Remove TODO message

## Summary

This PR implements [`use-implicit-booleaness-not-len` /
`C1802`](https://pylint.pycqa.org/en/latest/user_guide/messages/convention/use-implicit-booleaness-not-len.html)
> For sequences, (strings, lists, tuples), use the fact that empty
sequences are false.

---------

Co-authored-by: xbrtnik1 <524841@mail.muni.cz>
Co-authored-by: xbrtnik1 <xbrtnik1@mail.muni.cz>

.editorconfig

@@ -17,4 +17,7 @@ indent_size = 4
 trim_trailing_whitespace = false
 
 [*.md]
-max_line_length = 100
+max_line_length = 100
+
+[*.toml]
+indent_size = 4

.github/workflows/ci.yaml

@@ -115,7 +115,7 @@ jobs:
 
   cargo-test-linux:
     name: "cargo test (linux)"
-    runs-on: ubuntu-latest
+    runs-on: depot-ubuntu-22.04-16
     needs: determine_changes
     if: ${{ needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main' }}
     timeout-minutes: 20
@@ -157,9 +157,36 @@ jobs:
           name: ruff
           path: target/debug/ruff
 
+  cargo-test-linux-release:
+    name: "cargo test (linux, release)"
+    runs-on: depot-ubuntu-22.04-16
+    needs: determine_changes
+    if: ${{ needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main' }}
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v4
+      - name: "Install Rust toolchain"
+        run: rustup show
+      - name: "Install mold"
+        uses: rui314/setup-mold@v1
+      - name: "Install cargo nextest"
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
+      - name: "Install cargo insta"
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-insta
+      - uses: Swatinem/rust-cache@v2
+      - name: "Run tests"
+        shell: bash
+        env:
+          NEXTEST_PROFILE: "ci"
+        run: cargo insta test --release --all-features --unreferenced reject --test-runner nextest
+
   cargo-test-windows:
     name: "cargo test (windows)"
-    runs-on: windows-latest
+    runs-on: windows-latest-xlarge
     needs: determine_changes
     if: ${{ needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main' }}
     timeout-minutes: 20
@@ -197,6 +224,8 @@ jobs:
           cache: "npm"
           cache-dependency-path: playground/package-lock.json
       - uses: jetli/wasm-pack-action@v0.4.0
+        with:
+          version: v0.13.1
       - uses: Swatinem/rust-cache@v2
       - name: "Test ruff_wasm"
         run: |
@@ -210,8 +239,7 @@ jobs:
   cargo-build-release:
     name: "cargo build (release)"
     runs-on: macos-latest
-    needs: determine_changes
-    if: ${{ needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main' }}
+    if: ${{ github.ref == 'refs/heads/main' }}
     timeout-minutes: 20
     steps:
       - uses: actions/checkout@v4
@@ -255,11 +283,11 @@ jobs:
           NEXTEST_PROFILE: "ci"
         run: cargo +${{ steps.msrv.outputs.value }} insta test --all-features --unreferenced reject --test-runner nextest
 
-  cargo-fuzz:
-    name: "cargo fuzz"
+  cargo-fuzz-build:
+    name: "cargo fuzz build"
     runs-on: ubuntu-latest
     needs: determine_changes
-    if: ${{ needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main' }}
+    if: ${{ github.ref == 'refs/heads/main' }}
     timeout-minutes: 10
     steps:
       - uses: actions/checkout@v4
@@ -278,7 +306,7 @@ jobs:
       - run: cargo fuzz build -s none
 
   fuzz-parser:
-    name: "Fuzz the parser"
+    name: "fuzz parser"
     runs-on: ubuntu-latest
     needs:
       - cargo-test-linux
@@ -307,7 +335,7 @@ jobs:
           # Make executable, since artifact download doesn't preserve this
           chmod +x ${{ steps.download-cached-binary.outputs.download-path }}/ruff
 
-          python scripts/fuzz-parser/fuzz.py 0-500 --test-executable ${{ steps.download-cached-binary.outputs.download-path }}/ruff
+          python scripts/fuzz-parser/fuzz.py --bin ruff 0-500 --test-executable ${{ steps.download-cached-binary.outputs.download-path }}/ruff
 
   scripts:
     name: "test scripts"
@@ -331,7 +359,7 @@ jobs:
 
   ecosystem:
     name: "ecosystem"
-    runs-on: ubuntu-latest
+    runs-on: depot-ubuntu-latest-8
     needs:
       - cargo-test-linux
       - determine_changes
@@ -561,12 +589,12 @@ jobs:
         run: rustup show
       - name: "Cache rust"
         uses: Swatinem/rust-cache@v2
-      - name: "Formatter progress"
+      - name: "Run checks"
         run: scripts/formatter_ecosystem_checks.sh
       - name: "Github step summary"
-        run: cat target/progress_projects_stats.txt > $GITHUB_STEP_SUMMARY
+        run: cat target/formatter-ecosystem/stats.txt > $GITHUB_STEP_SUMMARY
       - name: "Remove checkouts from cache"
-        run: rm -r target/progress_projects
+        run: rm -r target/formatter-ecosystem
 
   check-ruff-lsp:
     name: "test ruff-lsp"

.github/workflows/daily_fuzz.yaml

@@ -49,7 +49,7 @@ jobs:
         # but this is outweighed by the fact that a release build takes *much* longer to compile in CI
         run: cargo build --locked
       - name: Fuzz
-        run: python scripts/fuzz-parser/fuzz.py $(shuf -i 0-9999999999999999999 -n 1000) --test-executable target/debug/ruff
+        run: python scripts/fuzz-parser/fuzz.py --bin ruff $(shuf -i 0-9999999999999999999 -n 1000) --test-executable target/debug/ruff
 
   create-issue-on-failure:
     name: Create an issue if the daily fuzz surfaced any bugs

.github/workflows/release.yml

@@ -1,4 +1,4 @@
-# This file was autogenerated by cargo-dist: https://opensource.axo.dev/cargo-dist/
+# This file was autogenerated by dist: https://opensource.axo.dev/cargo-dist/
 #
 # Copyright 2022-2024, axodotdev
 # SPDX-License-Identifier: MIT or Apache-2.0
@@ -6,7 +6,7 @@
 # CI that:
 #
 # * checks for a Git Tag that looks like a release
-# * builds artifacts with cargo-dist (archives, installers, hashes)
+# * builds artifacts with dist (archives, installers, hashes)
 # * uploads those artifacts to temporary workflow zip
 # * on success, uploads the artifacts to a GitHub Release
 #
@@ -24,10 +24,10 @@ permissions:
 # must be a Cargo-style SemVer Version (must have at least major.minor.patch).
 #
 # If PACKAGE_NAME is specified, then the announcement will be for that
-# package (erroring out if it doesn't have the given version or isn't cargo-dist-able).
+# package (erroring out if it doesn't have the given version or isn't dist-able).
 #
 # If PACKAGE_NAME isn't specified, then the announcement will be for all
-# (cargo-dist-able) packages in the workspace with that version (this mode is
+# (dist-able) packages in the workspace with that version (this mode is
 # intended for workspaces with only one dist-able package, or with all dist-able
 # packages versioned/released in lockstep).
 #
@@ -48,7 +48,7 @@ on:
         type: string
 
 jobs:
-  # Run 'cargo dist plan' (or host) to determine what tasks we need to do
+  # Run 'dist plan' (or host) to determine what tasks we need to do
   plan:
     runs-on: "ubuntu-20.04"
     outputs:
@@ -62,16 +62,16 @@ jobs:
       - uses: actions/checkout@v4
         with:
           submodules: recursive
-      - name: Install cargo-dist
+      - name: Install dist
         # we specify bash to get pipefail; it guards against the `curl` command
         # failing. otherwise `sh` won't catch that `curl` returned non-0
         shell: bash
-        run: "curl --proto '=https' --tlsv1.2 -LsSf https://github.com/axodotdev/cargo-dist/releases/download/v0.22.1/cargo-dist-installer.sh | sh"
-      - name: Cache cargo-dist
+        run: "curl --proto '=https' --tlsv1.2 -LsSf https://github.com/axodotdev/cargo-dist/releases/download/v0.25.2-prerelease.3/cargo-dist-installer.sh | sh"
+      - name: Cache dist
         uses: actions/upload-artifact@v4
         with:
           name: cargo-dist-cache
-          path: ~/.cargo/bin/cargo-dist
+          path: ~/.cargo/bin/dist
       # sure would be cool if github gave us proper conditionals...
       # so here's a doubly-nested ternary-via-truthiness to try to provide the best possible
       # functionality based on whether this is a pull_request, and whether it's from a fork.
@@ -79,8 +79,8 @@ jobs:
       # but also really annoying to build CI around when it needs secrets to work right.)
       - id: plan
         run: |
-          cargo dist ${{ (inputs.tag && inputs.tag != 'dry-run' && format('host --steps=create --tag={0}', inputs.tag)) || 'plan' }} --output-format=json > plan-dist-manifest.json
-          echo "cargo dist ran successfully"
+          dist ${{ (inputs.tag && inputs.tag != 'dry-run' && format('host --steps=create --tag={0}', inputs.tag)) || 'plan' }} --output-format=json > plan-dist-manifest.json
+          echo "dist ran successfully"
           cat plan-dist-manifest.json
           echo "manifest=$(jq -c "." plan-dist-manifest.json)" >> "$GITHUB_OUTPUT"
       - name: "Upload dist-manifest.json"
@@ -124,12 +124,12 @@ jobs:
       - uses: actions/checkout@v4
         with:
           submodules: recursive
-      - name: Install cached cargo-dist
+      - name: Install cached dist
         uses: actions/download-artifact@v4
         with:
           name: cargo-dist-cache
           path: ~/.cargo/bin/
-      - run: chmod +x ~/.cargo/bin/cargo-dist
+      - run: chmod +x ~/.cargo/bin/dist
       # Get all the local artifacts for the global tasks to use (for e.g. checksums)
       - name: Fetch local artifacts
         uses: actions/download-artifact@v4
@@ -140,8 +140,8 @@ jobs:
       - id: cargo-dist
         shell: bash
         run: |
-          cargo dist build ${{ needs.plan.outputs.tag-flag }} --output-format=json "--artifacts=global" > dist-manifest.json
-          echo "cargo dist ran successfully"
+          dist build ${{ needs.plan.outputs.tag-flag }} --output-format=json "--artifacts=global" > dist-manifest.json
+          echo "dist ran successfully"
 
           # Parse out what we just built and upload it to scratch storage
           echo "paths<<EOF" >> "$GITHUB_OUTPUT"
@@ -174,12 +174,12 @@ jobs:
       - uses: actions/checkout@v4
         with:
           submodules: recursive
-      - name: Install cached cargo-dist
+      - name: Install cached dist
         uses: actions/download-artifact@v4
         with:
           name: cargo-dist-cache
           path: ~/.cargo/bin/
-      - run: chmod +x ~/.cargo/bin/cargo-dist
+      - run: chmod +x ~/.cargo/bin/dist
       # Fetch artifacts from scratch-storage
       - name: Fetch artifacts
         uses: actions/download-artifact@v4
@@ -191,7 +191,7 @@ jobs:
       - id: host
         shell: bash
         run: |
-          cargo dist host ${{ needs.plan.outputs.tag-flag }} --steps=upload --steps=release --output-format=json > dist-manifest.json
+          dist host ${{ needs.plan.outputs.tag-flag }} --steps=upload --steps=release --output-format=json > dist-manifest.json
           echo "artifacts uploaded and released successfully"
           cat dist-manifest.json
           echo "manifest=$(jq -c "." dist-manifest.json)" >> "$GITHUB_OUTPUT"

.pre-commit-config.yaml

@@ -17,7 +17,7 @@ exclude: |
 
 repos:
   - repo: https://github.com/abravalheri/validate-pyproject
-    rev: v0.22
+    rev: v0.23
     hooks:
       - id: validate-pyproject
 
@@ -73,7 +73,7 @@ repos:
         pass_filenames: false # This makes it a lot faster
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.7.3
+    rev: v0.7.4
     hooks:
       - id: ruff-format
       - id: ruff

