More simplification

Extract check_call function so that we can use early-returns
Move reveal_type, assert_type handling out of CallOutcome
2025-02-17 15:28:10 +01:00 · 2025-02-17 15:28:09 +01:00 · 2025-02-17 15:28:07 +01:00 · 2025-02-17 08:38:26 +00:00 · 2025-02-17 07:58:54 +00:00 · 2025-02-17 08:37:33 +01:00
319 changed files with 13584 additions and 5034 deletions
--- a/.github/renovate.json5
+++ b/.github/renovate.json5
@@ -58,6 +58,12 @@
      description: "Disable PRs updating GitHub runners (e.g. 'runs-on: macos-14')",
      enabled: false,
    },
+    {
+      // TODO: Remove this once the codebase is upgrade to v4 (https://github.com/astral-sh/ruff/pull/16069)
+      matchPackageNames: ["tailwindcss"],
+      matchManagers: ["npm"],
+      enabled: false,
+    },
    {
      // Disable updates of `zip-rs`; intentionally pinned for now due to ownership change
      // See: https://github.com/astral-sh/uv/issues/3642
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -712,7 +712,7 @@ jobs:
          just test

  benchmarks:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
    needs: determine_changes
    if: ${{ github.repository == 'astral-sh/ruff' && !contains(github.event.pull_request.labels.*.name, 'no-test') && (needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main') }}
    timeout-minutes: 20
--- a/.github/workflows/publish-playground.yml
+++ b/.github/workflows/publish-playground.yml
@@ -49,7 +49,7 @@ jobs:
        working-directory: playground
      - name: "Deploy to Cloudflare Pages"
        if: ${{ env.CF_API_TOKEN_EXISTS == 'true' }}
-        uses: cloudflare/wrangler-action@v3.13.1
+        uses: cloudflare/wrangler-action@v3.14.0
        with:
          apiToken: ${{ secrets.CF_API_TOKEN }}
          accountId: ${{ secrets.CF_ACCOUNT_ID }}
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -60,7 +60,7 @@ repos:
          - black==25.1.0

  - repo: https://github.com/crate-ci/typos
-    rev: v1.29.5
+    rev: v1.29.7
    hooks:
      - id: typos

@@ -74,7 +74,7 @@ repos:
        pass_filenames: false # This makes it a lot faster

  - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.9.4
+    rev: v0.9.6
    hooks:
      - id: ruff-format
      - id: ruff
@@ -84,7 +84,7 @@ repos:

  # Prettier
  - repo: https://github.com/rbubley/mirrors-prettier
-    rev: v3.4.2
+    rev: v3.5.1
    hooks:
      - id: prettier
        types: [yaml]
@@ -92,7 +92,7 @@ repos:
  # zizmor detects security vulnerabilities in GitHub Actions workflows.
  # Additional configuration for the tool is found in `.github/zizmor.yml`
  - repo: https://github.com/woodruffw/zizmor-pre-commit
-    rev: v1.3.0
+    rev: v1.3.1
    hooks:
      - id: zizmor

--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,44 @@
 # Changelog

+## 0.9.6
+
+### Preview features
+
+- \[`airflow`\] Add `external_task.{ExternalTaskMarker, ExternalTaskSensor}` for `AIR302` ([#16014](https://github.com/astral-sh/ruff/pull/16014))
+- \[`flake8-builtins`\] Make strict module name comparison optional (`A005`) ([#15951](https://github.com/astral-sh/ruff/pull/15951))
+- \[`flake8-pyi`\] Extend fix to Python \<= 3.9 for `redundant-none-literal` (`PYI061`) ([#16044](https://github.com/astral-sh/ruff/pull/16044))
+- \[`pylint`\] Also report when the object isn't a literal (`PLE1310`) ([#15985](https://github.com/astral-sh/ruff/pull/15985))
+- \[`ruff`\] Implement `indented-form-feed` (`RUF054`) ([#16049](https://github.com/astral-sh/ruff/pull/16049))
+- \[`ruff`\] Skip type definitions for `missing-f-string-syntax` (`RUF027`) ([#16054](https://github.com/astral-sh/ruff/pull/16054))
+
+### Rule changes
+
+- \[`flake8-annotations`\] Correct syntax for `typing.Union` in suggested return type fixes for `ANN20x` rules ([#16025](https://github.com/astral-sh/ruff/pull/16025))
+- \[`flake8-builtins`\] Match upstream module name comparison (`A005`) ([#16006](https://github.com/astral-sh/ruff/pull/16006))
+- \[`flake8-comprehensions`\] Detect overshadowed `list`/`set`/`dict`, ignore variadics and named expressions (`C417`) ([#15955](https://github.com/astral-sh/ruff/pull/15955))
+- \[`flake8-pie`\] Remove following comma correctly when the unpacked dictionary is empty (`PIE800`) ([#16008](https://github.com/astral-sh/ruff/pull/16008))
+- \[`flake8-simplify`\] Only trigger `SIM401` on known dictionaries ([#15995](https://github.com/astral-sh/ruff/pull/15995))
+- \[`pylint`\] Do not report calls when object type and argument type mismatch, remove custom escape handling logic (`PLE1310`) ([#15984](https://github.com/astral-sh/ruff/pull/15984))
+- \[`pyupgrade`\] Comments within parenthesized value ranges should not affect applicability (`UP040`) ([#16027](https://github.com/astral-sh/ruff/pull/16027))
+- \[`pyupgrade`\] Don't introduce invalid syntax when upgrading old-style type aliases with parenthesized multiline values (`UP040`) ([#16026](https://github.com/astral-sh/ruff/pull/16026))
+- \[`pyupgrade`\] Ensure we do not rename two type parameters to the same name (`UP049`) ([#16038](https://github.com/astral-sh/ruff/pull/16038))
+- \[`pyupgrade`\] \[`ruff`\] Don't apply renamings if the new name is shadowed in a scope of one of the references to the binding (`UP049`, `RUF052`) ([#16032](https://github.com/astral-sh/ruff/pull/16032))
+- \[`ruff`\] Update `RUF009` to behave similar to `B008` and ignore attributes with immutable types ([#16048](https://github.com/astral-sh/ruff/pull/16048))
+
+### Server
+
+- Root exclusions in the server to project root ([#16043](https://github.com/astral-sh/ruff/pull/16043))
+
+### Bug fixes
+
+- \[`flake8-datetime`\] Ignore `.replace()` calls while looking for `.astimezone` ([#16050](https://github.com/astral-sh/ruff/pull/16050))
+- \[`flake8-type-checking`\] Avoid `TC004` false positive where the runtime definition is provided by `__getattr__` ([#16052](https://github.com/astral-sh/ruff/pull/16052))
+
+### Documentation
+
+- Improve `ruff-lsp` migration document ([#16072](https://github.com/astral-sh/ruff/pull/16072))
+- Undeprecate `ruff.nativeServer` ([#16039](https://github.com/astral-sh/ruff/pull/16039))
+
 ## 0.9.5

 ### Preview features
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -29,6 +29,12 @@ dependencies = [
 "memchr",
 ]

+[[package]]
+name = "allocator-api2"
+version = "0.2.21"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "683d7910e743518b0e34f1186f92494becacb047c7b6bf616c96772180fef923"
+
 [[package]]
 name = "android-tzdata"
 version = "0.1.1"
@@ -354,9 +360,9 @@ dependencies = [

 [[package]]
 name = "clap"
-version = "4.5.27"
+version = "4.5.29"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "769b0145982b4b48713e01ec42d61614425f27b7058bda7180a3a41f30104796"
+checksum = "8acebd8ad879283633b343856142139f2da2317c96b05b4dd6181c61e2480184"
 dependencies = [
 "clap_builder",
 "clap_derive",
@@ -364,9 +370,9 @@ dependencies = [

 [[package]]
 name = "clap_builder"
-version = "4.5.27"
+version = "4.5.29"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1b26884eb4b57140e4d2d93652abfa49498b938b3c9179f9fc487b0acc3edad7"
+checksum = "f6ba32cbda51c7e1dfd49acc1457ba1a7dec5b64fe360e828acb13ca8dc9c2f9"
 dependencies = [
 "anstream",
 "anstyle",
@@ -407,9 +413,9 @@ dependencies = [

 [[package]]
 name = "clap_derive"
-version = "4.5.24"
+version = "4.5.28"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "54b755194d6389280185988721fffba69495eed5ee9feeee9a599b53db80318c"
+checksum = "bf4ced95c6f4a675af3da73304b9ac4ed991640c36374e4b46795c49e17cf1ed"
 dependencies = [
 "heck",
 "proc-macro2",
@@ -438,20 +444,22 @@ dependencies = [

 [[package]]
 name = "codspeed"
-version = "2.7.2"
+version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "450a0e9df9df1c154156f4344f99d8f6f6e69d0fc4de96ef6e2e68b2ec3bce97"
+checksum = "25d2f5a6570db487f5258e0bded6352fa2034c2aeb46bb5cc3ff060a0fcfba2f"
 dependencies = [
 "colored 2.2.0",
 "libc",
+ "serde",
 "serde_json",
+ "uuid",
 ]

 [[package]]
 name = "codspeed-criterion-compat"
-version = "2.7.2"
+version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8eb1a6cb9c20e177fde58cdef97c1c7c9264eb1424fe45c4fccedc2fb078a569"
+checksum = "f53a55558dedec742b14aae3c5fec389361b8b5ca28c1aadf09dd91faf710074"
 dependencies = [
 "codspeed",
 "colored 2.2.0",
@@ -897,7 +905,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "33d852cb9b869c2a9b3df2f71a3074817f01e1844f839a144f5fcef059a4eb5d"
 dependencies = [
 "libc",
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]

 [[package]]
@@ -1031,10 +1039,8 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c4567c8db10ae91089c99af84c68c38da3ec2f087c3f82960bcdbf3656b6f4d7"
 dependencies = [
 "cfg-if",
- "js-sys",
 "libc",
 "wasi 0.11.0+wasi-snapshot-preview1",
- "wasm-bindgen",
 ]

 [[package]]
@@ -1098,6 +1104,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
 dependencies = [
 "ahash",
+ "allocator-api2",
 ]

 [[package]]
@@ -1475,7 +1482,7 @@ checksum = "e19b23d53f35ce9f56aebc7d1bb4e6ac1e9c0db7ac85c8d1760c04379edced37"
 dependencies = [
 "hermit-abi 0.4.0",
 "libc",
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]

 [[package]]
@@ -2049,7 +2056,7 @@ dependencies = [
 "once_cell",
 "pep440_rs",
 "regex",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "serde",
 "smallvec",
 "thiserror 1.0.69",
@@ -2409,6 +2416,7 @@ dependencies = [
 "red_knot_server",
 "regex",
 "ruff_db",
+ "ruff_python_ast",
 "ruff_python_trivia",
 "salsa",
 "tempfile",
@@ -2437,7 +2445,7 @@ dependencies = [
 "ruff_macros",
 "ruff_python_ast",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "salsa",
 "schemars",
 "serde",
@@ -2477,7 +2485,7 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "salsa",
 "schemars",
 "serde",
@@ -2505,7 +2513,7 @@ dependencies = [
 "ruff_python_ast",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "serde",
 "serde_json",
 "shellexpand",
@@ -2531,7 +2539,7 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "salsa",
 "serde",
 "smallvec",
@@ -2558,9 +2566,9 @@ dependencies = [
 "js-sys",
 "log",
 "red_knot_project",
- "red_knot_python_semantic",
 "ruff_db",
 "ruff_notebook",
+ "ruff_python_ast",
 "wasm-bindgen",
 "wasm-bindgen-test",
 ]
@@ -2642,7 +2650,7 @@ dependencies = [

 [[package]]
 name = "ruff"
-version = "0.9.5"
+version = "0.9.6"
 dependencies = [
 "anyhow",
 "argfile",
@@ -2682,7 +2690,7 @@ dependencies = [
 "ruff_source_file",
 "ruff_text_size",
 "ruff_workspace",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "serde",
 "serde_json",
 "shellexpand",
@@ -2721,14 +2729,13 @@ dependencies = [
 "mimalloc",
 "rayon",
 "red_knot_project",
- "red_knot_python_semantic",
 "ruff_db",
 "ruff_linter",
 "ruff_python_ast",
 "ruff_python_formatter",
 "ruff_python_parser",
 "ruff_python_trivia",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "tikv-jemallocator",
 ]

@@ -2750,10 +2757,10 @@ name = "ruff_db"
 version = "0.0.0"
 dependencies = [
 "camino",
- "colored 3.0.0",
 "countme",
 "dashmap 6.1.0",
 "dunce",
+ "etcetera",
 "filetime",
 "glob",
 "ignore",
@@ -2768,7 +2775,7 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "salsa",
 "schemars",
 "serde",
@@ -2839,7 +2846,7 @@ dependencies = [
 "ruff_cache",
 "ruff_macros",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "schemars",
 "serde",
 "static_assertions",
@@ -2871,12 +2878,13 @@ name = "ruff_index"
 version = "0.0.0"
 dependencies = [
 "ruff_macros",
+ "salsa",
 "static_assertions",
 ]

 [[package]]
 name = "ruff_linter"
-version = "0.9.5"
+version = "0.9.6"
 dependencies = [
 "aho-corasick",
 "anyhow",
@@ -2918,7 +2926,7 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "schemars",
 "serde",
 "serde_json",
@@ -2980,7 +2988,8 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
+ "salsa",
 "schemars",
 "serde",
 ]
@@ -3027,7 +3036,7 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "schemars",
 "serde",
 "serde_json",
@@ -3074,7 +3083,7 @@ dependencies = [
 "ruff_python_trivia",
 "ruff_source_file",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "static_assertions",
 "unicode-ident",
 "unicode-normalization",
@@ -3105,7 +3114,7 @@ dependencies = [
 "ruff_python_parser",
 "ruff_python_stdlib",
 "ruff_text_size",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "schemars",
 "serde",
 "smallvec",
@@ -3164,7 +3173,7 @@ dependencies = [
 "ruff_source_file",
 "ruff_text_size",
 "ruff_workspace",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "serde",
 "serde_json",
 "shellexpand",
@@ -3194,7 +3203,7 @@ dependencies = [

 [[package]]
 name = "ruff_wasm"
-version = "0.9.5"
+version = "0.9.6"
 dependencies = [
 "console_error_panic_hook",
 "console_log",
@@ -3246,7 +3255,7 @@ dependencies = [
 "ruff_python_semantic",
 "ruff_python_stdlib",
 "ruff_source_file",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "schemars",
 "serde",
 "shellexpand",
@@ -3273,9 +3282,9 @@ checksum = "08d43f7aa6b08d49f382cde6a7982047c3426db949b1424bc4b7ec9ae12c6ce2"

 [[package]]
 name = "rustc-hash"
-version = "2.1.0"
+version = "2.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c7fb8039b3032c191086b10f11f319a6e99e1e82889c5cc6046f515c9db1d497"
+checksum = "357703d41365b4b27c590e3ed91eabb1b663f07c4c084095e60cbed4362dff0d"

 [[package]]
 name = "rustix"
@@ -3287,7 +3296,7 @@ dependencies = [
 "errno",
 "libc",
 "linux-raw-sys",
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]

 [[package]]
@@ -3305,17 +3314,19 @@ checksum = "6ea1a2d0a644769cc99faa24c3ad26b379b786fe7c36fd3c546254801650e6dd"
 [[package]]
 name = "salsa"
 version = "0.18.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=88a1d7774d78f048fbd77d40abca9ebd729fd1f0#88a1d7774d78f048fbd77d40abca9ebd729fd1f0"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=351d9cf0037be949d17800d0c7b4838e533c2ed6#351d9cf0037be949d17800d0c7b4838e533c2ed6"
 dependencies = [
 "append-only-vec",
 "arc-swap",
+ "compact_str",
 "crossbeam",
 "dashmap 6.1.0",
+ "hashbrown 0.14.5",
 "hashlink",
 "indexmap",
 "parking_lot",
 "rayon",
- "rustc-hash 2.1.0",
+ "rustc-hash 2.1.1",
 "salsa-macro-rules",
 "salsa-macros",
 "smallvec",
@@ -3325,12 +3336,12 @@ dependencies = [
 [[package]]
 name = "salsa-macro-rules"
 version = "0.1.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=88a1d7774d78f048fbd77d40abca9ebd729fd1f0#88a1d7774d78f048fbd77d40abca9ebd729fd1f0"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=351d9cf0037be949d17800d0c7b4838e533c2ed6#351d9cf0037be949d17800d0c7b4838e533c2ed6"

 [[package]]
 name = "salsa-macros"
 version = "0.18.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=88a1d7774d78f048fbd77d40abca9ebd729fd1f0#88a1d7774d78f048fbd77d40abca9ebd729fd1f0"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=351d9cf0037be949d17800d0c7b4838e533c2ed6#351d9cf0037be949d17800d0c7b4838e533c2ed6"
 dependencies = [
 "heck",
 "proc-macro2",
@@ -3539,9 +3550,9 @@ checksum = "56199f7ddabf13fe5074ce809e7d3f42b42ae711800501b5b16ea82ad029c39d"

 [[package]]
 name = "smallvec"
-version = "1.13.2"
+version = "1.14.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3c5e1a9a646d36c3599cd173a41282daf47c44583ad367b8e6837255952e5c67"
+checksum = "7fcf8323ef1faaee30a44a340193b1ac6814fd9b7b4e88e9d4519a3e4abe1cfd"

 [[package]]
 name = "snapbox"
@@ -3601,18 +3612,18 @@ checksum = "7da8b5736845d9f2fcb837ea5d9e2628564b3b043a70948a3f0b778838c5fb4f"

 [[package]]
 name = "strum"
-version = "0.26.3"
+version = "0.27.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8fec0f0aef304996cf250b31b5a10dee7980c85da9d759361292b8bca5a18f06"
+checksum = "f64def088c51c9510a8579e3c5d67c65349dcf755e5479ad3d010aa6454e2c32"
 dependencies = [
 "strum_macros",
 ]

 [[package]]
 name = "strum_macros"
-version = "0.26.4"
+version = "0.27.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4c6bee85a5a24955dc440386795aa378cd9cf82acd5f764469152d2270e581be"
+checksum = "c77a8c5abcaf0f9ce05d62342b7d298c346515365c36b673df4ebe3ced01fde8"
 dependencies = [
 "heck",
 "proc-macro2",
@@ -3656,16 +3667,16 @@ dependencies = [

 [[package]]
 name = "tempfile"
-version = "3.16.0"
+version = "3.17.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "38c246215d7d24f48ae091a2902398798e05d978b24315d6efbc00ede9a8bb91"
+checksum = "a40f762a77d2afa88c2d919489e390a12bdd261ed568e60cfa7e48d4e20f0d33"
 dependencies = [
 "cfg-if",
 "fastrand",
 "getrandom 0.3.1",
 "once_cell",
 "rustix",
- "windows-sys 0.52.0",
+ "windows-sys 0.59.0",
 ]

 [[package]]
@@ -3854,9 +3865,9 @@ checksum = "1f3ccbac311fea05f86f61904b462b55fb3df8837a366dfc601a0161d0532f20"

 [[package]]
 name = "toml"
-version = "0.8.19"
+version = "0.8.20"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a1ed1f98e3fdc28d6d910e6737ae6ab1a93bf1985935a1193e68f93eeb68d24e"
+checksum = "cd87a5cdd6ffab733b2f74bc4fd7ee5fff6634124999ac278c35fc78c6120148"
 dependencies = [
 "serde",
 "serde_spanned",
@@ -4153,21 +4164,22 @@ checksum = "06abde3611657adf66d383f00b093d7faecc7fa57071cce2578660c9f1010821"

 [[package]]
 name = "uuid"
-version = "1.12.1"
+version = "1.13.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b3758f5e68192bb96cc8f9b7e2c2cfdabb435499a28499a42f8f984092adad4b"
+checksum = "ced87ca4be083373936a67f8de945faa23b6b42384bd5b64434850802c6dccd0"
 dependencies = [
- "getrandom 0.2.15",
- "rand 0.8.5",
+ "getrandom 0.3.1",
+ "js-sys",
+ "rand 0.9.0",
 "uuid-macro-internal",
 "wasm-bindgen",
 ]

