value? Otherwise, prepend `assert` or remove it.
+B015.py:24:5: B015 Pointless comparison. Did you mean to assign a value? Otherwise, prepend `assert` or remove it.
   |
-20 | def test2():
-21 |     1 in (1, 2)
-   |     ^^^^^^^^^^^ B015
-22 |     return
-   |
-
-B015.py:29:5: B015 Pointless comparison. Did you mean to assign a value? Otherwise, prepend `assert` or remove it.
-   |
-28 | class TestClass:
-29 |     1 == 1
+23 | class TestClass:
+24 |     1 == 1
   |     ^^^^^^ B015
   |
+
+
--- a/crates/ruff_linter/src/rules/flake8_import_conventions/mod.rs
+++ b/crates/ruff_linter/src/rules/flake8_import_conventions/mod.rs
@@ -7,7 +7,7 @@ mod tests {
    use std::path::Path;

    use anyhow::Result;
-    use rustc_hash::{FxHashMap, FxHashSet};
+    use foldhash::{HashMap, HashMapExt, HashSet};

    use crate::assert_messages;
    use crate::registry::Rule;
@@ -28,7 +28,7 @@ mod tests {
    #[test]
    fn custom() -> Result<()> {
        let mut aliases = default_aliases();
-        aliases.extend(FxHashMap::from_iter([
+        aliases.extend(HashMap::from_iter([
            ("dask.array".to_string(), "da".to_string()),
            ("dask.dataframe".to_string(), "dd".to_string()),
        ]));
@@ -37,8 +37,8 @@ mod tests {
            &LinterSettings {
                flake8_import_conventions: super::settings::Settings {
                    aliases,
-                    banned_aliases: FxHashMap::default(),
-                    banned_from: FxHashSet::default(),
+                    banned_aliases: HashMap::default(),
+                    banned_from: HashSet::default(),
                },
                ..LinterSettings::for_rule(Rule::UnconventionalImportAlias)
            },
@@ -54,7 +54,7 @@ mod tests {
            &LinterSettings {
                flake8_import_conventions: super::settings::Settings {
                    aliases: default_aliases(),
-                    banned_aliases: FxHashMap::from_iter([
+                    banned_aliases: HashMap::from_iter([
                        (
                            "typing".to_string(),
                            BannedAliases::from_iter(["t".to_string(), "ty".to_string()]),
@@ -72,7 +72,7 @@ mod tests {
                            BannedAliases::from_iter(["F".to_string()]),
                        ),
                    ]),
-                    banned_from: FxHashSet::default(),
+                    banned_from: HashSet::default(),
                },
                ..LinterSettings::for_rule(Rule::BannedImportAlias)
            },
@@ -88,8 +88,8 @@ mod tests {
            &LinterSettings {
                flake8_import_conventions: super::settings::Settings {
                    aliases: default_aliases(),
-                    banned_aliases: FxHashMap::default(),
-                    banned_from: FxHashSet::from_iter([
+                    banned_aliases: HashMap::default(),
+                    banned_from: HashSet::from_iter([
                        "logging.config".to_string(),
                        "typing".to_string(),
                        "pandas".to_string(),
@@ -108,14 +108,14 @@ mod tests {
            Path::new("flake8_import_conventions/remove_default.py"),
            &LinterSettings {
                flake8_import_conventions: super::settings::Settings {
-                    aliases: FxHashMap::from_iter([
+                    aliases: HashMap::from_iter([
                        ("altair".to_string(), "alt".to_string()),
                        ("matplotlib.pyplot".to_string(), "plt".to_string()),
                        ("pandas".to_string(), "pd".to_string()),
                        ("seaborn".to_string(), "sns".to_string()),
                    ]),
-                    banned_aliases: FxHashMap::default(),
-                    banned_from: FxHashSet::default(),
+                    banned_aliases: HashMap::default(),
+                    banned_from: HashSet::default(),
                },
                ..LinterSettings::for_rule(Rule::UnconventionalImportAlias)
            },
@@ -127,7 +127,7 @@ mod tests {
    #[test]
    fn override_defaults() -> Result<()> {
        let mut aliases = default_aliases();
-        aliases.extend(FxHashMap::from_iter([(
+        aliases.extend(HashMap::from_iter([(
            "numpy".to_string(),
            "nmp".to_string(),
        )]));
@@ -137,8 +137,8 @@ mod tests {
            &LinterSettings {
                flake8_import_conventions: super::settings::Settings {
                    aliases,
-                    banned_aliases: FxHashMap::default(),
-                    banned_from: FxHashSet::default(),
+                    banned_aliases: HashMap::default(),
+                    banned_from: HashSet::default(),
                },
                ..LinterSettings::for_rule(Rule::UnconventionalImportAlias)
            },
@@ -150,7 +150,7 @@ mod tests {
    #[test]
    fn from_imports() -> Result<()> {
        let mut aliases = default_aliases();
-        aliases.extend(FxHashMap::from_iter([
+        aliases.extend(HashMap::from_iter([
            ("xml.dom.minidom".to_string(), "md".to_string()),
            (
                "xml.dom.minidom.parseString".to_string(),
@@ -163,8 +163,8 @@ mod tests {
            &LinterSettings {
                flake8_import_conventions: super::settings::Settings {
                    aliases,
-                    banned_aliases: FxHashMap::default(),
-                    banned_from: FxHashSet::default(),
+                    banned_aliases: HashMap::default(),
+                    banned_from: HashSet::default(),
                },
                ..LinterSettings::for_rule(Rule::UnconventionalImportAlias)
            },
@@ -186,7 +186,7 @@ mod tests {
    #[test]
    fn same_name() -> Result<()> {
        let mut aliases = default_aliases();
-        aliases.extend(FxHashMap::from_iter([(
+        aliases.extend(HashMap::from_iter([(
            "django.conf.settings".to_string(),
            "settings".to_string(),
        )]));
@@ -195,8 +195,8 @@ mod tests {
            &LinterSettings {
                flake8_import_conventions: super::settings::Settings {
                    aliases,
-                    banned_aliases: FxHashMap::default(),
-                    banned_from: FxHashSet::default(),
+                    banned_aliases: HashMap::default(),
+                    banned_from: HashSet::default(),
                },
                ..LinterSettings::for_rule(Rule::UnconventionalImportAlias)
            },
--- a/Show More
+++ b/Show More