BREAKING_CHANGES.md

@@ -1,5 +1,30 @@
 # Breaking Changes
 
+## 0.8.0
+
+- **Default to Python 3.9**
+
+    Ruff now defaults to Python 3.9 instead of 3.8 if no explicit Python version is configured using [`ruff.target-version`](https://docs.astral.sh/ruff/settings/#target-version) or [`project.requires-python`](https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#python-requires) ([#13896](https://github.com/astral-sh/ruff/pull/13896))
+
+- **Changed location of `pydoclint` diagnostics**
+
+    [`pydoclint`](https://docs.astral.sh/ruff/rules/#pydoclint-doc) diagnostics now point to the first-line of the problematic docstring. Previously, this was not the case.
+
+    If you've opted into these preview rules but have them suppressed using
+    [`noqa`](https://docs.astral.sh/ruff/linter/#error-suppression) comments in
+    some places, this change may mean that you need to move the `noqa` suppression
+    comments. Most users should be unaffected by this change.
+
+- **Use XDG (i.e. `~/.local/bin`) instead of the Cargo home directory in the standalone installer**
+
+    Previously, Ruff's installer used `$CARGO_HOME` or `~/.cargo/bin` for its target install directory. Now, Ruff will be installed into `$XDG_BIN_HOME`, `$XDG_DATA_HOME/../bin`, or `~/.local/bin` (in that order).
+
+    This change is only relevant to users of the standalone Ruff installer (using the shell or PowerShell script). If you installed Ruff using uv or pip, you should be unaffected.
+
+- **Changes to the line width calculation**
+
+    Ruff now uses a new version of the [unicode-width](https://github.com/unicode-rs/unicode-width) Rust crate to calculate the line width. In very rare cases, this may lead to lines containing Unicode characters being reformatted, or being considered too long when they were not before ([`E501`](https://docs.astral.sh/ruff/rules/line-too-long/)).
+
 ## 0.7.0
 
 - The pytest rules `PT001` and `PT023` now default to omitting the decorator parentheses when there are no arguments

CHANGELOG.md

@@ -1,5 +1,104 @@
 # Changelog
 
+## 0.8.0
+
+Check out the [blog post](https://astral.sh/blog/ruff-v0.8.0) for a migration guide and overview of the changes!
+
+### Breaking changes
+
+See also, the "Remapped rules" section which may result in disabled rules.
+
+- **Default to Python 3.9**
+
+    Ruff now defaults to Python 3.9 instead of 3.8 if no explicit Python version is configured using [`ruff.target-version`](https://docs.astral.sh/ruff/settings/#target-version) or [`project.requires-python`](https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#python-requires) ([#13896](https://github.com/astral-sh/ruff/pull/13896))
+
+- **Changed location of `pydoclint` diagnostics**
+
+    [`pydoclint`](https://docs.astral.sh/ruff/rules/#pydoclint-doc) diagnostics now point to the first-line of the problematic docstring. Previously, this was not the case.
+
+    If you've opted into these preview rules but have them suppressed using
+    [`noqa`](https://docs.astral.sh/ruff/linter/#error-suppression) comments in
+    some places, this change may mean that you need to move the `noqa` suppression
+    comments. Most users should be unaffected by this change.
+
+- **Use XDG (i.e. `~/.local/bin`) instead of the Cargo home directory in the standalone installer**
+
+    Previously, Ruff's installer used `$CARGO_HOME` or `~/.cargo/bin` for its target install directory. Now, Ruff will be installed into `$XDG_BIN_HOME`, `$XDG_DATA_HOME/../bin`, or `~/.local/bin` (in that order).
+
+    This change is only relevant to users of the standalone Ruff installer (using the shell or PowerShell script). If you installed Ruff using uv or pip, you should be unaffected.
+
+- **Changes to the line width calculation**
+
+    Ruff now uses a new version of the [unicode-width](https://github.com/unicode-rs/unicode-width) Rust crate to calculate the line width. In very rare cases, this may lead to lines containing Unicode characters being reformatted, or being considered too long when they were not before ([`E501`](https://docs.astral.sh/ruff/rules/line-too-long/)).
+
+### Removed Rules
+
+The following deprecated rules have been removed:
+
+- [`missing-type-self`](https://docs.astral.sh/ruff/rules/missing-type-self/) (`ANN101`)
+- [`missing-type-cls`](https://docs.astral.sh/ruff/rules/missing-type-cls/) (`ANN102`)
+- [`syntax-error`](https://docs.astral.sh/ruff/rules/syntax-error/) (`E999`)
+- [`pytest-missing-fixture-name-underscore`](https://docs.astral.sh/ruff/rules/pytest-missing-fixture-name-underscore/) (`PT004`)
+- [`pytest-incorrect-fixture-name-underscore`](https://docs.astral.sh/ruff/rules/pytest-incorrect-fixture-name-underscore/) (`PT005`)
+- [`unpacked-list-comprehension`](https://docs.astral.sh/ruff/rules/unpacked-list-comprehension/) (`UP027`)
+
+### Remapped rules
+
+The following rules have been remapped to new rule codes:
+
+- [`flake8-type-checking`](https://docs.astral.sh/ruff/rules/#flake8-type-checking-tc): `TCH` to `TC`
+
+### Stabilization
+
+The following rules have been stabilized and are no longer in preview:
+
+- [`builtin-import-shadowing`](https://docs.astral.sh/ruff/rules/builtin-import-shadowing/) (`A004`)
+- [`mutable-contextvar-default`](https://docs.astral.sh/ruff/rules/mutable-contextvar-default/) (`B039`)
+- [`fast-api-redundant-response-model`](https://docs.astral.sh/ruff/rules/fast-api-redundant-response-model/) (`FAST001`)
+- [`fast-api-non-annotated-dependency`](https://docs.astral.sh/ruff/rules/fast-api-non-annotated-dependency/) (`FAST002`)
+- [`dict-index-missing-items`](https://docs.astral.sh/ruff/rules/dict-index-missing-items/) (`PLC0206`)
+- [`pep484-style-positional-only-parameter`](https://docs.astral.sh/ruff/rules/pep484-style-positional-only-parameter/) (`PYI063`)
+- [`redundant-final-literal`](https://docs.astral.sh/ruff/rules/redundant-final-literal/) (`PYI064`)
+- [`bad-version-info-order`](https://docs.astral.sh/ruff/rules/bad-version-info-order/) (`PYI066`)
+- [`parenthesize-chained-operators`](https://docs.astral.sh/ruff/rules/parenthesize-chained-operators/) (`RUF021`)
+- [`unsorted-dunder-all`](https://docs.astral.sh/ruff/rules/unsorted-dunder-all/) (`RUF022`)
+- [`unsorted-dunder-slots`](https://docs.astral.sh/ruff/rules/unsorted-dunder-slots/) (`RUF023`)
+- [`assert-with-print-message`](https://docs.astral.sh/ruff/rules/assert-with-print-message/) (`RUF030`)
+- [`unnecessary-default-type-args`](https://docs.astral.sh/ruff/rules/unnecessary-default-type-args/) (`UP043`)
+
+The following behaviors have been stabilized:
+
+- [`ambiguous-variable-name`](https://docs.astral.sh/ruff/rules/ambiguous-variable-name/) (`E741`): Violations in stub files are now ignored. Stub authors typically don't control variable names.
+- [`printf-string-formatting`](https://docs.astral.sh/ruff/rules/printf-string-formatting/) (`UP031`): Report all `printf`-like usages even if no autofix is available
+
+The following fixes have been stabilized:
+
+- [`zip-instead-of-pairwise`](https://docs.astral.sh/ruff/rules/zip-instead-of-pairwise/) (`RUF007`)
+
+### Preview features
+
+- \[`flake8-datetimez`\] Exempt `min.time()` and `max.time()` (`DTZ901`) ([#14394](https://github.com/astral-sh/ruff/pull/14394))
+- \[`flake8-pie`\] Mark fix as unsafe if the following statement is a string literal (`PIE790`) ([#14393](https://github.com/astral-sh/ruff/pull/14393))
+- \[`flake8-pyi`\] New rule `redundant-none-literal` (`PYI061`) ([#14316](https://github.com/astral-sh/ruff/pull/14316))
+- \[`flake8-pyi`\] Add autofix for `redundant-numeric-union` (`PYI041`) ([#14273](https://github.com/astral-sh/ruff/pull/14273))
+- \[`ruff`\] New rule `map-int-version-parsing` (`RUF048`) ([#14373](https://github.com/astral-sh/ruff/pull/14373))
+- \[`ruff`\] New rule `redundant-bool-literal` (`RUF038`) ([#14319](https://github.com/astral-sh/ruff/pull/14319))
+- \[`ruff`\] New rule `unraw-re-pattern` (`RUF039`) ([#14446](https://github.com/astral-sh/ruff/pull/14446))
+- \[`pycodestyle`\] Exempt `pytest.importorskip()` calls (`E402`) ([#14474](https://github.com/astral-sh/ruff/pull/14474))
+- \[`pylint`\] Autofix suggests using sets when possible (`PLR1714`) ([#14372](https://github.com/astral-sh/ruff/pull/14372))
+
+### Rule changes
+
+- [`invalid-pyproject-toml`](https://docs.astral.sh/ruff/rules/invalid-pyproject-toml/) (`RUF200`): Updated to reflect the provisionally accepted [PEP 639](https://peps.python.org/pep-0639/).
+- \[`flake8-pyi`\] Avoid panic in unfixable case (`PYI041`) ([#14402](https://github.com/astral-sh/ruff/pull/14402))
+- \[`flake8-type-checking`\] Correctly handle quotes in subscript expression when generating an autofix ([#14371](https://github.com/astral-sh/ruff/pull/14371))
+- \[`pylint`\] Suggest correct autofix for `__contains__` (`PLC2801`) ([#14424](https://github.com/astral-sh/ruff/pull/14424))
+
+### Configuration
+
+- Ruff now emits a warning instead of an error when a configuration [`ignore`](https://docs.astral.sh/ruff/settings/#lint_ignore)s a rule that has been removed ([#14435](https://github.com/astral-sh/ruff/pull/14435))
+- Ruff now validates that `lint.flake8-import-conventions.aliases` only uses valid module names and aliases ([#14477](https://github.com/astral-sh/ruff/pull/14477))
+
 ## 0.7.4
 
 ### Preview features
@@ -978,7 +1077,7 @@ The following deprecated CLI commands have been removed:
 ### Preview features
 
 - \[`flake8-bugbear`\] Implement `return-in-generator` (`B901`) ([#11644](https://github.com/astral-sh/ruff/pull/11644))
-- \[`flake8-pyi`\] Implement `PYI063` ([#11699](https://github.com/astral-sh/ruff/pull/11699))
+- \[`flake8-pyi`\] Implement `pep484-style-positional-only-parameter` (`PYI063`) ([#11699](https://github.com/astral-sh/ruff/pull/11699))
 - \[`pygrep_hooks`\] Check blanket ignores via file-level pragmas (`PGH004`) ([#11540](https://github.com/astral-sh/ruff/pull/11540))
 
 ### Rule changes

Cargo.lock

@@ -170,6 +170,12 @@ version = "1.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f1fdabc7756949593fe60f30ec81974b613357de856987752631dea1e3394c80"
 
+[[package]]
+name = "base64"
+version = "0.13.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9e1b586273c5702936fe7b7d6896644d8be71e6314cfe09d3167c95f712589e8"
+
 [[package]]
 name = "base64"
 version = "0.22.0"
@@ -207,10 +213,16 @@ dependencies = [
 ]
 
 [[package]]
-name = "bstr"
-version = "1.10.0"
+name = "boxcar"
+version = "0.2.7"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "40723b8fb387abc38f4f4a37c09073622e41dd12327033091ef8950659e6dc0c"
+checksum = "7f839cdf7e2d3198ac6ca003fd8ebc61715755f41c1cad15ff13df67531e00ed"
+
+[[package]]
+name = "bstr"
+version = "1.11.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1a68f1f47cdf0ec8ee4b941b2eee2a80cb796db73118c0dd09ac63fbe405be22"
 dependencies = [
  "memchr",
  "regex-automata 0.4.8",
@@ -341,9 +353,9 @@ dependencies = [
 
 [[package]]
 name = "clap"
-version = "4.5.20"
+version = "4.5.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b97f376d85a664d5837dbae44bf546e6477a679ff6610010f17276f686d867e8"
+checksum = "fb3b4b9e5a7c7514dfa52869339ee98b3156b0bfb4e8a77c4ff4babb64b1604f"
 dependencies = [
  "clap_builder",
  "clap_derive",
@@ -351,9 +363,9 @@ dependencies = [
 