 [[package]]
 name = "uuid-macro-internal"
-version = "1.12.1"
+version = "1.13.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f8a86d88347b61a0e17b9908a67efcc594130830bf1045653784358dd023e294"
+checksum = "d28dd23acb5f2fa7bd2155ab70b960e770596b3bb6395119b40476c3655dfba4"
 dependencies = [
 "proc-macro2",
 "quote",
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -123,7 +123,7 @@ rayon = { version = "1.10.0" }
 regex = { version = "1.10.2" }
 rustc-hash = { version = "2.0.0" }
 # When updating salsa, make sure to also update the revision in `fuzz/Cargo.toml`
-salsa = { git = "https://github.com/salsa-rs/salsa.git", rev = "88a1d7774d78f048fbd77d40abca9ebd729fd1f0" }
+salsa = { git = "https://github.com/salsa-rs/salsa.git", rev = "351d9cf0037be949d17800d0c7b4838e533c2ed6" }
 schemars = { version = "0.8.16" }
 seahash = { version = "4.1.0" }
 serde = { version = "1.0.197", features = ["derive"] }
@@ -143,8 +143,8 @@ snapbox = { version = "0.6.0", features = [
    "examples",
 ] }
 static_assertions = "1.1.0"
-strum = { version = "0.26.0", features = ["strum_macros"] }
-strum_macros = { version = "0.26.0" }
+strum = { version = "0.27.0", features = ["strum_macros"] }
+strum_macros = { version = "0.27.0" }
 syn = { version = "2.0.55" }
 tempfile = { version = "3.9.0" }
 test-case = { version = "3.3.1" }
--- a/README.md
+++ b/README.md
@@ -149,8 +149,8 @@ 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/0.9.5/install.sh | sh
-powershell -c "irm https://astral.sh/ruff/0.9.5/install.ps1 | iex"
+curl -LsSf https://astral.sh/ruff/0.9.6/install.sh | sh
+powershell -c "irm https://astral.sh/ruff/0.9.6/install.ps1 | iex"
 ```

 You can also install Ruff via [Homebrew](https://formulae.brew.sh/formula/ruff), [Conda](https://anaconda.org/conda-forge/ruff),
@@ -183,7 +183,7 @@ 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: v0.9.5
+  rev: v0.9.6
  hooks:
    # Run the linter.
    - id: ruff
--- a/crates/red_knot/Cargo.toml
+++ b/crates/red_knot/Cargo.toml
@@ -16,6 +16,7 @@ red_knot_python_semantic = { workspace = true }
 red_knot_project = { workspace = true, features = ["zstd"] }
 red_knot_server = { workspace = true }
 ruff_db = { workspace = true, features = ["os", "cache"] }
+ruff_python_ast = { workspace = true }

 anyhow = { workspace = true }
 chrono = { workspace = true }
--- a/crates/red_knot/src/args.rs
+++ b/crates/red_knot/src/args.rs
@@ -1,7 +1,7 @@
 use crate::logging::Verbosity;
 use crate::python_version::PythonVersion;
 use clap::{ArgAction, ArgMatches, Error, Parser};
-use red_knot_project::metadata::options::{EnvironmentOptions, Options};
+use red_knot_project::metadata::options::{EnvironmentOptions, Options, TerminalOptions};
 use red_knot_project::metadata::value::{RangedValue, RelativePathBuf};
 use red_knot_python_semantic::lint;
 use ruff_db::system::SystemPathBuf;
@@ -67,8 +67,8 @@ pub(crate) struct CheckCommand {
    pub(crate) rules: RulesArg,

    /// Use exit code 1 if there are any warning-level diagnostics.
-    #[arg(long, conflicts_with = "exit_zero")]
-    pub(crate) error_on_warning: bool,
+    #[arg(long, conflicts_with = "exit_zero", default_missing_value = "true", num_args=0..1)]
+    pub(crate) error_on_warning: Option<bool>,

    /// Always use exit code 0, even when there are error-level diagnostics.
    #[arg(long)]
@@ -107,6 +107,9 @@ impl CheckCommand {
                }),
                ..EnvironmentOptions::default()
            }),
+            terminal: Some(TerminalOptions {
+                error_on_warning: self.error_on_warning,
+            }),
            rules,
            ..Default::default()
        }
--- a/crates/red_knot/src/main.rs
+++ b/crates/red_knot/src/main.rs
@@ -11,18 +11,17 @@ use clap::Parser;
 use colored::Colorize;
 use crossbeam::channel as crossbeam_channel;
 use red_knot_project::metadata::options::Options;
-use red_knot_project::watch;
 use red_knot_project::watch::ProjectWatcher;
+use red_knot_project::{watch, Db};
 use red_knot_project::{ProjectDatabase, ProjectMetadata};
 use red_knot_server::run_server;
-use ruff_db::diagnostic::{Diagnostic, Severity};
+use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig, Severity};
 use ruff_db::system::{OsSystem, System, SystemPath, SystemPathBuf};
 use salsa::plumbing::ZalsaDatabase;

 mod args;
 mod logging;
 mod python_version;
-mod verbosity;
 mod version;

 #[allow(clippy::print_stdout, clippy::unnecessary_wraps, clippy::print_stderr)]
@@ -97,19 +96,15 @@ fn run_check(args: CheckCommand) -> anyhow::Result<ExitStatus> {
    let system = OsSystem::new(cwd);
    let watch = args.watch;
    let exit_zero = args.exit_zero;
-    let min_error_severity = if args.error_on_warning {
-        Severity::Warning
-    } else {
-        Severity::Error
-    };

    let cli_options = args.into_options();
-    let mut workspace_metadata = ProjectMetadata::discover(system.current_directory(), &system)?;
-    workspace_metadata.apply_cli_options(cli_options.clone());
+    let mut project_metadata = ProjectMetadata::discover(system.current_directory(), &system)?;
+    project_metadata.apply_cli_options(cli_options.clone());
+    project_metadata.apply_configuration_files(&system)?;

-    let mut db = ProjectDatabase::new(workspace_metadata, system)?;
+    let mut db = ProjectDatabase::new(project_metadata, system)?;

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

    // Listen to Ctrl+C and abort the watch mode.
    let main_loop_cancellation_token = Mutex::new(Some(main_loop_cancellation_token));
@@ -167,18 +162,10 @@ struct MainLoop {
    watcher: Option<ProjectWatcher>,

    cli_options: Options,
-
-    /// The minimum severity to consider an error when deciding the exit status.
-    ///
-    /// TODO(micha): Get from the terminal settings.
-    min_error_severity: Severity,
 }

 impl MainLoop {
-    fn new(
-        cli_options: Options,
-        min_error_severity: Severity,
-    ) -> (Self, MainLoopCancellationToken) {
+    fn new(cli_options: Options) -> (Self, MainLoopCancellationToken) {
        let (sender, receiver) = crossbeam_channel::bounded(10);

        (
@@ -187,7 +174,6 @@ impl MainLoop {
                receiver,
                watcher: None,
                cli_options,
-                min_error_severity,
            },
            MainLoopCancellationToken { sender },
        )
@@ -245,14 +231,24 @@ impl MainLoop {
                    result,
                    revision: check_revision,
                } => {
+                    let display_config = DisplayDiagnosticConfig::default()
+                        .color(colored::control::SHOULD_COLORIZE.should_colorize());
+
+                    let min_error_severity =
+                        if db.project().settings(db).terminal().error_on_warning {
+                            Severity::Warning
+                        } else {
+                            Severity::Error
+                        };
+
                    let failed = result
                        .iter()
-                        .any(|diagnostic| diagnostic.severity() >= self.min_error_severity);
+                        .any(|diagnostic| diagnostic.severity() >= min_error_severity);

                    if check_revision == revision {
                        #[allow(clippy::print_stdout)]
                        for diagnostic in result {
-                            println!("{}", diagnostic.display(db));
+                            println!("{}", diagnostic.display(db, &display_config));
                        }
                    } else {
                        tracing::debug!(
--- a/crates/red_knot/src/python_version.rs
+++ b/crates/red_knot/src/python_version.rs
@@ -40,7 +40,7 @@ impl std::fmt::Display for PythonVersion {
    }
 }

-impl From<PythonVersion> for red_knot_python_semantic::PythonVersion {
+impl From<PythonVersion> for ruff_python_ast::python_version::PythonVersion {
    fn from(value: PythonVersion) -> Self {
        match value {
            PythonVersion::Py37 => Self::PY37,
@@ -61,8 +61,8 @@ mod tests {
    #[test]
    fn same_default_as_python_version() {
        assert_eq!(
-            red_knot_python_semantic::PythonVersion::from(PythonVersion::default()),
-            red_knot_python_semantic::PythonVersion::default()
+            ruff_python_ast::python_version::PythonVersion::from(PythonVersion::default()),
+            ruff_python_ast::python_version::PythonVersion::default()
        );
    }
 }
--- a/crates/red_knot/src/verbosity.rs
+++ b/crates/red_knot/src/verbosity.rs
@@ -1 +0,0 @@
-
--- a/crates/red_knot/tests/cli.rs
+++ b/crates/red_knot/tests/cli.rs
@@ -98,7 +98,7 @@ fn cli_arguments_are_relative_to_the_current_directory() -> anyhow::Result<()> {
    ])?;

    // Make sure that the CLI fails when the `libs` directory is not in the search path.
-    assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")), @r###"
+    assert_cmd_snapshot!(case.command().current_dir(case.root().join("child")), @r###"
    success: false
    exit_code: 1
    ----- stdout -----
@@ -115,7 +115,7 @@ fn cli_arguments_are_relative_to_the_current_directory() -> anyhow::Result<()> {
    ----- stderr -----
    "###);

-    assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")).arg("--extra-search-path").arg("../libs"), @r"
+    assert_cmd_snapshot!(case.command().current_dir(case.root().join("child")).arg("--extra-search-path").arg("../libs"), @r"
        success: true
        exit_code: 0
        ----- stdout -----
@@ -167,7 +167,7 @@ fn paths_in_configuration_files_are_relative_to_the_project_root() -> anyhow::Re
        ),
    ])?;

-    assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")), @r"
+    assert_cmd_snapshot!(case.command().current_dir(case.root().join("child")), @r"
        success: true
        exit_code: 0
        ----- stdout -----
@@ -575,6 +575,37 @@ fn exit_code_no_errors_but_error_on_warning_is_true() -> anyhow::Result<()> {
    Ok(())
 }

+#[test]
+fn exit_code_no_errors_but_error_on_warning_is_enabled_in_configuration() -> anyhow::Result<()> {
+    let case = TestCase::with_files([
+        ("test.py", r"print(x)  # [unresolved-reference]"),
+        (
+            "knot.toml",
+            r#"
+            [terminal]
+            error-on-warning = true
+        "#,
+        ),
+    ])?;
+
+    assert_cmd_snapshot!(case.command(), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    warning: lint:unresolved-reference
+     --> <temp_dir>/test.py:1:7
+      |
+    1 | print(x)  # [unresolved-reference]
+      |       - Name `x` used when not defined
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
 #[test]
 fn exit_code_both_warnings_and_errors() -> anyhow::Result<()> {
    let case = TestCase::with_file(
@@ -686,6 +717,109 @@ fn exit_code_exit_zero_is_true() -> anyhow::Result<()> {
    Ok(())
 }

+#[test]
+fn user_configuration() -> anyhow::Result<()> {
+    let case = TestCase::with_files([
+        (
+            "project/knot.toml",
+            r#"
+            [rules]
+            division-by-zero = "warn"
+            "#,
+        ),
+        (
+            "project/main.py",
+            r#"
+            y = 4 / 0
+
+            for a in range(0, y):
+                x = a
+
+            print(x)
+            "#,
+        ),
+    ])?;
+
+    let config_directory = case.root().join("home/.config");
+    let config_env_var = if cfg!(windows) {
+        "APPDATA"
+    } else {
+        "XDG_CONFIG_HOME"
+    };
+
+    assert_cmd_snapshot!(
+        case.command().current_dir(case.root().join("project")).env(config_env_var, config_directory.as_os_str()),
+        @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: lint:division-by-zero
+     --> <temp_dir>/project/main.py:2:5
+      |
+    2 | y = 4 / 0
+      |     ----- Cannot divide object of type `Literal[4]` by zero
+    3 |
+    4 | for a in range(0, y):
+      |
+
+    warning: lint:possibly-unresolved-reference
+     --> <temp_dir>/project/main.py:7:7
+      |
+    5 |     x = a
+    6 |
+    7 | print(x)
+      |       - Name `x` used when possibly not defined
+      |
+
+
+    ----- stderr -----
+    "###
+    );
+
+    // The user-level configuration promotes `possibly-unresolved-reference` to an error.
+    // Changing the level for `division-by-zero` has no effect, because the project-level configuration
+    // has higher precedence.
+    case.write_file(
+        config_directory.join("knot/knot.toml"),
+        r#"
+        [rules]
+        division-by-zero = "error"
+        possibly-unresolved-reference = "error"
+        "#,
+    )?;
+
+    assert_cmd_snapshot!(
+        case.command().current_dir(case.root().join("project")).env(config_env_var, config_directory.as_os_str()),
+        @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    warning: lint:division-by-zero
+     --> <temp_dir>/project/main.py:2:5
+      |
+    2 | y = 4 / 0
+      |     ----- Cannot divide object of type `Literal[4]` by zero
+    3 |
+    4 | for a in range(0, y):
+      |
+
+    error: lint:possibly-unresolved-reference
+     --> <temp_dir>/project/main.py:7:7
+      |
+    5 |     x = a
+    6 |
+    7 | print(x)
+      |       ^ Name `x` used when possibly not defined
+      |
+
+
+    ----- stderr -----
+    "###
+    );
+
+    Ok(())
+}
+
 struct TestCase {
    _temp_dir: TempDir,
    _settings_scope: SettingsBindDropGuard,
@@ -753,7 +887,7 @@ impl TestCase {
        Ok(())
    }

-    fn project_dir(&self) -> &Path {
+    fn root(&self) -> &Path {
        &self.project_dir
    }

--- a/crates/red_knot/tests/file_watching.rs
+++ b/crates/red_knot/tests/file_watching.rs
@@ -9,11 +9,14 @@ use red_knot_project::metadata::pyproject::{PyProject, Tool};
 use red_knot_project::metadata::value::{RangedValue, RelativePathBuf};
 use red_knot_project::watch::{directory_watcher, ChangeEvent, ProjectWatcher};
 use red_knot_project::{Db, ProjectDatabase, ProjectMetadata};
-use red_knot_python_semantic::{resolve_module, ModuleName, PythonPlatform, PythonVersion};
+use red_knot_python_semantic::{resolve_module, ModuleName, PythonPlatform};
 use ruff_db::files::{system_path_to_file, File, FileError};
 use ruff_db::source::source_text;
-use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf};
+use ruff_db::system::{
+    OsSystem, System, SystemPath, SystemPathBuf, UserConfigDirectoryOverrideGuard,
+};
 use ruff_db::Upcast;
+use ruff_python_ast::python_version::PythonVersion;

 struct TestCase {
    db: ProjectDatabase,
@@ -220,17 +223,44 @@ where
 }

 trait SetupFiles {
-    fn setup(self, root_path: &SystemPath, project_path: &SystemPath) -> anyhow::Result<()>;
+    fn setup(self, context: &SetupContext) -> anyhow::Result<()>;
+}
+
+struct SetupContext<'a> {
+    system: &'a OsSystem,
+    root_path: &'a SystemPath,
+}
+
+impl<'a> SetupContext<'a> {
+    fn system(&self) -> &'a OsSystem {
+        self.system
+    }
+
+    fn join_project_path(&self, relative: impl AsRef<SystemPath>) -> SystemPathBuf {
+        self.project_path().join(relative)
+    }
+
+    fn project_path(&self) -> &SystemPath {
+        self.system.current_directory()
+    }
+
+    fn root_path(&self) -> &'a SystemPath {
+        self.root_path
+    }
+
+    fn join_root_path(&self, relative: impl AsRef<SystemPath>) -> SystemPathBuf {
+        self.root_path().join(relative)
+    }
 }

 impl<const N: usize, P> SetupFiles for [(P, &'static str); N]
 where
    P: AsRef<SystemPath>,
 {
-    fn setup(self, _root_path: &SystemPath, project_path: &SystemPath) -> anyhow::Result<()> {
+    fn setup(self, context: &SetupContext) -> anyhow::Result<()> {
        for (relative_path, content) in self {
            let relative_path = relative_path.as_ref();
-            let absolute_path = project_path.join(relative_path);
+            let absolute_path = context.join_project_path(relative_path);
            if let Some(parent) = absolute_path.parent() {
                std::fs::create_dir_all(parent).with_context(|| {
                    format!("Failed to create parent directory for file `{relative_path}`")
@@ -250,10 +280,10 @@ where

 impl<F> SetupFiles for F
 where
-    F: FnOnce(&SystemPath, &SystemPath) -> anyhow::Result<()>,
+    F: FnOnce(&SetupContext) -> anyhow::Result<()>,
 {
-    fn setup(self, root_path: &SystemPath, project_path: &SystemPath) -> anyhow::Result<()> {
-        self(root_path, project_path)
+    fn setup(self, context: &SetupContext) -> anyhow::Result<()> {
+        self(context)
    }
 }

@@ -261,13 +291,12 @@ fn setup<F>(setup_files: F) -> anyhow::Result<TestCase>
 where
    F: SetupFiles,
 {
-    setup_with_options(setup_files, |_root, _project_path| None)
+    setup_with_options(setup_files, |_context| None)
 }

-// TODO: Replace with configuration?
 fn setup_with_options<F>(
    setup_files: F,
-    create_options: impl FnOnce(&SystemPath, &SystemPath) -> Option<Options>,
+    create_options: impl FnOnce(&SetupContext) -> Option<Options>,
 ) -> anyhow::Result<TestCase>
 where
    F: SetupFiles,
@@ -295,13 +324,17 @@ where
    std::fs::create_dir_all(project_path.as_std_path())
        .with_context(|| format!("Failed to create project directory `{project_path}`"))?;

+    let system = OsSystem::new(&project_path);
+    let setup_context = SetupContext {
+        system: &system,
+        root_path: &root_path,
+    };
+
    setup_files
-        .setup(&root_path, &project_path)
+        .setup(&setup_context)
        .context("Failed to setup test files")?;

-    let system = OsSystem::new(&project_path);
-
-    if let Some(options) = create_options(&root_path, &project_path) {
+    if let Some(options) = create_options(&setup_context) {
        std::fs::write(
            project_path.join("pyproject.toml").as_std_path(),
            toml::to_string(&PyProject {
@@ -315,7 +348,9 @@ where
        .context("Failed to write configuration")?;
    }

-    let project = ProjectMetadata::discover(&project_path, &system)?;
+    let mut project = ProjectMetadata::discover(&project_path, &system)?;
+    project.apply_configuration_files(&system)?;
+
    let program_settings = project.to_program_settings(&system);

    for path in program_settings
@@ -789,10 +824,12 @@ fn directory_deleted() -> anyhow::Result<()> {

 #[test]
 fn search_path() -> anyhow::Result<()> {
-    let mut case = setup_with_options([("bar.py", "import sub.a")], |root_path, _project_path| {
+    let mut case = setup_with_options([("bar.py", "import sub.a")], |context| {
        Some(Options {
            environment: Some(EnvironmentOptions {
-                extra_paths: Some(vec![RelativePathBuf::cli(root_path.join("site_packages"))]),
+                extra_paths: Some(vec![RelativePathBuf::cli(
+                    context.join_root_path("site_packages"),
+                )]),
                ..EnvironmentOptions::default()
            }),
            ..Options::default()
@@ -853,10 +890,12 @@ fn add_search_path() -> anyhow::Result<()> {

 #[test]
 fn remove_search_path() -> anyhow::Result<()> {
-    let mut case = setup_with_options([("bar.py", "import sub.a")], |root_path, _project_path| {
+    let mut case = setup_with_options([("bar.py", "import sub.a")], |context| {
        Some(Options {
            environment: Some(EnvironmentOptions {
-                extra_paths: Some(vec![RelativePathBuf::cli(root_path.join("site_packages"))]),
+                extra_paths: Some(vec![RelativePathBuf::cli(
+                    context.join_root_path("site_packages"),
+                )]),
                ..EnvironmentOptions::default()
            }),
            ..Options::default()
@@ -894,7 +933,7 @@ import os
 print(sys.last_exc, os.getegid())
 "#,
        )],
-        |_root_path, _project_path| {
+        |_context| {
            Some(Options {
                environment: Some(EnvironmentOptions {
                    python_version: Some(RangedValue::cli(PythonVersion::PY311)),
@@ -942,21 +981,31 @@ print(sys.last_exc, os.getegid())
 #[test]
 fn changed_versions_file() -> anyhow::Result<()> {
    let mut case = setup_with_options(
-        |root_path: &SystemPath, project_path: &SystemPath| {
-            std::fs::write(project_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(), "")?;
+        |context: &SetupContext| {
            std::fs::write(
-                root_path.join("typeshed/stdlib/os.pyi").as_std_path(),
+                context.join_project_path("bar.py").as_std_path(),
+                "import sub.a",
+            )?;
+            std::fs::create_dir_all(context.join_root_path("typeshed/stdlib").as_std_path())?;
+            std::fs::write(
+                context
+                    .join_root_path("typeshed/stdlib/VERSIONS")
+                    .as_std_path(),
+                "",
+            )?;
+            std::fs::write(
+                context
+                    .join_root_path("typeshed/stdlib/os.pyi")
+                    .as_std_path(),
                "# not important",
            )?;

            Ok(())
        },
-        |root_path, _project_path| {
+        |context| {
            Some(Options {
                environment: Some(EnvironmentOptions {
-                    typeshed: Some(RelativePathBuf::cli(root_path.join("typeshed"))),
+                    typeshed: Some(RelativePathBuf::cli(context.join_root_path("typeshed"))),
                    ..EnvironmentOptions::default()
                }),
                ..Options::default()
@@ -1007,12 +1056,12 @@ fn changed_versions_file() -> anyhow::Result<()> {
 /// we're seeing is that Windows only emits a single event, similar to Linux.
 #[test]
 fn hard_links_in_project() -> anyhow::Result<()> {
-    let mut case = setup(|_root: &SystemPath, project: &SystemPath| {
-        let foo_path = project.join("foo.py");
+    let mut case = setup(|context: &SetupContext| {
+        let foo_path = context.join_project_path("foo.py");
        std::fs::write(foo_path.as_std_path(), "print('Version 1')")?;

        // Create a hardlink to `foo`
-        let bar_path = project.join("bar.py");
+        let bar_path = context.join_project_path("bar.py");
        std::fs::hard_link(foo_path.as_std_path(), bar_path.as_std_path())
            .context("Failed to create hard link from foo.py -> bar.py")?;

@@ -1078,12 +1127,12 @@ fn hard_links_in_project() -> anyhow::Result<()> {
    ignore = "windows doesn't support observing changes to hard linked files."
 )]
 fn hard_links_to_target_outside_project() -> anyhow::Result<()> {
-    let mut case = setup(|root: &SystemPath, project: &SystemPath| {
-        let foo_path = root.join("foo.py");
+    let mut case = setup(|context: &SetupContext| {
+        let foo_path = context.join_root_path("foo.py");
        std::fs::write(foo_path.as_std_path(), "print('Version 1')")?;

        // Create a hardlink to `foo`
-        let bar_path = project.join("bar.py");
+        let bar_path = context.join_project_path("bar.py");
        std::fs::hard_link(foo_path.as_std_path(), bar_path.as_std_path())
            .context("Failed to create hard link from foo.py -> bar.py")?;

@@ -1186,9 +1235,9 @@ mod unix {
        ignore = "FSEvents doesn't emit change events for symlinked directories outside of the watched paths."
    )]
    fn symlink_target_outside_watched_paths() -> anyhow::Result<()> {
-        let mut case = setup(|root: &SystemPath, project: &SystemPath| {
+        let mut case = setup(|context: &SetupContext| {
            // Set up the symlink target.
-            let link_target = root.join("bar");
+            let link_target = context.join_root_path("bar");
            std::fs::create_dir_all(link_target.as_std_path())
                .context("Failed to create link target directory")?;
            let baz_original = link_target.join("baz.py");
@@ -1196,7 +1245,7 @@ mod unix {
                .context("Failed to write link target file")?;

            // Create a symlink inside the project
-            let bar = project.join("bar");
+            let bar = context.join_project_path("bar");
            std::os::unix::fs::symlink(link_target.as_std_path(), bar.as_std_path())
                .context("Failed to create symlink to bar package")?;

@@ -1267,9 +1316,9 @@ mod unix {
    /// ```
    #[test]
    fn symlink_inside_project() -> anyhow::Result<()> {
-        let mut case = setup(|_root: &SystemPath, project: &SystemPath| {
+        let mut case = setup(|context: &SetupContext| {
            // Set up the symlink target.
-            let link_target = project.join("patched/bar");
+            let link_target = context.join_project_path("patched/bar");
            std::fs::create_dir_all(link_target.as_std_path())
                .context("Failed to create link target directory")?;
            let baz_original = link_target.join("baz.py");
@@ -1277,7 +1326,7 @@ mod unix {
                .context("Failed to write link target file")?;

            // Create a symlink inside site-packages
-            let bar_in_project = project.join("bar");
+            let bar_in_project = context.join_project_path("bar");
            std::os::unix::fs::symlink(link_target.as_std_path(), bar_in_project.as_std_path())
                .context("Failed to create symlink to bar package")?;

@@ -1358,9 +1407,9 @@ mod unix {
    #[test]
    fn symlinked_module_search_path() -> anyhow::Result<()> {
        let mut case = setup_with_options(
-            |root: &SystemPath, project: &SystemPath| {
+            |context: &SetupContext| {
                // Set up the symlink target.
-                let site_packages = root.join("site-packages");
+                let site_packages = context.join_root_path("site-packages");
                let bar = site_packages.join("bar");
                std::fs::create_dir_all(bar.as_std_path())
                    .context("Failed to create bar directory")?;
@@ -1369,7 +1418,8 @@ mod unix {
                    .context("Failed to write baz.py")?;

                // Symlink the site packages in the venv to the global site packages
-                let venv_site_packages = project.join(".venv/lib/python3.12/site-packages");
+                let venv_site_packages =
+                    context.join_project_path(".venv/lib/python3.12/site-packages");
                std::fs::create_dir_all(venv_site_packages.parent().unwrap())
                    .context("Failed to create .venv directory")?;
                std::os::unix::fs::symlink(
@@ -1380,7 +1430,7 @@ mod unix {

                Ok(())
            },
-            |_root, _project| {
+            |_context| {
                Some(Options {
                    environment: Some(EnvironmentOptions {
                        extra_paths: Some(vec![RelativePathBuf::cli(
@@ -1450,9 +1500,9 @@ mod unix {

 #[test]
 fn nested_projects_delete_root() -> anyhow::Result<()> {
-    let mut case = setup(|root: &SystemPath, project_root: &SystemPath| {
+    let mut case = setup(|context: &SetupContext| {
        std::fs::write(
-            project_root.join("pyproject.toml").as_std_path(),
+            context.join_project_path("pyproject.toml").as_std_path(),
            r#"
            [project]
            name = "inner"
@@ -1462,7 +1512,7 @@ fn nested_projects_delete_root() -> anyhow::Result<()> {
        )?;

        std::fs::write(
-            root.join("pyproject.toml").as_std_path(),
+            context.join_root_path("pyproject.toml").as_std_path(),
            r#"
            [project]
            name = "outer"
@@ -1487,3 +1537,79 @@ fn nested_projects_delete_root() -> anyhow::Result<()> {

    Ok(())
 }
+
+#[test]
+fn changes_to_user_configuration() -> anyhow::Result<()> {
+    let mut _config_dir_override: Option<UserConfigDirectoryOverrideGuard> = None;
+
+    let mut case = setup(|context: &SetupContext| {
+        std::fs::write(
+            context.join_project_path("pyproject.toml").as_std_path(),
+            r#"
+            [project]
+            name = "test"
+            "#,
+        )?;
+
+        std::fs::write(
+            context.join_project_path("foo.py").as_std_path(),
+            "a = 10 / 0",
+        )?;
+
+        let config_directory = context.join_root_path("home/.config");
+        std::fs::create_dir_all(config_directory.join("knot").as_std_path())?;
+        std::fs::write(
+            config_directory.join("knot/knot.toml").as_std_path(),
+            r#"
+            [rules]
+            division-by-zero = "ignore"
+            "#,
+        )?;
+
+        _config_dir_override = Some(
+            context
+                .system()
+                .with_user_config_directory(Some(config_directory)),
+        );
+
+        Ok(())
+    })?;
+
+    let foo = case
+        .system_file(case.project_path("foo.py"))
+        .expect("foo.py to exist");
+    let diagnostics = case
+        .db()
+        .check_file(foo)
+        .context("Failed to check project.")?;
+
+    assert!(
+        diagnostics.is_empty(),
+        "Expected no diagnostics but got: {diagnostics:#?}"
+    );
+
+    // Enable division-by-zero in the user configuration with warning severity
+    update_file(
+        case.root_path().join("home/.config/knot/knot.toml"),
+        r#"
+        [rules]
+        division-by-zero = "warn"
+        "#,
+    )?;
+
+    let changes = case.stop_watch(event_for_file("knot.toml"));
+
+    case.apply_changes(changes);
+
+    let diagnostics = case
+        .db()
+        .check_file(foo)
+        .context("Failed to check project.")?;
+
+    assert!(
+        diagnostics.len() == 1,
+        "Expected exactly one diagnostic but got: {diagnostics:#?}"
+    );
+
+    Ok(())
+}
--- a/crates/red_knot_project/Cargo.toml
+++ b/crates/red_knot_project/Cargo.toml
@@ -13,7 +13,7 @@ license.workspace = true

 [dependencies]
 ruff_cache = { workspace = true }
-ruff_db = { workspace = true, features = ["os", "cache", "serde"] }
+ruff_db = { workspace = true, features = ["cache", "serde"] }
 ruff_macros = { workspace = true }
 ruff_python_ast = { workspace = true, features = ["serde"] }
 ruff_text_size = { workspace = true }
@@ -24,7 +24,7 @@ anyhow = { workspace = true }
 crossbeam = { workspace = true }
 glob = { workspace = true }
 notify = { workspace = true }
-pep440_rs = { workspace = true }
+pep440_rs = { workspace = true, features = ["version-ranges"] }
 rayon = { workspace = true }
 rustc-hash = { workspace = true }
 salsa = { workspace = true }
--- a/crates/red_knot_project/src/combine.rs
+++ b/crates/red_knot_project/src/combine.rs
@@ -1,7 +1,8 @@
 use std::{collections::HashMap, hash::BuildHasher};

-use red_knot_python_semantic::{PythonPlatform, PythonVersion, SitePackages};
+use red_knot_python_semantic::{PythonPlatform, SitePackages};
 use ruff_db::system::SystemPathBuf;
+use ruff_python_ast::python_version::PythonVersion;

 /// Combine two values, preferring the values in `self`.
 ///
--- a/crates/red_knot_project/src/db.rs
+++ b/crates/red_knot_project/src/db.rs
@@ -114,8 +114,8 @@ impl SemanticDb for ProjectDatabase {
        project.is_file_open(self, file)
    }

-    fn rule_selection(&self) -> &RuleSelection {
-        self.project().rule_selection(self)
+    fn rule_selection(&self) -> Arc<RuleSelection> {
+        self.project().rules(self)
    }

    fn lint_registry(&self) -> &LintRegistry {
@@ -186,7 +186,6 @@ pub(crate) mod tests {
        files: Files,
        system: TestSystem,
        vendored: VendoredFileSystem,
-        rule_selection: RuleSelection,
        project: Option<Project>,
    }

@@ -198,7 +197,6 @@ pub(crate) mod tests {
                vendored: red_knot_vendored::file_system().clone(),
                files: Files::default(),
                events: Arc::default(),
-                rule_selection: RuleSelection::from_registry(&DEFAULT_LINT_REGISTRY),
                project: None,
            };

@@ -270,8 +268,8 @@ pub(crate) mod tests {
            !file.path(self).is_vendored_path()
        }

-        fn rule_selection(&self) -> &RuleSelection {
-            &self.rule_selection
+        fn rule_selection(&self) -> Arc<RuleSelection> {
+            self.project().rules(self)
        }

        fn lint_registry(&self) -> &LintRegistry {
--- a/crates/red_knot_project/src/db/changes.rs
+++ b/crates/red_knot_project/src/db/changes.rs
@@ -8,6 +8,7 @@ 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 as _;
+use ruff_python_ast::PySourceType;
 use rustc_hash::FxHashSet;

 impl ProjectDatabase {
@@ -47,7 +48,7 @@ impl ProjectDatabase {
            if let Some(path) = change.system_path() {
                if matches!(
                    path.file_name(),
-                    Some(".gitignore" | ".ignore" | "ruff.toml" | ".ruff.toml" | "pyproject.toml")
+                    Some(".gitignore" | ".ignore" | "knot.toml" | "pyproject.toml")
                ) {
                    // Changes to ignore files or settings can change the project structure or add/remove files.
                    project_changed = true;
@@ -144,6 +145,12 @@ impl ProjectDatabase {
                        metadata.apply_cli_options(cli_options.clone());
                    }

+                    if let Err(error) = metadata.apply_configuration_files(self.system()) {
+                        tracing::error!(
+                            "Failed to apply configuration files, continuing without applying them: {error}"
+                        );
+                    }
+
                    let program_settings = metadata.to_program_settings(self.system());

                    let program = Program::get(self);
@@ -201,9 +208,16 @@ impl ProjectDatabase {
                        return WalkState::Continue;
                    }

-                    let mut paths = added_paths.lock().unwrap();
+                    if entry
+                        .path()
+                        .extension()
+                        .and_then(PySourceType::try_from_extension)
+                        .is_some()
+                    {
+                        let mut paths = added_paths.lock().unwrap();

-                    paths.push(entry.into_path());
+                        paths.push(entry.into_path());
+                    }

                    WalkState::Continue
                })
--- a/crates/red_knot_project/src/lib.rs
+++ b/crates/red_knot_project/src/lib.rs
@@ -3,18 +3,18 @@
 use crate::metadata::options::OptionDiagnostic;
 pub use db::{Db, ProjectDatabase};
 use files::{Index, Indexed, IndexedFiles};
+use metadata::settings::Settings;
 pub use metadata::{ProjectDiscoveryError, ProjectMetadata};
 use red_knot_python_semantic::lint::{LintRegistry, LintRegistryBuilder, RuleSelection};
 use red_knot_python_semantic::register_lints;
 use red_knot_python_semantic::types::check_types;
-use ruff_db::diagnostic::{Diagnostic, DiagnosticId, ParseDiagnostic, Severity};
+use ruff_db::diagnostic::{Diagnostic, DiagnosticId, ParseDiagnostic, Severity, Span};
 use ruff_db::files::{system_path_to_file, File};
 use ruff_db::parsed::parsed_module;
 use ruff_db::source::{source_text, SourceTextError};
 use ruff_db::system::walk_directory::WalkState;
 use ruff_db::system::{FileType, SystemPath};
 use ruff_python_ast::PySourceType;
-use ruff_text_size::TextRange;
 use rustc_hash::{FxBuildHasher, FxHashSet};
 use salsa::Durability;
 use salsa::Setter;
@@ -66,12 +66,22 @@ pub struct Project {
    /// The metadata describing the project, including the unresolved options.
    #[return_ref]
    pub metadata: ProjectMetadata,
+
+    /// The resolved project settings.
+    #[return_ref]
+    pub settings: Settings,
+
+    /// Diagnostics that were generated when resolving the project settings.
+    #[return_ref]
+    settings_diagnostics: Vec<OptionDiagnostic>,
 }

 #[salsa::tracked]
 impl Project {
    pub fn from_metadata(db: &dyn Db, metadata: ProjectMetadata) -> Self {
-        Project::builder(metadata)
+        let (settings, settings_diagnostics) = metadata.options().to_settings(db);
+
+        Project::builder(metadata, settings, settings_diagnostics)
            .durability(Durability::MEDIUM)
            .open_fileset_durability(Durability::LOW)
            .file_set_durability(Durability::LOW)
@@ -86,30 +96,37 @@ impl Project {
        self.metadata(db).name()
    }

+    /// Returns the resolved linter rules for the project.
+    ///
+    /// This is a salsa query to prevent re-computing queries if other, unrelated
+    /// settings change. For example, we don't want that changing the terminal settings
+    /// invalidates any type checking queries.
+    #[salsa::tracked]
+    pub fn rules(self, db: &dyn Db) -> Arc<RuleSelection> {
+        self.settings(db).to_rules()
+    }
+
    pub fn reload(self, db: &mut dyn Db, metadata: ProjectMetadata) {
        tracing::debug!("Reloading project");
        assert_eq!(self.root(db), metadata.root());

        if &metadata != self.metadata(db) {
+            let (settings, settings_diagnostics) = metadata.options().to_settings(db);
+
+            if self.settings(db) != &settings {
+                self.set_settings(db).to(settings);
+            }
+
+            if self.settings_diagnostics(db) != &settings_diagnostics {
+                self.set_settings_diagnostics(db).to(settings_diagnostics);
+            }
+
            self.set_metadata(db).to(metadata);
        }

        self.reload_files(db);
    }

-    pub fn rule_selection(self, db: &dyn Db) -> &RuleSelection {
-        let (selection, _) = self.rule_selection_with_diagnostics(db);
-        selection
-    }
-
-    #[salsa::tracked(return_ref)]
-    fn rule_selection_with_diagnostics(
-        self,
-        db: &dyn Db,
-    ) -> (RuleSelection, Vec<OptionDiagnostic>) {
-        self.metadata(db).options().to_rule_selection(db)
-    }
-
    /// Checks all open files in the project and its dependencies.
    pub(crate) fn check(self, db: &ProjectDatabase) -> Vec<Box<dyn Diagnostic>> {
        let project_span = tracing::debug_span!("Project::check");
@@ -118,8 +135,7 @@ impl Project {
        tracing::debug!("Checking project '{name}'", name = self.name(db));

        let mut diagnostics: Vec<Box<dyn Diagnostic>> = Vec::new();
-        let (_, options_diagnostics) = self.rule_selection_with_diagnostics(db);
-        diagnostics.extend(options_diagnostics.iter().map(|diagnostic| {
+        diagnostics.extend(self.settings_diagnostics(db).iter().map(|diagnostic| {
            let diagnostic: Box<dyn Diagnostic> = Box::new(diagnostic.clone());
            diagnostic
        }));
@@ -151,9 +167,8 @@ impl Project {
    }

    pub(crate) fn check_file(self, db: &dyn Db, file: File) -> Vec<Box<dyn Diagnostic>> {
-        let (_, options_diagnostics) = self.rule_selection_with_diagnostics(db);
-
-        let mut file_diagnostics: Vec<_> = options_diagnostics
+        let mut file_diagnostics: Vec<_> = self
+            .settings_diagnostics(db)
            .iter()
            .map(|diagnostic| {
                let diagnostic: Box<dyn Diagnostic> = Box::new(diagnostic.clone());
@@ -329,7 +344,13 @@ fn check_file_impl(db: &dyn Db, file: File) -> Vec<Box<dyn Diagnostic>> {
        boxed
    }));

-    diagnostics.sort_unstable_by_key(|diagnostic| diagnostic.range().unwrap_or_default().start());
+    diagnostics.sort_unstable_by_key(|diagnostic| {
+        diagnostic
+            .span()
+            .and_then(|span| span.range())
+            .unwrap_or_default()
+            .start()
+    });

    diagnostics
 }
@@ -442,12 +463,8 @@ impl Diagnostic for IOErrorDiagnostic {
        self.error.to_string().into()
    }

-    fn file(&self) -> Option<File> {
-        Some(self.file)
-    }
-
-    fn range(&self) -> Option<TextRange> {
-        None
+    fn span(&self) -> Option<Span> {
+        Some(Span::from(self.file))
    }

    fn severity(&self) -> Severity {
--- a/crates/red_knot_project/src/metadata.rs
+++ b/crates/red_knot_project/src/metadata.rs
@@ -1,3 +1,4 @@
+use configuration_file::{ConfigurationFile, ConfigurationFileError};
 use red_knot_python_semantic::ProgramSettings;
 use ruff_db::system::{System, SystemPath, SystemPathBuf};
 use ruff_python_ast::name::Name;
@@ -5,13 +6,15 @@ use std::sync::Arc;
 use thiserror::Error;

 use crate::combine::Combine;
-use crate::metadata::pyproject::{Project, PyProject, PyProjectError};
+use crate::metadata::pyproject::{Project, PyProject, PyProjectError, ResolveRequiresPythonError};
 use crate::metadata::value::ValueSource;
 use options::KnotTomlError;
 use options::Options;

+mod configuration_file;
 pub mod options;
 pub mod pyproject;
+pub mod settings;
 pub mod value;

 #[derive(Debug, PartialEq, Eq)]
@@ -23,6 +26,15 @@ pub struct ProjectMetadata {

    /// The raw options
    pub(super) options: Options,
+
+    /// Paths of configurations other than the project's configuration that were combined into [`Self::options`].
+    ///
+    /// This field stores the paths of the configuration files, mainly for
+    /// knowing which files to watch for changes.
+    ///
+    /// The path ordering doesn't imply precedence.
+    #[cfg_attr(test, serde(skip_serializing_if = "Vec::is_empty"))]
+    pub(super) extra_configuration_paths: Vec<SystemPathBuf>,
 }

 impl ProjectMetadata {
@@ -31,12 +43,16 @@ impl ProjectMetadata {
        Self {
            name,
            root,
+            extra_configuration_paths: Vec::default(),
            options: Options::default(),
        }
    }

    /// Loads a project from a `pyproject.toml` file.
-    pub(crate) fn from_pyproject(pyproject: PyProject, root: SystemPathBuf) -> Self {
+    pub(crate) fn from_pyproject(
+        pyproject: PyProject,
+        root: SystemPathBuf,
+    ) -> Result<Self, ResolveRequiresPythonError> {
        Self::from_options(
            pyproject
                .tool
@@ -49,21 +65,37 @@ impl ProjectMetadata {

    /// Loads a project from a set of options with an optional pyproject-project table.
    pub(crate) fn from_options(
-        options: Options,
+        mut options: Options,
        root: SystemPathBuf,
        project: Option<&Project>,
-    ) -> Self {
+    ) -> Result<Self, ResolveRequiresPythonError> {
        let name = project
-            .and_then(|project| project.name.as_ref())
-            .map(|name| Name::new(&***name))
+            .and_then(|project| project.name.as_deref())
+            .map(|name| Name::new(&**name))
            .unwrap_or_else(|| Name::new(root.file_name().unwrap_or("root")));

-        // TODO(https://github.com/astral-sh/ruff/issues/15491): Respect requires-python
-        Self {
+        // If the `options` don't specify a python version but the `project.requires-python` field is set,
+        // use that as a lower bound instead.
+        if let Some(project) = project {
+            if !options
+                .environment
+                .as_ref()
+                .is_some_and(|env| env.python_version.is_some())
+            {
+                if let Some(requires_python) = project.resolve_requires_python_lower_bound()? {
+                    let mut environment = options.environment.unwrap_or_default();
+                    environment.python_version = Some(requires_python);
+                    options.environment = Some(environment);
+                }
+            }
+        }
+
+        Ok(Self {
            name,
            root,
            options,
-        }
+            extra_configuration_paths: Vec::new(),
+        })
    }

    /// Discovers the closest project at `path` and returns its metadata.
@@ -131,19 +163,34 @@ impl ProjectMetadata {
                }

                tracing::debug!("Found project at '{}'", project_root);
-                return Ok(ProjectMetadata::from_options(
+
+                let metadata = ProjectMetadata::from_options(
                    options,
                    project_root.to_path_buf(),
                    pyproject
                        .as_ref()
                        .and_then(|pyproject| pyproject.project.as_ref()),
-                ));
+                )
+                .map_err(|err| {
+                    ProjectDiscoveryError::InvalidRequiresPythonConstraint {
+                        source: err,
+                        path: pyproject_path,
+                    }
+                })?;
+
+                return Ok(metadata);
            }

            if let Some(pyproject) = pyproject {
                let has_knot_section = pyproject.knot().is_some();
                let metadata =
-                    ProjectMetadata::from_pyproject(pyproject, project_root.to_path_buf());
+                    ProjectMetadata::from_pyproject(pyproject, project_root.to_path_buf())
+                        .map_err(
+                            |err| ProjectDiscoveryError::InvalidRequiresPythonConstraint {
+                                source: err,
+                                path: pyproject_path,
+                            },
+                        )?;

                if has_knot_section {
                    tracing::debug!("Found project at '{}'", project_root);
@@ -191,6 +238,10 @@ impl ProjectMetadata {
        &self.options
    }

+    pub fn extra_configuration_paths(&self) -> &[SystemPathBuf] {
+        &self.extra_configuration_paths
+    }
+
    pub fn to_program_settings(&self, system: &dyn System) -> ProgramSettings {
        self.options.to_program_settings(self.root(), system)
    }
@@ -200,9 +251,31 @@ impl ProjectMetadata {
        self.options = options.combine(std::mem::take(&mut self.options));
    }

-    /// Combine the project options with the user options where project options take precedence.
-    pub fn apply_user_options(&mut self, options: Options) {
-        self.options.combine_with(options);
+    /// Applies the options from the configuration files to the project's options.
+    ///
+    /// This includes:
+    ///
+    /// * The user-level configuration
+    pub fn apply_configuration_files(
+        &mut self,
+        system: &dyn System,
+    ) -> Result<(), ConfigurationFileError> {
+        if let Some(user) = ConfigurationFile::user(system)? {
+            tracing::debug!(
+                "Applying user-level configuration loaded from `{path}`.",
+                path = user.path()
+            );
+            self.apply_configuration_file(user);
+        }
+
+        Ok(())
+    }
+
+    /// Applies a lower-precedence configuration files to the project's options.
+    fn apply_configuration_file(&mut self, options: ConfigurationFile) {
+        self.extra_configuration_paths
+            .push(options.path().to_owned());
+        self.options.combine_with(options.into_options());
    }
 }

@@ -222,16 +295,22 @@ pub enum ProjectDiscoveryError {
        source: Box<KnotTomlError>,
        path: SystemPathBuf,
    },
+
+    #[error("Invalid `requires-python` version specifier (`{path}`): {source}")]
+    InvalidRequiresPythonConstraint {
+        source: ResolveRequiresPythonError,
+        path: SystemPathBuf,
+    },
 }

 #[cfg(test)]
 mod tests {
    //! Integration tests for project discovery

-    use crate::snapshot_project;
    use anyhow::{anyhow, Context};
    use insta::assert_ron_snapshot;
    use ruff_db::system::{SystemPathBuf, TestSystem};
+    use ruff_python_ast::python_version::PythonVersion;

    use crate::{ProjectDiscoveryError, ProjectMetadata};

@@ -250,7 +329,15 @@ mod tests {

        assert_eq!(project.root(), &*root);

-        snapshot_project!(project);
+        with_escaped_paths(|| {
+            assert_ron_snapshot!(&project, @r#"
+                ProjectMetadata(
+                  name: Name("app"),
+                  root: "/app",
+                  options: Options(),
+                )
+            "#);
+        });

        Ok(())
    }
@@ -279,7 +366,16 @@ mod tests {
            ProjectMetadata::discover(&root, &system).context("Failed to discover project")?;

        assert_eq!(project.root(), &*root);
-        snapshot_project!(project);
+
+        with_escaped_paths(|| {
+            assert_ron_snapshot!(&project, @r#"
+                ProjectMetadata(
+                  name: Name("backend"),
+                  root: "/app",
+                  options: Options(),
+                )
+            "#);
+        });

        // Discovering the same package from a subdirectory should give the same result
        let from_src = ProjectMetadata::discover(&root.join("db"), &system)
@@ -362,7 +458,19 @@ expected `.`, `]`

        let sub_project = ProjectMetadata::discover(&root.join("packages/a"), &system)?;

-        snapshot_project!(sub_project);
+        with_escaped_paths(|| {
+            assert_ron_snapshot!(sub_project, @r#"
+            ProjectMetadata(
+              name: Name("nested-project"),
+              root: "/app/packages/a",
+              options: Options(
+                src: Some(SrcOptions(
+                  root: Some("src"),
+                )),
+              ),
+            )
+            "#);
+        });

        Ok(())
    }
@@ -400,7 +508,19 @@ expected `.`, `]`

        let root = ProjectMetadata::discover(&root, &system)?;

-        snapshot_project!(root);
+        with_escaped_paths(|| {
+            assert_ron_snapshot!(root, @r#"
+                              ProjectMetadata(
+                                name: Name("project-root"),
+                                root: "/app",
+                                options: Options(
+                                  src: Some(SrcOptions(
+                                    root: Some("src"),
+                                  )),
+                                ),
+                              )
+                              "#);
+        });

        Ok(())
    }
@@ -432,7 +552,15 @@ expected `.`, `]`

        let sub_project = ProjectMetadata::discover(&root.join("packages/a"), &system)?;

-        snapshot_project!(sub_project);
+        with_escaped_paths(|| {
+            assert_ron_snapshot!(sub_project, @r#"
+            ProjectMetadata(
+              name: Name("nested-project"),
+              root: "/app/packages/a",
+              options: Options(),
+            )
+            "#);
+        });

        Ok(())
    }
@@ -467,7 +595,19 @@ expected `.`, `]`

        let root = ProjectMetadata::discover(&root.join("packages/a"), &system)?;

-        snapshot_project!(root);
+        with_escaped_paths(|| {
+            assert_ron_snapshot!(root, @r#"
+            ProjectMetadata(
+              name: Name("project-root"),
+              root: "/app",
+              options: Options(
+                environment: Some(EnvironmentOptions(
+                  r#python-version: Some("3.10"),
+                )),
+              ),
+            )
+            "#);
+        });

        Ok(())
    }
@@ -487,27 +627,304 @@ expected `.`, `]`
                (
                    root.join("pyproject.toml"),
                    r#"
-                        [project]
-                        name = "super-app"
-                        requires-python = ">=3.12"
+                    [project]
+                    name = "super-app"
+                    requires-python = ">=3.12"

-                        [tool.knot.src]
-                        root = "this_option_is_ignored"
-                        "#,
+                    [tool.knot.src]
+                    root = "this_option_is_ignored"
+                    "#,
                ),
                (
                    root.join("knot.toml"),
                    r#"
-                        [src]
-                        root = "src"
-                        "#,
+                    [src]
+                    root = "src"
+                    "#,
                ),
            ])
            .context("Failed to write files")?;

        let root = ProjectMetadata::discover(&root, &system)?;

-        snapshot_project!(root);
+        with_escaped_paths(|| {
+            assert_ron_snapshot!(root, @r#"
+            ProjectMetadata(
+              name: Name("super-app"),
+              root: "/app",
+              options: Options(
+                environment: Some(EnvironmentOptions(
+                  r#python-version: Some("3.12"),
+                )),
+                src: Some(SrcOptions(
+                  root: Some("src"),
+                )),
+              ),
+            )
+            "#);
+        });
+
+        Ok(())
+    }
+    #[test]
+    fn requires_python_major_minor() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = ">=3.12"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let root = ProjectMetadata::discover(&root, &system)?;
+
+        assert_eq!(
+            root.options
+                .environment
+                .unwrap_or_default()
+                .python_version
+                .as_deref(),
+            Some(&PythonVersion::PY312)
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn requires_python_major_only() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = ">=3"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let root = ProjectMetadata::discover(&root, &system)?;
+
+        assert_eq!(
+            root.options
+                .environment
+                .unwrap_or_default()
+                .python_version
+                .as_deref(),
+            Some(&PythonVersion::from((3, 0)))
+        );
+
+        Ok(())
+    }
+
+    /// A `requires-python` constraint with major, minor and patch can be simplified
+    /// to major and minor (e.g. 3.12.1 -> 3.12).
+    #[test]
+    fn requires_python_major_minor_patch() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = ">=3.12.8"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let root = ProjectMetadata::discover(&root, &system)?;
+
+        assert_eq!(
+            root.options
+                .environment
+                .unwrap_or_default()
+                .python_version
+                .as_deref(),
+            Some(&PythonVersion::PY312)
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn requires_python_beta_version() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = ">= 3.13.0b0"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let root = ProjectMetadata::discover(&root, &system)?;
+
+        assert_eq!(
+            root.options
+                .environment
+                .unwrap_or_default()
+                .python_version
+                .as_deref(),
+            Some(&PythonVersion::PY313)
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn requires_python_greater_than_major_minor() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                # This is somewhat nonsensical because 3.12.1 > 3.12 is true.
+                # That's why simplifying the constraint to >= 3.12 is correct
+                requires-python = ">3.12"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let root = ProjectMetadata::discover(&root, &system)?;
+
+        assert_eq!(
+            root.options
+                .environment
+                .unwrap_or_default()
+                .python_version
+                .as_deref(),
+            Some(&PythonVersion::PY312)
+        );
+
+        Ok(())
+    }
+
+    /// `python-version` takes precedence if both `requires-python` and `python-version` are configured.
+    #[test]
+    fn requires_python_and_python_version() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = ">=3.12"
+
+                [tool.knot.environment]
+                python-version = "3.10"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let root = ProjectMetadata::discover(&root, &system)?;
+
+        assert_eq!(
+            root.options
+                .environment
+                .unwrap_or_default()
+                .python_version
+                .as_deref(),
+            Some(&PythonVersion::PY310)
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn requires_python_less_than() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = "<3.12"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let Err(error) = ProjectMetadata::discover(&root, &system) else {
+            return Err(anyhow!("Expected project discovery to fail because the `requires-python` doesn't specify a lower bound (it only specifies an upper bound)."));
+        };
+
+        assert_error_eq(&error, "Invalid `requires-python` version specifier (`/app/pyproject.toml`): value `<3.12` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `environment.python-version`.");
+
+        Ok(())
+    }
+
+    #[test]
+    fn requires_python_no_specifiers() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = ""
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let Err(error) = ProjectMetadata::discover(&root, &system) else {
+            return Err(anyhow!("Expected project discovery to fail because the `requires-python` specifiers are empty and don't define a lower bound."));
+        };
+
+        assert_error_eq(&error, "Invalid `requires-python` version specifier (`/app/pyproject.toml`): value `` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `environment.python-version`.");
+
+        Ok(())
+    }
+
+    #[test]
+    fn requires_python_too_large_major_version() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_file(
+                root.join("pyproject.toml"),
+                r#"
+                [project]
+                requires-python = ">=999.0"
+                "#,
+            )
+            .context("Failed to write file")?;
+
+        let Err(error) = ProjectMetadata::discover(&root, &system) else {
+            return Err(anyhow!("Expected project discovery to fail because of the requires-python major version that is larger than 255."));
+        };
+
+        assert_error_eq(&error, "Invalid `requires-python` version specifier (`/app/pyproject.toml`): The major version `999` is larger than the maximum supported value 255");

        Ok(())
    }
@@ -517,15 +934,12 @@ expected `.`, `]`
        assert_eq!(error.to_string().replace('\\', "/"), message);
    }

-    /// Snapshots a project but with all paths using unix separators.
-    #[macro_export]
-    macro_rules! snapshot_project {
-    ($project:expr) => {{
-        assert_ron_snapshot!($project,{
-            ".root" => insta::dynamic_redaction(|content, _content_path| {
-                content.as_str().unwrap().replace("\\", "/")
-            }),
+    fn with_escaped_paths<R>(f: impl FnOnce() -> R) -> R {
+        let mut settings = insta::Settings::clone_current();
+        settings.add_dynamic_redaction(".root", |content, _path| {
+            content.as_str().unwrap().replace('\\', "/")
        });
-    }};
-}
+
+        settings.bind(f)
+    }
 }
--- a/crates/red_knot_project/src/metadata/configuration_file.rs
+++ b/crates/red_knot_project/src/metadata/configuration_file.rs
@@ -0,0 +1,69 @@
+use std::sync::Arc;
+
+use ruff_db::system::{System, SystemPath, SystemPathBuf};
+use thiserror::Error;
+
+use crate::metadata::value::ValueSource;
+
+use super::options::{KnotTomlError, Options};
+
+/// A `knot.toml` configuration file with the options it contains.
+pub(crate) struct ConfigurationFile {
+    path: SystemPathBuf,
+    options: Options,
+}
+
+impl ConfigurationFile {
+    /// Loads the user-level configuration file if it exists.
+    ///
+    /// Returns `None` if the file does not exist or if the concept of user-level configurations
+    /// doesn't exist on `system`.
+    pub(crate) fn user(system: &dyn System) -> Result<Option<Self>, ConfigurationFileError> {
+        let Some(configuration_directory) = system.user_config_directory() else {
+            return Ok(None);
+        };
+
+        let knot_toml_path = configuration_directory.join("knot").join("knot.toml");
+
+        tracing::debug!(
+            "Searching for a user-level configuration at `{path}`",
+            path = &knot_toml_path
+        );
+
+        let Ok(knot_toml_str) = system.read_to_string(&knot_toml_path) else {
+            return Ok(None);
+        };
+
+        match Options::from_toml_str(
+            &knot_toml_str,
+            ValueSource::File(Arc::new(knot_toml_path.clone())),
+        ) {
+            Ok(options) => Ok(Some(Self {
+                path: knot_toml_path,
+                options,
+            })),
+            Err(error) => Err(ConfigurationFileError::InvalidKnotToml {
+                source: Box::new(error),
+                path: knot_toml_path,
+            }),
+        }
+    }
+
+    /// Returns the path to the configuration file.
+    pub(crate) fn path(&self) -> &SystemPath {
+        &self.path
+    }
+
+    pub(crate) fn into_options(self) -> Options {
+        self.options
+    }
+}
+
+#[derive(Debug, Error)]
+pub enum ConfigurationFileError {
+    #[error("{path} is not a valid `knot.toml`: {source}")]
+    InvalidKnotToml {
+        source: Box<KnotTomlError>,
+        path: SystemPathBuf,
+    },
+}
--- a/crates/red_knot_project/src/metadata/options.rs
+++ b/crates/red_knot_project/src/metadata/options.rs
@@ -1,20 +1,20 @@
 use crate::metadata::value::{RangedValue, RelativePathBuf, ValueSource, ValueSourceGuard};
 use crate::Db;
 use red_knot_python_semantic::lint::{GetLintError, Level, LintSource, RuleSelection};
-use red_knot_python_semantic::{
-    ProgramSettings, PythonPlatform, PythonVersion, SearchPathSettings, SitePackages,
-};
-use ruff_db::diagnostic::{Diagnostic, DiagnosticId, Severity};
-use ruff_db::files::{system_path_to_file, File};
+use red_knot_python_semantic::{ProgramSettings, PythonPlatform, SearchPathSettings, SitePackages};
+use ruff_db::diagnostic::{Diagnostic, DiagnosticId, Severity, Span};
+use ruff_db::files::system_path_to_file;
 use ruff_db::system::{System, SystemPath};
 use ruff_macros::Combine;
-use ruff_text_size::TextRange;
+use ruff_python_ast::python_version::PythonVersion;
 use rustc_hash::FxHashMap;
 use serde::{Deserialize, Serialize};
 use std::borrow::Cow;
 use std::fmt::Debug;
 use thiserror::Error;

+use super::settings::{Settings, TerminalSettings};
+
 /// The options for the project.
 #[derive(Debug, Default, Clone, PartialEq, Eq, Combine, Serialize, Deserialize)]
 #[serde(rename_all = "kebab-case", deny_unknown_fields)]
@@ -30,6 +30,9 @@ pub struct Options {
    /// Configures the enabled lints and their severity.
    #[serde(skip_serializing_if = "Option::is_none")]
    pub rules: Option<Rules>,
+
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub terminal: Option<TerminalOptions>,
 }

 impl Options {
@@ -110,7 +113,22 @@ impl Options {
    }

    #[must_use]
-    pub(crate) fn to_rule_selection(&self, db: &dyn Db) -> (RuleSelection, Vec<OptionDiagnostic>) {
+    pub(crate) fn to_settings(&self, db: &dyn Db) -> (Settings, Vec<OptionDiagnostic>) {
+        let (rules, diagnostics) = self.to_rule_selection(db);
+
+        let mut settings = Settings::new(rules);
+
+        if let Some(terminal) = self.terminal.as_ref() {
+            settings.set_terminal(TerminalSettings {
+                error_on_warning: terminal.error_on_warning.unwrap_or_default(),
+            });
+        }
+
+        (settings, diagnostics)
+    }
+
+    #[must_use]
+    fn to_rule_selection(&self, db: &dyn Db) -> (RuleSelection, Vec<OptionDiagnostic>) {
        let registry = db.lint_registry();
        let mut diagnostics = Vec::new();

@@ -169,7 +187,14 @@ impl Options {
                        ),
                    };

-                    diagnostics.push(diagnostic.with_file(file).with_range(rule_name.range()));
+                    let span = file.map(Span::from).map(|span| {
+                        if let Some(range) = rule_name.range() {
+                            span.with_range(range)
+                        } else {
+                            span
+                        }
+                    });
+                    diagnostics.push(diagnostic.with_span(span));
                }
            }
        }
@@ -244,6 +269,16 @@ impl FromIterator<(RangedValue<String>, RangedValue<Level>)> for Rules {
    }
 }

+#[derive(Debug, Default, Clone, Eq, PartialEq, Combine, Serialize, Deserialize)]
+#[serde(rename_all = "kebab-case", deny_unknown_fields)]
+#[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
+pub struct TerminalOptions {
+    /// Use exit code 1 if there are any warning-level diagnostics.
+    ///
+    /// Defaults to `false`.
+    pub error_on_warning: Option<bool>,
+}
+
 #[cfg(feature = "schemars")]
 mod schema {
    use crate::DEFAULT_LINT_REGISTRY;
@@ -318,8 +353,7 @@ pub struct OptionDiagnostic {
    id: DiagnosticId,
    message: String,
    severity: Severity,
-    file: Option<File>,
-    range: Option<TextRange>,
+    span: Option<Span>,
 }

 impl OptionDiagnostic {
@@ -328,21 +362,13 @@ impl OptionDiagnostic {
            id,
            message,
            severity,
-            file: None,
-            range: None,
+            span: None,
        }
    }

    #[must_use]
-    fn with_file(mut self, file: Option<File>) -> Self {
-        self.file = file;
-        self
-    }
-
-    #[must_use]
-    fn with_range(mut self, range: Option<TextRange>) -> Self {
-        self.range = range;
-        self
+    fn with_span(self, span: Option<Span>) -> Self {
+        OptionDiagnostic { span, ..self }
    }
 }

@@ -355,12 +381,8 @@ impl Diagnostic for OptionDiagnostic {
        Cow::Borrowed(&self.message)
    }

-    fn file(&self) -> Option<File> {
-        self.file
-    }
-
-    fn range(&self) -> Option<TextRange> {
-        self.range
+    fn span(&self) -> Option<Span> {
+        self.span.clone()
    }

    fn severity(&self) -> Severity {
--- a/crates/red_knot_project/src/metadata/pyproject.rs
+++ b/crates/red_knot_project/src/metadata/pyproject.rs
@@ -1,10 +1,11 @@
-use pep440_rs::{Version, VersionSpecifiers};
-use serde::{Deserialize, Deserializer, Serialize};
-use std::ops::Deref;
-use thiserror::Error;
-
 use crate::metadata::options::Options;
 use crate::metadata::value::{RangedValue, ValueSource, ValueSourceGuard};
+use pep440_rs::{release_specifiers_to_ranges, Version, VersionSpecifiers};
+use ruff_python_ast::python_version::PythonVersion;
+use serde::{Deserialize, Deserializer, Serialize};
+use std::collections::Bound;
+use std::ops::Deref;
+use thiserror::Error;

 /// A `pyproject.toml` as specified in PEP 517.
 #[derive(Deserialize, Serialize, Debug, Default, Clone)]
@@ -55,6 +56,73 @@ pub struct Project {
    pub requires_python: Option<RangedValue<VersionSpecifiers>>,
 }

+impl Project {
+    pub(super) fn resolve_requires_python_lower_bound(
+        &self,
+    ) -> Result<Option<RangedValue<PythonVersion>>, ResolveRequiresPythonError> {
+        let Some(requires_python) = self.requires_python.as_ref() else {
+            return Ok(None);
+        };
+
+        tracing::debug!("Resolving requires-python constraint: `{requires_python}`");
+
+        let ranges = release_specifiers_to_ranges((**requires_python).clone());
+        let Some((lower, _)) = ranges.bounding_range() else {
+            return Ok(None);
+        };
+
+        let version = match lower {
+            // Ex) `>=3.10.1` -> `>=3.10`
+            Bound::Included(version) => version,
+
+            // Ex) `>3.10.1` -> `>=3.10` or `>3.10` -> `>=3.10`
+            // The second example looks obscure at first but it is required because
+            // `3.10.1 > 3.10` is true but we only have two digits here. So including 3.10 is the
+            // right move. Overall, using `>` without a patch release is most likely bogus.
+            Bound::Excluded(version) => version,
+
+            // Ex) `<3.10` or ``
+            Bound::Unbounded => {
+                return Err(ResolveRequiresPythonError::NoLowerBound(
+                    requires_python.to_string(),
+                ))
+            }
+        };
+
+        // Take the major and minor version
+        let mut versions = version.release().iter().take(2);
+
+        let Some(major) = versions.next().copied() else {
+            return Ok(None);
+        };
+
+        let minor = versions.next().copied().unwrap_or_default();
+
+        tracing::debug!("Resolved requires-python constraint to: {major}.{minor}");
+
+        let major =
+            u8::try_from(major).map_err(|_| ResolveRequiresPythonError::TooLargeMajor(major))?;
+        let minor =
+            u8::try_from(minor).map_err(|_| ResolveRequiresPythonError::TooLargeMajor(minor))?;
+
+        Ok(Some(
+            requires_python
+                .clone()
+                .map_value(|_| PythonVersion::from((major, minor))),
+        ))
+    }
+}
+
+#[derive(Debug, Error)]
+pub enum ResolveRequiresPythonError {
+    #[error("The major version `{0}` is larger than the maximum supported value 255")]
+    TooLargeMajor(u64),
+    #[error("The minor version `{0}` is larger than the maximum supported value 255")]
+    TooLargeMinor(u64),
+    #[error("value `{0}` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `environment.python-version`.")]
+    NoLowerBound(String),
+}
+
 #[derive(Deserialize, Serialize, Debug, Clone, PartialEq, Eq)]
 #[serde(rename_all = "kebab-case")]
 pub struct Tool {
--- a/crates/red_knot_project/src/metadata/settings.rs
+++ b/crates/red_knot_project/src/metadata/settings.rs
@@ -0,0 +1,53 @@
+use std::sync::Arc;
+
+use red_knot_python_semantic::lint::RuleSelection;
+
+/// The resolved [`super::Options`] for the project.
+///
+/// Unlike [`super::Options`], the struct has default values filled in and
+/// uses representations that are optimized for reads (instead of preserving the source representation).
+/// It's also not required that this structure precisely resembles the TOML schema, although
+/// it's encouraged to use a similar structure.
+///
+/// It's worth considering to adding a salsa query for specific settings to
+/// limit the blast radius when only some settings change. For example,
+/// changing the terminal settings shouldn't invalidate any core type-checking queries.
+/// This can be achieved by adding a salsa query for the type checking specific settings.
+///
+/// Settings that are part of [`red_knot_python_semantic::ProgramSettings`] are not included here.
+#[derive(Clone, Debug, Eq, PartialEq)]
+pub struct Settings {
+    rules: Arc<RuleSelection>,
+
+    terminal: TerminalSettings,
+}
+
+impl Settings {
+    pub fn new(rules: RuleSelection) -> Self {
+        Self {
+            rules: Arc::new(rules),
+            terminal: TerminalSettings::default(),
+        }
+    }
+
+    pub fn rules(&self) -> &RuleSelection {
+        &self.rules
+    }
+
+    pub fn to_rules(&self) -> Arc<RuleSelection> {
+        self.rules.clone()
+    }
+
+    pub fn terminal(&self) -> &TerminalSettings {
+        &self.terminal
+    }
+
+    pub fn set_terminal(&mut self, terminal: TerminalSettings) {
+        self.terminal = terminal;
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Default)]
+pub struct TerminalSettings {
+    pub error_on_warning: bool,
+}
--- a/crates/red_knot_project/src/metadata/value.rs
+++ b/crates/red_knot_project/src/metadata/value.rs
@@ -118,6 +118,15 @@ impl<T> RangedValue<T> {
        self
    }

+    #[must_use]
+    pub fn map_value<R>(self, f: impl FnOnce(T) -> R) -> RangedValue<R> {
+        RangedValue {
+            value: f(self.value),
+            source: self.source,
+            range: self.range,
+        }
+    }
+
    pub fn into_inner(self) -> T {
        self.value
    }
--- a/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_in_root_project.snap
+++ b/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_in_root_project.snap
@@ -1,13 +0,0 @@
---
-source: crates/red_knot_project/src/metadata.rs
-expression: root
---
-ProjectMetadata(
-  name: Name("project-root"),
-  root: "/app",
-  options: Options(
-    src: Some(SrcOptions(
-      root: Some("src"),
-    )),
-  ),
-)
--- a/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_in_sub_project.snap
+++ b/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_in_sub_project.snap
@@ -1,13 +0,0 @@
---
-source: crates/red_knot_project/src/metadata.rs
-expression: sub_project
---
-ProjectMetadata(
-  name: Name("nested-project"),
-  root: "/app/packages/a",
-  options: Options(
-    src: Some(SrcOptions(
-      root: Some("src"),
-    )),
-  ),
-)
--- a/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_with_outer_knot_section.snap
+++ b/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_with_outer_knot_section.snap
@@ -1,13 +0,0 @@
---
-source: crates/red_knot_project/src/metadata.rs
-expression: root
---
-ProjectMetadata(
-  name: Name("project-root"),
-  root: "/app",
-  options: Options(
-    environment: Some(EnvironmentOptions(
-      r#python-version: Some("3.10"),
-    )),
-  ),
-)
--- a/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_without_knot_sections.snap
+++ b/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__nested_projects_without_knot_sections.snap
@@ -1,9 +0,0 @@
---
-source: crates/red_knot_project/src/metadata.rs
-expression: sub_project
---
-ProjectMetadata(
-  name: Name("nested-project"),
-  root: "/app/packages/a",
-  options: Options(),
-)
--- a/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__project_with_knot_and_pyproject_toml.snap
+++ b/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__project_with_knot_and_pyproject_toml.snap
@@ -1,13 +0,0 @@
---
-source: crates/red_knot_project/src/metadata.rs
-expression: root
---
-ProjectMetadata(
-  name: Name("super-app"),
-  root: "/app",
-  options: Options(
-    src: Some(SrcOptions(
-      root: Some("src"),
-    )),
-  ),
-)
--- a/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__project_with_pyproject.snap
+++ b/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__project_with_pyproject.snap
@@ -1,9 +0,0 @@
---
-source: crates/red_knot_project/src/metadata.rs
-expression: project
---
-ProjectMetadata(
-  name: Name("backend"),
-  root: "/app",
-  options: Options(),
-)
--- a/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__project_without_pyproject.snap
+++ b/crates/red_knot_project/src/snapshots/red_knot_projectmetadatatests__project_without_pyproject.snap
@@ -1,9 +0,0 @@
---
-source: crates/red_knot_project/src/metadata.rs
-expression: project
---
-ProjectMetadata(
-  name: Name("app"),
-  root: "/app",
-  options: Options(),
-)
--- a/crates/red_knot_project/src/watch/project_watcher.rs
+++ b/crates/red_knot_project/src/watch/project_watcher.rs
@@ -73,6 +73,13 @@ impl ProjectWatcher {
            .canonicalize_path(&project_path)
            .unwrap_or(project_path);

+        let config_paths = db
+            .project()
+            .metadata(db)
+            .extra_configuration_paths()
+            .iter()
+            .cloned();
+
        // Find the non-overlapping module search paths and filter out paths that are already covered by the project.
        // Module search paths are already canonicalized.
        let unique_module_paths = ruff_db::system::deduplicate_nested_paths(
@@ -83,8 +90,11 @@ impl ProjectWatcher {
        .map(SystemPath::to_path_buf);

        // Now add the new paths, first starting with the project path and then
-        // adding the library search paths.
-        for path in std::iter::once(project_path).chain(unique_module_paths) {
+        // adding the library search paths, and finally the paths for configurations.
+        for path in std::iter::once(project_path)
+            .chain(unique_module_paths)
+            .chain(config_paths)
+        {
            // Log a warning. It's not worth aborting if registering a single folder fails because
            // Ruff otherwise stills works as expected.
            if let Err(error) = self.watcher.watch(&path) {
--- a/crates/red_knot_python_semantic/Cargo.toml
+++ b/crates/red_knot_python_semantic/Cargo.toml
@@ -12,9 +12,9 @@ license = { workspace = true }

 [dependencies]
 ruff_db = { workspace = true }
-ruff_index = { workspace = true }
+ruff_index = { workspace = true, features = ["salsa"] }
 ruff_macros = { workspace = true }
-ruff_python_ast = { workspace = true }
+ruff_python_ast = { workspace = true, features = ["salsa"] }
 ruff_python_parser = { workspace = true }
 ruff_python_stdlib = { workspace = true }
 ruff_source_file = { workspace = true }
@@ -31,7 +31,7 @@ drop_bomb = { workspace = true }
 indexmap = { workspace = true }
 itertools = { workspace = true }
 ordermap = { workspace = true }
-salsa = { workspace = true }
+salsa = { workspace = true, features = ["compact_str"] }
 thiserror = { workspace = true }
 tracing = { workspace = true }
 rustc-hash = { workspace = true }
@@ -44,7 +44,7 @@ test-case = { workspace = true }
 memchr = { workspace = true }

 [dev-dependencies]
-ruff_db = { workspace = true, features = ["os", "testing"] }
+ruff_db = { workspace = true, features = ["testing", "os"] }
 ruff_python_parser = { workspace = true }
 red_knot_test = { workspace = true }
 red_knot_vendored = { workspace = true }
@@ -57,7 +57,7 @@ quickcheck = { version = "1.0.3", default-features = false }
 quickcheck_macros = { version = "1.0.0" }

 [features]
-serde = ["ruff_db/serde", "dep:serde"]
+serde = ["ruff_db/serde", "dep:serde", "ruff_python_ast/serde"]

 [lints]
 workspace = true
--- a/crates/red_knot_python_semantic/resources/mdtest/annotations/int_float_complex.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/annotations/int_float_complex.md
@@ -0,0 +1,90 @@
+# Special cases for int/float/complex in annotations
+
+In order to support common use cases, an annotation of `float` actually means `int | float`, and an
+annotation of `complex` actually means `int | float | complex`. See
+[the specification](https://typing.readthedocs.io/en/latest/spec/special-types.html#special-cases-for-float-and-complex)
+
+## float
+
+An annotation of `float` means `int | float`, so `int` is assignable to it:
+
+```py
+def takes_float(x: float):
+    pass
+
+def passes_int_to_float(x: int):
+    # no error!
+    takes_float(x)
+```
+
+It also applies to variable annotations:
+
+```py
+def assigns_int_to_float(x: int):
+    # no error!
+    y: float = x
+```
+
+It doesn't work the other way around:
+
+```py
+def takes_int(x: int):
+    pass
+
+def passes_float_to_int(x: float):
+    # error: [invalid-argument-type]
+    takes_int(x)
+
+def assigns_float_to_int(x: float):
+    # error: [invalid-assignment]
+    y: int = x
+```
+
+Unlike other type checkers, we choose not to obfuscate this special case by displaying `int | float`
+as just `float`; we display the actual type:
+
+```py
+def f(x: float):
+    reveal_type(x)  # revealed: int | float
+```
+
+## complex
+
+An annotation of `complex` means `int | float | complex`, so `int` and `float` are both assignable
+to it (but not the other way around):
+
+```py
+def takes_complex(x: complex):
+    pass
+
+def passes_to_complex(x: float, y: int):
+    # no errors!
+    takes_complex(x)
+    takes_complex(y)
+
+def assigns_to_complex(x: float, y: int):
+    # no errors!
+    a: complex = x
+    b: complex = y
+
+def takes_int(x: int):
+    pass
+
+def takes_float(x: float):
+    pass
+
+def passes_complex(x: complex):
+    # error: [invalid-argument-type]
+    takes_int(x)
+    # error: [invalid-argument-type]
+    takes_float(x)
+
+def assigns_complex(x: complex):
+    # error: [invalid-assignment]
+    y: int = x
+    # error: [invalid-assignment]
+    z: float = x
+
+def f(x: complex):
+    reveal_type(x)  # revealed: int | float | complex
+```
--- a/crates/red_knot_python_semantic/resources/mdtest/annotations/union.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/annotations/union.md
@@ -9,9 +9,9 @@ from typing import Union

 a: Union[int, str]
 a1: Union[int, bool]
-a2: Union[int, Union[float, str]]
+a2: Union[int, Union[bytes, str]]
 a3: Union[int, None]
-a4: Union[Union[float, str]]
+a4: Union[Union[bytes, str]]
 a5: Union[int]
 a6: Union[()]

@@ -21,11 +21,11 @@ def f():
    # Since bool is a subtype of int we simplify to int here. But we do allow assigning boolean values (see below).
    # revealed: int
    reveal_type(a1)
-    # revealed: int | float | str
+    # revealed: int | bytes | str
    reveal_type(a2)
    # revealed: int | None
    reveal_type(a3)
-    # revealed: float | str
+    # revealed: bytes | str
    reveal_type(a4)
    # revealed: int
    reveal_type(a5)
--- a/crates/red_knot_python_semantic/resources/mdtest/assignment/augmented.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/assignment/augmented.md
@@ -9,7 +9,7 @@ reveal_type(x)  # revealed: Literal[2]

 x = 1.0
 x /= 2
-reveal_type(x)  # revealed: float
+reveal_type(x)  # revealed: int | float
 ```