 [[package]]
 name = "clap_builder"
-version = "4.5.20"
+version = "4.5.21"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "19bc80abd44e4bed93ca373a0704ccbd1b710dc5749406201bb018272808dc54"
+checksum = "b17a95aa67cc7b5ebd32aa5370189aa0d79069ef1c64ce893bd30fb24bff20ec"
 dependencies = [
  "anstream",
  "anstyle",
@@ -401,7 +413,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -681,7 +693,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "strsim 0.10.0",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -692,7 +704,7 @@ checksum = "a668eda54683121533a393014d8692171709ff57a7d61f187b6e782719f8933f"
 dependencies = [
  "darling_core",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -746,23 +758,23 @@ dependencies = [
 
 [[package]]
 name = "dir-test"
-version = "0.3.0"
+version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5c44bdf9319ad5223afb7eb15a7110452b0adf0373ea6756561b2c708eef0dd1"
+checksum = "b12781621d53fd9087021f5a338df5c57c04f84a6231c1f4726f45e2e333470b"
 dependencies = [
  "dir-test-macros",
 ]
 
 [[package]]
 name = "dir-test-macros"
-version = "0.3.0"
+version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "644f96047137dfaa7a09e34d4623f9e52a1926ecc25ba32ad2ba3fc422536b25"
+checksum = "1340852f50b2285d01a7f598cc5d08b572669c3e09e614925175cc3c26787b91"
 dependencies = [
  "glob",
  "proc-macro2",
  "quote",
- "syn 1.0.109",
+ "syn",
 ]
 
 [[package]]
@@ -814,7 +826,7 @@ checksum = "97369cbbc041bc366949bc74d34658d6cda5621039731c6310521892a3a20ae0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -829,6 +841,12 @@ version = "0.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9bda8e21c04aca2ae33ffc2fd8c23134f3cac46db123ba97bd9d3f3b8a4a85e1"
 
+[[package]]
+name = "dunce"
+version = "1.0.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "92773504d58c093f6de2459af4af33faa518c13451eb8f2b5698ed3d36e7c813"
+
 [[package]]
 name = "dyn-clone"
 version = "1.0.17"
@@ -1050,9 +1068,9 @@ dependencies = [
 
 [[package]]
 name = "hashbrown"
-version = "0.15.1"
+version = "0.15.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3a9bfc1af68b1726ea47d3d5109de126281def866b33970e10fbab11b5dafab3"
+checksum = "bf151400ff0baff5465007dd2f3e717f3fe502074ca563069ce3a6629d07b289"
 
 [[package]]
 name = "hashlink"
@@ -1228,7 +1246,7 @@ checksum = "1ec89e9337638ecdc08744df490b221a7399bf8d164eb52a665454e60e075ad6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -1301,22 +1319,22 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "707907fe3c25f5424cce2cb7e1cbcafee6bdbe735ca90ef77c29e84591e5b9da"
 dependencies = [
  "equivalent",
- "hashbrown 0.15.1",
+ "hashbrown 0.15.2",
  "serde",
 ]
 
 [[package]]
 name = "indicatif"
-version = "0.17.8"
+version = "0.17.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "763a5a8f45087d6bcea4222e7b72c291a054edf80e4ef6efd2a4979878c7bea3"
+checksum = "cbf675b85ed934d3c67b5c5469701eec7db22689d0a2139d856e0925fa28b281"
 dependencies = [
  "console",
- "instant",
  "number_prefix",
  "portable-atomic",
- "unicode-width 0.1.13",
+ "unicode-width 0.2.0",
  "vt100",
+ "web-time",
 ]
 
 [[package]]
@@ -1358,6 +1376,7 @@ dependencies = [
  "pest",
  "pest_derive",
  "regex",
+ "ron",
  "serde",
  "similar",
  "walkdir",
@@ -1401,7 +1420,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -1501,15 +1520,15 @@ checksum = "e2abad23fbc42b3700f2f279844dc832adb2b2eb069b2df918f455c4e18cc646"
 
 [[package]]
 name = "libc"
-version = "0.2.162"
+version = "0.2.164"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "18d287de67fe55fd7e1581fe933d965a5a9477b38e949cfa9f8574ef01506398"
+checksum = "433bfe06b8c75da9b2e3fbea6e5329ff87748f0b144ef75306e674c3f6f7c13f"
 
 [[package]]
 name = "libcst"
-version = "1.5.0"
+version = "1.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "1586dd7a857d8a61a577afde1a24cc9573ff549eff092d5ce968b1ec93cc61b6"
+checksum = "fa3e60579a8cba3d86aa4a5f7fc98973cc0fd2ac270bf02f85a9bef09700b075"
 dependencies = [
  "chic",
  "libcst_derive",
@@ -1527,7 +1546,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a2ae40017ac09cd2c6a53504cb3c871c7f2b41466eac5bc66ba63f39073b467b"
 dependencies = [
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -1691,9 +1710,9 @@ checksum = "308d96db8debc727c3fd9744aac51751243420e46edf401010908da7f8d5e57c"
 
 [[package]]
 name = "newtype-uuid"
-version = "1.1.0"
+version = "1.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3526cb7c660872e401beaf3297f95f548ce3b4b4bdd8121b7c0713771d7c4a6e"
+checksum = "4c8781e2ef64806278a55ad223f0bc875772fd40e1fe6e73e8adbf027817229d"
 dependencies = [
  "uuid",
 ]
@@ -1922,18 +1941,6 @@ version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e3aeb8f54c078314c2065ee649a7241f46b9d8e418e1a9581ba0546657d7aa3a"
 
-[[package]]
-name = "pep440_rs"
-version = "0.4.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e0c29f9c43de378b4e4e0cd7dbcce0e5cfb80443de8c05620368b2948bc936a1"
-dependencies = [
- "once_cell",
- "regex",
- "serde",
- "unicode-width 0.1.13",
-]
-
 [[package]]
 name = "pep440_rs"
 version = "0.7.2"
@@ -1943,22 +1950,29 @@ dependencies = [
  "serde",
  "unicode-width 0.2.0",
  "unscanny",
+ "version-ranges",
 ]
 
 [[package]]
 name = "pep508_rs"
-version = "0.3.0"
+version = "0.9.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "910c513bea0f4f833122321c0f20e8c704e01de98692f6989c2ec21f43d88b1e"
+checksum = "8c2feee999fa547bacab06a4881bacc74688858b92fa8ef1e206c748b0a76048"
 dependencies = [
+ "boxcar",
+ "indexmap",
+ "itertools 0.13.0",
  "once_cell",
- "pep440_rs 0.4.0",
+ "pep440_rs",
  "regex",
+ "rustc-hash 2.0.0",
  "serde",
+ "smallvec",
  "thiserror 1.0.67",
- "tracing",
- "unicode-width 0.1.13",
+ "unicode-width 0.2.0",
  "url",
+ "urlencoding",
+ "version-ranges",
 ]
 
 [[package]]
@@ -1998,7 +2012,7 @@ dependencies = [
  "pest_meta",
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -2113,46 +2127,47 @@ dependencies = [
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.89"
+version = "1.0.92"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f139b0662de085916d1fb67d2b4169d1addddda1919e696f3252b740b629986e"
+checksum = "37d3544b3f2748c54e147655edb5025752e2303145b5aefb3c3ea2c78b973bb0"
 dependencies = [
  "unicode-ident",
 ]
 
 [[package]]
 name = "pyproject-toml"
-version = "0.9.0"
+version = "0.13.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "95c3dd745f99aa3c554b7bb00859f7d18c2f1d6afd749ccc86d60b61e702abd9"
+checksum = "643af57c3f36ba90a8b53e972727d8092f7408a9ebfbaf4c3d2c17b07c58d835"
 dependencies = [
  "indexmap",
- "pep440_rs 0.4.0",
+ "pep440_rs",
  "pep508_rs",
  "serde",
+ "thiserror 1.0.67",
  "toml",
 ]
 
 [[package]]
 name = "quick-junit"
-version = "0.5.0"
+version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "62ffd2f9a162cfae131bed6d9d1ed60adced33be340a94f96952897d7cb0c240"
+checksum = "3ed1a693391a16317257103ad06a88c6529ac640846021da7c435a06fffdacd7"
 dependencies = [
  "chrono",
  "indexmap",
  "newtype-uuid",
  "quick-xml",
  "strip-ansi-escapes",
- "thiserror 1.0.67",
+ "thiserror 2.0.3",
  "uuid",
 ]
 
 [[package]]
 name = "quick-xml"
-version = "0.36.1"
+version = "0.37.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "96a05e2e8efddfa51a84ca47cec303fac86c8541b686d37cac5efc0e094417bc"
+checksum = "f22f29bdff3987b4d8632ef95fd6424ec7e4e0a57e2f4fc63e489e75357f6a03"
 dependencies = [
  "memchr",
 ]
@@ -2251,7 +2266,7 @@ dependencies = [
  "compact_str",
  "countme",
  "dir-test",
- "hashbrown 0.15.1",
+ "hashbrown 0.15.2",
  "indexmap",
  "insta",
  "itertools 0.13.0",
@@ -2269,6 +2284,7 @@ dependencies = [
  "ruff_text_size",
  "rustc-hash 2.0.0",
  "salsa",
+ "serde",
  "smallvec",
  "static_assertions",
  "tempfile",
@@ -2353,7 +2369,10 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "crossbeam",
+ "glob",
+ "insta",
  "notify",
+ "pep440_rs",
  "rayon",
  "red_knot_python_semantic",
  "red_knot_vendored",
@@ -2363,7 +2382,9 @@ dependencies = [
  "ruff_text_size",
  "rustc-hash 2.0.0",
  "salsa",
- "tempfile",
+ "serde",
+ "thiserror 2.0.3",
+ "toml",
  "tracing",
 ]
 
@@ -2455,9 +2476,20 @@ dependencies = [
  "windows-sys 0.52.0",
 ]
 
+[[package]]
+name = "ron"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "88073939a61e5b7680558e6be56b419e208420c2adb92be54921fa6b72283f1a"
+dependencies = [
+ "base64 0.13.1",
+ "bitflags 1.3.2",
+ "serde",
+]
+
 [[package]]
 name = "ruff"
-version = "0.7.4"
+version = "0.8.0"
 dependencies = [
  "anyhow",
  "argfile",
@@ -2556,7 +2588,9 @@ dependencies = [
  "camino",
  "countme",
  "dashmap 6.1.0",
+ "dunce",
  "filetime",
+ "glob",
  "ignore",
  "insta",
  "matchit",
@@ -2642,7 +2676,7 @@ dependencies = [
  "serde",
  "static_assertions",
  "tracing",
- "unicode-width 0.1.13",
+ "unicode-width 0.2.0",
 ]
 
 [[package]]
@@ -2674,7 +2708,7 @@ dependencies = [
 
 [[package]]
 name = "ruff_linter"
-version = "0.7.4"
+version = "0.8.0"
 dependencies = [
  "aho-corasick",
  "annotate-snippets 0.9.2",
@@ -2697,7 +2731,7 @@ dependencies = [
  "natord",
  "path-absolutize",
  "pathdiff",
- "pep440_rs 0.7.2",
+ "pep440_rs",
  "pyproject-toml",
  "quick-junit",
  "regex",
@@ -2728,7 +2762,7 @@ dependencies = [
  "toml",
  "typed-arena",
  "unicode-normalization",
- "unicode-width 0.1.13",
+ "unicode-width 0.2.0",
  "unicode_names2",
  "url",
 ]
@@ -2741,7 +2775,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "ruff_python_trivia",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -2778,7 +2812,6 @@ dependencies = [
  "ruff_source_file",
  "ruff_text_size",
  "rustc-hash 2.0.0",
- "salsa",
  "schemars",
  "serde",
 ]
@@ -2990,7 +3023,7 @@ dependencies = [
 