 ## Dunder methods
@@ -24,12 +24,12 @@ x -= 1
 reveal_type(x)  # revealed: str

 class C:
-    def __iadd__(self, other: str) -> float:
-        return 1.0
+    def __iadd__(self, other: str) -> int:
+        return 1

 x = C()
 x += "Hello"
-reveal_type(x)  # revealed: float
+reveal_type(x)  # revealed: int
 ```

 ## Unsupported types
@@ -130,10 +130,10 @@ def _(flag: bool):
    if flag:
        f = Foo()
    else:
-        f = 42.0
+        f = 42
    f += 12

-    reveal_type(f)  # revealed: str | float
+    reveal_type(f)  # revealed: str | Literal[54]
 ```

 ## Partially bound target union with `__add__`
--- a/crates/red_knot_python_semantic/resources/mdtest/attributes.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/attributes.md
@@ -804,6 +804,67 @@ def _(flag: bool, flag1: bool, flag2: bool):
    reveal_type(C.x)  # revealed: Unknown | Literal[1, 2, 3]
 ```

+### Attribute possibly unbound on a subclass but not on a superclass
+
+```py
+def _(flag: bool):
+    class Foo:
+        x = 1
+
+    class Bar(Foo):
+        if flag:
+            x = 2
+
+    reveal_type(Bar.x)  # revealed: Unknown | Literal[2, 1]
+```
+
+### Attribute possibly unbound on a subclass and on a superclass
+
+```py
+def _(flag: bool):
+    class Foo:
+        if flag:
+            x = 1
+
+    class Bar(Foo):
+        if flag:
+            x = 2
+
+    # error: [possibly-unbound-attribute]
+    reveal_type(Bar.x)  # revealed: Unknown | Literal[2, 1]
+```
+
+### Attribute access on `Any`
+
+The union of the set of types that `Any` could materialise to is equivalent to `object`. It follows
+from this that attribute access on `Any` resolves to `Any` if the attribute does not exist on
+`object` -- but if the attribute *does* exist on `object`, the type of the attribute is
+`<type as it exists on object> & Any`.
+
+```py
+from typing import Any
+
+class Foo(Any): ...
+
+reveal_type(Foo.bar)  # revealed: Any
+reveal_type(Foo.__repr__)  # revealed: Literal[__repr__] & Any
+```
+
+Similar principles apply if `Any` appears in the middle of an inheritance hierarchy:
+
+```py
+from typing import ClassVar, Literal
+
+class A:
+    x: ClassVar[Literal[1]] = 1
+
+class B(Any): ...
+class C(B, A): ...
+
+reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Literal[B], Any, Literal[A], Literal[object]]
+reveal_type(C.x)  # revealed: Literal[1] & Any
+```
+
 ### Unions with all paths unbound

 If the symbol is unbound in all elements of the union, we detect that:
--- a/crates/red_knot_python_semantic/resources/mdtest/binary/booleans.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/binary/booleans.md
@@ -56,7 +56,7 @@ def _(a: bool):
        reveal_type(x - a)  # revealed: int
        reveal_type(x * a)  # revealed: int
        reveal_type(x // a)  # revealed: int
-        reveal_type(x / a)  # revealed: float
+        reveal_type(x / a)  # revealed: int | float
        reveal_type(x % a)  # revealed: int

    def rhs_is_int(x: int):
@@ -64,7 +64,7 @@ def _(a: bool):
        reveal_type(a - x)  # revealed: int
        reveal_type(a * x)  # revealed: int
        reveal_type(a // x)  # revealed: int
-        reveal_type(a / x)  # revealed: float
+        reveal_type(a / x)  # revealed: int | float
        reveal_type(a % x)  # revealed: int

    def lhs_is_bool(x: bool):
@@ -72,7 +72,7 @@ def _(a: bool):
        reveal_type(x - a)  # revealed: int
        reveal_type(x * a)  # revealed: int
        reveal_type(x // a)  # revealed: int
-        reveal_type(x / a)  # revealed: float
+        reveal_type(x / a)  # revealed: int | float
        reveal_type(x % a)  # revealed: int

    def rhs_is_bool(x: bool):
@@ -80,7 +80,7 @@ def _(a: bool):
        reveal_type(a - x)  # revealed: int
        reveal_type(a * x)  # revealed: int
        reveal_type(a // x)  # revealed: int
-        reveal_type(a / x)  # revealed: float
+        reveal_type(a / x)  # revealed: int | float
        reveal_type(a % x)  # revealed: int

    def both_are_bool(x: bool, y: bool):
@@ -88,6 +88,6 @@ def _(a: bool):
        reveal_type(x - y)  # revealed: int
        reveal_type(x * y)  # revealed: int
        reveal_type(x // y)  # revealed: int
-        reveal_type(x / y)  # revealed: float
+        reveal_type(x / y)  # revealed: int | float
        reveal_type(x % y)  # revealed: int
 ```
--- a/crates/red_knot_python_semantic/resources/mdtest/binary/instances.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/binary/instances.md
@@ -268,23 +268,28 @@ reveal_type(B() + B())  # revealed: Unknown | int

 ## Integration test: numbers from typeshed

+We get less precise results from binary operations on float/complex literals due to the special case
+for annotations of `float` or `complex`, which applies also to return annotations for typeshed
+dunder methods. Perhaps we could have a special-case on the special-case, to exclude these typeshed
+return annotations from the widening, and preserve a bit more precision here?
+
 ```py
-reveal_type(3j + 3.14)  # revealed: complex
-reveal_type(4.2 + 42)  # revealed: float
-reveal_type(3j + 3)  # revealed: complex
+reveal_type(3j + 3.14)  # revealed: int | float | complex
+reveal_type(4.2 + 42)  # revealed: int | float
+reveal_type(3j + 3)  # revealed: int | float | complex

-# TODO should be complex, need to check arg type and fall back to `rhs.__radd__`
-reveal_type(3.14 + 3j)  # revealed: float
+# TODO should be int | float | complex, need to check arg type and fall back to `rhs.__radd__`
+reveal_type(3.14 + 3j)  # revealed: int | float

-# TODO should be float, need to check arg type and fall back to `rhs.__radd__`
+# TODO should be int | float, need to check arg type and fall back to `rhs.__radd__`
 reveal_type(42 + 4.2)  # revealed: int

-# TODO should be complex, need to check arg type and fall back to `rhs.__radd__`
+# TODO should be int | float | complex, need to check arg type and fall back to `rhs.__radd__`
 reveal_type(3 + 3j)  # revealed: int

 def _(x: bool, y: int):
    reveal_type(x + y)  # revealed: int
-    reveal_type(4.2 + x)  # revealed: float
+    reveal_type(4.2 + x)  # revealed: int | float

    # TODO should be float, need to check arg type and fall back to `rhs.__radd__`
    reveal_type(y + 4.12)  # revealed: int
--- a/crates/red_knot_python_semantic/resources/mdtest/binary/integers.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/binary/integers.md
@@ -19,7 +19,7 @@ def lhs(x: int):
    reveal_type(x - 4)  # revealed: int
    reveal_type(x * -1)  # revealed: int
    reveal_type(x // 3)  # revealed: int
-    reveal_type(x / 3)  # revealed: float
+    reveal_type(x / 3)  # revealed: int | float
    reveal_type(x % 3)  # revealed: int

 def rhs(x: int):
@@ -27,7 +27,7 @@ def rhs(x: int):
    reveal_type(3 - x)  # revealed: int
    reveal_type(3 * x)  # revealed: int
    reveal_type(-3 // x)  # revealed: int
-    reveal_type(-3 / x)  # revealed: float
+    reveal_type(-3 / x)  # revealed: int | float
    reveal_type(5 % x)  # revealed: int

 def both(x: int):
@@ -35,7 +35,7 @@ def both(x: int):
    reveal_type(x - x)  # revealed: int
    reveal_type(x * x)  # revealed: int
    reveal_type(x // x)  # revealed: int
-    reveal_type(x / x)  # revealed: float
+    reveal_type(x / x)  # revealed: int | float
    reveal_type(x % x)  # revealed: int
 ```

@@ -80,24 +80,20 @@ c = 3 % 0  # error: "Cannot reduce object of type `Literal[3]` modulo zero"
 reveal_type(c)  # revealed: int

 # error: "Cannot divide object of type `int` by zero"
-# revealed: float
-reveal_type(int() / 0)
+reveal_type(int() / 0)  # revealed: int | float

 # error: "Cannot divide object of type `Literal[1]` by zero"
-# revealed: float
-reveal_type(1 / False)
+reveal_type(1 / False)  # revealed: float
 # error: [division-by-zero] "Cannot divide object of type `Literal[True]` by zero"
 True / False
 # error: [division-by-zero] "Cannot divide object of type `Literal[True]` by zero"
 bool(1) / False

 # error: "Cannot divide object of type `float` by zero"
-# revealed: float
-reveal_type(1.0 / 0)
+reveal_type(1.0 / 0)  # revealed: int | float

 class MyInt(int): ...

 # No error for a subclass of int
-# revealed: float
-reveal_type(MyInt(3) / 0)
+reveal_type(MyInt(3) / 0)  # revealed: int | float
 ```
--- a/crates/red_knot_python_semantic/resources/mdtest/call/callable_instance.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/call/callable_instance.md
@@ -4,14 +4,14 @@

 ```py
 class Multiplier:
-    def __init__(self, factor: float):
+    def __init__(self, factor: int):
        self.factor = factor

-    def __call__(self, number: float) -> float:
+    def __call__(self, number: int) -> int:
        return number * self.factor

-a = Multiplier(2.0)(3.0)
-reveal_type(a)  # revealed: float
+a = Multiplier(2)(3)
+reveal_type(a)  # revealed: int

 class Unit: ...

--- a/crates/red_knot_python_semantic/resources/mdtest/comparison/instances/rich_comparison.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/comparison/instances/rich_comparison.md
@@ -20,8 +20,8 @@ class A:
    def __eq__(self, other: A) -> int:
        return 42

-    def __ne__(self, other: A) -> float:
-        return 42.0
+    def __ne__(self, other: A) -> bytearray:
+        return bytearray()

    def __lt__(self, other: A) -> str:
        return "42"
@@ -36,7 +36,7 @@ class A:
        return {42}

 reveal_type(A() == A())  # revealed: int
-reveal_type(A() != A())  # revealed: float
+reveal_type(A() != A())  # revealed: bytearray
 reveal_type(A() < A())  # revealed: str
 reveal_type(A() <= A())  # revealed: bytes
 reveal_type(A() > A())  # revealed: list
@@ -55,8 +55,8 @@ class A:
    def __eq__(self, other: B) -> int:
        return 42

-    def __ne__(self, other: B) -> float:
-        return 42.0
+    def __ne__(self, other: B) -> bytearray:
+        return bytearray()

    def __lt__(self, other: B) -> str:
        return "42"
@@ -73,7 +73,7 @@ class A:
 class B: ...

 reveal_type(A() == B())  # revealed: int
-reveal_type(A() != B())  # revealed: float
+reveal_type(A() != B())  # revealed: bytearray
 reveal_type(A() < B())  # revealed: str
 reveal_type(A() <= B())  # revealed: bytes
 reveal_type(A() > B())  # revealed: list
@@ -93,8 +93,8 @@ class A:
    def __eq__(self, other: B) -> int:
        return 42

-    def __ne__(self, other: B) -> float:
-        return 42.0
+    def __ne__(self, other: B) -> bytearray:
+        return bytearray()

    def __lt__(self, other: B) -> str:
        return "42"
@@ -117,7 +117,7 @@ class B:
    def __ne__(self, other: str) -> B:
        return B()

-# TODO: should be `int` and `float`.
+# TODO: should be `int` and `bytearray`.
 # Need to check arg type and fall back to `rhs.__eq__` and `rhs.__ne__`.
 #
 # Because `object.__eq__` and `object.__ne__` accept `object` in typeshed,
@@ -136,11 +136,11 @@ class C:
    def __gt__(self, other: C) -> int:
        return 42

-    def __ge__(self, other: C) -> float:
-        return 42.0
+    def __ge__(self, other: C) -> bytearray:
+        return bytearray()

 reveal_type(C() < C())  # revealed: int
-reveal_type(C() <= C())  # revealed: float
+reveal_type(C() <= C())  # revealed: bytearray
 ```

 ## Reflected Comparisons with Subclasses
@@ -175,8 +175,8 @@ class B(A):
    def __eq__(self, other: A) -> int:
        return 42

-    def __ne__(self, other: A) -> float:
-        return 42.0
+    def __ne__(self, other: A) -> bytearray:
+        return bytearray()

    def __lt__(self, other: A) -> str:
        return "42"
@@ -191,7 +191,7 @@ class B(A):
        return {42}

 reveal_type(A() == B())  # revealed: int
-reveal_type(A() != B())  # revealed: float
+reveal_type(A() != B())  # revealed: bytearray

 reveal_type(A() < B())  # revealed: list
 reveal_type(A() <= B())  # revealed: set
--- a/crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md
@@ -151,11 +151,11 @@ class A:
    def __ne__(self, o: object) -> bytes:
        return b"world"

-    def __lt__(self, o: A) -> float:
-        return 3.14
+    def __lt__(self, o: A) -> bytearray:
+        return bytearray()

-    def __le__(self, o: A) -> complex:
-        return complex(0.5, -0.5)
+    def __le__(self, o: A) -> memoryview:
+        return memoryview(b"")

    def __gt__(self, o: A) -> tuple:
        return (1, 2, 3)
@@ -167,8 +167,8 @@ a = (A(), A())

 reveal_type(a == a)  # revealed: bool
 reveal_type(a != a)  # revealed: bool
-reveal_type(a < a)  # revealed: float | Literal[False]
-reveal_type(a <= a)  # revealed: complex | Literal[True]
+reveal_type(a < a)  # revealed: bytearray | Literal[False]
+reveal_type(a <= a)  # revealed: memoryview | Literal[True]
 reveal_type(a > a)  # revealed: tuple | Literal[False]
 reveal_type(a >= a)  # revealed: list | Literal[True]

@@ -187,7 +187,7 @@ class B:
    def __lt__(self, o: B) -> set:
        return set()

-reveal_type((A(), B()) < (A(), B()))  # revealed: float | set | Literal[False]
+reveal_type((A(), B()) < (A(), B()))  # revealed: bytearray | set | Literal[False]
 ```

 #### Special Handling of Eq and NotEq in Lexicographic Comparisons
--- a/crates/red_knot_python_semantic/resources/mdtest/doc/README.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/doc/README.md
@@ -0,0 +1,2 @@
+This directory contains user-facing documentation, but also doubles as an extended test suite that
+makes sure that our documentation stays up to date.
--- a/crates/red_knot_python_semantic/resources/mdtest/doc/public_type_undeclared_symbols.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/doc/public_type_undeclared_symbols.md
@@ -0,0 +1,125 @@
+# Public type of undeclared symbols
+
+## Summary
+
+One major deviation from the behavior of existing Python type checkers is our handling of 'public'
+types for undeclared symbols. This is best illustrated with an example:
+
+```py
+class Wrapper:
+    value = None
+
+wrapper = Wrapper()
+
+reveal_type(wrapper.value)  # revealed: Unknown | None
+
+wrapper.value = 1
+```
+
+Mypy and Pyright both infer a type of `None` for the type of `wrapper.value`. Consequently, both
+tools emit an error when trying to assign `1` to `wrapper.value`. But there is nothing wrong with
+this program. Emitting an error here violates the [gradual guarantee] which states that *"Removing
+type annotations (making the program more dynamic) should not result in additional static type
+errors."*: If `value` were annotated with `int | None` here, Mypy and Pyright would not emit any
+errors.
+
+By inferring `Unknown | None` instead, we allow arbitrary values to be assigned to `wrapper.value`.
+This is a deliberate choice to prevent false positive errors on untyped code.
+
+More generally, we infer `Unknown | T_inferred` for undeclared symbols, where `T_inferred` is the
+inferred type of the right-hand side of the assignment. This gradual type represents an *unknown*
+fully-static type that is *at least as large as* `T_inferred`. It accurately describes our static
+knowledge about this type. In the example above, we don't know what values `wrapper.value` could
+possibly contain, but we *do know* that `None` is a possibility. This allows us to catch errors
+where `wrapper.value` is used in a way that is incompatible with `None`:
+
+```py
+def accepts_int(i: int) -> None:
+    pass
+
+def f(w: Wrapper) -> None:
+    # This is fine
+    v: int | None = w.value
+
+    # This function call is incorrect, because `w.value` could be `None`. We therefore emit the following
+    # error: "`Unknown | None` cannot be assigned to parameter 1 (`i`) of function `accepts_int`; expected type `int`"
+    c = accepts_int(w.value)
+```
+
+## Explicit lack of knowledge
+
+The following example demonstrates how Mypy and Pyright's type inference of fully-static types in
+these situations can lead to false-negatives, even though everything appears to be (statically)
+typed. To make this a bit more realistic, imagine that `OptionalInt` is imported from an external,
+untyped module:
+
+`optional_int.py`:
+
+```py
+class OptionalInt:
+    value = 10
+
+def reset(o):
+    o.value = None
+```
+
+It is then used like this:
+
+```py
+from optional_int import OptionalInt, reset
+
+o = OptionalInt()
+reset(o)  # Oh no...
+
+# Mypy and Pyright infer a fully-static type of `int` here, which appears to make the
+# subsequent division operation safe -- but it is not. We infer the following type:
+reveal_type(o.value)  # revealed: Unknown | Literal[10]
+
+print(o.value // 2)  # Runtime error!
+```
+
+We do not catch this mistake either, but we accurately reflect our lack of knowledge about
+`o.value`. Together with a possible future type-checker mode that would detect the prevalence of
+dynamic types, this could help developers catch such mistakes.
+
+## Stricter behavior
+
+Users can always opt in to stricter behavior by adding type annotations. For the `OptionalInt`
+class, this would probably be:
+
+```py
+class OptionalInt:
+    value: int | None = 10
+
+o = OptionalInt()
+
+# The following public type is now
+# revealed: int | None
+reveal_type(o.value)
+
+# Incompatible assignments are now caught:
+# error: "Object of type `Literal["a"]` is not assignable to attribute `value` of type `int | None`"
+o.value = "a"
+```
+
+## What is meant by 'public' type?
+
+We apply different semantics depending on whether a symbol is accessed from the same scope in which
+it was originally defined, or whether it is accessed from an external scope. External scopes will
+see the symbol's "public type", which has been discussed above. But within the same scope the symbol
+was defined in, we use a narrower type of `T_inferred` for undeclared symbols. This is because, from
+the perspective of this scope, there is no way that the value of the symbol could have been
+reassigned from external scopes. For example:
+
+```py
+class Wrapper:
+    value = None
+
+    # Type as seen from the same scope:
+    reveal_type(value)  # revealed: None
+
+# Type as seen from another scope:
+reveal_type(Wrapper.value)  # revealed: Unknown | None
+```
+
+[gradual guarantee]: https://typing.readthedocs.io/en/latest/spec/concepts.html#the-gradual-guarantee
--- a/crates/red_knot_python_semantic/resources/mdtest/exception/control_flow.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/exception/control_flow.md
@@ -303,8 +303,8 @@ An example with multiple `except` branches and a `finally` branch:
 def could_raise_returns_memoryview() -> memoryview:
    return memoryview(b"")

-def could_raise_returns_float() -> float:
-    return 3.14
+def could_raise_returns_bytearray() -> bytearray:
+    return bytearray()

 x = 1

@@ -322,13 +322,13 @@ except ValueError:
    reveal_type(x)  # revealed: Literal[1] | str
    x = could_raise_returns_memoryview()
    reveal_type(x)  # revealed: memoryview
-    x = could_raise_returns_float()
-    reveal_type(x)  # revealed: float
+    x = could_raise_returns_bytearray()
+    reveal_type(x)  # revealed: bytearray
 finally:
-    # TODO: should be `Literal[1] | str | bytes | bool | memoryview | float`
-    reveal_type(x)  # revealed: str | bool | float
+    # TODO: should be `Literal[1] | str | bytes | bool | memoryview | bytearray`
+    reveal_type(x)  # revealed: str | bool | bytearray

-reveal_type(x)  # revealed: str | bool | float
+reveal_type(x)  # revealed: str | bool | bytearray
 ```

 ## Combining `except`, `else` and `finally` branches
@@ -350,8 +350,8 @@ def could_raise_returns_bool() -> bool:
 def could_raise_returns_memoryview() -> memoryview:
    return memoryview(b"")

-def could_raise_returns_float() -> float:
-    return 3.14
+def could_raise_returns_bytearray() -> bytearray:
+    return bytearray()

 x = 1

@@ -369,13 +369,13 @@ else:
    reveal_type(x)  # revealed: str
    x = could_raise_returns_memoryview()
    reveal_type(x)  # revealed: memoryview
-    x = could_raise_returns_float()
-    reveal_type(x)  # revealed: float
+    x = could_raise_returns_bytearray()
+    reveal_type(x)  # revealed: bytearray
 finally:
-    # TODO: should be `Literal[1] | str | bytes | bool | memoryview | float`
-    reveal_type(x)  # revealed: bool | float
+    # TODO: should be `Literal[1] | str | bytes | bool | memoryview | bytearray`
+    reveal_type(x)  # revealed: bool | bytearray

-reveal_type(x)  # revealed: bool | float
+reveal_type(x)  # revealed: bool | bytearray
 ```

 The same again, this time with multiple `except` branches:
@@ -403,8 +403,8 @@ except ValueError:
    reveal_type(x)  # revealed: Literal[1] | str
    x = could_raise_returns_memoryview()
    reveal_type(x)  # revealed: memoryview
-    x = could_raise_returns_float()
-    reveal_type(x)  # revealed: float
+    x = could_raise_returns_bytearray()
+    reveal_type(x)  # revealed: bytearray
 else:
    reveal_type(x)  # revealed: str
    x = could_raise_returns_range()
@@ -412,10 +412,10 @@ else:
    x = could_raise_returns_slice()
    reveal_type(x)  # revealed: slice
 finally:
-    # TODO: should be `Literal[1] | str | bytes | bool | memoryview | float | range | slice`
-    reveal_type(x)  # revealed: bool | float | slice
+    # TODO: should be `Literal[1] | str | bytes | bool | memoryview | bytearray | range | slice`
+    reveal_type(x)  # revealed: bool | bytearray | slice

-reveal_type(x)  # revealed: bool | float | slice
+reveal_type(x)  # revealed: bool | bytearray | slice
 ```

 ## Nested `try`/`except` blocks
@@ -441,8 +441,8 @@ def could_raise_returns_bool() -> bool:
 def could_raise_returns_memoryview() -> memoryview:
    return memoryview(b"")

-def could_raise_returns_float() -> float:
-    return 3.14
+def could_raise_returns_property() -> property:
+    return property()

 def could_raise_returns_range() -> range:
    return range(42)
@@ -450,8 +450,8 @@ def could_raise_returns_range() -> range:
 def could_raise_returns_slice() -> slice:
    return slice(None)

-def could_raise_returns_complex() -> complex:
-    return 3j
+def could_raise_returns_super() -> super:
+    return super()

 def could_raise_returns_bytearray() -> bytearray:
    return bytearray()
@@ -482,8 +482,8 @@ try:
        reveal_type(x)  # revealed: Literal[1] | str
        x = could_raise_returns_memoryview()
        reveal_type(x)  # revealed: memoryview
-        x = could_raise_returns_float()
-        reveal_type(x)  # revealed: float
+        x = could_raise_returns_property()
+        reveal_type(x)  # revealed: property
    else:
        reveal_type(x)  # revealed: str
        x = could_raise_returns_range()
@@ -491,15 +491,15 @@ try:
        x = could_raise_returns_slice()
        reveal_type(x)  # revealed: slice
    finally:
-        # TODO: should be `Literal[1] | str | bytes | bool | memoryview | float | range | slice`
-        reveal_type(x)  # revealed: bool | float | slice
+        # TODO: should be `Literal[1] | str | bytes | bool | memoryview | property | range | slice`
+        reveal_type(x)  # revealed: bool | property | slice
        x = 2
        reveal_type(x)  # revealed: Literal[2]
    reveal_type(x)  # revealed: Literal[2]
 except:
-    reveal_type(x)  # revealed: Literal[1, 2] | str | bytes | bool | memoryview | float | range | slice
-    x = could_raise_returns_complex()
-    reveal_type(x)  # revealed: complex
+    reveal_type(x)  # revealed: Literal[1, 2] | str | bytes | bool | memoryview | property | range | slice
+    x = could_raise_returns_super()
+    reveal_type(x)  # revealed: super
    x = could_raise_returns_bytearray()
    reveal_type(x)  # revealed: bytearray
 else:
@@ -509,7 +509,7 @@ else:
    x = could_raise_returns_Bar()
    reveal_type(x)  # revealed: Bar
 finally:
-    # TODO: should be `Literal[1, 2] | str | bytes | bool | memoryview | float | range | slice | complex | bytearray | Foo | Bar`
+    # TODO: should be `Literal[1, 2] | str | bytes | bool | memoryview | property | range | slice | super | bytearray | Foo | Bar`
    reveal_type(x)  # revealed: bytearray | Bar

 # Either one `except` branch or the `else`
@@ -535,8 +535,8 @@ def could_raise_returns_range() -> range:
 def could_raise_returns_bytearray() -> bytearray:
    return bytearray()

-def could_raise_returns_float() -> float:
-    return 3.14
+def could_raise_returns_memoryview() -> memoryview:
+    return memoryview(b"")

 x = 1

@@ -553,12 +553,12 @@ try:
            reveal_type(x)  # revealed: str | bytes
            x = could_raise_returns_bytearray()
            reveal_type(x)  # revealed: bytearray
-            x = could_raise_returns_float()
-            reveal_type(x)  # revealed: float
+            x = could_raise_returns_memoryview()
+            reveal_type(x)  # revealed: memoryview
        finally:
-            # TODO: should be `str | bytes | bytearray | float`
-            reveal_type(x)  # revealed: bytes | float
-        reveal_type(x)  # revealed: bytes | float
+            # TODO: should be `str | bytes | bytearray | memoryview`
+            reveal_type(x)  # revealed: bytes | memoryview
+        reveal_type(x)  # revealed: bytes | memoryview
    x = foo
    reveal_type(x)  # revealed: Literal[foo]
 except:
--- a/crates/red_knot_python_semantic/resources/mdtest/import/conventions.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/import/conventions.md
@@ -0,0 +1,371 @@
+# Import conventions
+
+This document describes the conventions for importing symbols.
+
+Reference:
+
+- <https://typing.readthedocs.io/en/latest/spec/distributing.html#import-conventions>
+
+## Builtins scope
+
+When looking up for a name, red knot will fallback to using the builtins scope if the name is not
+found in the global scope. The `builtins.pyi` file, that will be used to resolve any symbol in the
+builtins scope, contains multiple symbols from other modules (e.g., `typing`) but those are not
+re-exported.
+
+```py
+# These symbols are being imported in `builtins.pyi` but shouldn't be considered as being
+# available in the builtins scope.
+
+# error: "Name `Literal` used when not defined"
+reveal_type(Literal)  # revealed: Unknown
+
+# error: "Name `sys` used when not defined"
+reveal_type(sys)  # revealed: Unknown
+```
+
+## Builtins import
+
+Similarly, trying to import the symbols from the builtins module which aren't re-exported should
+also raise an error.
+
+```py
+# error: "Module `builtins` has no member `Literal`"
+# error: "Module `builtins` has no member `sys`"
+from builtins import Literal, sys
+
+reveal_type(Literal)  # revealed: Unknown
+reveal_type(sys)  # revealed: Unknown
+
+# error: "Module `math` has no member `Iterable`"
+from math import Iterable
+
+reveal_type(Iterable)  # revealed: Unknown
+```
+
+## Re-exported symbols in stub files
+
+When a symbol is re-exported, importing it should not raise an error. This tests both `import ...`
+and `from ... import ...` forms.
+
+Note: Submodule imports in `import ...` form doesn't work because it's a syntax error. For example,
+in `import os.path as os.path` the `os.path` is not a valid identifier.
+
+```py
+from b import Any, Literal, foo
+
+reveal_type(Any)  # revealed: typing.Any
+reveal_type(Literal)  # revealed: typing.Literal
+reveal_type(foo)  # revealed: <module 'foo'>
+```
+
+`b.pyi`:
+
+```pyi
+import foo as foo
+from typing import Any as Any, Literal as Literal
+```
+
+`foo.py`:
+
+```py
+```
+
+## Non-exported symbols in stub files
+
+Here, none of the symbols are being re-exported in the stub file.
+
+```py
+# error: 15 [unresolved-import] "Module `b` has no member `foo`"
+# error: 20 [unresolved-import] "Module `b` has no member `Any`"
+# error: 25 [unresolved-import] "Module `b` has no member `Literal`"
+from b import foo, Any, Literal
+
+reveal_type(Any)  # revealed: Unknown
+reveal_type(Literal)  # revealed: Unknown
+reveal_type(foo)  # revealed: Unknown
+```
+
+`b.pyi`:
+
+```pyi
+import foo
+from typing import Any, Literal
+```
+
+`foo.pyi`:
+
+```pyi
+```
+
+## Nested non-exports
+
+Here, a chain of modules all don't re-export an import.
+
+```py
+# error: "Module `a` has no member `Any`"
+from a import Any
+
+reveal_type(Any)  # revealed: Unknown
+```
+
+`a.pyi`:
+
+```pyi
+# error: "Module `b` has no member `Any`"
+from b import Any
+
+reveal_type(Any)  # revealed: Unknown
+```
+
+`b.pyi`:
+
+```pyi
+# error: "Module `c` has no member `Any`"
+from c import Any
+
+reveal_type(Any)  # revealed: Unknown
+```
+
+`c.pyi`:
+
+```pyi
+from typing import Any
+
+reveal_type(Any)  # revealed: typing.Any
+```
+
+## Nested mixed re-export and not
+
+But, if the symbol is being re-exported explicitly in one of the modules in the chain, it should not
+raise an error at that step in the chain.
+
+```py
+# error: "Module `a` has no member `Any`"
+from a import Any
+
+reveal_type(Any)  # revealed: Unknown
+```
+
+`a.pyi`:
+
+```pyi
+from b import Any
+
+reveal_type(Any)  # revealed: Unknown
+```
+
+`b.pyi`:
+
+```pyi
+# error: "Module `c` has no member `Any`"
+from c import Any as Any
+
+reveal_type(Any)  # revealed: Unknown
+```
+
+`c.pyi`:
+
+```pyi
+from typing import Any
+
+reveal_type(Any)  # revealed: typing.Any
+```
+
+## Exported as different name
+
+The re-export convention only works when the aliased name is exactly the same as the original name.
+
+```py
+# error: "Module `a` has no member `Foo`"
+from a import Foo
+
+reveal_type(Foo)  # revealed: Unknown
+```
+
+`a.pyi`:
+
+```pyi
+from b import AnyFoo as Foo
+
+reveal_type(Foo)  # revealed: Literal[AnyFoo]
+```
+
+`b.pyi`:
+
+```pyi
+class AnyFoo: ...
+```
+
+## Exported using `__all__`
+
+Here, the symbol is re-exported using the `__all__` variable.
+
+```py
+# TODO: This should *not* be an error but we don't understand `__all__` yet.
+# error: "Module `a` has no member `Foo`"
+from a import Foo
+```
+
+`a.pyi`:
+
+```pyi
+from b import Foo
+
+__all__ = ['Foo']
+```
+
+`b.pyi`:
+
+```pyi
+class Foo: ...
+```
+
+## Re-exports in `__init__.pyi`
+
+Similarly, for an `__init__.pyi` (stub) file, importing a non-exported name should raise an error
+but the inference would be `Unknown`.
+
+```py
+# error: 15 "Module `a` has no member `Foo`"
+# error: 20 "Module `a` has no member `c`"
+from a import Foo, c, foo
+
+reveal_type(Foo)  # revealed: Unknown
+reveal_type(c)  # revealed: Unknown
+reveal_type(foo)  # revealed: <module 'a.foo'>
+```
+
+`a/__init__.pyi`:
+
+```pyi
+from .b import c
+from .foo import Foo
+```
+
+`a/foo.pyi`:
+
+```pyi
+class Foo: ...
+```
+
+`a/b/__init__.pyi`:
+
+```pyi
+```
+
+`a/b/c.pyi`:
+
+```pyi
+```
+
+## Conditional re-export in stub file
+
+The following scenarios are when a re-export happens conditionally in a stub file.
+
+### Global import
+
+```py
+# error: "Member `Foo` of module `a` is possibly unbound"
+from a import Foo
+
+reveal_type(Foo)  # revealed: str
+```
+
+`a.pyi`:
+
+```pyi
+from b import Foo
+
+def coinflip() -> bool: ...
+
+if coinflip():
+    Foo: str = ...
+
+reveal_type(Foo)  # revealed: Literal[Foo] | str
+```
+
+`b.pyi`:
+
+```pyi
+class Foo: ...
+```
+
+### Both branch is an import
+
+Here, both the branches of the condition are import statements where one of them re-exports while
+the other does not.
+
+```py
+# error: "Member `Foo` of module `a` is possibly unbound"
+from a import Foo
+
+reveal_type(Foo)  # revealed: Literal[Foo]
+```
+
+`a.pyi`:
+
+```pyi
+def coinflip() -> bool: ...
+
+if coinflip():
+    from b import Foo
+else:
+    from b import Foo as Foo
+
+reveal_type(Foo)  # revealed: Literal[Foo]
+```
+
+`b.pyi`:
+
+```pyi
+class Foo: ...
+```
+
+### Re-export in one branch
+
+```py
+# error: "Member `Foo` of module `a` is possibly unbound"
+from a import Foo
+
+reveal_type(Foo)  # revealed: Literal[Foo]
+```
+
+`a.pyi`:
+
+```pyi
+def coinflip() -> bool: ...
+
+if coinflip():
+    from b import Foo as Foo
+```
+
+`b.pyi`:
+
+```pyi
+class Foo: ...
+```
+
+### Non-export in one branch
+
+```py
+# error: "Module `a` has no member `Foo`"
+from a import Foo
+
+reveal_type(Foo)  # revealed: Unknown
+```
+
+`a.pyi`:
+
+```pyi
+def coinflip() -> bool: ...
+
+if coinflip():
+    from b import Foo
+```
+
+`b.pyi`:
+
+```pyi
+class Foo: ...
+```
--- a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals/is.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals/is.md
@@ -64,3 +64,39 @@ def _(flag1: bool, flag2: bool):
    else:
        reveal_type(x)  # revealed: Literal[1]
 ```
+
+## `is` for `EllipsisType` (Python 3.10+)
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+from types import EllipsisType
+
+def _(x: int | EllipsisType):
+    if x is ...:
+        reveal_type(x)  # revealed: EllipsisType
+    else:
+        reveal_type(x)  # revealed: int
+```
+
+## `is` for `EllipsisType` (Python 3.9 and below)
+
+```toml
+[environment]
+python-version = "3.9"
+```
+
+```py
+def _(flag: bool):
+    x = ... if flag else 42
+
+    reveal_type(x)  # revealed: ellipsis | Literal[42]
+
+    if x is ...:
+        reveal_type(x)  # revealed: ellipsis
+    else:
+        reveal_type(x)  # revealed: Literal[42]
+```
--- a/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_disjoint_from.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_disjoint_from.md
@@ -132,27 +132,6 @@ static_assert(not is_disjoint_from(Intersection[X, Z], Y))
 static_assert(not is_disjoint_from(Intersection[Y, Z], X))
 ```

-## Negation / complement
-
-The complement of a type `T` is disjoint from `T`. In fact, it is disjoint from every subtype of
-`T`:
-
-```py
-from knot_extensions import Not, Intersection, is_disjoint_from, static_assert
-
-class T: ...
-class S(T): ...
-
-static_assert(is_disjoint_from(Not[T], T))
-static_assert(is_disjoint_from(Not[T], S))
-
-static_assert(is_disjoint_from(Intersection[T, Any], Not[T]))
-static_assert(is_disjoint_from(Not[T], Intersection[T, Any]))
-
-static_assert(is_disjoint_from(Intersection[S, Any], Not[T]))
-static_assert(is_disjoint_from(Not[T], Intersection[S, Any]))
-```
-
 ## Special types

 ### `Never`
@@ -265,7 +244,7 @@ static_assert(not is_disjoint_from(TypeOf[f], object))
 ### `AlwaysTruthy` and `AlwaysFalsy`

 ```py
-from knot_extensions import AlwaysFalsy, AlwaysTruthy, Intersection, Not, is_disjoint_from, static_assert
+from knot_extensions import AlwaysFalsy, AlwaysTruthy, is_disjoint_from, static_assert
 from typing import Literal

 static_assert(is_disjoint_from(None, AlwaysTruthy))
@@ -277,14 +256,6 @@ static_assert(not is_disjoint_from(str, AlwaysTruthy))

 static_assert(is_disjoint_from(Literal[1, 2], AlwaysFalsy))
 static_assert(not is_disjoint_from(Literal[0, 1], AlwaysTruthy))
-
-type Truthy = Not[AlwaysFalsy]
-type Falsy = Not[AlwaysTruthy]
-
-type AmbiguousTruthiness = Intersection[Truthy, Falsy]
-
-static_assert(is_disjoint_from(AlwaysTruthy, AmbiguousTruthiness))
-static_assert(is_disjoint_from(AlwaysFalsy, AmbiguousTruthiness))
 ```

 ### Instance types versus `type[T]` types
--- a/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_singleton.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_singleton.md
@@ -54,3 +54,41 @@ from knot_extensions import is_singleton, static_assert

 static_assert(is_singleton(_NoDefaultType))
 ```
+
+## `builtins.ellipsis`/`types.EllipsisType`
+
+### All Python versions
+
+The type of the builtin symbol `Ellipsis` is the same as the type of an ellipsis literal (`...`).
+The type is not actually exposed from the standard library on Python \<3.10, but we still recognise
+the type as a singleton on any Python version.
+
+```toml
+[environment]
+python-version = "3.9"
+```
+
+```py
+import sys
+from knot_extensions import is_singleton, static_assert
+
+static_assert(is_singleton(Ellipsis.__class__))
+static_assert(is_singleton((...).__class__))
+```
+
+### Python 3.10+
+
+On Python 3.10+, the standard library exposes the type of `...` as `types.EllipsisType`, and we also
+recognise this as a singleton type when it is referenced directly:
+
+```toml
+[environment]
+python-version = "3.10"
+```
+
+```py
+import types
+from knot_extensions import static_assert, is_singleton
+
+static_assert(is_singleton(types.EllipsisType))
+```
--- a/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_subtype_of.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/type_properties/is_subtype_of.md
@@ -11,11 +11,15 @@ See the [typing documentation] for more information.

 - `bool` is a subtype of `int`. This is modeled after Python's runtime behavior, where `int` is a
    supertype of `bool` (present in `bool`s bases and MRO).
- `int` is not a subtype of `float`/`complex`, even though `float`/`complex` can be used in place of
-    `int` in some contexts (see [special case for float and complex]).
+- `int` is not a subtype of `float`/`complex`, although this is muddied by the
+    [special case for float and complex] where annotations of `float` and `complex` are interpreted
+    as `int | float` and `int | float | complex`, respectively.

 ```py
-from knot_extensions import is_subtype_of, static_assert
+from knot_extensions import is_subtype_of, static_assert, TypeOf
+
+type JustFloat = TypeOf[1.0]
+type JustComplex = TypeOf[1j]

 static_assert(is_subtype_of(bool, bool))
 static_assert(is_subtype_of(bool, int))
@@ -30,8 +34,8 @@ static_assert(not is_subtype_of(int, bool))
 static_assert(not is_subtype_of(int, str))
 static_assert(not is_subtype_of(object, int))

-static_assert(not is_subtype_of(int, float))
-static_assert(not is_subtype_of(int, complex))
+static_assert(not is_subtype_of(int, JustFloat))
+static_assert(not is_subtype_of(int, JustComplex))

 static_assert(is_subtype_of(TypeError, Exception))
 static_assert(is_subtype_of(FloatingPointError, Exception))
@@ -79,7 +83,9 @@ static_assert(is_subtype_of(C, object))

 ```py
 from typing_extensions import Literal, LiteralString
-from knot_extensions import is_subtype_of, static_assert
+from knot_extensions import is_subtype_of, static_assert, TypeOf
+
+type JustFloat = TypeOf[1.0]

 # Boolean literals
 static_assert(is_subtype_of(Literal[True], bool))
@@ -92,8 +98,7 @@ static_assert(is_subtype_of(Literal[1], object))

 static_assert(not is_subtype_of(Literal[1], bool))

-# See the note above (or link below) concerning int and float/complex
-static_assert(not is_subtype_of(Literal[1], float))
+static_assert(not is_subtype_of(Literal[1], JustFloat))

 # String literals
 static_assert(is_subtype_of(Literal["foo"], LiteralString))
--- a/crates/red_knot_python_semantic/resources/mdtest/union_types.md
+++ b/crates/red_knot_python_semantic/resources/mdtest/union_types.md
@@ -37,6 +37,31 @@ def noreturn(u1: int | NoReturn, u2: int | NoReturn | str) -> None:
    reveal_type(u2)  # revealed:  int | str
 ```

+## `object` subsumes everything
+
+Unions with `object` can be simplified to `object`:
+
+```py
+from typing_extensions import Never, Any
+
+def _(
+    u1: int | object,
+    u2: object | int,
+    u3: Any | object,
+    u4: object | Any,
+    u5: object | Never,
+    u6: Never | object,
+    u7: int | str | object | bytes | Any,
+) -> None:
+    reveal_type(u1)  # revealed: object
+    reveal_type(u2)  # revealed: object
+    reveal_type(u3)  # revealed: object
+    reveal_type(u4)  # revealed: object
+    reveal_type(u5)  # revealed: object
+    reveal_type(u6)  # revealed: object
+    reveal_type(u7)  # revealed: object
+```
+
 ## Flattening of nested unions

 ```py
@@ -45,11 +70,11 @@ from typing import Literal
 def _(
    u1: (int | str) | bytes,
    u2: int | (str | bytes),
-    u3: int | (str | (bytes | complex)),
+    u3: int | (str | (bytes | bytearray)),
 ) -> None:
    reveal_type(u1)  # revealed: int | str | bytes
    reveal_type(u2)  # revealed: int | str | bytes
-    reveal_type(u3)  # revealed: int | str | bytes | complex
+    reveal_type(u3)  # revealed: int | str | bytes | bytearray
 ```

 ## Simplification using subtyping
@@ -120,8 +145,8 @@ Simplifications still apply when `Unknown` is present.
 ```py
 from knot_extensions import Unknown