 [[package]]
 name = "ruff_wasm"
-version = "0.7.4"
+version = "0.8.0"
 dependencies = [
  "console_error_panic_hook",
  "console_log",
@@ -3029,7 +3062,7 @@ dependencies = [
  "matchit",
  "path-absolutize",
  "path-slash",
- "pep440_rs 0.7.2",
+ "pep440_rs",
  "regex",
  "ruff_cache",
  "ruff_formatter",
@@ -3039,6 +3072,7 @@ dependencies = [
  "ruff_python_ast",
  "ruff_python_formatter",
  "ruff_python_semantic",
+ "ruff_python_stdlib",
  "ruff_source_file",
  "rustc-hash 2.0.0",
  "schemars",
@@ -3161,7 +3195,7 @@ dependencies = [
  "heck",
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
  "synstructure",
 ]
 
@@ -3195,7 +3229,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "serde_derive_internals",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3218,9 +3252,9 @@ checksum = "1c107b6f4780854c8b126e228ea8869f4d7b71260f962fefb57b996b8959ba6b"
 
 [[package]]
 name = "serde"
-version = "1.0.214"
+version = "1.0.215"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f55c3193aca71c12ad7890f1785d2b73e1b9f63a0bbc353c08ef26fe03fc56b5"
+checksum = "6513c1ad0b11a9376da888e3e0baa0077f1aed55c17f50e7b2397136129fb88f"
 dependencies = [
  "serde_derive",
 ]
@@ -3238,13 +3272,13 @@ dependencies = [
 
 [[package]]
 name = "serde_derive"
-version = "1.0.214"
+version = "1.0.215"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "de523f781f095e28fa605cdce0f8307e451cc0fd14e2eb4cd2e98a355b147766"
+checksum = "ad1e866f866923f252f05c889987993144fb74e722403468a4ebd70c3cd756c0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3255,14 +3289,14 @@ checksum = "330f01ce65a3a5fe59a60c82f3c9a024b573b8a6e875bd233fe5f934e71d54e3"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
 name = "serde_json"
-version = "1.0.132"
+version = "1.0.133"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d726bfaff4b320266d395898905d0eba0345aae23b54aee3a737e260fd46db03"
+checksum = "c7fceb2473b9166b2294ef05efcb65a3db80803f0b03ef86a5fc88a2b85ee377"
 dependencies = [
  "itoa",
  "memchr",
@@ -3278,7 +3312,7 @@ checksum = "6c64451ba24fc7a6a2d60fc75dd9c83c90903b19028d4eff35e88fc1e86564e9"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3319,7 +3353,7 @@ dependencies = [
  "darling",
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3427,7 +3461,7 @@ dependencies = [
  "proc-macro2",
  "quote",
  "rustversion",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3438,20 +3472,9 @@ checksum = "81cdd64d312baedb58e21336b31bc043b77e01cc99033ce76ef539f78e965ebc"
 
 [[package]]
 name = "syn"
-version = "1.0.109"
+version = "2.0.89"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "72b64191b275b66ffe2469e8af2c1cfe3bafa67b529ead792a6d0160888b4237"
-dependencies = [
- "proc-macro2",
- "quote",
- "unicode-ident",
-]
-
-[[package]]
-name = "syn"
-version = "2.0.87"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "25aa4ce346d03a6dcd68dd8b4010bcb74e54e62c90c573f394c46eae99aba32d"
+checksum = "44d46482f1c1c87acd84dea20c1bf5ebff4c757009ed6bf19cfd36fb10e92c4e"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -3466,7 +3489,7 @@ checksum = "c8af7666ab7b6390ab78131fb5b0fce11d6b7a6951602017c35fa82800708971"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3529,7 +3552,7 @@ dependencies = [
  "cfg-if",
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3540,7 +3563,7 @@ checksum = "5c89e72a01ed4c579669add59014b9a524d609c0c88c6a585ce37485879f6ffb"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
  "test-case-core",
 ]
 
@@ -3570,7 +3593,7 @@ checksum = "b607164372e89797d78b8e23a6d67d5d1038c1c65efd52e1389ef8b77caba2a6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3581,7 +3604,7 @@ checksum = "f077553d607adc1caf65430528a576c757a71ed73944b66ebb58ef2bbd243568"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3703,7 +3726,7 @@ checksum = "34704c8d6ebcbc939824180af020566b01a7c01f80641264eba0999f6c2b6be7"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3842,9 +3865,9 @@ dependencies = [
 
 [[package]]
 name = "unicode-ident"
-version = "1.0.13"
+version = "1.0.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e91b56cd4cadaeb79bbf1a5645f6b4f8dc5bde8834ad5894a8db35fda9efa1fe"
+checksum = "adb9e6ca4f869e1180728b7950e35922a7fc6397f7b641499e8f3ef06e50dc83"
 
 [[package]]
 name = "unicode-normalization"
@@ -3907,7 +3930,7 @@ version = "2.10.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b74fc6b57825be3373f7054754755f03ac3a8f5d70015ccad699ba2029956f4a"
 dependencies = [
- "base64",
+ "base64 0.22.0",
  "flate2",
  "log",
  "once_cell",
@@ -3919,9 +3942,9 @@ dependencies = [
 
 [[package]]
 name = "url"
-version = "2.5.3"
+version = "2.5.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8d157f1b96d14500ffdc1f10ba712e780825526c03d9a49b4d0324b0d9113ada"
+checksum = "32f8b686cadd1473f4bd0117a5d28d36b1ade384ea9b5069a1c40aefed7fda60"
 dependencies = [
  "form_urlencoded",
  "idna",
@@ -3929,6 +3952,12 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "urlencoding"
+version = "2.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "daf8dba3b7eb870caf1ddeed7bc9d2a049f3cfdfae7cb521b087cc33ae4c49da"
+
 [[package]]
 name = "utf16_iter"
 version = "1.0.5"
@@ -3967,7 +3996,7 @@ checksum = "6b91f57fe13a38d0ce9e28a03463d8d3c2468ed03d75375110ec71d93b449a08"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -3976,6 +4005,15 @@ version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "830b7e5d4d90034032940e4ace0d9a9a057e7a45cd94e6c007832e39edb82f6d"
 
+[[package]]
+name = "version-ranges"
+version = "0.1.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f8d079415ceb2be83fc355adbadafe401307d5c309c7e6ade6638e6f9f42f42d"
+dependencies = [
+ "smallvec",
+]
+
 [[package]]
 name = "version_check"
 version = "0.9.4"
@@ -4053,7 +4091,7 @@ dependencies = [
  "once_cell",
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
  "wasm-bindgen-shared",
 ]
 
@@ -4087,7 +4125,7 @@ checksum = "26c6ab57572f7a24a4985830b120de1594465e5d500f24afe89e16b4e833ef68"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
  "wasm-bindgen-backend",
  "wasm-bindgen-shared",
 ]
@@ -4121,7 +4159,7 @@ checksum = "c97b2ef2c8d627381e51c071c2ab328eac606d3f69dd82bcbca20a9e389d95f0"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -4424,7 +4462,7 @@ checksum = "28cc31741b18cb6f1d5ff12f5b7523e3d6eb0852bbbad19d73905511d9849b95"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
  "synstructure",
 ]
 
@@ -4445,7 +4483,7 @@ checksum = "9ce1b18ccd8e73a9321186f97e46f9f04b778851177567b1975109d26a08d2a6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]
@@ -4465,7 +4503,7 @@ checksum = "0ea7b4a3637ea8669cedf0f1fd5c286a17f3de97b8dd5a70a6c167a1730e63a5"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
  "synstructure",
 ]
 
@@ -4494,7 +4532,7 @@ checksum = "6eafa6dfb17584ea3e2bd6e76e0cc15ad7af12b09abdd1ca55961bed9b1063c6"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn 2.0.87",
+ "syn",
 ]
 
 [[package]]

Cargo.toml

@@ -65,7 +65,8 @@ compact_str = "0.8.0"
 criterion = { version = "0.5.1", default-features = false }
 crossbeam = { version = "0.8.4" }
 dashmap = { version = "6.0.1" }
-dir-test = { version = "0.3.0" }
+dir-test = { version = "0.4.0" }
+dunce = { version = "1.0.5" }
 drop_bomb = { version = "0.1.5" }
 env_logger = { version = "0.11.0" }
 etcetera = { version = "0.8.0" }
@@ -81,7 +82,7 @@ hashbrown = { version = "0.15.0", default-features = false, features = [
 ignore = { version = "0.4.22" }
 imara-diff = { version = "0.1.5" }
 imperative = { version = "1.0.4" }
-indexmap = {version = "2.6.0" }
+indexmap = { version = "2.6.0" }
 indicatif = { version = "0.17.8" }
 indoc = { version = "2.0.4" }
 insta = { version = "1.35.1" }
@@ -110,7 +111,7 @@ pathdiff = { version = "0.2.1" }
 pep440_rs = { version = "0.7.1" }
 pretty_assertions = "1.3.0"
 proc-macro2 = { version = "1.0.79" }
-pyproject-toml = { version = "0.9.0" }
+pyproject-toml = { version = "0.13.4" }
 quick-junit = { version = "0.5.0" }
 quote = { version = "1.0.23" }
 rand = { version = "0.8.5" }
@@ -150,7 +151,7 @@ tracing-tree = { version = "0.4.0" }
 typed-arena = { version = "2.0.2" }
 unic-ucd-category = { version = "0.9" }
 unicode-ident = { version = "1.0.12" }
-unicode-width = { version = "0.1.11" }
+unicode-width = { version = "0.2.0" }
 unicode_names2 = { version = "1.2.2" }
 unicode-normalization = { version = "0.1.23" }
 ureq = { version = "2.9.6" }
@@ -247,10 +248,10 @@ debug = 1
 [profile.dist]
 inherits = "release"
 
-# Config for 'cargo dist'
+# Config for 'dist'
 [workspace.metadata.dist]
-# The preferred cargo-dist version to use in CI (Cargo.toml SemVer syntax)
-cargo-dist-version = "0.22.1"
+# The preferred dist version to use in CI (Cargo.toml SemVer syntax)
+cargo-dist-version = "0.25.2-prerelease.3"
 # CI backends to support
 ci = "github"
 # The installers to generate for each app
@@ -281,13 +282,13 @@ targets = [
 ]
 # Whether to auto-include files like READMEs, LICENSEs, and CHANGELOGs (default true)
 auto-includes = false
-# Whether cargo-dist should create a GitHub Release or use an existing draft
+# Whether dist should create a Github Release or use an existing draft
 create-release = true
 # Which actions to run on pull requests
 pr-run-mode = "skip"
 # Whether CI should trigger releases with dispatches instead of tag pushes
 dispatch-releases = true
-# Which phase cargo-dist should use to create the GitHub release
+# Which phase dist should use to create the GitHub release
 github-release = "announce"
 # Whether CI should include auto-generated code to build local artifacts
 build-local-artifacts = false
@@ -296,14 +297,10 @@ local-artifacts-jobs = ["./build-binaries", "./build-docker"]
 # Publish jobs to run in CI
 publish-jobs = ["./publish-pypi", "./publish-wasm"]
 # Post-announce jobs to run in CI
-post-announce-jobs = [
-    "./notify-dependents",
-    "./publish-docs",
-    "./publish-playground",
-]
+post-announce-jobs = ["./notify-dependents", "./publish-docs", "./publish-playground"]
 # Custom permissions for GitHub Jobs
 github-custom-job-permissions = { "build-docker" = { packages = "write", contents = "read" }, "publish-wasm" = { contents = "read", id-token = "write", packages = "write" } }
 # Whether to install an updater program
 install-updater = false
 # Path that installers should place binaries in
-install-path = "CARGO_HOME"
+install-path = ["$XDG_BIN_HOME/", "$XDG_DATA_HOME/../bin", "~/.local/bin"]

README.md

@@ -136,8 +136,8 @@ curl -LsSf https://astral.sh/ruff/install.sh | sh
 powershell -c "irm https://astral.sh/ruff/install.ps1 | iex"
 
 # For a specific version.
-curl -LsSf https://astral.sh/ruff/0.7.4/install.sh | sh
-powershell -c "irm https://astral.sh/ruff/0.7.4/install.ps1 | iex"
+curl -LsSf https://astral.sh/ruff/0.8.0/install.sh | sh
+powershell -c "irm https://astral.sh/ruff/0.8.0/install.ps1 | iex"
 ```
 
 You can also install Ruff via [Homebrew](https://formulae.brew.sh/formula/ruff), [Conda](https://anaconda.org/conda-forge/ruff),
@@ -170,7 +170,7 @@ Ruff can also be used as a [pre-commit](https://pre-commit.com/) hook via [`ruff
 ```yaml
 - repo: https://github.com/astral-sh/ruff-pre-commit
   # Ruff version.
-  rev: v0.7.4
+  rev: v0.8.0
   hooks:
     # Run the linter.
     - id: ruff
@@ -238,8 +238,8 @@ exclude = [
 line-length = 88
 indent-width = 4
 
-# Assume Python 3.8
-target-version = "py38"
+# Assume Python 3.9
+target-version = "py39"
 
 [lint]
 # Enable Pyflakes (`F`) and a subset of the pycodestyle (`E`)  codes by default.