-def _(u1: str | Unknown | int | object):
-    reveal_type(u1)  # revealed: Unknown | object
+def _(u1: int | Unknown | bool) -> None:
+    reveal_type(u1)  # revealed: int | Unknown
 ```

 ## Union of intersections
--- a/crates/red_knot_python_semantic/src/ast_node_ref.rs
+++ b/crates/red_knot_python_semantic/src/ast_node_ref.rs
@@ -12,14 +12,32 @@ use ruff_db::parsed::ParsedModule;
 /// Holding on to any [`AstNodeRef`] prevents the [`ParsedModule`] from being released.
 ///
 /// ## Equality
-/// Two `AstNodeRef` are considered equal if their wrapped nodes are equal.
+/// Two `AstNodeRef` are considered equal if their pointer addresses are equal.
+///
+/// ## Usage in salsa tracked structs
+/// It's important that [`AstNodeRef`] fields in salsa tracked structs are tracked fields
+/// (attributed with `#[tracked`]). It prevents that the tracked struct gets a new ID
+/// everytime the AST changes, which in turn, invalidates the result of any query
+/// that takes said tracked struct as a query argument or returns the tracked struct as part of its result.
+///
+/// For example, marking the [`AstNodeRef`] as tracked on `Expression`
+/// has the effect that salsa will consider the expression as "unchanged" for as long as it:
+///
+/// * belongs to the same file
+/// * belongs to the same scope
+/// * has the same kind
+/// * was created in the same order
+///
+/// This means that changes to expressions in other scopes don't invalidate the expression's id, giving
+/// us some form of scope-stable identity for expressions. Only queries accessing the node field
+/// run on every AST change. All other queries only run when the expression's identity changes.
 #[derive(Clone)]
 pub struct AstNodeRef<T> {
    /// Owned reference to the node's [`ParsedModule`].
    ///
    /// The node's reference is guaranteed to remain valid as long as it's enclosing
    /// [`ParsedModule`] is alive.
-    _parsed: ParsedModule,
+    parsed: ParsedModule,

    /// Pointer to the referenced node.
    node: std::ptr::NonNull<T>,
@@ -37,7 +55,7 @@ impl<T> AstNodeRef<T> {
    /// the invariant `node belongs to parsed` is upheld.
    pub(super) unsafe fn new(parsed: ParsedModule, node: &T) -> Self {
        Self {
-            _parsed: parsed,
+            parsed,
            node: std::ptr::NonNull::from(node),
        }
    }
@@ -72,7 +90,14 @@ where
    T: PartialEq,
 {
    fn eq(&self, other: &Self) -> bool {
-        self.node().eq(other.node())
+        if self.parsed == other.parsed {
+            // Comparing the pointer addresses is sufficient to determine equality
+            // if the parsed are the same.
+            self.node.eq(&other.node)
+        } else {
+            // Otherwise perform a deep comparison.
+            self.node().eq(other.node())
+        }
    }
 }

@@ -87,6 +112,20 @@ where
    }
 }

+#[allow(unsafe_code)]
+unsafe impl<T> salsa::Update for AstNodeRef<T> {
+    unsafe fn maybe_update(old_pointer: *mut Self, new_value: Self) -> bool {
+        let old_ref = &mut (*old_pointer);
+
+        if old_ref.parsed == new_value.parsed && old_ref.node.eq(&new_value.node) {
+            false
+        } else {
+            *old_ref = new_value;
+            true
+        }
+    }
+}
+
 #[allow(unsafe_code)]
 unsafe impl<T> Send for AstNodeRef<T> where T: Send {}
 #[allow(unsafe_code)]
--- a/crates/red_knot_python_semantic/src/db.rs
+++ b/crates/red_knot_python_semantic/src/db.rs
@@ -1,3 +1,5 @@
+use std::sync::Arc;
+
 use crate::lint::{LintRegistry, RuleSelection};
 use ruff_db::files::File;
 use ruff_db::{Db as SourceDb, Upcast};
@@ -7,7 +9,7 @@ use ruff_db::{Db as SourceDb, Upcast};
 pub trait Db: SourceDb + Upcast<dyn SourceDb> {
    fn is_file_open(&self, file: File) -> bool;

-    fn rule_selection(&self) -> &RuleSelection;
+    fn rule_selection(&self) -> Arc<RuleSelection>;

    fn lint_registry(&self) -> &LintRegistry;
 }
@@ -17,7 +19,6 @@ pub(crate) mod tests {
    use std::sync::Arc;

    use crate::program::{Program, SearchPathSettings};
-    use crate::python_version::PythonVersion;
    use crate::{default_lint_registry, ProgramSettings, PythonPlatform};

    use super::Db;
@@ -27,6 +28,7 @@ pub(crate) mod tests {
    use ruff_db::system::{DbWithTestSystem, System, SystemPathBuf, TestSystem};
    use ruff_db::vendored::VendoredFileSystem;
    use ruff_db::{Db as SourceDb, Upcast};
+    use ruff_python_ast::python_version::PythonVersion;

    #[salsa::db]
    #[derive(Clone)]
@@ -111,8 +113,8 @@ pub(crate) mod tests {
            !file.path(self).is_vendored_path()
        }

-        fn rule_selection(&self) -> &RuleSelection {
-            &self.rule_selection
+        fn rule_selection(&self) -> Arc<RuleSelection> {
+            self.rule_selection.clone()
        }

        fn lint_registry(&self) -> &LintRegistry {
--- a/crates/red_knot_python_semantic/src/lib.rs
+++ b/crates/red_knot_python_semantic/src/lib.rs
@@ -9,7 +9,6 @@ pub use module_name::ModuleName;
 pub use module_resolver::{resolve_module, system_module_search_paths, KnownModule, Module};
 pub use program::{Program, ProgramSettings, SearchPathSettings, SitePackages};
 pub use python_platform::PythonPlatform;
-pub use python_version::PythonVersion;
 pub use semantic_model::{HasType, SemanticModel};

 pub mod ast_node_ref;
@@ -20,7 +19,6 @@ mod module_resolver;
 mod node_key;
 mod program;
 mod python_platform;
-mod python_version;
 pub mod semantic_index;
 mod semantic_model;
 pub(crate) mod site_packages;
--- a/crates/red_knot_python_semantic/src/module_resolver/path.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/path.rs
@@ -631,10 +631,10 @@ impl PartialEq<SearchPath> for VendoredPathBuf {
 #[cfg(test)]
 mod tests {
    use ruff_db::Db;
+    use ruff_python_ast::python_version::PythonVersion;

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

    use super::*;

--- a/crates/red_knot_python_semantic/src/module_resolver/resolver.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/resolver.rs
@@ -6,12 +6,13 @@ 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_python_ast::python_version::PythonVersion;

 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 crate::{Program, SearchPathSettings, SitePackages};

 use super::module::{Module, ModuleKind};
 use super::path::{ModulePath, SearchPath, SearchPathValidationError};
@@ -133,7 +134,7 @@ pub(crate) fn search_paths(db: &dyn Db) -> SearchPathIterator {
 }

 #[derive(Debug, PartialEq, Eq)]
-pub(crate) struct SearchPaths {
+pub 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.
    static_paths: Vec<SearchPath>,
@@ -724,12 +725,12 @@ mod tests {
        assert_const_function_query_was_not_run, assert_function_query_was_not_run,
    };
    use ruff_db::Db;
+    use ruff_python_ast::python_version::PythonVersion;

    use crate::db::tests::TestDb;
    use crate::module_name::ModuleName;
    use crate::module_resolver::module::ModuleKind;
    use crate::module_resolver::testing::{FileSpec, MockedTypeshed, TestCase, TestCaseBuilder};
-    use crate::PythonVersion;
    use crate::{ProgramSettings, PythonPlatform};

    use super::*;
--- a/crates/red_knot_python_semantic/src/module_resolver/testing.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/testing.rs
@@ -1,9 +1,9 @@
 use ruff_db::system::{DbWithTestSystem, SystemPath, SystemPathBuf};
 use ruff_db::vendored::VendoredPathBuf;
+use ruff_python_ast::python_version::PythonVersion;

 use crate::db::tests::TestDb;
 use crate::program::{Program, SearchPathSettings};
-use crate::python_version::PythonVersion;
 use crate::{ProgramSettings, PythonPlatform, SitePackages};

 /// A test case for the module resolver.
--- a/crates/red_knot_python_semantic/src/module_resolver/typeshed.rs
+++ b/crates/red_knot_python_semantic/src/module_resolver/typeshed.rs
@@ -4,11 +4,12 @@ use std::num::{NonZeroU16, NonZeroUsize};
 use std::ops::{RangeFrom, RangeInclusive};
 use std::str::FromStr;

+use ruff_python_ast::python_version::PythonVersion;
 use rustc_hash::FxHashMap;

 use crate::db::Db;
 use crate::module_name::ModuleName;
-use crate::{Program, PythonVersion};
+use crate::Program;

 pub(in crate::module_resolver) fn vendored_typeshed_versions(db: &dyn Db) -> TypeshedVersions {
    TypeshedVersions::from_str(
@@ -278,12 +279,12 @@ impl FromStr for PyVersionRange {
        let mut parts = s.split('-').map(str::trim);
        match (parts.next(), parts.next(), parts.next()) {
            (Some(lower), Some(""), None) => {
-                let lower = PythonVersion::from_versions_file_string(lower)?;
+                let lower = python_version_from_versions_file_string(lower)?;
                Ok(Self::AvailableFrom(lower..))
            }
            (Some(lower), Some(upper), None) => {
-                let lower = PythonVersion::from_versions_file_string(lower)?;
-                let upper = PythonVersion::from_versions_file_string(upper)?;
+                let lower = python_version_from_versions_file_string(lower)?;
+                let upper = python_version_from_versions_file_string(upper)?;
                Ok(Self::AvailableWithin(lower..=upper))
            }
            _ => Err(TypeshedVersionsParseErrorKind::UnexpectedNumberOfHyphens),
@@ -302,21 +303,21 @@ impl fmt::Display for PyVersionRange {
    }
 }

-impl PythonVersion {
-    fn from_versions_file_string(s: &str) -> Result<Self, TypeshedVersionsParseErrorKind> {
-        let mut parts = s.split('.').map(str::trim);
-        let (Some(major), Some(minor), None) = (parts.next(), parts.next(), parts.next()) else {
-            return Err(TypeshedVersionsParseErrorKind::UnexpectedNumberOfPeriods(
-                s.to_string(),
-            ));
-        };
-        PythonVersion::try_from((major, minor)).map_err(|int_parse_error| {
-            TypeshedVersionsParseErrorKind::IntegerParsingFailure {
-                version: s.to_string(),
-                err: int_parse_error,
-            }
-        })
-    }
+fn python_version_from_versions_file_string(
+    s: &str,
+) -> Result<PythonVersion, TypeshedVersionsParseErrorKind> {
+    let mut parts = s.split('.').map(str::trim);
+    let (Some(major), Some(minor), None) = (parts.next(), parts.next(), parts.next()) else {
+        return Err(TypeshedVersionsParseErrorKind::UnexpectedNumberOfPeriods(
+            s.to_string(),
+        ));
+    };
+    PythonVersion::try_from((major, minor)).map_err(|int_parse_error| {
+        TypeshedVersionsParseErrorKind::IntegerParsingFailure {
+            version: s.to_string(),
+            err: int_parse_error,
+        }
+    })
 }

 #[cfg(test)]
--- a/crates/red_knot_python_semantic/src/program.rs
+++ b/crates/red_knot_python_semantic/src/program.rs
@@ -1,10 +1,10 @@
 use crate::module_resolver::SearchPaths;
 use crate::python_platform::PythonPlatform;
-use crate::python_version::PythonVersion;
 use crate::Db;

 use anyhow::Context;
 use ruff_db::system::{SystemPath, SystemPathBuf};
+use ruff_python_ast::python_version::PythonVersion;
 use salsa::Durability;
 use salsa::Setter;

--- a/crates/red_knot_python_semantic/src/semantic_index.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index.rs
@@ -1,13 +1,14 @@
 use std::iter::FusedIterator;
 use std::sync::Arc;

-use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
-use salsa::plumbing::AsId;
-
 use ruff_db::files::File;
 use ruff_db::parsed::parsed_module;
 use ruff_index::{IndexSlice, IndexVec};

+use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
+use salsa::plumbing::AsId;
+use salsa::Update;
+
 use crate::module_name::ModuleName;
 use crate::semantic_index::ast_ids::node_key::ExpressionNodeKey;
 use crate::semantic_index::ast_ids::AstIds;
@@ -123,7 +124,7 @@ pub(crate) fn global_scope(db: &dyn Db, file: File) -> ScopeId<'_> {
 }

 /// The symbol tables and use-def maps for all scopes in a file.
-#[derive(Debug)]
+#[derive(Debug, Update)]
 pub(crate) struct SemanticIndex<'db> {
    /// List of all symbol tables in this file, indexed by scope.
    symbol_tables: IndexVec<FileScopeId, Arc<SymbolTable>>,
--- a/crates/red_knot_python_semantic/src/semantic_index/ast_ids.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/ast_ids.rs
@@ -24,7 +24,7 @@ use crate::Db;
 ///
 /// x = foo()
 /// ```
-#[derive(Debug)]
+#[derive(Debug, salsa::Update)]
 pub(crate) struct AstIds {
    /// Maps expressions to their expression id.
    expressions_map: FxHashMap<ExpressionNodeKey, ScopedExpressionId>,
@@ -74,6 +74,7 @@ impl HasScopedUseId for ast::ExprRef<'_> {

 /// Uniquely identifies an [`ast::Expr`] in a [`crate::semantic_index::symbol::FileScopeId`].
 #[newtype_index]
+#[derive(salsa::Update)]
 pub struct ScopedExpressionId;

 pub trait HasScopedExpressionId {
@@ -181,7 +182,7 @@ pub(crate) mod node_key {

    use crate::node_key::NodeKey;

-    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+    #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, salsa::Update)]
    pub(crate) struct ExpressionNodeKey(NodeKey);

    impl From<ast::ExprRef<'_>> for ExpressionNodeKey {
--- a/crates/red_knot_python_semantic/src/semantic_index/attribute_assignment.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/attribute_assignment.rs
@@ -9,7 +9,7 @@ use rustc_hash::FxHashMap;

 /// Describes an (annotated) attribute assignment that we discovered in a method
 /// body, typically of the form `self.x: int`, `self.x: int = …` or `self.x = …`.
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, salsa::Update)]
 pub(crate) enum AttributeAssignment<'db> {
    /// An attribute assignment with an explicit type annotation, either
    /// `self.x: <annotation>` or `self.x: <annotation> = …`.
--- a/crates/red_knot_python_semantic/src/semantic_index/builder.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/builder.rs
@@ -33,8 +33,8 @@ use crate::Db;

 use super::constraint::{Constraint, ConstraintNode, PatternConstraint};
 use super::definition::{
-    DefinitionCategory, ExceptHandlerDefinitionNodeRef, MatchPatternDefinitionNodeRef,
-    WithItemDefinitionNodeRef,
+    DefinitionCategory, ExceptHandlerDefinitionNodeRef, ImportDefinitionNodeRef,
+    MatchPatternDefinitionNodeRef, WithItemDefinitionNodeRef,
 };

 mod except_handlers;
@@ -886,22 +886,28 @@ where
                        self.imported_modules.extend(module_name.ancestors());
                    }

-                    let symbol_name = if let Some(asname) = &alias.asname {
-                        asname.id.clone()
+                    let (symbol_name, is_reexported) = if let Some(asname) = &alias.asname {
+                        (asname.id.clone(), asname.id == alias.name.id)
                    } else {
-                        Name::new(alias.name.id.split('.').next().unwrap())
+                        (Name::new(alias.name.id.split('.').next().unwrap()), false)
                    };

                    let symbol = self.add_symbol(symbol_name);
-                    self.add_definition(symbol, alias);
+                    self.add_definition(
+                        symbol,
+                        ImportDefinitionNodeRef {
+                            alias,
+                            is_reexported,
+                        },
+                    );
                }
            }
            ast::Stmt::ImportFrom(node) => {
                for (alias_index, alias) in node.names.iter().enumerate() {
-                    let symbol_name = if let Some(asname) = &alias.asname {
-                        &asname.id
+                    let (symbol_name, is_reexported) = if let Some(asname) = &alias.asname {
+                        (&asname.id, asname.id == alias.name.id)
                    } else {
-                        &alias.name.id
+                        (&alias.name.id, false)
                    };

                    // Look for imports `from __future__ import annotations`, ignore `as ...`
@@ -914,7 +920,14 @@ where

                    let symbol = self.add_symbol(symbol_name.clone());

-                    self.add_definition(symbol, ImportFromDefinitionNodeRef { node, alias_index });
+                    self.add_definition(
+                        symbol,
+                        ImportFromDefinitionNodeRef {
+                            node,
+                            alias_index,
+                            is_reexported,
+                        },
+                    );
                }
            }
            ast::Stmt::Assign(node) => {
--- a/crates/red_knot_python_semantic/src/semantic_index/constraint.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/constraint.rs
@@ -5,20 +5,20 @@ use crate::db::Db;
 use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{FileScopeId, ScopeId};

-#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, salsa::Update)]
 pub(crate) struct Constraint<'db> {
    pub(crate) node: ConstraintNode<'db>,
    pub(crate) is_positive: bool,
 }

-#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq, salsa::Update)]
 pub(crate) enum ConstraintNode<'db> {
    Expression(Expression<'db>),
    Pattern(PatternConstraint<'db>),
 }

 /// Pattern kinds for which we support type narrowing and/or static visibility analysis.
-#[derive(Debug, Clone, Hash, PartialEq)]
+#[derive(Debug, Clone, Hash, PartialEq, salsa::Update)]
 pub(crate) enum PatternConstraintKind<'db> {
    Singleton(Singleton, Option<Expression<'db>>),
    Value(Expression<'db>, Option<Expression<'db>>),
@@ -28,21 +28,15 @@ pub(crate) enum PatternConstraintKind<'db> {

 #[salsa::tracked]
 pub(crate) struct PatternConstraint<'db> {
-    #[id]
    pub(crate) file: File,

-    #[id]
    pub(crate) file_scope: FileScopeId,

-    #[no_eq]
-    #[return_ref]
    pub(crate) subject: Expression<'db>,

-    #[no_eq]
    #[return_ref]
    pub(crate) kind: PatternConstraintKind<'db>,

-    #[no_eq]
    count: countme::Count<PatternConstraint<'static>>,
 }

--- a/crates/red_knot_python_semantic/src/semantic_index/definition.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/definition.rs
@@ -25,22 +25,19 @@ use crate::Db;
 #[salsa::tracked]
 pub struct Definition<'db> {
    /// The file in which the definition occurs.
-    #[id]
    pub(crate) file: File,

    /// The scope in which the definition occurs.
-    #[id]
    pub(crate) file_scope: FileScopeId,

    /// The symbol defined.
-    #[id]
    pub(crate) symbol: ScopedSymbolId,

    #[no_eq]
    #[return_ref]
+    #[tracked]
    pub(crate) kind: DefinitionKind<'db>,

-    #[no_eq]
    count: countme::Count<Definition<'static>>,
 }

@@ -60,11 +57,15 @@ impl<'db> Definition<'db> {
    pub(crate) fn is_binding(self, db: &'db dyn Db) -> bool {
        self.kind(db).category().is_binding()
    }
+
+    pub(crate) fn is_reexported(self, db: &'db dyn Db) -> bool {
+        self.kind(db).is_reexported()
+    }
 }

 #[derive(Copy, Clone, Debug)]
 pub(crate) enum DefinitionNodeRef<'a> {
-    Import(&'a ast::Alias),
+    Import(ImportDefinitionNodeRef<'a>),
    ImportFrom(ImportFromDefinitionNodeRef<'a>),
    For(ForStmtDefinitionNodeRef<'a>),
    Function(&'a ast::StmtFunctionDef),
@@ -122,12 +123,6 @@ impl<'a> From<&'a ast::StmtAugAssign> for DefinitionNodeRef<'a> {
    }
 }

-impl<'a> From<&'a ast::Alias> for DefinitionNodeRef<'a> {
-    fn from(node_ref: &'a ast::Alias) -> Self {
-        Self::Import(node_ref)
-    }
-}
-
 impl<'a> From<&'a ast::TypeParamTypeVar> for DefinitionNodeRef<'a> {
    fn from(value: &'a ast::TypeParamTypeVar) -> Self {
        Self::TypeVar(value)
@@ -146,6 +141,12 @@ impl<'a> From<&'a ast::TypeParamTypeVarTuple> for DefinitionNodeRef<'a> {
    }
 }

+impl<'a> From<ImportDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
+    fn from(node_ref: ImportDefinitionNodeRef<'a>) -> Self {
+        Self::Import(node_ref)
+    }
+}
+
 impl<'a> From<ImportFromDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
    fn from(node_ref: ImportFromDefinitionNodeRef<'a>) -> Self {
        Self::ImportFrom(node_ref)
@@ -188,10 +189,17 @@ impl<'a> From<MatchPatternDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
    }
 }

+#[derive(Copy, Clone, Debug)]
+pub(crate) struct ImportDefinitionNodeRef<'a> {
+    pub(crate) alias: &'a ast::Alias,
+    pub(crate) is_reexported: bool,
+}
+
 #[derive(Copy, Clone, Debug)]
 pub(crate) struct ImportFromDefinitionNodeRef<'a> {
    pub(crate) node: &'a ast::StmtImportFrom,
    pub(crate) alias_index: usize,
+    pub(crate) is_reexported: bool,
 }

 #[derive(Copy, Clone, Debug)]
@@ -247,15 +255,22 @@ impl<'db> DefinitionNodeRef<'db> {
    #[allow(unsafe_code)]
    pub(super) unsafe fn into_owned(self, parsed: ParsedModule) -> DefinitionKind<'db> {
        match self {
-            DefinitionNodeRef::Import(alias) => {
-                DefinitionKind::Import(AstNodeRef::new(parsed, alias))
-            }
-            DefinitionNodeRef::ImportFrom(ImportFromDefinitionNodeRef { node, alias_index }) => {
-                DefinitionKind::ImportFrom(ImportFromDefinitionKind {
-                    node: AstNodeRef::new(parsed, node),
-                    alias_index,
-                })
-            }
+            DefinitionNodeRef::Import(ImportDefinitionNodeRef {
+                alias,
+                is_reexported,
+            }) => DefinitionKind::Import(ImportDefinitionKind {
+                alias: AstNodeRef::new(parsed, alias),
+                is_reexported,
+            }),
+            DefinitionNodeRef::ImportFrom(ImportFromDefinitionNodeRef {
+                node,
+                alias_index,
+                is_reexported,
+            }) => DefinitionKind::ImportFrom(ImportFromDefinitionKind {
+                node: AstNodeRef::new(parsed, node),
+                alias_index,
+                is_reexported,
+            }),
            DefinitionNodeRef::Function(function) => {
                DefinitionKind::Function(AstNodeRef::new(parsed, function))
            }
@@ -357,10 +372,15 @@ impl<'db> DefinitionNodeRef<'db> {

    pub(super) fn key(self) -> DefinitionNodeKey {
        match self {
-            Self::Import(node) => node.into(),
-            Self::ImportFrom(ImportFromDefinitionNodeRef { node, alias_index }) => {
-                (&node.names[alias_index]).into()
-            }
+            Self::Import(ImportDefinitionNodeRef {
+                alias,
+                is_reexported: _,
+            }) => alias.into(),
+            Self::ImportFrom(ImportFromDefinitionNodeRef {
+                node,
+                alias_index,
+                is_reexported: _,
+            }) => (&node.names[alias_index]).into(),
            Self::Function(node) => node.into(),
            Self::Class(node) => node.into(),
            Self::TypeAlias(node) => node.into(),
@@ -435,9 +455,16 @@ impl DefinitionCategory {
    }
 }

+/// The kind of a definition.
+///
+/// ## Usage in salsa tracked structs
+///
+/// [`DefinitionKind`] fields in salsa tracked structs should be tracked (attributed with `#[tracked]`)
+/// because the kind is a thin wrapper around [`AstNodeRef`]. See the [`AstNodeRef`] documentation
+/// for an in-depth explanation of why this is necessary.
 #[derive(Clone, Debug)]
 pub enum DefinitionKind<'db> {
-    Import(AstNodeRef<ast::Alias>),
+    Import(ImportDefinitionKind),
    ImportFrom(ImportFromDefinitionKind),
    Function(AstNodeRef<ast::StmtFunctionDef>),
    Class(AstNodeRef<ast::StmtClassDef>),
@@ -460,6 +487,14 @@ pub enum DefinitionKind<'db> {
 }

 impl DefinitionKind<'_> {
+    pub(crate) fn is_reexported(&self) -> bool {
+        match self {
+            DefinitionKind::Import(import) => import.is_reexported(),
+            DefinitionKind::ImportFrom(import) => import.is_reexported(),
+            _ => true,
+        }
+    }
+
    /// Returns the [`TextRange`] of the definition target.
    ///
    /// A definition target would mainly be the node representing the symbol being defined i.e.,
@@ -468,7 +503,7 @@ impl DefinitionKind<'_> {
    /// This is mainly used for logging and debugging purposes.
    pub(crate) fn target_range(&self) -> TextRange {
        match self {
-            DefinitionKind::Import(alias) => alias.range(),
+            DefinitionKind::Import(import) => import.alias().range(),
            DefinitionKind::ImportFrom(import) => import.alias().range(),
            DefinitionKind::Function(function) => function.name.range(),
            DefinitionKind::Class(class) => class.name.range(),
@@ -540,7 +575,7 @@ impl DefinitionKind<'_> {
    }
 }

-#[derive(Copy, Clone, Debug, PartialEq)]
+#[derive(Copy, Clone, Debug, PartialEq, Hash)]
 pub(crate) enum TargetKind<'db> {
    Sequence(Unpack<'db>),
    Name,
@@ -599,10 +634,27 @@ impl ComprehensionDefinitionKind {
    }
 }

+#[derive(Clone, Debug)]
+pub struct ImportDefinitionKind {
+    alias: AstNodeRef<ast::Alias>,
+    is_reexported: bool,
+}
+
+impl ImportDefinitionKind {
+    pub(crate) fn alias(&self) -> &ast::Alias {
+        self.alias.node()
+    }
+
+    pub(crate) fn is_reexported(&self) -> bool {
+        self.is_reexported
+    }
+}
+
 #[derive(Clone, Debug)]
 pub struct ImportFromDefinitionKind {
    node: AstNodeRef<ast::StmtImportFrom>,
    alias_index: usize,
+    is_reexported: bool,
 }

 impl ImportFromDefinitionKind {
@@ -613,6 +665,10 @@ impl ImportFromDefinitionKind {
    pub(crate) fn alias(&self) -> &ast::Alias {
        &self.node.node().names[self.alias_index]
    }
+
+    pub(crate) fn is_reexported(&self) -> bool {
+        self.is_reexported
+    }
 }

 #[derive(Clone, Debug)]
@@ -713,7 +769,7 @@ impl ExceptHandlerDefinitionKind {
    }
 }

-#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, salsa::Update)]
 pub(crate) struct DefinitionNodeKey(NodeKey);

 impl From<&ast::Alias> for DefinitionNodeKey {
--- a/crates/red_knot_python_semantic/src/semantic_index/expression.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/expression.rs
@@ -33,23 +33,20 @@ pub(crate) enum ExpressionKind {
 #[salsa::tracked]
 pub(crate) struct Expression<'db> {
    /// The file in which the expression occurs.
-    #[id]
    pub(crate) file: File,

    /// The scope in which the expression occurs.
-    #[id]
    pub(crate) file_scope: FileScopeId,

    /// The expression node.
    #[no_eq]
+    #[tracked]
    #[return_ref]
    pub(crate) node_ref: AstNodeRef<ast::Expr>,

    /// Should this expression be inferred as a normal expression or a type expression?
-    #[id]
    pub(crate) kind: ExpressionKind,

-    #[no_eq]
    count: countme::Count<Expression<'static>>,
 }

--- a/crates/red_knot_python_semantic/src/semantic_index/symbol.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/symbol.rs
@@ -96,18 +96,16 @@ impl From<FileSymbolId> for ScopedSymbolId {

 /// Symbol ID that uniquely identifies a symbol inside a [`Scope`].
 #[newtype_index]
+#[derive(salsa::Update)]
 pub struct ScopedSymbolId;

 /// A cross-module identifier of a scope that can be used as a salsa query parameter.
 #[salsa::tracked]
 pub struct ScopeId<'db> {
-    #[id]
    pub file: File,

-    #[id]
    pub file_scope_id: FileScopeId,

-    #[no_eq]
    count: countme::Count<ScopeId<'static>>,
 }

@@ -159,6 +157,7 @@ impl<'db> ScopeId<'db> {

 /// ID that uniquely identifies a scope inside of a module.
 #[newtype_index]
+#[derive(salsa::Update)]
 pub struct FileScopeId;

 impl FileScopeId {
@@ -177,7 +176,7 @@ impl FileScopeId {
    }
 }

-#[derive(Debug)]
+#[derive(Debug, salsa::Update)]
 pub struct Scope {
    pub(super) parent: Option<FileScopeId>,
    pub(super) node: NodeWithScopeKind,
@@ -216,7 +215,7 @@ impl ScopeKind {
 }

 /// Symbol table for a specific [`Scope`].
-#[derive(Debug, Default)]
+#[derive(Debug, Default, salsa::Update)]
 pub struct SymbolTable {
    /// The symbols in this scope.
    symbols: IndexVec<ScopedSymbolId, Symbol>,
@@ -424,7 +423,7 @@ impl NodeWithScopeRef<'_> {
 }

 /// Node that introduces a new scope.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, salsa::Update)]
 pub enum NodeWithScopeKind {
    Module,
    Class(AstNodeRef<ast::StmtClassDef>),
--- a/crates/red_knot_python_semantic/src/semantic_index/use_def.rs
+++ b/crates/red_knot_python_semantic/src/semantic_index/use_def.rs
@@ -278,7 +278,7 @@ mod symbol_state;
 type AllConstraints<'db> = IndexVec<ScopedConstraintId, Constraint<'db>>;

 /// Applicable definitions and constraints for every use of a name.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, salsa::Update)]
 pub(crate) struct UseDefMap<'db> {
    /// Array of [`Definition`] in this scope. Only the first entry should be `None`;
    /// this represents the implicit "unbound"/"undeclared" definition of every symbol.
@@ -384,7 +384,7 @@ impl<'db> UseDefMap<'db> {
 }

 /// Either live bindings or live declarations for a symbol.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, salsa::Update)]
 enum SymbolDefinitions {
    Bindings(SymbolBindings),
    Declarations(SymbolDeclarations),
--- a/crates/red_knot_python_semantic/src/site_packages.rs
+++ b/crates/red_knot_python_semantic/src/site_packages.rs
@@ -14,8 +14,7 @@ use std::num::NonZeroUsize;
 use std::ops::Deref;

 use ruff_db::system::{System, SystemPath, SystemPathBuf};
-
-use crate::PythonVersion;
+use ruff_python_ast::python_version::PythonVersion;

 type SitePackagesDiscoveryResult<T> = Result<T, SitePackagesDiscoveryError>;

--- a/crates/red_knot_python_semantic/src/stdlib.rs
+++ b/crates/red_knot_python_semantic/src/stdlib.rs
@@ -2,7 +2,7 @@ use crate::module_resolver::{resolve_module, KnownModule};
 use crate::semantic_index::global_scope;
 use crate::semantic_index::symbol::ScopeId;
 use crate::symbol::Symbol;
-use crate::types::global_symbol;
+use crate::types::imported_symbol;
 use crate::Db;

 /// Lookup the type of `symbol` in a given known module
@@ -14,18 +14,10 @@ pub(crate) fn known_module_symbol<'db>(
    symbol: &str,
 ) -> Symbol<'db> {
    resolve_module(db, &known_module.name())
-        .map(|module| global_symbol(db, module.file(), symbol))
+        .map(|module| imported_symbol(db, &module, symbol))
        .unwrap_or(Symbol::Unbound)
 }

-/// Lookup the type of `symbol` in the builtins namespace.
-///
-/// Returns `Symbol::Unbound` if the `builtins` module isn't available for some reason.
-#[inline]
-pub(crate) fn builtins_symbol<'db>(db: &'db dyn Db, symbol: &str) -> Symbol<'db> {
-    known_module_symbol(db, KnownModule::Builtins, symbol)
-}
-
 /// Lookup the type of `symbol` in the `typing` module namespace.
 ///
 /// Returns `Symbol::Unbound` if the `typing` module isn't available for some reason.
--- a/crates/red_knot_python_semantic/src/symbol.rs
+++ b/crates/red_knot_python_semantic/src/symbol.rs
@@ -1,5 +1,5 @@
 use crate::{
-    types::{Type, UnionType},
+    types::{todo_type, Type, UnionType},
    Db,
 };

@@ -26,13 +26,25 @@ pub(crate) enum Boundness {
 /// possibly_unbound:  Symbol::Type(Type::IntLiteral(2), Boundness::PossiblyUnbound),
 /// non_existent:      Symbol::Unbound,
 /// ```
-#[derive(Debug, Clone, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq, salsa::Update)]
 pub(crate) enum Symbol<'db> {
    Type(Type<'db>, Boundness),
    Unbound,
 }

 impl<'db> Symbol<'db> {
+    /// Constructor that creates a `Symbol` with boundness [`Boundness::Bound`].
+    pub(crate) fn bound(ty: impl Into<Type<'db>>) -> Self {
+        Symbol::Type(ty.into(), Boundness::Bound)
+    }
+
+    /// Constructor that creates a [`Symbol`] with a [`crate::types::TodoType`] type
+    /// and boundness [`Boundness::Bound`].
+    #[allow(unused_variables)] // Only unused in release builds
+    pub(crate) fn todo(message: &'static str) -> Self {
+        Symbol::Type(todo_type!(message), Boundness::Bound)
+    }
+
    pub(crate) fn is_unbound(&self) -> bool {
        matches!(self, Symbol::Unbound)
    }
@@ -55,6 +67,30 @@ impl<'db> Symbol<'db> {
            .expect("Expected a (possibly unbound) type, not an unbound symbol")
    }

+    /// Transform the symbol into a [`LookupResult`],
+    /// a [`Result`] type in which the `Ok` variant represents a definitely bound symbol
+    /// and the `Err` variant represents a symbol that is either definitely or possibly unbound.
+    pub(crate) fn into_lookup_result(self) -> LookupResult<'db> {
+        match self {
+            Symbol::Type(ty, Boundness::Bound) => Ok(ty),
+            Symbol::Type(ty, Boundness::PossiblyUnbound) => Err(LookupError::PossiblyUnbound(ty)),
+            Symbol::Unbound => Err(LookupError::Unbound),
+        }
+    }
+
+    /// Safely unwrap the symbol into a [`Type`].
+    ///
+    /// If the symbol is definitely unbound or possibly unbound, it will be transformed into a
+    /// [`LookupError`] and `diagnostic_fn` will be applied to the error value before returning
+    /// the result of `diagnostic_fn` (which will be a [`Type`]). This allows the caller to ensure
+    /// that a diagnostic is emitted if the symbol is possibly or definitely unbound.
+    pub(crate) fn unwrap_with_diagnostic(
+        self,
+        diagnostic_fn: impl FnOnce(LookupError<'db>) -> Type<'db>,
+    ) -> Type<'db> {
+        self.into_lookup_result().unwrap_or_else(diagnostic_fn)
+    }
+
    /// Fallback (partially or fully) to another symbol if `self` is partially or fully unbound.
    ///
    /// 1. If `self` is definitely bound, return `self` without evaluating `fallback_fn()`.
@@ -71,17 +107,9 @@ impl<'db> Symbol<'db> {
        db: &'db dyn Db,
        fallback_fn: impl FnOnce() -> Self,
    ) -> Self {
-        match self {
-            Symbol::Type(_, Boundness::Bound) => self,
-            Symbol::Unbound => fallback_fn(),
-            Symbol::Type(self_ty, Boundness::PossiblyUnbound) => match fallback_fn() {
-                Symbol::Unbound => self,
-                Symbol::Type(fallback_ty, fallback_boundness) => Symbol::Type(
-                    UnionType::from_elements(db, [self_ty, fallback_ty]),
-                    fallback_boundness,
-                ),
-            },
-        }
+        self.into_lookup_result()
+            .or_else(|lookup_error| lookup_error.or_fall_back_to(db, fallback_fn()))
+            .into()
    }

    #[must_use]
@@ -93,6 +121,51 @@ impl<'db> Symbol<'db> {
    }
 }

+impl<'db> From<LookupResult<'db>> for Symbol<'db> {
+    fn from(value: LookupResult<'db>) -> Self {
+        match value {
+            Ok(ty) => Symbol::Type(ty, Boundness::Bound),
+            Err(LookupError::Unbound) => Symbol::Unbound,
+            Err(LookupError::PossiblyUnbound(ty)) => Symbol::Type(ty, Boundness::PossiblyUnbound),
+        }
+    }
+}
+
+/// Possible ways in which a symbol lookup can (possibly or definitely) fail.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub(crate) enum LookupError<'db> {
+    Unbound,
+    PossiblyUnbound(Type<'db>),
+}
+
+impl<'db> LookupError<'db> {
+    /// Fallback (wholly or partially) to `fallback` to create a new [`LookupResult`].
+    pub(crate) fn or_fall_back_to(
+        self,
+        db: &'db dyn Db,
+        fallback: Symbol<'db>,
+    ) -> LookupResult<'db> {
+        let fallback = fallback.into_lookup_result();
+        match (&self, &fallback) {
+            (LookupError::Unbound, _) => fallback,
+            (LookupError::PossiblyUnbound { .. }, Err(LookupError::Unbound)) => Err(self),
+            (LookupError::PossiblyUnbound(ty), Ok(ty2)) => {
+                Ok(UnionType::from_elements(db, [ty, ty2]))
+            }
+            (LookupError::PossiblyUnbound(ty), Err(LookupError::PossiblyUnbound(ty2))) => Err(
+                LookupError::PossiblyUnbound(UnionType::from_elements(db, [ty, ty2])),
+            ),
+        }
+    }
+}
+
+/// A [`Result`] type in which the `Ok` variant represents a definitely bound symbol
+/// and the `Err` variant represents a symbol that is either definitely or possibly unbound.
+///
+/// Note that this type is exactly isomorphic to [`Symbol`].
+/// In the future, we could possibly consider removing `Symbol` and using this type everywhere instead.
+pub(crate) type LookupResult<'db> = Result<Type<'db>, LookupError<'db>>;
+
 #[cfg(test)]
 mod tests {
    use super::*;
--- a/crates/red_knot_python_semantic/src/types.rs
+++ b/crates/red_knot_python_semantic/src/types.rs
--- a/crates/red_knot_python_semantic/src/types/builder.rs
+++ b/crates/red_knot_python_semantic/src/types/builder.rs
@@ -43,6 +43,13 @@ impl<'db> UnionBuilder<'db> {
        }
    }

+    /// Collapse the union to a single type: `object`.
+    fn collapse_to_object(mut self) -> Self {
+        self.elements.clear();
+        self.elements.push(Type::object(self.db));
+        self
+    }
+
    /// Adds a type to this union.
    pub(crate) fn add(mut self, ty: Type<'db>) -> Self {
        match ty {
@@ -53,7 +60,12 @@ impl<'db> UnionBuilder<'db> {
                    self = self.add(*element);
                }
            }
+            // Adding `Never` to a union is a no-op.
            Type::Never => {}
+            // Adding `object` to a union results in `object`.
+            ty if ty.is_object(self.db) => {
+                return self.collapse_to_object();
+            }
            _ => {
                let bool_pair = if let Type::BooleanLiteral(b) = ty {
                    Some(Type::BooleanLiteral(!b))
@@ -76,7 +88,10 @@ impl<'db> UnionBuilder<'db> {
                        break;
                    }

-                    if ty.is_same_gradual_form(*element) || ty.is_subtype_of(self.db, *element) {
+                    if ty.is_same_gradual_form(*element)
+                        || ty.is_subtype_of(self.db, *element)
+                        || element.is_object(self.db)
+                    {
                        return self;
                    } else if element.is_subtype_of(self.db, ty) {
                        to_remove.push(index);
@@ -88,9 +103,7 @@ impl<'db> UnionBuilder<'db> {
                        // `element | ty` must be `object` (object has no other supertypes). This means we can simplify
                        // the whole union to just `object`, since all other potential elements would also be subtypes of
                        // `object`.
-                        self.elements.clear();
-                        self.elements.push(KnownClass::Object.to_instance(self.db));
-                        return self;
+                        return self.collapse_to_object();
                    }
                }
                match to_remove[..] {
@@ -416,7 +429,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
            Type::Never => {
                // Adding ~Never to an intersection is a no-op.
            }
-            Type::Instance(instance) if instance.class.is_known(db, KnownClass::Object) => {
+            Type::Instance(instance) if instance.class.is_object(db) => {
                // Adding ~object to an intersection results in Never.
                *self = Self::default();
                self.positive.insert(Type::Never);
@@ -481,7 +494,7 @@ impl<'db> InnerIntersectionBuilder<'db> {

    fn build(mut self, db: &'db dyn Db) -> Type<'db> {
        match (self.positive.len(), self.negative.len()) {
-            (0, 0) => KnownClass::Object.to_instance(db),
+            (0, 0) => Type::object(db),
            (1, 0) => self.positive[0],
            _ => {
                self.positive.shrink_to_fit();
@@ -534,7 +547,7 @@ mod tests {
        let db = setup_db();

        let intersection = IntersectionBuilder::new(&db).build();
-        assert_eq!(intersection, KnownClass::Object.to_instance(&db));
+        assert_eq!(intersection, Type::object(&db));
    }

    #[test_case(Type::BooleanLiteral(true))]
@@ -548,7 +561,7 @@ mod tests {
        // We add t_object in various orders (in first or second position) in
        // the tests below to ensure that the boolean simplification eliminates
        // everything from the intersection, not just `bool`.
-        let t_object = KnownClass::Object.to_instance(&db);
+        let t_object = Type::object(&db);
        let t_bool = KnownClass::Bool.to_instance(&db);

        let ty = IntersectionBuilder::new(&db)
--- a/crates/red_knot_python_semantic/src/types/call.rs
+++ b/crates/red_knot_python_semantic/src/types/call.rs
@@ -1,9 +1,7 @@
 use super::context::InferContext;
-use super::diagnostic::{CALL_NON_CALLABLE, TYPE_ASSERTION_FAILURE};
-use super::{Severity, Signature, Type, TypeArrayDisplay, UnionBuilder};
-use crate::types::diagnostic::STATIC_ASSERT_ERROR;
+use super::diagnostic::CALL_NON_CALLABLE;
+use super::{Signature, Type, TypeArrayDisplay, UnionBuilder};
 use crate::Db;
-use ruff_db::diagnostic::DiagnosticId;
 use ruff_python_ast as ast;

 mod arguments;
@@ -13,22 +11,10 @@ pub(super) use arguments::{Argument, CallArguments};
 pub(super) use bind::{bind_call, CallBinding};

 #[derive(Debug, Clone, PartialEq, Eq)]
-pub(super) enum StaticAssertionErrorKind<'db> {
-    ArgumentIsFalse,
-    ArgumentIsFalsy(Type<'db>),
-    ArgumentTruthinessIsAmbiguous(Type<'db>),
-    CustomError(&'db str),
-}
-
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub(super) enum CallOutcome<'db> {
+pub(crate) enum CallOutcome<'db> {
    Callable {
        binding: CallBinding<'db>,
    },
-    RevealType {
-        binding: CallBinding<'db>,
-        revealed_ty: Type<'db>,
-    },
    NotCallable {
        not_callable_ty: Type<'db>,
    },
@@ -40,14 +26,6 @@ pub(super) enum CallOutcome<'db> {
        called_ty: Type<'db>,
        call_outcome: Box<CallOutcome<'db>>,
    },
-    StaticAssertionError {
-        binding: CallBinding<'db>,
-        error_kind: StaticAssertionErrorKind<'db>,
-    },
-    AssertType {
-        binding: CallBinding<'db>,
-        asserted_ty: Type<'db>,
-    },
 }

 impl<'db> CallOutcome<'db> {
@@ -61,14 +39,6 @@ impl<'db> CallOutcome<'db> {
        CallOutcome::NotCallable { not_callable_ty }
    }

-    /// Create a new `CallOutcome::RevealType` with given revealed and return types.
-    pub(super) fn revealed(binding: CallBinding<'db>, revealed_ty: Type<'db>) -> CallOutcome<'db> {
-        CallOutcome::RevealType {
-            binding,
-            revealed_ty,
-        }
-    }
-
    /// Create a new `CallOutcome::Union` with given wrapped outcomes.
    pub(super) fn union(
        called_ty: Type<'db>,
@@ -80,22 +50,10 @@ impl<'db> CallOutcome<'db> {
        }
    }

-    /// Create a new `CallOutcome::AssertType` with given asserted and return types.
-    pub(super) fn asserted(binding: CallBinding<'db>, asserted_ty: Type<'db>) -> CallOutcome<'db> {
-        CallOutcome::AssertType {
-            binding,
-            asserted_ty,
-        }
-    }
-
    /// Get the return type of the call, or `None` if not callable.
    pub(super) fn return_type(&self, db: &'db dyn Db) -> Option<Type<'db>> {
        match self {
            Self::Callable { binding } => Some(binding.return_type()),
-            Self::RevealType {
-                binding,
-                revealed_ty: _,
-            } => Some(binding.return_type()),
            Self::NotCallable { not_callable_ty: _ } => None,
            Self::Union {
                outcomes,
@@ -114,11 +72,6 @@ impl<'db> CallOutcome<'db> {
                })
                .map(UnionBuilder::build),
            Self::PossiblyUnboundDunderCall { call_outcome, .. } => call_outcome.return_type(db),
-            Self::StaticAssertionError { .. } => Some(Type::none(db)),
-            Self::AssertType {
-                binding,
-                asserted_ty: _,
-            } => Some(binding.return_type()),
        }
    }

@@ -204,22 +157,12 @@ impl<'db> CallOutcome<'db> {
        // only non-callable diagnostics in the union case, which is inconsistent.
        match self {
            Self::Callable { binding } => {
+                // TODO: Move this out of the `CallOutcome` and into `TypeInferenceBuilder`?
+                // This check is required everywhere where we call `return_type_result`
+                // from the TypeInferenceBuilder.
                binding.report_diagnostics(context, node);
                Ok(binding.return_type())
            }
-            Self::RevealType {
-                binding,
-                revealed_ty,
-            } => {
-                binding.report_diagnostics(context, node);
-                context.report_diagnostic(
-                    node,
-                    DiagnosticId::RevealedType,
-                    Severity::Info,
-                    format_args!("Revealed type is `{}`", revealed_ty.display(context.db())),
-                );
-                Ok(binding.return_type())
-            }
            Self::NotCallable { not_callable_ty } => Err(NotCallableError::Type {
                not_callable_ty: *not_callable_ty,
                return_ty: Type::unknown(),
@@ -239,24 +182,12 @@ impl<'db> CallOutcome<'db> {
            } => {
                let mut not_callable = vec![];
                let mut union_builder = UnionBuilder::new(context.db());
-                let mut revealed = false;
                for outcome in outcomes {
                    let return_ty = match outcome {
                        Self::NotCallable { not_callable_ty } => {
                            not_callable.push(*not_callable_ty);
                            Type::unknown()
                        }
-                        Self::RevealType {
-                            binding,
-                            revealed_ty: _,
-                        } => {
-                            if revealed {
-                                binding.return_type()
-                            } else {
-                                revealed = true;
-                                outcome.unwrap_with_diagnostic(context, node)
-                            }
-                        }
                        _ => outcome.unwrap_with_diagnostic(context, node),
                    };
                    union_builder = union_builder.add(return_ty);
@@ -280,73 +211,6 @@ impl<'db> CallOutcome<'db> {
                    }),
                }
            }
-            Self::StaticAssertionError {
-                binding,
-                error_kind,
-            } => {
-                binding.report_diagnostics(context, node);
-
-                match error_kind {
-                    StaticAssertionErrorKind::ArgumentIsFalse => {
-                        context.report_lint(
-                            &STATIC_ASSERT_ERROR,
-                            node,
-                            format_args!("Static assertion error: argument evaluates to `False`"),
-                        );
-                    }
-                    StaticAssertionErrorKind::ArgumentIsFalsy(parameter_ty) => {
-                        context.report_lint(
-                            &STATIC_ASSERT_ERROR,
-                            node,
-                            format_args!(
-                                "Static assertion error: argument of type `{parameter_ty}` is statically known to be falsy",
-                                parameter_ty=parameter_ty.display(context.db())
-                            ),
-                        );
-                    }
-                    StaticAssertionErrorKind::ArgumentTruthinessIsAmbiguous(parameter_ty) => {
-                        context.report_lint(
-                            &STATIC_ASSERT_ERROR,
-                            node,
-                            format_args!(
-                                "Static assertion error: argument of type `{parameter_ty}` has an ambiguous static truthiness",
-                                parameter_ty=parameter_ty.display(context.db())
-                            ),
-                        );
-                    }
-                    StaticAssertionErrorKind::CustomError(message) => {
-                        context.report_lint(
-                            &STATIC_ASSERT_ERROR,
-                            node,
-                            format_args!("Static assertion error: {message}"),
-                        );
-                    }
-                }
-
-                Ok(Type::unknown())
-            }
-            Self::AssertType {
-                binding,
-                asserted_ty,
-            } => {
-                let [actual_ty, _asserted] = binding.parameter_types() else {
-                    return Ok(binding.return_type());
-                };
-
-                if !actual_ty.is_gradual_equivalent_to(context.db(), *asserted_ty) {
-                    context.report_lint(
-                        &TYPE_ASSERTION_FAILURE,
-                        node,
-                        format_args!(
-                            "Actual type `{}` is not the same as asserted type `{}`",
-                            actual_ty.display(context.db()),
-                            asserted_ty.display(context.db()),
-                        ),
-                    );
-                }
-
-                Ok(binding.return_type())
-            }
        }
    }
 }
--- a/crates/red_knot_python_semantic/src/types/call/bind.rs
+++ b/crates/red_knot_python_semantic/src/types/call/bind.rs
@@ -5,7 +5,7 @@ use crate::types::diagnostic::{
    TOO_MANY_POSITIONAL_ARGUMENTS, UNKNOWN_ARGUMENT,
 };
 use crate::types::signatures::Parameter;
-use crate::types::UnionType;
+use crate::types::{todo_type, UnionType};
 use ruff_python_ast as ast;

 /// Bind a [`CallArguments`] against a callable [`Signature`].
@@ -16,7 +16,7 @@ pub(crate) fn bind_call<'db>(
    db: &'db dyn Db,
    arguments: &CallArguments<'_, 'db>,
    signature: &Signature<'db>,
-    callable_ty: Option<Type<'db>>,
+    callable_ty: Type<'db>,
 ) -> CallBinding<'db> {
    let parameters = signature.parameters();
    // The type assigned to each parameter at this call site.
@@ -138,7 +138,7 @@ pub(crate) fn bind_call<'db>(
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) struct CallBinding<'db> {
    /// Type of the callable object (function, class...)
-    callable_ty: Option<Type<'db>>,
+    callable_ty: Type<'db>,

    /// Return type of the call.
    return_ty: Type<'db>,
@@ -154,7 +154,7 @@ impl<'db> CallBinding<'db> {
    // TODO remove this constructor and construct always from `bind_call`
    pub(crate) fn from_return_type(return_ty: Type<'db>) -> Self {
        Self {
-            callable_ty: None,
+            callable_ty: todo_type!("CallBinding::from_return_type"),
            return_ty,
            parameter_tys: Box::default(),
            errors: vec![],
@@ -189,8 +189,8 @@ impl<'db> CallBinding<'db> {

    fn callable_name(&self, db: &'db dyn Db) -> Option<&str> {
        match self.callable_ty {
-            Some(Type::FunctionLiteral(function)) => Some(function.name(db)),
-            Some(Type::ClassLiteral(class_type)) => Some(class_type.class.name(db)),
+            Type::FunctionLiteral(function) => Some(function.name(db)),
+            Type::ClassLiteral(class_type) => Some(class_type.class.name(db)),
            _ => None,
        }
    }
--- a/crates/red_knot_python_semantic/src/types/diagnostic.rs
+++ b/crates/red_knot_python_semantic/src/types/diagnostic.rs
@@ -8,7 +8,7 @@ use crate::types::string_annotation::{
 };
 use crate::types::{ClassLiteralType, KnownInstanceType, Type};
 use crate::{declare_lint, Db};
-use ruff_db::diagnostic::{Diagnostic, DiagnosticId, Severity};
+use ruff_db::diagnostic::{Diagnostic, DiagnosticId, Severity, Span};
 use ruff_db::files::File;
 use ruff_python_ast::{self as ast, AnyNodeRef};
 use ruff_text_size::{Ranged, TextRange};
@@ -802,12 +802,8 @@ impl Diagnostic for TypeCheckDiagnostic {
        TypeCheckDiagnostic::message(self).into()
    }

-    fn file(&self) -> Option<File> {
-        Some(TypeCheckDiagnostic::file(self))
-    }
-
-    fn range(&self) -> Option<TextRange> {
-        Some(Ranged::range(self))
+    fn span(&self) -> Option<Span> {
+        Some(Span::from(self.file).with_range(self.range))
    }

    fn severity(&self) -> Severity {
--- a/crates/red_knot_python_semantic/src/types/infer.rs
+++ b/crates/red_knot_python_semantic/src/types/infer.rs
@@ -29,6 +29,7 @@
 use std::num::NonZeroU32;

 use itertools::{Either, Itertools};
+use ruff_db::diagnostic::{DiagnosticId, Severity};
 use ruff_db::files::File;
 use ruff_db::parsed::parsed_module;
 use ruff_python_ast::{self as ast, AnyNodeRef, ExprContext};
@@ -49,7 +50,8 @@ use crate::semantic_index::semantic_index;
 use crate::semantic_index::symbol::{NodeWithScopeKind, NodeWithScopeRef, ScopeId};
 use crate::semantic_index::SemanticIndex;
 use crate::stdlib::builtins_module_scope;
-use crate::types::call::{Argument, CallArguments};
+use crate::symbol::LookupError;
+use crate::types::call::{Argument, CallArguments, CallOutcome};
 use crate::types::diagnostic::{
    report_invalid_arguments_to_annotated, report_invalid_assignment,
    report_invalid_attribute_assignment, report_unresolved_module, TypeCheckDiagnostics,
@@ -58,19 +60,19 @@ use crate::types::diagnostic::{
    INCONSISTENT_MRO, INVALID_ATTRIBUTE_ACCESS, INVALID_BASE, INVALID_CONTEXT_MANAGER,
    INVALID_DECLARATION, INVALID_PARAMETER_DEFAULT, INVALID_TYPE_FORM,
    INVALID_TYPE_VARIABLE_CONSTRAINTS, POSSIBLY_UNBOUND_ATTRIBUTE, POSSIBLY_UNBOUND_IMPORT,
-    UNDEFINED_REVEAL, UNRESOLVED_ATTRIBUTE, UNRESOLVED_IMPORT, UNSUPPORTED_OPERATOR,
+    STATIC_ASSERT_ERROR, TYPE_ASSERTION_FAILURE, UNDEFINED_REVEAL, UNRESOLVED_ATTRIBUTE,
+    UNRESOLVED_IMPORT, UNSUPPORTED_OPERATOR,
 };
 use crate::types::mro::MroErrorKind;
-use crate::types::statistics::TypeStatistics;
 use crate::types::unpacker::{UnpackResult, Unpacker};
 use crate::types::{
-    builtins_symbol, global_symbol, symbol, symbol_from_bindings, symbol_from_declarations,
-    todo_type, typing_extensions_symbol, Boundness, CallDunderResult, Class, ClassLiteralType,
-    DynamicType, FunctionType, InstanceType, IntersectionBuilder, IntersectionType,
-    IterationOutcome, KnownClass, KnownFunction, KnownInstanceType, MetaclassCandidate,
-    MetaclassErrorKind, SliceLiteralType, SubclassOfType, Symbol, SymbolAndQualifiers, Truthiness,
-    TupleType, Type, TypeAliasType, TypeAndQualifiers, TypeArrayDisplay, TypeQualifiers,
-    TypeVarBoundOrConstraints, TypeVarInstance, UnionBuilder, UnionType,
+    builtins_symbol, symbol, symbol_from_bindings, symbol_from_declarations, todo_type,
+    typing_extensions_symbol, Boundness, CallDunderResult, Class, ClassLiteralType, DynamicType,
+    FunctionType, InstanceType, IntersectionBuilder, IntersectionType, IterationOutcome,
+    KnownClass, KnownFunction, KnownInstanceType, MetaclassCandidate, MetaclassErrorKind,
+    RequiresExplicitReExport, SliceLiteralType, SubclassOfType, Symbol, SymbolAndQualifiers,
+    Truthiness, TupleType, Type, TypeAliasType, TypeAndQualifiers, TypeArrayDisplay,
+    TypeQualifiers, TypeVarBoundOrConstraints, TypeVarInstance, UnionBuilder, UnionType,
 };
 use crate::unpack::Unpack;
 use crate::util::subscript::{PyIndex, PySlice};
@@ -87,7 +89,7 @@ use super::slots::check_class_slots;
 use super::string_annotation::{
    parse_string_annotation, BYTE_STRING_TYPE_ANNOTATION, FSTRING_TYPE_ANNOTATION,
 };
-use super::{ParameterExpectation, ParameterExpectations};
+use super::{global_symbol, ParameterExpectation, ParameterExpectations};

 /// Infer all types for a [`ScopeId`], including all definitions and expressions in that scope.
 /// Use when checking a scope, or needing to provide a type for an arbitrary expression in the
@@ -117,7 +119,9 @@ fn infer_definition_types_cycle_recovery<'db>(
    let mut inference = TypeInference::empty(input.scope(db));
    let category = input.category(db);
    if category.is_declaration() {
-        inference.declarations.insert(input, Type::unknown().into());
+        inference
+            .declarations
+            .insert(input, TypeAndQualifiers::unknown());
    }
    if category.is_binding() {
        inference.bindings.insert(input, Type::unknown());
@@ -237,7 +241,7 @@ impl<'db> InferenceRegion<'db> {
 }

 /// The inferred types for a single region.
-#[derive(Debug, Eq, PartialEq)]
+#[derive(Debug, Eq, PartialEq, salsa::Update)]
 pub(crate) struct TypeInference<'db> {
    /// The types of every expression in this region.
    expressions: FxHashMap<ScopedExpressionId, Type<'db>>,
@@ -300,14 +304,6 @@ impl<'db> TypeInference<'db> {
        self.diagnostics.shrink_to_fit();
        self.deferred.shrink_to_fit();
    }
-
-    pub(super) fn statistics(&self) -> TypeStatistics {
-        let mut statistics = TypeStatistics::default();
-        for ty in self.expressions.values() {
-            statistics.increment(*ty);
-        }
-        statistics
-    }
 }

 impl WithDiagnostics for TypeInference<'_> {
@@ -742,7 +738,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                self.infer_type_alias_definition(type_alias.node(), definition);
            }
            DefinitionKind::Import(import) => {
-                self.infer_import_definition(import.node(), definition);
+                self.infer_import_definition(import.alias(), definition);
            }
            DefinitionKind::ImportFrom(import_from) => {
                self.infer_import_from_definition(
@@ -878,22 +874,25 @@ impl<'db> TypeInferenceBuilder<'db> {
        let use_def = self.index.use_def_map(binding.file_scope(self.db()));
        let declarations = use_def.declarations_at_binding(binding);
        let mut bound_ty = ty;
-        let declared_ty = symbol_from_declarations(self.db(), declarations)
-            .map(|SymbolAndQualifiers(s, _)| s.ignore_possibly_unbound().unwrap_or(Type::unknown()))
-            .unwrap_or_else(|(ty, conflicting)| {
-                // TODO point out the conflicting declarations in the diagnostic?
-                let symbol_table = self.index.symbol_table(binding.file_scope(self.db()));
-                let symbol_name = symbol_table.symbol(binding.symbol(self.db())).name();
-                self.context.report_lint(
-                    &CONFLICTING_DECLARATIONS,
-                    node,
-                    format_args!(
-                        "Conflicting declared types for `{symbol_name}`: {}",
-                        conflicting.display(self.db())
-                    ),
-                );
-                ty.inner_type()
-            });
+        let declared_ty =
+            symbol_from_declarations(self.db(), declarations, RequiresExplicitReExport::No)
+                .map(|SymbolAndQualifiers(s, _)| {
+                    s.ignore_possibly_unbound().unwrap_or(Type::unknown())
+                })
+                .unwrap_or_else(|(ty, conflicting)| {
+                    // TODO point out the conflicting declarations in the diagnostic?
+                    let symbol_table = self.index.symbol_table(binding.file_scope(self.db()));
+                    let symbol_name = symbol_table.symbol(binding.symbol(self.db())).name();
+                    self.context.report_lint(
+                        &CONFLICTING_DECLARATIONS,
+                        node,
+                        format_args!(
+                            "Conflicting declared types for `{symbol_name}`: {}",
+                            conflicting.display(self.db())
+                        ),
+                    );
+                    ty.inner_type()
+                });
        if !bound_ty.is_assignable_to(self.db(), declared_ty) {
            report_invalid_assignment(&self.context, node, declared_ty, bound_ty);
            // allow declarations to override inference in case of invalid assignment
@@ -913,9 +912,10 @@ impl<'db> TypeInferenceBuilder<'db> {
        let use_def = self.index.use_def_map(declaration.file_scope(self.db()));
        let prior_bindings = use_def.bindings_at_declaration(declaration);
        // unbound_ty is Never because for this check we don't care about unbound
-        let inferred_ty = symbol_from_bindings(self.db(), prior_bindings)
-            .ignore_possibly_unbound()
-            .unwrap_or(Type::Never);
+        let inferred_ty =
+            symbol_from_bindings(self.db(), prior_bindings, RequiresExplicitReExport::No)
+                .ignore_possibly_unbound()
+                .unwrap_or(Type::Never);
        let ty = if inferred_ty.is_assignable_to(self.db(), ty.inner_type()) {
            ty
        } else {
@@ -928,7 +928,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                    inferred_ty.display(self.db())
                ),
            );
-            Type::unknown().into()
+            TypeAndQualifiers::unknown()
        };
        self.types.declarations.insert(declaration, ty);
    }
@@ -2816,17 +2816,15 @@ impl<'db> TypeInferenceBuilder<'db> {

    fn infer_number_literal_expression(&mut self, literal: &ast::ExprNumberLiteral) -> Type<'db> {
        let ast::ExprNumberLiteral { range: _, value } = literal;
+        let db = self.db();

        match value {
            ast::Number::Int(n) => n
                .as_i64()
                .map(Type::IntLiteral)
-                .unwrap_or_else(|| KnownClass::Int.to_instance(self.db())),
-            ast::Number::Float(_) => KnownClass::Float.to_instance(self.db()),
-            ast::Number::Complex { .. } => builtins_symbol(self.db(), "complex")
-                .ignore_possibly_unbound()
-                .unwrap_or(Type::unknown())
-                .to_instance(self.db()),
+                .unwrap_or_else(|| KnownClass::Int.to_instance(db)),
+            ast::Number::Float(_) => KnownClass::Float.to_instance(db),
+            ast::Number::Complex { .. } => KnownClass::Complex.to_instance(db),
        }
    }

@@ -2910,9 +2908,7 @@ impl<'db> TypeInferenceBuilder<'db> {
        &mut self,
        _literal: &ast::ExprEllipsisLiteral,
    ) -> Type<'db> {
-        builtins_symbol(self.db(), "Ellipsis")
-            .ignore_possibly_unbound()
-            .unwrap_or(Type::unknown())
+        KnownClass::EllipsisType.to_instance(self.db())
    }

    fn infer_tuple_expression(&mut self, tuple: &ast::ExprTuple) -> Type<'db> {
@@ -3252,9 +3248,107 @@ impl<'db> TypeInferenceBuilder<'db> {
            .unwrap_or_default();

        let call_arguments = self.infer_arguments(arguments, parameter_expectations);
-        function_type
-            .call(self.db(), &call_arguments)
-            .unwrap_with_diagnostic(&self.context, call_expression.into())
+        let call_outcome = self.check_call(function_type, &call_arguments, call_expression);
+
+        call_outcome.unwrap_with_diagnostic(&self.context, call_expression.into())
+    }
+
+    fn check_call(
+        &self,
+        callee: Type<'db>,
+        arguments: &CallArguments<'_, 'db>,
+        call_expression: &ast::ExprCall,
+    ) -> CallOutcome<'db> {
+        let call = callee.call(self.db(), arguments);
+
+        let Type::FunctionLiteral(function_type) = callee else {
+            return call;
+        };
+
+        let CallOutcome::Callable { binding } = &call else {
+            return call;
+        };
+
+        let Some(known) = function_type.known(self.db()) else {
+            return call;
+        };
+
+        match known {
+            KnownFunction::RevealType => {
+                let revealed_ty = binding.one_parameter_type().unwrap_or(Type::unknown());
+                self.context.report_diagnostic(
+                    call_expression.into(),
+                    DiagnosticId::RevealedType,
+                    Severity::Info,
+                    format_args!("Revealed type is `{}`", revealed_ty.display(self.db())),
+                );
+            }
+
+            KnownFunction::AssertType => {
+                let [actual_ty, asserted_ty] = binding.parameter_types() else {
+                    return call;
+                };
+
+                if !actual_ty.is_gradual_equivalent_to(self.db(), *asserted_ty) {
+                    self.context.report_lint(
+                        &TYPE_ASSERTION_FAILURE,
+                        call_expression.into(),
+                        format_args!(
+                            "Actual type `{}` is not the same as asserted type `{}`",
+                            actual_ty.display(self.db()),
+                            asserted_ty.display(self.db()),
+                        ),
+                    );
+                }
+            }
+
+            KnownFunction::StaticAssert => {
+                let Some((parameter_ty, message)) = binding.two_parameter_types() else {
+                    return call;
+                };
+
+                let truthiness = parameter_ty.bool(self.db());
+
+                if !truthiness.is_always_true() {
+                    if let Some(message) =
+                        message.into_string_literal().map(|s| &**s.value(self.db()))
+                    {
+                        self.context.report_lint(
+                            &STATIC_ASSERT_ERROR,
+                            call_expression.into(),
+                            format_args!("Static assertion error: {message}"),
+                        );
+                    } else if parameter_ty == Type::BooleanLiteral(false) {
+                        self.context.report_lint(
+                            &STATIC_ASSERT_ERROR,
+                            call_expression.into(),
+                            format_args!("Static assertion error: argument evaluates to `False`"),
+                        );
+                    } else if truthiness.is_always_false() {
+                        self.context.report_lint(
+                            &STATIC_ASSERT_ERROR,
+                            call_expression.into(),
+                            format_args!(
+                                "Static assertion error: argument of type `{parameter_ty}` is statically known to be falsy",
+                                parameter_ty=parameter_ty.display(self.db())
+                            ),
+                        );
+                    } else {
+                        self.context.report_lint(
+                            &STATIC_ASSERT_ERROR,
+                            call_expression.into(),
+                            format_args!(
+                                "Static assertion error: argument of type `{parameter_ty}` has an ambiguous static truthiness",
+                                parameter_ty=parameter_ty.display(self.db())
+                            ),
+                        );
+                    };
+                }
+            }
+            _ => {}
+        }
+
+        call
    }

    fn infer_starred_expression(&mut self, starred: &ast::ExprStarred) -> Type<'db> {
@@ -3297,38 +3391,84 @@ impl<'db> TypeInferenceBuilder<'db> {
        todo_type!("generic `typing.Awaitable` type")
    }

-    /// Look up a name reference that isn't bound in the local scope.
-    fn lookup_name(&mut self, name_node: &ast::ExprName) -> Symbol<'db> {
+    /// Infer the type of a [`ast::ExprName`] expression, assuming a load context.
+    fn infer_name_load(&mut self, name_node: &ast::ExprName) -> Type<'db> {
+        let ast::ExprName {
+            range: _,
+            id: symbol_name,
+            ctx: _,
+        } = name_node;
+
        let db = self.db();
-        let ast::ExprName { id: name, .. } = name_node;
-        let file_scope_id = self.scope().file_scope_id(db);
-        let is_bound =
-            if let Some(symbol) = self.index.symbol_table(file_scope_id).symbol_by_name(name) {
-                symbol.is_bound()
+        let scope = self.scope();
+        let file_scope_id = scope.file_scope_id(db);
+        let symbol_table = self.index.symbol_table(file_scope_id);
+        let use_def = self.index.use_def_map(file_scope_id);
+
+        // If we're inferring types of deferred expressions, always treat them as public symbols
+        let local_scope_symbol = if self.is_deferred() {
+            if let Some(symbol_id) = symbol_table.symbol_id_by_name(symbol_name) {
+                symbol_from_bindings(
+                    db,
+                    use_def.public_bindings(symbol_id),
+                    RequiresExplicitReExport::No,
+                )
            } else {
                assert!(
                    self.deferred_state.in_string_annotation(),
                    "Expected the symbol table to create a symbol for every Name node"
                );
-                false
+                Symbol::Unbound
+            }
+        } else {
+            let use_id = name_node.scoped_use_id(db, scope);
+            symbol_from_bindings(
+                db,
+                use_def.bindings_at_use(use_id),
+                RequiresExplicitReExport::No,
+            )
+        };
+
+        let symbol = local_scope_symbol.or_fall_back_to(db, || {
+            let has_bindings_in_this_scope = match symbol_table.symbol_by_name(symbol_name) {
+                Some(symbol) => symbol.is_bound(),
+                None => {
+                    assert!(
+                        self.deferred_state.in_string_annotation(),
+                        "Expected the symbol table to create a symbol for every Name node"
+                    );
+                    false
+                }
            };

-        // In function-like scopes, any local variable (symbol that is bound in this scope) can
-        // only have a definition in this scope, or error; it never references another scope.
-        // (At runtime, it would use the `LOAD_FAST` opcode.)
-        if !is_bound || !self.scope().is_function_like(db) {
+            // If it's a function-like scope and there is one or more binding in this scope (but
+            // none of those bindings are visible from where we are in the control flow), we cannot
+            // fallback to any bindings in enclosing scopes. As such, we can immediately short-circuit
+            // here and return `Symbol::Unbound`.
+            //
+            // This is because Python is very strict in its categorisation of whether a variable is
+            // a local variable or not in function-like scopes. If a variable has any bindings in a
+            // function-like scope, it is considered a local variable; it never references another
+            // scope. (At runtime, it would use the `LOAD_FAST` opcode.)
+            if has_bindings_in_this_scope && scope.is_function_like(db) {
+                return Symbol::Unbound;
+            }
+
+            let current_file = self.file();
+
            // Walk up parent scopes looking for a possible enclosing scope that may have a
            // definition of this name visible to us (would be `LOAD_DEREF` at runtime.)