_typos.toml

@@ -1,6 +1,11 @@
 [files]
 # https://github.com/crate-ci/typos/issues/868
-extend-exclude = ["crates/red_knot_vendored/vendor/**/*", "**/resources/**/*", "**/snapshots/**/*"]
+extend-exclude = [
+  "crates/red_knot_vendored/vendor/**/*",
+  "**/resources/**/*",
+  "**/snapshots/**/*",
+  "crates/red_knot_workspace/src/workspace/pyproject/package_name.rs"
+]
 
 [default.extend-words]
 "arange" = "arange"  # e.g. `numpy.arange`

crates/red_knot/Cargo.toml

@@ -34,6 +34,7 @@ tracing-tree = { workspace = true }
 [dev-dependencies]
 filetime = { workspace = true }
 tempfile = { workspace = true }
+ruff_db = { workspace = true, features = ["testing"] }
 
 [lints]
 workspace = true

crates/red_knot/src/main.rs

@@ -183,10 +183,10 @@ fn run() -> anyhow::Result<ExitStatus> {
 
     let system = OsSystem::new(cwd.clone());
     let cli_configuration = args.to_configuration(&cwd);
-    let workspace_metadata = WorkspaceMetadata::from_path(
+    let workspace_metadata = WorkspaceMetadata::discover(
         system.current_directory(),
         &system,
-        Some(cli_configuration.clone()),
+        Some(&cli_configuration),
     )?;
 
     // TODO: Use the `program_settings` to compute the key for the database's persistent

crates/red_knot/src/target_version.rs

@@ -4,8 +4,8 @@
 #[derive(Copy, Clone, Hash, Debug, PartialEq, Eq, PartialOrd, Ord, Default, clap::ValueEnum)]
 pub enum TargetVersion {
     Py37,
-    #[default]
     Py38,
+    #[default]
     Py39,
     Py310,
     Py311,
@@ -46,3 +46,17 @@ impl From<TargetVersion> for red_knot_python_semantic::PythonVersion {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::target_version::TargetVersion;
+    use red_knot_python_semantic::PythonVersion;
+
+    #[test]
+    fn same_default_as_python_version() {
+        assert_eq!(
+            PythonVersion::from(TargetVersion::default()),
+            PythonVersion::default()
+        );
+    }
+}

crates/red_knot/tests/file_watching.rs

@@ -4,9 +4,8 @@ use std::io::Write;
 use std::time::Duration;
 
 use anyhow::{anyhow, Context};
-
 use red_knot_python_semantic::{resolve_module, ModuleName, Program, PythonVersion, SitePackages};
-use red_knot_workspace::db::RootDatabase;
+use red_knot_workspace::db::{Db, RootDatabase};
 use red_knot_workspace::watch;
 use red_knot_workspace::watch::{directory_watcher, WorkspaceWatcher};
 use red_knot_workspace::workspace::settings::{Configuration, SearchPathConfiguration};
@@ -14,6 +13,7 @@ use red_knot_workspace::workspace::WorkspaceMetadata;
 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::testing::{setup_logging, setup_logging_with_filter};
 use ruff_db::Upcast;
 
 struct TestCase {
@@ -46,6 +46,8 @@ impl TestCase {
     }
 
     fn try_stop_watch(&mut self, timeout: Duration) -> Option<Vec<watch::ChangeEvent>> {
+        tracing::debug!("Try stopping watch with timeout {:?}", timeout);
+
         let watcher = self
             .watcher
             .take()
@@ -55,8 +57,11 @@ impl TestCase {
             .changes_receiver
             .recv_timeout(timeout)
             .unwrap_or_default();
+
         watcher.flush();
+        tracing::debug!("Flushed file watcher");
         watcher.stop();
+        tracing::debug!("Stopping file watcher");
 
         for event in &self.changes_receiver {
             all_events.extend(event);
@@ -69,7 +74,6 @@ impl TestCase {
         Some(all_events)
     }
 
-    #[cfg(unix)]
     fn take_watch_changes(&self) -> Vec<watch::ChangeEvent> {
         self.try_take_watch_changes(Duration::from_secs(10))
             .expect("Expected watch changes but observed none")
@@ -110,8 +114,8 @@ impl TestCase {
     ) -> anyhow::Result<()> {
         let program = Program::get(self.db());
 
-        self.configuration.search_paths = configuration.clone();
-        let new_settings = configuration.into_settings(self.db.workspace().root(&self.db));
+        let new_settings = configuration.to_settings(self.db.workspace().root(&self.db));
+        self.configuration.search_paths = configuration;
 
         program.update_search_paths(&mut self.db, &new_settings)?;
 
@@ -204,7 +208,9 @@ where
             .as_utf8_path()
             .canonicalize_utf8()
             .with_context(|| "Failed to canonicalize root path.")?,
-    );
+    )
+    .simplified()
+    .to_path_buf();
 
     let workspace_path = root_path.join("workspace");
 
@@ -241,8 +247,7 @@ where
         search_paths,
     };
 
-    let workspace =
-        WorkspaceMetadata::from_path(&workspace_path, &system, Some(configuration.clone()))?;
+    let workspace = WorkspaceMetadata::discover(&workspace_path, &system, Some(&configuration))?;
 
     let db = RootDatabase::new(workspace, system)?;
 
@@ -599,6 +604,8 @@ fn directory_moved_to_trash() -> anyhow::Result<()> {
 
 #[test]
 fn directory_renamed() -> anyhow::Result<()> {
+    let _tracing = setup_logging_with_filter("file_watching=TRACE,red_knot=TRACE");
+
     let mut case = setup([
         ("bar.py", "import sub.a"),
         ("sub/__init__.py", ""),
@@ -639,6 +646,10 @@ fn directory_renamed() -> anyhow::Result<()> {
 
     let changes = case.stop_watch();
 
+    for event in &changes {
+        tracing::debug!("Event: {:?}", event);
+    }
+
     case.apply_changes(changes);
 
     // `import sub.a` should no longer resolve
@@ -1311,3 +1322,138 @@ mod unix {
         Ok(())
     }
 }
+
+#[test]
+fn nested_packages_delete_root() -> anyhow::Result<()> {
+    let mut case = setup(|root: &SystemPath, workspace_root: &SystemPath| {
+        std::fs::write(
+            workspace_root.join("pyproject.toml").as_std_path(),
+            r#"
+            [project]
+            name = "inner"
+            "#,
+        )?;
+
+        std::fs::write(
+            root.join("pyproject.toml").as_std_path(),
+            r#"
+            [project]
+            name = "outer"
+            "#,
+        )?;
+
+        Ok(())
+    })?;
+
+    assert_eq!(
+        case.db().workspace().root(case.db()),
+        &*case.workspace_path("")
+    );
+
+    std::fs::remove_file(case.workspace_path("pyproject.toml").as_std_path())?;
+
+    let changes = case.stop_watch();
+
+    case.apply_changes(changes);
+
+    // It should now pick up the outer workspace.
+    assert_eq!(case.db().workspace().root(case.db()), case.root_path());
+
+    Ok(())
+}
+
+#[test]
+fn added_package() -> anyhow::Result<()> {
+    let _ = setup_logging();
+    let mut case = setup([
+        (
+            "pyproject.toml",
+            r#"
+            [project]
+            name = "inner"
+
+            [tool.knot.workspace]
+            members = ["packages/*"]
+            "#,
+        ),
+        (
+            "packages/a/pyproject.toml",
+            r#"
+            [project]
+            name = "a"
+            "#,
+        ),
+    ])?;
+
+    assert_eq!(case.db().workspace().packages(case.db()).len(), 2);
+
+    std::fs::create_dir(case.workspace_path("packages/b").as_std_path())
+        .context("failed to create folder for package 'b'")?;
+
+    // It seems that the file watcher won't pick up on file changes shortly after the folder
+    // was created... I suspect this is because most file watchers don't support recursive
+    // file watching. Instead, file-watching libraries manually implement recursive file watching
+    // by setting a watcher for each directory. But doing this obviously "lags" behind.
+    case.take_watch_changes();
+
+    std::fs::write(
+        case.workspace_path("packages/b/pyproject.toml")
+            .as_std_path(),
+        r#"
+            [project]
+            name = "b"
+            "#,
+    )
+    .context("failed to write pyproject.toml for package b")?;
+
+    let changes = case.stop_watch();
+
+    case.apply_changes(changes);
+
+    assert_eq!(case.db().workspace().packages(case.db()).len(), 3);
+
+    Ok(())
+}
+
+#[test]
+fn removed_package() -> anyhow::Result<()> {
+    let mut case = setup([
+        (
+            "pyproject.toml",
+            r#"
+            [project]
+            name = "inner"
+
+            [tool.knot.workspace]
+            members = ["packages/*"]
+            "#,
+        ),
+        (
+            "packages/a/pyproject.toml",
+            r#"
+            [project]
+            name = "a"
+            "#,
+        ),
+        (
+            "packages/b/pyproject.toml",
+            r#"
+            [project]
+            name = "b"
+            "#,
+        ),
+    ])?;
+
+    assert_eq!(case.db().workspace().packages(case.db()).len(), 3);
+
+    std::fs::remove_dir_all(case.workspace_path("packages/b").as_std_path())
+        .context("failed to remove package 'b'")?;
+
+    let changes = case.stop_watch();
+
+    case.apply_changes(changes);
+
+    assert_eq!(case.db().workspace().packages(case.db()).len(), 2);
+
+    Ok(())
+}

crates/red_knot_python_semantic/Cargo.toml

@@ -13,7 +13,7 @@ license = { workspace = true }
 [dependencies]
 ruff_db = { workspace = true }
 ruff_index = { workspace = true }
-ruff_python_ast = { workspace = true, features = ["salsa"] }
+ruff_python_ast = { workspace = true }
 ruff_python_parser = { workspace = true }
 ruff_python_stdlib = { workspace = true }
 ruff_source_file = { workspace = true }
@@ -33,6 +33,7 @@ thiserror = { workspace = true }
 tracing = { workspace = true }
 rustc-hash = { workspace = true }
 hashbrown = { workspace = true }
+serde = { workspace = true, optional = true }
 smallvec = { workspace = true }
 static_assertions = { workspace = true }
 test-case = { workspace = true }

crates/red_knot_python_semantic/resources/mdtest/annotations/never.md

@@ -0,0 +1,62 @@
+# NoReturn & Never
+
+`NoReturn` is used to annotate the return type for functions that never return. `Never` is the
+bottom type, representing the empty set of Python objects. These two annotations can be used
+interchangeably.
+
+## Function Return Type Annotation
+
+```py
+from typing import NoReturn
+
+def stop() -> NoReturn:
+    raise RuntimeError("no way")
+
+# revealed: Never
+reveal_type(stop())
+```
+
+## Assignment
+
+```py
+from typing import NoReturn, Never, Any
+
+# error: [invalid-type-parameter] "Type `typing.Never` expected no type parameter"
+x: Never[int]
+a1: NoReturn
+# TODO: Test `Never` is only available in python >= 3.11
+a2: Never
+b1: Any
+b2: int
+
+def f():
+    # revealed: Never
+    reveal_type(a1)
+    # revealed: Never
+    reveal_type(a2)
+
+    # Never is assignable to all types.
+    v1: int = a1
+    v2: str = a1
+    # Other types are not assignable to Never except for Never (and Any).
+    v3: Never = b1
+    v4: Never = a2
+    v5: Any = b2
+    # error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to `Never`"
+    v6: Never = 1
+```
+
+## Typing Extensions
+
+```py
+from typing_extensions import NoReturn, Never
+
+x: NoReturn
+y: Never
+
+def f():
+    # revealed: Never
+    reveal_type(x)
+    # revealed: Never
+    reveal_type(y)
+```

crates/red_knot_python_semantic/resources/mdtest/annotations/optional.md

@@ -0,0 +1,47 @@
+# Optional
+
+## Annotation
+
+`typing.Optional` is equivalent to using the type with a None in a Union.
+
+```py
+from typing import Optional
+
+a: Optional[int]
+a1: Optional[bool]
+a2: Optional[Optional[bool]]
+a3: Optional[None]
+
+def f():
+    # revealed: int | None
+    reveal_type(a)
+    # revealed: bool | None
+    reveal_type(a1)
+    # revealed: bool | None
+    reveal_type(a2)
+    # revealed: None
+    reveal_type(a3)
+```
+
+## Assignment
+
+```py
+from typing import Optional
+
+a: Optional[int] = 1
+a = None
+# error: [invalid-assignment] "Object of type `Literal[""]` is not assignable to `int | None`"
+a = ""
+```
+
+## Typing Extensions
+
+```py
+from typing_extensions import Optional
+
+a: Optional[int]
+
+def f():
+    # revealed: int | None
+    reveal_type(a)
+```

crates/red_knot_python_semantic/resources/mdtest/annotations/starred.md

@@ -9,10 +9,10 @@ Ts = TypeVarTuple("Ts")
 
 def append_int(*args: *Ts) -> tuple[*Ts, int]:
     # TODO: should show some representation of the variadic generic type
-    reveal_type(args)  # revealed: @Todo
+    reveal_type(args)  # revealed: @Todo(function parameter type)
 
     return (*args, 1)
 