            for (enclosing_scope_file_id, _) in self.index.ancestor_scopes(file_scope_id) {
                // Class scopes are not visible to nested scopes, and we need to handle global
                // scope differently (because an unbound name there falls back to builtins), so
                // check only function-like scopes.
-                let enclosing_scope_id = enclosing_scope_file_id.to_scope_id(db, self.file());
+                let enclosing_scope_id = enclosing_scope_file_id.to_scope_id(db, current_file);
                if !enclosing_scope_id.is_function_like(db) {
                    continue;
                }
                let enclosing_symbol_table = self.index.symbol_table(enclosing_scope_file_id);
-                let Some(enclosing_symbol) = enclosing_symbol_table.symbol_by_name(name) else {
+                let Some(enclosing_symbol) = enclosing_symbol_table.symbol_by_name(symbol_name)
+                else {
                    continue;
                };
                if enclosing_symbol.is_bound() {
@@ -3337,7 +3477,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                    // runtime, it is the scope that creates the cell for our closure.) If the name
                    // isn't bound in that scope, we should get an unbound name, not continue
                    // falling back to other scopes / globals / builtins.
-                    return symbol(db, enclosing_scope_id, name);
+                    return symbol(db, enclosing_scope_id, symbol_name);
                }
            }

@@ -3348,7 +3488,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                    if file_scope_id.is_global() {
                        Symbol::Unbound
                    } else {
-                        global_symbol(db, self.file(), name)
+                        global_symbol(db, self.file(), symbol_name)
                    }
                })
                // Not found in globals? Fallback to builtins
@@ -3357,12 +3497,12 @@ impl<'db> TypeInferenceBuilder<'db> {
                    if Some(self.scope()) == builtins_module_scope(db) {
                        Symbol::Unbound
                    } else {
-                        builtins_symbol(db, name)
+                        builtins_symbol(db, symbol_name)
                    }
                })
                // Still not found? It might be `reveal_type`...
                .or_fall_back_to(db, || {
-                    if name == "reveal_type" {
+                    if symbol_name == "reveal_type" {
                        self.context.report_lint(
                            &UNDEFINED_REVEAL,
                            name_node.into(),
@@ -3371,70 +3511,23 @@ impl<'db> TypeInferenceBuilder<'db> {
                            this is allowed for debugging convenience but will fail at runtime"
                            ),
                        );
-                        typing_extensions_symbol(db, name)
+                        typing_extensions_symbol(db, symbol_name)
                    } else {
                        Symbol::Unbound
                    }
                })
-        } else {
-            Symbol::Unbound
-        }
-    }
+        });

-    /// Infer the type of a [`ast::ExprName`] expression, assuming a load context.
-    fn infer_name_load(&mut self, name: &ast::ExprName) -> Type<'db> {
-        let ast::ExprName {
-            range: _,
-            id,
-            ctx: _,
-        } = name;
-
-        let file_scope_id = self.scope().file_scope_id(self.db());
-        let use_def = self.index.use_def_map(file_scope_id);
-
-        // If we're inferring types of deferred expressions, always treat them as public symbols
-        let inferred = if self.is_deferred() {
-            if let Some(symbol) = self.index.symbol_table(file_scope_id).symbol_id_by_name(id) {
-                symbol_from_bindings(self.db(), use_def.public_bindings(symbol))
-            } else {
-                assert!(
-                    self.deferred_state.in_string_annotation(),
-                    "Expected the symbol table to create a symbol for every Name node"
-                );
-                Symbol::Unbound
+        symbol.unwrap_with_diagnostic(|lookup_error| match lookup_error {
+            LookupError::Unbound => {
+                report_unresolved_reference(&self.context, name_node);
+                Type::unknown()
            }
-        } else {
-            let use_id = name.scoped_use_id(self.db(), self.scope());
-            symbol_from_bindings(self.db(), use_def.bindings_at_use(use_id))
-        };
-
-        if let Symbol::Type(ty, Boundness::Bound) = inferred {
-            ty
-        } else {
-            match self.lookup_name(name) {
-                Symbol::Type(looked_up_ty, looked_up_boundness) => {
-                    if looked_up_boundness == Boundness::PossiblyUnbound {
-                        report_possibly_unresolved_reference(&self.context, name);
-                    }
-
-                    inferred
-                        .ignore_possibly_unbound()
-                        .map(|ty| UnionType::from_elements(self.db(), [ty, looked_up_ty]))
-                        .unwrap_or(looked_up_ty)
-                }
-                Symbol::Unbound => match inferred {
-                    Symbol::Type(ty, Boundness::PossiblyUnbound) => {
-                        report_possibly_unresolved_reference(&self.context, name);
-                        ty
-                    }
-                    Symbol::Unbound => {
-                        report_unresolved_reference(&self.context, name);
-                        Type::unknown()
-                    }
-                    Symbol::Type(_, Boundness::Bound) => unreachable!("Handled above"),
-                },
+            LookupError::PossiblyUnbound(type_when_bound) => {
+                report_possibly_unresolved_reference(&self.context, name_node);
+                type_when_bound
            }
-        }
+        })
    }

    fn infer_name_expression(&mut self, name: &ast::ExprName) -> Type<'db> {
@@ -3454,36 +3547,37 @@ impl<'db> TypeInferenceBuilder<'db> {
            ctx: _,
        } = attribute;

-        let value_ty = self.infer_expression(value);
-        match value_ty.member(self.db(), &attr.id) {
-            Symbol::Type(member_ty, boundness) => {
-                if boundness == Boundness::PossiblyUnbound {
+        let value_type = self.infer_expression(value);
+        let db = self.db();
+
+        value_type
+            .member(db, &attr.id)
+            .unwrap_with_diagnostic(|lookup_error| match lookup_error {
+                LookupError::Unbound => {
+                    self.context.report_lint(
+                        &UNRESOLVED_ATTRIBUTE,
+                        attribute.into(),
+                        format_args!(
+                            "Type `{}` has no attribute `{}`",
+                            value_type.display(db),
+                            attr.id
+                        ),
+                    );
+                    Type::unknown()
+                }
+                LookupError::PossiblyUnbound(type_when_bound) => {
                    self.context.report_lint(
                        &POSSIBLY_UNBOUND_ATTRIBUTE,
                        attribute.into(),
                        format_args!(
                            "Attribute `{}` on type `{}` is possibly unbound",
                            attr.id,
-                            value_ty.display(self.db()),
+                            value_type.display(db),
                        ),
                    );
+                    type_when_bound
                }
-
-                member_ty
-            }
-            Symbol::Unbound => {
-                self.context.report_lint(
-                    &UNRESOLVED_ATTRIBUTE,
-                    attribute.into(),
-                    format_args!(
-                        "Type `{}` has no attribute `{}`",
-                        value_ty.display(self.db()),
-                        attr.id
-                    ),
-                );
-                Type::unknown()
-            }
-        }
+            })
    }

    fn infer_attribute_expression(&mut self, attribute: &ast::ExprAttribute) -> Type<'db> {
@@ -3841,6 +3935,8 @@ impl<'db> TypeInferenceBuilder<'db> {
                if left_ty != right_ty && right_ty.is_subtype_of(self.db(), left_ty) {
                    let reflected_dunder = op.reflected_dunder();
                    let rhs_reflected = right_class.member(self.db(), reflected_dunder);
+                    // TODO: if `rhs_reflected` is possibly unbound, we should union the two possible
+                    // CallOutcomes together
                    if !rhs_reflected.is_unbound()
                        && rhs_reflected != left_class.member(self.db(), reflected_dunder)
                    {
@@ -4862,7 +4958,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                    bytes.into(),
                    format_args!("Type expressions cannot use bytes literal"),
                );
-                Type::unknown().into()
+                TypeAndQualifiers::unknown()
            }

            ast::Expr::FString(fstring) => {
@@ -4872,7 +4968,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                    format_args!("Type expressions cannot use f-strings"),
                );
                self.infer_fstring_expression(fstring);
-                Type::unknown().into()
+                TypeAndQualifiers::unknown()
            }

            ast::Expr::Name(name) => match name.ctx {
@@ -4893,7 +4989,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                            .into(),
                    }
                }
-                ast::ExprContext::Invalid => Type::unknown().into(),
+                ast::ExprContext::Invalid => TypeAndQualifiers::unknown(),
                ast::ExprContext::Store | ast::ExprContext::Del => todo_type!().into(),
            },

@@ -4931,7 +5027,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                                inner_annotation_ty
                            } else {
                                self.infer_type_expression(slice);
-                                Type::unknown().into()
+                                TypeAndQualifiers::unknown()
                            }
                        } else {
                            report_invalid_arguments_to_annotated(
@@ -5000,7 +5096,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                    DeferredExpressionState::InStringAnnotation,
                )
            }
-            None => Type::unknown().into(),
+            None => TypeAndQualifiers::unknown(),
        }
    }
 }
@@ -6093,7 +6189,7 @@ mod tests {
        let mut db = setup_db();
        let content = format!(
            r#"
-            from typing_extensions import assert_type
+            from typing_extensions import Literal, assert_type

            assert_type(not "{y}", bool)
            assert_type(not 10*"{y}", bool)
@@ -6115,7 +6211,7 @@ mod tests {
        let mut db = setup_db();
        let content = format!(
            r#"
-            from typing_extensions import assert_type
+            from typing_extensions import Literal, LiteralString, assert_type

            assert_type(2 * "hello", Literal["hellohello"])
            assert_type("goodbye" * 3, Literal["goodbyegoodbyegoodbye"])
@@ -6140,7 +6236,7 @@ mod tests {
        let mut db = setup_db();
        let content = format!(
            r#"
-            from typing_extensions import assert_type
+            from typing_extensions import Literal, LiteralString, assert_type

            assert_type("{y}", LiteralString)
            assert_type(10*"{y}", LiteralString)
@@ -6162,7 +6258,7 @@ mod tests {
        let mut db = setup_db();
        let content = format!(
            r#"
-            from typing_extensions import assert_type
+            from typing_extensions import LiteralString, assert_type

            assert_type("{y}", LiteralString)
            assert_type("a" + "{z}", LiteralString)
@@ -6182,7 +6278,7 @@ mod tests {
        let mut db = setup_db();
        let content = format!(
            r#"
-            from typing_extensions import assert_type
+            from typing_extensions import LiteralString, assert_type

            assert_type("{y}", LiteralString)
            assert_type("{y}" + "a", LiteralString)
@@ -6441,6 +6537,7 @@ mod tests {
            assert_eq!(attr_ty.display(&db).to_string(), "Unknown | str | None");
            db.take_salsa_events()
        };
+
        assert_function_query_was_not_run(
            &db,
            infer_expression_types,
--- a/crates/red_knot_python_semantic/src/types/mro.rs
+++ b/crates/red_knot_python_semantic/src/types/mro.rs
@@ -4,13 +4,13 @@ use std::ops::Deref;
 use rustc_hash::FxHashSet;

 use crate::types::class_base::ClassBase;
-use crate::types::{Class, KnownClass, Type};
+use crate::types::{Class, Type};
 use crate::Db;

 /// The inferred method resolution order of a given class.
 ///
 /// See [`Class::iter_mro`] for more details.
-#[derive(PartialEq, Eq, Clone, Debug)]
+#[derive(PartialEq, Eq, Clone, Debug, salsa::Update)]
 pub(super) struct Mro<'db>(Box<[ClassBase<'db>]>);

 impl<'db> Mro<'db> {
@@ -52,9 +52,7 @@ impl<'db> Mro<'db> {
        match class_bases {
            // `builtins.object` is the special case:
            // the only class in Python that has an MRO with length <2
-            [] if class.is_known(db, KnownClass::Object) => {
-                Ok(Self::from([ClassBase::Class(class)]))
-            }
+            [] if class.is_object(db) => Ok(Self::from([ClassBase::Class(class)])),

            // All other classes in Python have an MRO with length >=2.
            // Even if a class has no explicit base classes,
@@ -238,7 +236,7 @@ impl<'db> Iterator for MroIterator<'db> {

 impl std::iter::FusedIterator for MroIterator<'_> {}

-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, salsa::Update)]
 pub(super) struct MroError<'db> {
    kind: MroErrorKind<'db>,
    fallback_mro: Mro<'db>,
@@ -258,7 +256,7 @@ impl<'db> MroError<'db> {
 }

 /// Possible ways in which attempting to resolve the MRO of a class might fail.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, salsa::Update)]
 pub(super) enum MroErrorKind<'db> {
    /// The class inherits from one or more invalid bases.
    ///
--- a/crates/red_knot_python_semantic/src/types/narrow.rs
+++ b/crates/red_knot_python_semantic/src/types/narrow.rs
@@ -152,13 +152,13 @@ fn merge_constraints_or<'db>(
                *entry.get_mut() = UnionBuilder::new(db).add(*entry.get()).add(*value).build();
            }
            Entry::Vacant(entry) => {
-                entry.insert(KnownClass::Object.to_instance(db));
+                entry.insert(Type::object(db));
            }
        }
    }
    for (key, value) in into.iter_mut() {
        if !from.contains_key(key) {
-            *value = KnownClass::Object.to_instance(db);
+            *value = Type::object(db);
        }
    }
 }
@@ -231,10 +231,10 @@ impl<'db> NarrowingConstraintsBuilder<'db> {

        match pattern.kind(self.db) {
            PatternConstraintKind::Singleton(singleton, _guard) => {
-                self.evaluate_match_pattern_singleton(*subject, *singleton)
+                self.evaluate_match_pattern_singleton(subject, *singleton)
            }
            PatternConstraintKind::Class(cls, _guard) => {
-                self.evaluate_match_pattern_class(*subject, *cls)
+                self.evaluate_match_pattern_class(subject, *cls)
            }
            // TODO: support more pattern kinds
            PatternConstraintKind::Value(..) | PatternConstraintKind::Unsupported => None,
--- a/crates/red_knot_python_semantic/src/types/property_tests.rs
+++ b/crates/red_knot_python_semantic/src/types/property_tests.rs
@@ -329,7 +329,7 @@ fn union<'db>(db: &'db TestDb, tys: impl IntoIterator<Item = Type<'db>>) -> Type

 mod stable {
    use super::union;
-    use crate::types::{KnownClass, Type};
+    use crate::types::Type;

    // Reflexivity: `T` is equivalent to itself.
    type_property_test!(
@@ -419,13 +419,13 @@ mod stable {
    // All types should be assignable to `object`
    type_property_test!(
        all_types_assignable_to_object, db,
-        forall types t. t.is_assignable_to(db, KnownClass::Object.to_instance(db))
+        forall types t. t.is_assignable_to(db, Type::object(db))
    );

    // And for fully static types, they should also be subtypes of `object`
    type_property_test!(
        all_fully_static_types_subtype_of_object, db,
-        forall types t. t.is_fully_static(db) => t.is_subtype_of(db, KnownClass::Object.to_instance(db))
+        forall types t. t.is_fully_static(db) => t.is_subtype_of(db, Type::object(db))
    );

    // Never should be assignable to every type
--- a/crates/red_knot_python_semantic/src/types/signatures.rs
+++ b/crates/red_knot_python_semantic/src/types/signatures.rs
@@ -4,7 +4,7 @@ use crate::{semantic_index::definition::Definition, types::todo_type};
 use ruff_python_ast::{self as ast, name::Name};

 /// A typed callable signature.
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, salsa::Update)]
 pub(crate) struct Signature<'db> {
    /// Parameters, in source order.
    ///
@@ -60,7 +60,7 @@ impl<'db> Signature<'db> {
 }

 // TODO: use SmallVec here once invariance bug is fixed
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, salsa::Update)]
 pub(crate) struct Parameters<'db>(Vec<Parameter<'db>>);

 impl<'db> Parameters<'db> {
@@ -218,7 +218,7 @@ impl<'db> std::ops::Index<usize> for Parameters<'db> {
    }
 }

-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, salsa::Update)]
 pub(crate) struct Parameter<'db> {
    /// Parameter name.
    ///
@@ -304,7 +304,7 @@ impl<'db> Parameter<'db> {
    }
 }

-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, salsa::Update)]
 pub(crate) enum ParameterKind<'db> {
    /// Positional-only parameter, e.g. `def f(x, /): ...`
    PositionalOnly { default_ty: Option<Type<'db>> },
@@ -357,6 +357,8 @@ mod tests {
        db.write_dedented(
            "/src/a.py",
            "
+            from typing import Literal
+
            def f(a, b: int, c = 1, d: int = 2, /,
                  e = 3, f: Literal[4] = 4, *args: object,
                  g = 5, h: Literal[6] = 6, **kwargs: str) -> bytes: ...
@@ -411,7 +413,7 @@ mod tests {
                },
                Parameter {
                    name: Some(Name::new_static("args")),
-                    annotated_ty: Some(KnownClass::Object.to_instance(&db)),
+                    annotated_ty: Some(Type::object(&db)),
                    kind: ParameterKind::Variadic,
                },
                Parameter {
--- a/crates/red_knot_python_semantic/src/types/statistics.rs
+++ b/crates/red_knot_python_semantic/src/types/statistics.rs
@@ -1,121 +0,0 @@
-use crate::types::{infer_scope_types, semantic_index, Type};
-use crate::Db;
-use ruff_db::files::File;
-use rustc_hash::FxHashMap;
-
-/// Get type-coverage statistics for a file.
-#[salsa::tracked(return_ref)]
-pub fn type_statistics<'db>(db: &'db dyn Db, file: File) -> TypeStatistics<'db> {
-    let _span = tracing::trace_span!("type_statistics", file=?file.path(db)).entered();
-
-    tracing::debug!(
-        "Gathering statistics for file '{path}'",
-        path = file.path(db)
-    );
-
-    let index = semantic_index(db, file);
-    let mut statistics = TypeStatistics::default();
-
-    for scope_id in index.scope_ids() {
-        let result = infer_scope_types(db, scope_id);
-        statistics.extend(&result.statistics());
-    }
-
-    statistics
-}
-
-/// Map each type to count of expressions with that type.
-#[derive(Debug, Default, Eq, PartialEq)]
-pub(super) struct TypeStatistics<'db>(FxHashMap<Type<'db>, u32>);
-
-impl<'db> TypeStatistics<'db> {
-    fn extend(&mut self, other: &TypeStatistics<'db>) {
-        for (ty, count) in &other.0 {
-            self.0
-                .entry(*ty)
-                .and_modify(|my_count| *my_count += count)
-                .or_insert(*count);
-        }
-    }
-
-    pub(super) fn increment(&mut self, ty: Type<'db>) {
-        self.0
-            .entry(ty)
-            .and_modify(|count| *count += 1)
-            .or_insert(1);
-    }
-
-    #[allow(unused)]
-    fn expression_count(&self) -> u32 {
-        self.0.values().sum()
-    }
-
-    #[allow(unused)]
-    fn todo_count(&self) -> u32 {
-        self.0
-            .iter()
-            .filter(|(key, _)| key.is_todo())
-            .map(|(_, count)| count)
-            .sum()
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-    use crate::db::tests::{setup_db, TestDb};
-    use ruff_db::files::system_path_to_file;
-    use ruff_db::system::DbWithTestSystem;
-
-    fn get_stats<'db>(
-        db: &'db mut TestDb,
-        filename: &str,
-        source: &str,
-    ) -> &'db TypeStatistics<'db> {
-        db.write_dedented(filename, source).unwrap();
-
-        type_statistics(db, system_path_to_file(db, filename).unwrap())
-    }
-
-    #[test]
-    fn all_static() {
-        let mut db = setup_db();
-
-        let stats = get_stats(&mut db, "src/foo.py", "1");
-
-        assert_eq!(stats.0, FxHashMap::from_iter([(Type::IntLiteral(1), 1)]));
-    }
-
-    #[test]
-    fn todo_and_expression_count() {
-        let mut db = setup_db();
-
-        let stats = get_stats(
-            &mut db,
-            "src/foo.py",
-            r#"
-                x = [x for x in [1]]
-            "#,
-        );
-
-        assert_eq!(stats.todo_count(), 4);
-        assert_eq!(stats.expression_count(), 6);
-    }
-
-    #[test]
-    fn sum() {
-        let mut db = setup_db();
-
-        let stats = get_stats(
-            &mut db,
-            "src/foo.py",
-            r#"
-                1
-                def f():
-                    1
-            "#,
-        );
-
-        assert_eq!(stats.0[&Type::IntLiteral(1)], 2);
-    }
-}
--- a/crates/red_knot_python_semantic/src/types/string_annotation.rs
+++ b/crates/red_knot_python_semantic/src/types/string_annotation.rs
@@ -1,5 +1,4 @@
 use ruff_db::source::source_text;
-use ruff_python_ast::str::raw_contents;
 use ruff_python_ast::{self as ast, ModExpression};
 use ruff_python_parser::Parsed;
 use ruff_text_size::Ranged;
@@ -138,9 +137,8 @@ pub(crate) fn parse_string_annotation(
    let _span = tracing::trace_span!("parse_string_annotation", string=?string_expr.range(), file=%file.path(db)).entered();

    let source = source_text(db.upcast(), file);
-    let node_text = &source[string_expr.range()];

-    if let [string_literal] = string_expr.value.as_slice() {
+    if let Some(string_literal) = string_expr.as_unconcatenated_literal() {
        let prefix = string_literal.flags.prefix();
        if prefix.is_raw() {
            context.report_lint(
@@ -150,9 +148,7 @@ pub(crate) fn parse_string_annotation(
            );
        // Compare the raw contents (without quotes) of the expression with the parsed contents
        // contained in the string literal.
-        } else if raw_contents(node_text)
-            .is_some_and(|raw_contents| raw_contents == string_literal.as_str())
-        {
+        } else if &source[string_literal.content_range()] == string_literal.as_str() {
            match ruff_python_parser::parse_string_annotation(source.as_str(), string_literal) {
                Ok(parsed) => return Some(parsed),
                Err(parse_error) => context.report_lint(
--- a/crates/red_knot_python_semantic/src/types/subclass_of.rs
+++ b/crates/red_knot_python_semantic/src/types/subclass_of.rs
@@ -26,7 +26,7 @@ impl<'db> SubclassOfType<'db> {
            ClassBase::Class(class) => {
                if class.is_final(db) {
                    Type::ClassLiteral(ClassLiteralType { class })
-                } else if class.is_known(db, KnownClass::Object) {
+                } else if class.is_object(db) {
                    KnownClass::Type.to_instance(db)
                } else {
                    Type::SubclassOf(Self { subclass_of })
--- a/crates/red_knot_python_semantic/src/types/unpacker.rs
+++ b/crates/red_knot_python_semantic/src/types/unpacker.rs
@@ -261,7 +261,7 @@ impl<'db> Unpacker<'db> {
    }
 }

-#[derive(Debug, Default, PartialEq, Eq)]
+#[derive(Debug, Default, PartialEq, Eq, salsa::Update)]
 pub(crate) struct UnpackResult<'db> {
    targets: FxHashMap<ScopedExpressionId, Type<'db>>,
    diagnostics: TypeCheckDiagnostics,
--- a/crates/red_knot_python_semantic/src/unpack.rs
+++ b/crates/red_knot_python_semantic/src/unpack.rs
@@ -28,16 +28,15 @@ use crate::Db;
 /// * an argument of a cross-module query
 #[salsa::tracked]
 pub(crate) struct Unpack<'db> {
-    #[id]
    pub(crate) file: File,

-    #[id]
    pub(crate) file_scope: FileScopeId,

    /// The target expression that is being unpacked. For example, in `(a, b) = (1, 2)`, the target
    /// expression is `(a, b)`.
    #[no_eq]
    #[return_ref]
+    #[tracked]
    pub(crate) target: AstNodeRef<ast::Expr>,

    /// The ingredient representing the value expression of the unpacking. For example, in
@@ -45,7 +44,6 @@ pub(crate) struct Unpack<'db> {
    #[no_eq]
    pub(crate) value: UnpackValue<'db>,

-    #[no_eq]
    count: countme::Count<Unpack<'static>>,
 }

@@ -62,7 +60,7 @@ impl<'db> Unpack<'db> {
 }

 /// The expression that is being unpacked.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, Hash)]
 pub(crate) enum UnpackValue<'db> {
    /// An iterable expression like the one in a `for` loop or a comprehension.
    Iterable(Expression<'db>),
--- a/crates/red_knot_python_semantic/src/visibility_constraints.rs
+++ b/crates/red_knot_python_semantic/src/visibility_constraints.rs
@@ -338,7 +338,7 @@ const SMALLEST_TERMINAL: ScopedVisibilityConstraintId = ALWAYS_FALSE;

 /// A collection of visibility constraints. This is currently stored in `UseDefMap`, which means we
 /// maintain a separate set of visibility constraints for each scope in file.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Eq, salsa::Update)]
 pub(crate) struct VisibilityConstraints<'db> {
    constraints: IndexVec<Atom, Constraint<'db>>,
    interiors: IndexVec<ScopedVisibilityConstraintId, InteriorNode>,
@@ -627,7 +627,7 @@ impl<'db> VisibilityConstraints<'db> {
            ConstraintNode::Pattern(inner) => match inner.kind(db) {
                PatternConstraintKind::Value(value, guard) => {
                    let subject_expression = inner.subject(db);
-                    let inference = infer_expression_types(db, *subject_expression);
+                    let inference = infer_expression_types(db, subject_expression);
                    let scope = subject_expression.scope(db);
                    let subject_ty = inference.expression_type(
                        subject_expression
--- a/crates/red_knot_server/Cargo.toml
+++ b/crates/red_knot_server/Cargo.toml
@@ -12,7 +12,7 @@ license = { workspace = true }

 [dependencies]
 red_knot_project = { workspace = true }
-ruff_db = { workspace = true }
+ruff_db = { workspace = true, features = ["os"] }
 ruff_notebook = { workspace = true }
 ruff_python_ast = { workspace = true }
 ruff_source_file = { workspace = true }
--- a/crates/red_knot_server/src/server/api/requests/diagnostic.rs
+++ b/crates/red_knot_server/src/server/api/requests/diagnostic.rs
@@ -75,11 +75,13 @@ fn to_lsp_diagnostic(
    diagnostic: &dyn ruff_db::diagnostic::Diagnostic,
    encoding: crate::PositionEncoding,
 ) -> Diagnostic {
-    let range = if let (Some(file), Some(range)) = (diagnostic.file(), diagnostic.range()) {
-        let index = line_index(db.upcast(), file);
-        let source = source_text(db.upcast(), file);
+    let range = if let Some(span) = diagnostic.span() {
+        let index = line_index(db.upcast(), span.file());
+        let source = source_text(db.upcast(), span.file());

-        range.to_range(&source, &index, encoding)
+        span.range()
+            .map(|range| range.to_range(&source, &index, encoding))
+            .unwrap_or_default()
    } else {
        Range::default()
    };
--- a/crates/red_knot_server/src/session.rs
+++ b/crates/red_knot_server/src/session.rs
@@ -68,7 +68,9 @@ impl Session {
            let system = LSPSystem::new(index.clone());

            // TODO(dhruvmanila): Get the values from the client settings
-            let metadata = ProjectMetadata::discover(system_path, &system)?;
+            let mut metadata = ProjectMetadata::discover(system_path, &system)?;
+            metadata.apply_configuration_files(&system)?;
+
            // TODO(micha): Handle the case where the program settings are incorrect more gracefully.
            workspaces.insert(path, ProjectDatabase::new(metadata, system)?);
        }
--- a/crates/red_knot_server/src/system.rs
+++ b/crates/red_knot_server/src/system.rs
@@ -187,6 +187,10 @@ impl System for LSPSystem {
        self.os_system.current_directory()
    }

+    fn user_config_directory(&self) -> Option<SystemPathBuf> {
+        self.os_system.user_config_directory()
+    }
+
    fn read_directory<'a>(
        &'a self,
        path: &SystemPath,
--- a/crates/red_knot_test/src/config.rs
+++ b/crates/red_knot_test/src/config.rs
@@ -9,7 +9,8 @@
 //! ```

 use anyhow::Context;
-use red_knot_python_semantic::{PythonPlatform, PythonVersion};
+use red_knot_python_semantic::PythonPlatform;
+use ruff_python_ast::python_version::PythonVersion;
 use serde::Deserialize;

 #[derive(Deserialize, Debug, Default, Clone)]
--- a/crates/red_knot_test/src/db.rs
+++ b/crates/red_knot_test/src/db.rs
@@ -1,12 +1,15 @@
+use std::sync::Arc;
+
 use red_knot_python_semantic::lint::{LintRegistry, RuleSelection};
 use red_knot_python_semantic::{
    default_lint_registry, Db as SemanticDb, Program, ProgramSettings, PythonPlatform,
-    PythonVersion, SearchPathSettings,
+    SearchPathSettings,
 };
 use ruff_db::files::{File, Files};
 use ruff_db::system::{DbWithTestSystem, System, SystemPath, SystemPathBuf, TestSystem};
 use ruff_db::vendored::VendoredFileSystem;
 use ruff_db::{Db as SourceDb, Upcast};
+use ruff_python_ast::python_version::PythonVersion;

 #[salsa::db]
 #[derive(Clone)]
@@ -16,7 +19,7 @@ pub(crate) struct Db {
    files: Files,
    system: TestSystem,
    vendored: VendoredFileSystem,
-    rule_selection: RuleSelection,
+    rule_selection: Arc<RuleSelection>,
 }

 impl Db {
@@ -29,7 +32,7 @@ impl Db {
            system: TestSystem::default(),
            vendored: red_knot_vendored::file_system().clone(),
            files: Files::default(),
-            rule_selection,
+            rule_selection: Arc::new(rule_selection),
        };

        db.memory_file_system()
@@ -94,8 +97,8 @@ impl SemanticDb for Db {
        !file.path(self).is_vendored_path()
    }

-    fn rule_selection(&self) -> &RuleSelection {
-        &self.rule_selection
+    fn rule_selection(&self) -> Arc<RuleSelection> {
+        self.rule_selection.clone()
    }

    fn lint_registry(&self) -> &LintRegistry {
--- a/crates/red_knot_test/src/diagnostic.rs
+++ b/crates/red_knot_test/src/diagnostic.rs
@@ -26,7 +26,8 @@ where
            .into_iter()
            .map(|diagnostic| DiagnosticWithLine {
                line_number: diagnostic
-                    .range()
+                    .span()
+                    .and_then(|span| span.range())
                    .map_or(OneIndexed::from_zero_indexed(0), |range| {
                        line_index.line_index(range.start())
                    }),
@@ -144,7 +145,7 @@ struct DiagnosticWithLine<T> {
 mod tests {
    use crate::db::Db;
    use crate::diagnostic::Diagnostic;
-    use ruff_db::diagnostic::{DiagnosticId, LintName, Severity};
+    use ruff_db::diagnostic::{DiagnosticId, LintName, Severity, Span};
    use ruff_db::files::{system_path_to_file, File};
    use ruff_db::source::line_index;
    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
@@ -198,12 +199,8 @@ mod tests {
            "dummy".into()
        }

-        fn file(&self) -> Option<File> {
-            Some(self.file)
-        }
-
-        fn range(&self) -> Option<TextRange> {
-            Some(self.range)
+        fn span(&self) -> Option<Span> {
+            Some(Span::from(self.file).with_range(self.range))
        }

        fn severity(&self) -> Severity {
--- a/crates/red_knot_test/src/lib.rs
+++ b/crates/red_knot_test/src/lib.rs
@@ -5,7 +5,7 @@ use colored::Colorize;
 use parser as test_parser;
 use red_knot_python_semantic::types::check_types;
 use red_knot_python_semantic::{Program, ProgramSettings, SearchPathSettings, SitePackages};
-use ruff_db::diagnostic::{Diagnostic, ParseDiagnostic};
+use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig, ParseDiagnostic};
 use ruff_db::files::{system_path_to_file, File, Files};
 use ruff_db::panic::catch_unwind;
 use ruff_db::parsed::parsed_module;
@@ -300,9 +300,7 @@ fn create_diagnostic_snapshot<D: Diagnostic>(
    test: &parser::MarkdownTest,
    diagnostics: impl IntoIterator<Item = D>,
 ) -> String {
-    // TODO(ag): Do something better than requiring this
-    // global state to be twiddled everywhere.
-    colored::control::set_override(false);
+    let display_config = DisplayDiagnosticConfig::default().color(false);

    let mut snapshot = String::new();
    writeln!(snapshot).unwrap();
@@ -340,7 +338,7 @@ fn create_diagnostic_snapshot<D: Diagnostic>(
            writeln!(snapshot).unwrap();
        }
        writeln!(snapshot, "```").unwrap();
-        writeln!(snapshot, "{}", diag.display(db)).unwrap();
+        writeln!(snapshot, "{}", diag.display(db, &display_config)).unwrap();
        writeln!(snapshot, "```").unwrap();
    }
    snapshot
--- a/crates/red_knot_test/src/matcher.rs
+++ b/crates/red_knot_test/src/matcher.rs
@@ -257,7 +257,8 @@ impl Matcher {

    fn column<T: Diagnostic>(&self, diagnostic: &T) -> OneIndexed {
        diagnostic
-            .range()
+            .span()
+            .and_then(|span| span.range())
            .map(|range| {
                self.line_index
                    .source_location(range.start(), &self.source)
@@ -334,7 +335,7 @@ impl Matcher {
 #[cfg(test)]
 mod tests {
    use super::FailuresByLine;
-    use ruff_db::diagnostic::{Diagnostic, DiagnosticId, Severity};
+    use ruff_db::diagnostic::{Diagnostic, DiagnosticId, Severity, Span};
    use ruff_db::files::{system_path_to_file, File};
    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
    use ruff_python_trivia::textwrap::dedent;
@@ -385,12 +386,8 @@ mod tests {
            self.message.into()
        }

-        fn file(&self) -> Option<File> {
-            Some(self.file)
-        }
-
-        fn range(&self) -> Option<TextRange> {
-            Some(self.range)
+        fn span(&self) -> Option<Span> {
+            Some(Span::from(self.file).with_range(self.range))
        }

        fn severity(&self) -> Severity {
--- a/crates/red_knot_vendored/vendor/typeshed/source_commit.txt
+++ b/crates/red_knot_vendored/vendor/typeshed/source_commit.txt
@@ -1 +1 @@
-c193cd2a36839c8e6336f350397f51ce52fedd5e
+cc8ca939c0477a49fcce0554fa1743bd5c656a11
--- a/Show More
+++ b/Show More