 # TODO should be tuple[Literal[True], Literal["a"], int]
-reveal_type(append_int(True, "a"))  # revealed: @Todo
+reveal_type(append_int(True, "a"))  # revealed: @Todo(full tuple[...] support)
 ```

crates/red_knot_python_semantic/resources/mdtest/annotations/string.md

@@ -125,7 +125,6 @@ def f7() -> "\x69nt":
 def f8() -> """int""":
     return 1
 
-# error: [annotation-byte-string] "Type expressions cannot use bytes literal"
 def f9() -> "b'int'":
     return 1
 
@@ -189,3 +188,31 @@ reveal_type(d)  # revealed: Foo
 ## Parameter
 
 TODO: Add tests once parameter inference is supported
+
+## Invalid expressions
+
+The expressions in these string annotations aren't valid expressions in this context but we
+shouldn't panic.
+
+```py
+a: "1 or 2"
+b: "(x := 1)"
+c: "1 + 2"
+d: "lambda x: x"
+e: "x if True else y"
+f: "{'a': 1, 'b': 2}"
+g: "{1, 2}"
+h: "[i for i in range(5)]"
+i: "{i for i in range(5)}"
+j: "{i: i for i in range(5)}"
+k: "(i for i in range(5))"
+l: "await 1"
+# error: [forward-annotation-syntax-error]
+m: "yield 1"
+# error: [forward-annotation-syntax-error]
+n: "yield from 1"
+o: "1 < 2"
+p: "call()"
+r: "[1, 2]"
+s: "(1, 2)"
+```

crates/red_knot_python_semantic/resources/mdtest/annotations/union.md

@@ -0,0 +1,61 @@
+# Union
+
+## Annotation
+
+`typing.Union` can be used to construct union types same as `|` operator.
+
+```py
+from typing import Union
+
+a: Union[int, str]
+a1: Union[int, bool]
+a2: Union[int, Union[float, str]]
+a3: Union[int, None]
+a4: Union[Union[float, str]]
+a5: Union[int]
+a6: Union[()]
+
+def f():
+    # revealed: int | str
+    reveal_type(a)
+    # 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
+    reveal_type(a2)
+    # revealed: int | None
+    reveal_type(a3)
+    # revealed: float | str
+    reveal_type(a4)
+    # revealed: int
+    reveal_type(a5)
+    # revealed: Never
+    reveal_type(a6)
+```
+
+## Assignment
+
+```py
+from typing import Union
+
+a: Union[int, str]
+a = 1
+a = ""
+a1: Union[int, bool]
+a1 = 1
+a1 = True
+# error: [invalid-assignment] "Object of type `Literal[b""]` is not assignable to `int | str`"
+a = b""
+```
+
+## Typing Extensions
+
+```py
+from typing_extensions import Union
+
+a: Union[int, str]
+
+def f():
+    # revealed: int | str
+    reveal_type(a)
+```

crates/red_knot_python_semantic/resources/mdtest/assignment/annotations.md

@@ -51,12 +51,12 @@ reveal_type(c)  # revealed: tuple[str, int]
 reveal_type(d)  # revealed: tuple[tuple[str, str], tuple[int, int]]
 
 # TODO: homogenous tuples, PEP-646 tuples
-reveal_type(e)  # revealed: @Todo
-reveal_type(f)  # revealed: @Todo
-reveal_type(g)  # revealed: @Todo
+reveal_type(e)  # revealed: @Todo(full tuple[...] support)
+reveal_type(f)  # revealed: @Todo(full tuple[...] support)
+reveal_type(g)  # revealed: @Todo(full tuple[...] support)
 
 # TODO: support more kinds of type expressions in annotations
-reveal_type(h)  # revealed: @Todo
+reveal_type(h)  # revealed: @Todo(full tuple[...] support)
 
 reveal_type(i)  # revealed: tuple[str | int, str | int]
 reveal_type(j)  # revealed: tuple[str | int]

crates/red_knot_python_semantic/resources/mdtest/binary/instances.md

@@ -317,7 +317,7 @@ reveal_type(1 + A())  # revealed: int
 reveal_type(A() + "foo")  # revealed: A
 # TODO should be `A` since `str.__add__` doesn't support `A` instances
 # TODO overloads
-reveal_type("foo" + A())  # revealed: @Todo
+reveal_type("foo" + A())  # revealed: @Todo(return type)
 
 reveal_type(A() + b"foo")  # revealed: A
 # TODO should be `A` since `bytes.__add__` doesn't support `A` instances
@@ -325,7 +325,7 @@ reveal_type(b"foo" + A())  # revealed: bytes
 
 reveal_type(A() + ())  # revealed: A
 # TODO this should be `A`, since `tuple.__add__` doesn't support `A` instances
-reveal_type(() + A())  # revealed: @Todo
+reveal_type(() + A())  # revealed: @Todo(return type)
 
 literal_string_instance = "foo" * 1_000_000_000
 # the test is not testing what it's meant to be testing if this isn't a `LiteralString`:
@@ -334,7 +334,7 @@ reveal_type(literal_string_instance)  # revealed: LiteralString
 reveal_type(A() + literal_string_instance)  # revealed: A
 # TODO should be `A` since `str.__add__` doesn't support `A` instances
 # TODO overloads
-reveal_type(literal_string_instance + A())  # revealed: @Todo
+reveal_type(literal_string_instance + A())  # revealed: @Todo(return type)
 ```
 
 ## Operations involving instances of classes inheriting from `Any`

crates/red_knot_python_semantic/resources/mdtest/call/function.md

@@ -16,7 +16,7 @@ async def get_int_async() -> int:
     return 42
 
 # TODO: we don't yet support `types.CoroutineType`, should be generic `Coroutine[Any, Any, int]`
-reveal_type(get_int_async())  # revealed: @Todo
+reveal_type(get_int_async())  # revealed: @Todo(generic types.CoroutineType)
 ```
 
 ## Generic
@@ -36,6 +36,8 @@ from typing import Callable
 def foo() -> int:
     return 42
 
+# TODO: This should be resolved once we understand `Callable` annotations
+# error: [annotation-with-invalid-expression]
 def decorator(func) -> Callable[[], int]:
     return foo
 
@@ -44,7 +46,7 @@ def bar() -> str:
     return "bar"
 
 # TODO: should reveal `int`, as the decorator replaces `bar` with `foo`
-reveal_type(bar())  # revealed: @Todo
+reveal_type(bar())  # revealed: @Todo(return type)
 ```
 
 ## Invalid callable

crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md

@@ -58,7 +58,9 @@ reveal_type(c >= d)  # revealed: Literal[True]
 #### Results with Ambiguity
 
 ```py
-def bool_instance() -> bool: ...
+def bool_instance() -> bool:
+    return True
+
 def int_instance() -> int:
     return 42
 
@@ -134,23 +136,158 @@ reveal_type(c >= c)  # revealed: Literal[True]
 
 #### Non Boolean Rich Comparisons
 
+Rich comparison methods defined in a class affect tuple comparisons as well. Proper type inference
+should be possible even in cases where these methods return non-boolean types.
+
+Note: Tuples use lexicographic comparisons. If the `==` result for all paired elements in the tuple
+is True, the comparison then considers the tuple’s length. Regardless of the return type of the
+dunder methods, the final result can still be a boolean value.
+
+(+cpython: For tuples, `==` and `!=` always produce boolean results, regardless of the return type
+of the dunder methods.)
+
 ```py
+from __future__ import annotations
+
 class A:
-    def __eq__(self, o) -> str: ...
-    def __ne__(self, o) -> int: ...
-    def __lt__(self, o) -> float: ...
-    def __le__(self, o) -> object: ...
-    def __gt__(self, o) -> tuple: ...
-    def __ge__(self, o) -> list: ...
+    def __eq__(self, o: object) -> str:
+        return "hello"
+
+    def __ne__(self, o: object) -> bytes:
+        return b"world"
+
+    def __lt__(self, o: A) -> float:
+        return 3.14
+
+    def __le__(self, o: A) -> complex:
+        return complex(0.5, -0.5)
+
+    def __gt__(self, o: A) -> tuple:
+        return (1, 2, 3)
+
+    def __ge__(self, o: A) -> list:
+        return [1, 2, 3]
 
 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: tuple | Literal[False]
+reveal_type(a >= a)  # revealed: list | Literal[True]
+
+# If lexicographic comparison is finished before comparing A()
+b = ("1_foo", A())
+c = ("2_bar", A())
+
+reveal_type(b == c)  # revealed: Literal[False]
+reveal_type(b != c)  # revealed: Literal[True]
+reveal_type(b < c)  # revealed: Literal[True]
+reveal_type(b <= c)  # revealed: Literal[True]
+reveal_type(b > c)  # revealed: Literal[False]
+reveal_type(b >= c)  # revealed: Literal[False]
+
+class B:
+    def __lt__(self, o: B) -> set:
+        return set()
+
+reveal_type((A(), B()) < (A(), B()))  # revealed: float | set | Literal[False]
+```
+
+#### Special Handling of Eq and NotEq in Lexicographic Comparisons
+
+> Example: `(int_instance(), "foo") == (int_instance(), "bar")`
+
+`Eq` and `NotEq` have unique behavior compared to other operators in lexicographic comparisons.
+Specifically, for `Eq`, if any non-equal pair exists within the tuples being compared, we can
+immediately conclude that the tuples are not equal. Conversely, for `NotEq`, if any non-equal pair
+exists, we can determine that the tuples are unequal.
+
+In contrast, with operators like `<` and `>`, the comparison must consider each pair of elements
+sequentially, and the final outcome might remain ambiguous until all pairs are compared.
+
+```py
+def str_instance() -> str:
+    return "hello"
+
+def int_instance() -> int:
+    return 42
+
+reveal_type("foo" == "bar")  # revealed: Literal[False]
+reveal_type(("foo",) == ("bar",))  # revealed: Literal[False]
+reveal_type((4, "foo") == (4, "bar"))  # revealed: Literal[False]
+reveal_type((int_instance(), "foo") == (int_instance(), "bar"))  # revealed: Literal[False]
+
+a = (str_instance(), int_instance(), "foo")
+
 reveal_type(a == a)  # revealed: bool
 reveal_type(a != a)  # revealed: bool
 reveal_type(a < a)  # revealed: bool
 reveal_type(a <= a)  # revealed: bool
 reveal_type(a > a)  # revealed: bool
 reveal_type(a >= a)  # revealed: bool
+
+b = (str_instance(), int_instance(), "bar")
+
+reveal_type(a == b)  # revealed: Literal[False]
+reveal_type(a != b)  # revealed: Literal[True]
+reveal_type(a < b)  # revealed: bool
+reveal_type(a <= b)  # revealed: bool
+reveal_type(a > b)  # revealed: bool
+reveal_type(a >= b)  # revealed: bool
+
+c = (str_instance(), int_instance(), "foo", "different_length")
+reveal_type(a == c)  # revealed: Literal[False]
+reveal_type(a != c)  # revealed: Literal[True]
+reveal_type(a < c)  # revealed: bool
+reveal_type(a <= c)  # revealed: bool
+reveal_type(a > c)  # revealed: bool
+reveal_type(a >= c)  # revealed: bool
+```
+
+#### Error Propagation
+
+Errors occurring within a tuple comparison should propagate outward. However, if the tuple
+comparison can clearly conclude before encountering an error, the error should not be raised.
+
+```py
+def int_instance() -> int:
+    return 42
+
+def str_instance() -> str:
+    return "hello"
+
+class A: ...
+
+# error: [unsupported-operator] "Operator `<` is not supported for types `A` and `A`"
+A() < A()
+# error: [unsupported-operator] "Operator `<=` is not supported for types `A` and `A`"
+A() <= A()
+# error: [unsupported-operator] "Operator `>` is not supported for types `A` and `A`"
+A() > A()
+# error: [unsupported-operator] "Operator `>=` is not supported for types `A` and `A`"
+A() >= A()
+
+a = (0, int_instance(), A())
+
+# error: [unsupported-operator] "Operator `<` is not supported for types `A` and `A`, in comparing `tuple[Literal[0], int, A]` with `tuple[Literal[0], int, A]`"
+reveal_type(a < a)  # revealed: Unknown
+# error: [unsupported-operator] "Operator `<=` is not supported for types `A` and `A`, in comparing `tuple[Literal[0], int, A]` with `tuple[Literal[0], int, A]`"
+reveal_type(a <= a)  # revealed: Unknown
+# error: [unsupported-operator] "Operator `>` is not supported for types `A` and `A`, in comparing `tuple[Literal[0], int, A]` with `tuple[Literal[0], int, A]`"
+reveal_type(a > a)  # revealed: Unknown
+# error: [unsupported-operator] "Operator `>=` is not supported for types `A` and `A`, in comparing `tuple[Literal[0], int, A]` with `tuple[Literal[0], int, A]`"
+reveal_type(a >= a)  # revealed: Unknown
+
+# Comparison between `a` and `b` should only involve the first elements, `Literal[0]` and `Literal[99999]`,
+# and should terminate immediately.
+b = (99999, int_instance(), A())
+
+reveal_type(a < b)  # revealed: Literal[True]
+reveal_type(a <= b)  # revealed: Literal[True]
+reveal_type(a > b)  # revealed: Literal[False]
+reveal_type(a >= b)  # revealed: Literal[False]
 ```
 
 ### Membership Test Comparisons

crates/red_knot_python_semantic/resources/mdtest/comparison/unsupported.md

@@ -4,6 +4,8 @@
 def bool_instance() -> bool:
     return True
 
+class A: ...
+
 a = 1 in 7  # error: "Operator `in` is not supported for types `Literal[1]` and `Literal[7]`"
 reveal_type(a)  # revealed: bool
 
@@ -33,4 +35,8 @@ reveal_type(e)  # revealed: bool
 f = (1, 2) < (1, "hello")
 # TODO: should be Unknown, once operand type check is implemented
 reveal_type(f)  # revealed: bool
+
+# error: [unsupported-operator] "Operator `<` is not supported for types `A` and `A`, in comparing `tuple[bool, A]` with `tuple[bool, A]`"
+g = (bool_instance(), A()) < (bool_instance(), A())
+reveal_type(g)  # revealed: Unknown
 ```

crates/red_knot_python_semantic/resources/mdtest/exception/basic.md

@@ -50,11 +50,11 @@ def foo(
         help()
     except x as e:
         # TODO: should be `AttributeError`
-        reveal_type(e)  # revealed: @Todo
+        reveal_type(e)  # revealed: @Todo(exception type)
     except y as f:
         # TODO: should be `OSError | RuntimeError`
-        reveal_type(f)  # revealed: @Todo
+        reveal_type(f)  # revealed: @Todo(exception type)
     except z as g:
         # TODO: should be `BaseException`
-        reveal_type(g)  # revealed: @Todo
+        reveal_type(g)  # revealed: @Todo(exception type)
 ```

crates/red_knot_python_semantic/resources/mdtest/expression/if.md

@@ -22,3 +22,22 @@ reveal_type(1 if None else 2)  # revealed: Literal[2]
 reveal_type(1 if "" else 2)  # revealed: Literal[2]
 reveal_type(1 if 0 else 2)  # revealed: Literal[2]
 ```
+
+## Leaked Narrowing Constraint
+
+(issue #14588)
+
+The test inside an if expression should not affect code outside of the expression.
+
+```py
+def bool_instance() -> bool:
+    return True
+
+x: Literal[42, "hello"] = 42 if bool_instance() else "hello"
+
+reveal_type(x)  # revealed: Literal[42] | Literal["hello"]
+
+_ = ... if isinstance(x, str) else ...
+
+reveal_type(x)  # revealed: Literal[42] | Literal["hello"]
+```

crates/red_knot_python_semantic/resources/mdtest/generics.md

@@ -18,7 +18,7 @@ box: MyBox[int] = MyBox(5)
 wrong_innards: MyBox[int] = MyBox("five")
 
 # TODO reveal int
-reveal_type(box.data)  # revealed: @Todo
+reveal_type(box.data)  # revealed: @Todo(instance attributes)
 
 reveal_type(MyBox.box_model_number)  # revealed: Literal[695]
 ```
@@ -39,7 +39,7 @@ class MySecureBox[T](MyBox[T]): ...
 secure_box: MySecureBox[int] = MySecureBox(5)
 reveal_type(secure_box)  # revealed: MySecureBox
 # TODO reveal int
-reveal_type(secure_box.data)  # revealed: @Todo
+reveal_type(secure_box.data)  # revealed: @Todo(instance attributes)
 ```
 
 ## Cyclical class definition
@@ -60,52 +60,20 @@ reveal_type(S)  # revealed: Literal[S]
 
 ## Type params
 
-A PEP695 type variable defines a value of type `typing.TypeVar` with attributes `__name__`,
-`__bounds__`, `__constraints__`, and `__default__` (the latter three all lazily evaluated):
+A PEP695 type variable defines a value of type `typing.TypeVar`.
 
 ```py
-def f[T, U: A, V: (A, B), W = A, X: A = A1]():
+def f[T]():
     reveal_type(T)  # revealed: T
     reveal_type(T.__name__)  # revealed: Literal["T"]
-    reveal_type(T.__bound__)  # revealed: None
-    reveal_type(T.__constraints__)  # revealed: tuple[()]
-    reveal_type(T.__default__)  # revealed: NoDefault
-
-    reveal_type(U)  # revealed: U
-    reveal_type(U.__name__)  # revealed: Literal["U"]
-    reveal_type(U.__bound__)  # revealed: type[A]
-    reveal_type(U.__constraints__)  # revealed: tuple[()]
-    reveal_type(U.__default__)  # revealed: NoDefault
-
-    reveal_type(V)  # revealed: V
-    reveal_type(V.__name__)  # revealed: Literal["V"]
-    reveal_type(V.__bound__)  # revealed: None
-    reveal_type(V.__constraints__)  # revealed: tuple[type[A], type[B]]
-    reveal_type(V.__default__)  # revealed: NoDefault
-
-    reveal_type(W)  # revealed: W
-    reveal_type(W.__name__)  # revealed: Literal["W"]
-    reveal_type(W.__bound__)  # revealed: None
-    reveal_type(W.__constraints__)  # revealed: tuple[()]
-    reveal_type(W.__default__)  # revealed: type[A]
-
-    reveal_type(X)  # revealed: X
-    reveal_type(X.__name__)  # revealed: Literal["X"]
-    reveal_type(X.__bound__)  # revealed: type[A]
-    reveal_type(X.__constraints__)  # revealed: tuple[()]
-    reveal_type(X.__default__)  # revealed: type[A1]
-
-class A: ...
-class B: ...
-class A1(A): ...
 ```
 
 ## Minimum two constraints
 
-A typevar with less than two constraints emits a diagnostic and is treated as unconstrained:
+A typevar with less than two constraints emits a diagnostic:
 
 ```py
 # error: [invalid-typevar-constraints] "TypeVar must have at least two constrained types"
 def f[T: (int,)]():
-    reveal_type(T.__constraints__)  # revealed: tuple[()]
+    pass
 ```

crates/red_knot_python_semantic/resources/mdtest/literal/literal.md

@@ -51,6 +51,8 @@ invalid1: Literal[3 + 4]
 invalid2: Literal[4 + 3j]
 # error: [invalid-literal-parameter]
 invalid3: Literal[(3, 4)]
+
+hello = "hello"
 invalid4: Literal[
     1 + 2,  # error: [invalid-literal-parameter]
     "foo",
@@ -73,10 +75,11 @@ Literal: _SpecialForm
 ```py
 from other import Literal
 
+# error: [annotation-with-invalid-expression]
 a1: Literal[26]
 
 def f():
-    reveal_type(a1)  # revealed: @Todo
+    reveal_type(a1)  # revealed: @Todo(generics)
 ```
 
 ## Detecting typing_extensions.Literal

crates/red_knot_python_semantic/resources/mdtest/loops/async_for.md

@@ -18,7 +18,7 @@ async def foo():
         pass
 
     # TODO: should reveal `Unknown` because `__aiter__` is not defined
-    # revealed: @Todo
+    # revealed: @Todo(async iterables/iterators)
     # error: [possibly-unresolved-reference]
     reveal_type(x)
 ```
@@ -40,6 +40,6 @@ async def foo():
         pass
 
     # error: [possibly-unresolved-reference]
-    # revealed: @Todo
+    # revealed: @Todo(async iterables/iterators)
     reveal_type(x)
 ```

crates/red_knot_python_semantic/resources/mdtest/loops/while_loop.md

@@ -52,3 +52,29 @@ else:
 reveal_type(x)  # revealed: Literal[2, 3]
 reveal_type(y)  # revealed: Literal[1, 2, 4]
 ```
+
+## Nested while loops
+
+```py
+def flag() -> bool:
+    return True
+
+x = 1
+
+while flag():
+    x = 2
+
+    while flag():
+        x = 3
+        if flag():
+            break
+    else:
+        x = 4
+
+    if flag():
+        break
+else:
+    x = 5
+
+reveal_type(x)  # revealed: Literal[3, 4, 5]
+```

crates/red_knot_python_semantic/resources/mdtest/metaclass.md

@@ -171,7 +171,7 @@ def f(*args, **kwargs) -> int: ...
 class A(metaclass=f): ...
 
 # TODO should be `type[int]`
-reveal_type(A.__class__)  # revealed: @Todo
+reveal_type(A.__class__)  # revealed: @Todo(metaclass not a class)
 ```
 
 ## Cyclic

crates/red_knot_python_semantic/resources/mdtest/narrow/type.md

@@ -0,0 +1,152 @@
+# Narrowing for checks involving `type(x)`
+
+## `type(x) is C`
+
+```py
+class A: ...
+class B: ...
+
+def get_a_or_b() -> A | B:
+    return A()
+
+x = get_a_or_b()
+
+if type(x) is A:
+    reveal_type(x)  # revealed: A
+else:
+    # It would be wrong to infer `B` here. The type
+    # of `x` could be a subclass of `A`, so we need
+    # to infer the full union type:
+    reveal_type(x)  # revealed: A | B
+```
+
+## `type(x) is not C`
+
+```py
+class A: ...
+class B: ...
+
+def get_a_or_b() -> A | B:
+    return A()
+
+x = get_a_or_b()
+
+if type(x) is not A:
+    # Same reasoning as above: no narrowing should occur here.
+    reveal_type(x)  # revealed: A | B
+else:
+    reveal_type(x)  # revealed: A
+```
+
+## `type(x) == C`, `type(x) != C`
+
+No narrowing can occur for equality comparisons, since there might be a custom `__eq__`
+implementation on the metaclass.
+
+TODO: Narrowing might be possible in some cases where the classes themselves are `@final` or their
+metaclass is `@final`.
+
+```py
+class IsEqualToEverything(type):
+    def __eq__(cls, other):
+        return True
+
+class A(metaclass=IsEqualToEverything): ...
+class B(metaclass=IsEqualToEverything): ...
+
+def get_a_or_b() -> A | B:
+    return B()
+
+x = get_a_or_b()
+
+if type(x) == A:
+    reveal_type(x)  # revealed: A | B
+
+if type(x) != A:
+    reveal_type(x)  # revealed: A | B
+```
+
+## No narrowing for custom `type` callable
+
+```py
+class A: ...
+class B: ...
+
+def type(x):
+    return int
+
+def get_a_or_b() -> A | B:
+    return A()
+
+x = get_a_or_b()
+
+if type(x) is A:
+    reveal_type(x)  # revealed: A | B
+else:
+    reveal_type(x)  # revealed: A | B
+```
+
+## No narrowing for multiple arguments
+
+No narrowing should occur if `type` is used to dynamically create a class:
+
+```py
+def get_str_or_int() -> str | int:
+    return "test"
+
+x = get_str_or_int()
+
+if type(x, (), {}) is str:
+    reveal_type(x)  # revealed: str | int
+else:
+    reveal_type(x)  # revealed: str | int
+```
+
+## No narrowing for keyword arguments
+
+`type` can't be used with a keyword argument:
+
+```py
+def get_str_or_int() -> str | int:
+    return "test"
+
+x = get_str_or_int()
+
+# TODO: we could issue a diagnostic here
+if type(object=x) is str:
+    reveal_type(x)  # revealed: str | int
+```
+
+## Narrowing if `type` is aliased
+
+```py
+class A: ...
+class B: ...
+
+alias_for_type = type
+
+def get_a_or_b() -> A | B:
+    return A()
+
+x = get_a_or_b()
+
+if alias_for_type(x) is A:
+    reveal_type(x)  # revealed: A
+```
+
+## Limitations
+
+```py
+class Base: ...
+class Derived(Base): ...
+
+def get_base() -> Base:
+    return Base()
+
+x = get_base()
+
+if type(x) is Base:
+    # Ideally, this could be narrower, but there is now way to
+    # express a constraint like `Base & ~ProperSubtypeOf[Base]`.
+    reveal_type(x)  # revealed: Base
+```

crates/red_knot_python_semantic/resources/mdtest/scopes/moduletype_attrs.md

@@ -17,8 +17,7 @@ reveal_type(__doc__)  # revealed: str | None
 # (needs support for `*` imports)
 reveal_type(__spec__)  # revealed: Unknown | None
 
-# TODO: generics
-reveal_type(__path__)  # revealed: @Todo
+reveal_type(__path__)  # revealed: @Todo(generics)
 
 class X:
     reveal_type(__name__)  # revealed: str
@@ -64,7 +63,7 @@ reveal_type(typing.__class__)  # revealed: Literal[type]
 
 # TODO: needs support for attribute access on instances, properties and generics;
 # should be `dict[str, Any]`
-reveal_type(typing.__dict__)  # revealed: @Todo
+reveal_type(typing.__dict__)  # revealed: @Todo(instance attributes)
 ```
 
 Typeshed includes a fake `__getattr__` method in the stub for `types.ModuleType` to help out with
@@ -96,8 +95,8 @@ from foo import __dict__ as foo_dict
 
 # TODO: needs support for attribute access on instances, properties, and generics;
 # should be `dict[str, Any]` for both of these:
-reveal_type(foo.__dict__)  # revealed: @Todo
-reveal_type(foo_dict)  # revealed: @Todo
+reveal_type(foo.__dict__)  # revealed: @Todo(instance attributes)
+reveal_type(foo_dict)  # revealed: @Todo(instance attributes)
 ```
 
 ## Conditionally global or `ModuleType` attribute

crates/red_knot_python_semantic/resources/mdtest/subscript/bytes.md

@@ -27,7 +27,7 @@ def int_instance() -> int:
 
 a = b"abcde"[int_instance()]
 # TODO: Support overloads... Should be `bytes`
-reveal_type(a)  # revealed: @Todo
+reveal_type(a)  # revealed: @Todo(return type)
 ```
 
 ## Slices
@@ -47,11 +47,11 @@ def int_instance() -> int: ...
 
 byte_slice1 = b[int_instance() : int_instance()]
 # TODO: Support overloads... Should be `bytes`
-reveal_type(byte_slice1)  # revealed: @Todo
+reveal_type(byte_slice1)  # revealed: @Todo(return type)
 
 def bytes_instance() -> bytes: ...
 
 byte_slice2 = bytes_instance()[0:5]
 # TODO: Support overloads... Should be `bytes`
-reveal_type(byte_slice2)  # revealed: @Todo
+reveal_type(byte_slice2)  # revealed: @Todo(return type)
 ```

crates/red_knot_python_semantic/resources/mdtest/subscript/lists.md

@@ -12,13 +12,13 @@ x = [1, 2, 3]
 reveal_type(x)  # revealed: list
 
 # TODO reveal int
-reveal_type(x[0])  # revealed: @Todo
+reveal_type(x[0])  # revealed: @Todo(return type)
 
 # TODO reveal list
-reveal_type(x[0:1])  # revealed: @Todo
+reveal_type(x[0:1])  # revealed: @Todo(return type)
 
 # TODO error
-reveal_type(x["a"])  # revealed: @Todo
+reveal_type(x["a"])  # revealed: @Todo(return type)
 ```
 
 ## Assignments within list assignment

crates/red_knot_python_semantic/resources/mdtest/subscript/string.md

@@ -23,7 +23,7 @@ def int_instance() -> int: ...
 
 a = "abcde"[int_instance()]
 # TODO: Support overloads... Should be `str`
-reveal_type(a)  # revealed: @Todo
+reveal_type(a)  # revealed: @Todo(return type)
 ```
 
 ## Slices
@@ -78,13 +78,13 @@ def int_instance() -> int: ...
 
 substring1 = s[int_instance() : int_instance()]
 # TODO: Support overloads... Should be `LiteralString`
-reveal_type(substring1)  # revealed: @Todo
+reveal_type(substring1)  # revealed: @Todo(return type)
 
 def str_instance() -> str: ...
 
 substring2 = str_instance()[0:5]
 # TODO: Support overloads... Should be `str`
-reveal_type(substring2)  # revealed: @Todo
+reveal_type(substring2)  # revealed: @Todo(return type)
 ```
 
 ## Unsupported slice types

crates/red_knot_python_semantic/resources/mdtest/subscript/tuple.md

@@ -71,5 +71,5 @@ def int_instance() -> int: ...
 
 tuple_slice = t[int_instance() : int_instance()]
 # TODO: Support overloads... Should be `tuple[Literal[1, 'a', b"b"] | None, ...]`
-reveal_type(tuple_slice)  # revealed: @Todo
+reveal_type(tuple_slice)  # revealed: @Todo(return type)
 ```

crates/red_knot_python_semantic/resources/mdtest/sys_version_info.md

@@ -22,23 +22,23 @@ type:
 ```py
 import sys
 
-reveal_type(sys.version_info >= (3, 8))  # revealed: Literal[True]
-reveal_type((3, 8) <= sys.version_info)  # revealed: Literal[True]
+reveal_type(sys.version_info >= (3, 9))  # revealed: Literal[True]
+reveal_type((3, 9) <= sys.version_info)  # revealed: Literal[True]
 
-reveal_type(sys.version_info > (3, 8))  # revealed: Literal[True]
-reveal_type((3, 8) < sys.version_info)  # revealed: Literal[True]
+reveal_type(sys.version_info > (3, 9))  # revealed: Literal[True]
+reveal_type((3, 9) < sys.version_info)  # revealed: Literal[True]
 
-reveal_type(sys.version_info < (3, 8))  # revealed: Literal[False]
-reveal_type((3, 8) > sys.version_info)  # revealed: Literal[False]
+reveal_type(sys.version_info < (3, 9))  # revealed: Literal[False]
+reveal_type((3, 9) > sys.version_info)  # revealed: Literal[False]
 
-reveal_type(sys.version_info <= (3, 8))  # revealed: Literal[False]
-reveal_type((3, 8) >= sys.version_info)  # revealed: Literal[False]
+reveal_type(sys.version_info <= (3, 9))  # revealed: Literal[False]
+reveal_type((3, 9) >= sys.version_info)  # revealed: Literal[False]
 
-reveal_type(sys.version_info == (3, 8))  # revealed: Literal[False]
-reveal_type((3, 8) == sys.version_info)  # revealed: Literal[False]
+reveal_type(sys.version_info == (3, 9))  # revealed: Literal[False]
+reveal_type((3, 9) == sys.version_info)  # revealed: Literal[False]
 
-reveal_type(sys.version_info != (3, 8))  # revealed: Literal[True]
-reveal_type((3, 8) != sys.version_info)  # revealed: Literal[True]
+reveal_type(sys.version_info != (3, 9))  # revealed: Literal[True]
+reveal_type((3, 9) != sys.version_info)  # revealed: Literal[True]
 ```
 
 ## Non-literal types from comparisons
@@ -49,17 +49,16 @@ sometimes not:
 ```py
 import sys
 
-reveal_type(sys.version_info >= (3, 8, 1))  # revealed: bool
-reveal_type(sys.version_info >= (3, 8, 1, "final", 0))  # revealed: bool
+reveal_type(sys.version_info >= (3, 9, 1))  # revealed: bool
+reveal_type(sys.version_info >= (3, 9, 1, "final", 0))  # revealed: bool
 
 # TODO: While this won't fail at runtime, the user has probably made a mistake
 # if they're comparing a tuple of length >5 with `sys.version_info`
 # (`sys.version_info` is a tuple of length 5). It might be worth
 # emitting a lint diagnostic of some kind warning them about the probable error?
-reveal_type(sys.version_info >= (3, 8, 1, "final", 0, 5))  # revealed: bool
+reveal_type(sys.version_info >= (3, 9, 1, "final", 0, 5))  # revealed: bool
 
-# TODO: this should be `Literal[False]`; see #14279
-reveal_type(sys.version_info == (3, 8, 1, "finallllll", 0))  # revealed: bool
+reveal_type(sys.version_info == (3, 8, 1, "finallllll", 0))  # revealed: Literal[False]
 ```
 
 ## Imports and aliases
@@ -71,11 +70,11 @@ another name:
 from sys import version_info
 from sys import version_info as foo
 
-reveal_type(version_info >= (3, 8))  # revealed: Literal[True]
-reveal_type(foo >= (3, 8))  # revealed: Literal[True]
+reveal_type(version_info >= (3, 9))  # revealed: Literal[True]
+reveal_type(foo >= (3, 9))  # revealed: Literal[True]
 
 bar = version_info
-reveal_type(bar >= (3, 8))  # revealed: Literal[True]
+reveal_type(bar >= (3, 9))  # revealed: Literal[True]
 ```
 
 ## Non-stdlib modules named `sys`
@@ -92,7 +91,7 @@ version_info: tuple[int, int] = (4, 2)
 ```py path=package/script.py
 from .sys import version_info
 
-reveal_type(version_info >= (3, 8))  # revealed: bool
+reveal_type(version_info >= (3, 9))  # revealed: bool
 ```
 
 ## Accessing fields by name
@@ -103,8 +102,8 @@ The fields of `sys.version_info` can be accessed by name:
 import sys
 
 reveal_type(sys.version_info.major >= 3)  # revealed: Literal[True]
-reveal_type(sys.version_info.minor >= 8)  # revealed: Literal[True]
-reveal_type(sys.version_info.minor >= 9)  # revealed: Literal[False]
+reveal_type(sys.version_info.minor >= 9)  # revealed: Literal[True]
+reveal_type(sys.version_info.minor >= 10)  # revealed: Literal[False]
 ```
 
 But the `micro`, `releaselevel` and `serial` fields are inferred as `@Todo` until we support
@@ -113,9 +112,9 @@ properties on instance types:
 ```py path=b.py
 import sys
 
-reveal_type(sys.version_info.micro)  # revealed: @Todo
-reveal_type(sys.version_info.releaselevel)  # revealed: @Todo
-reveal_type(sys.version_info.serial)  # revealed: @Todo
+reveal_type(sys.version_info.micro)  # revealed: @Todo(instance attributes)
+reveal_type(sys.version_info.releaselevel)  # revealed: @Todo(instance attributes)
+reveal_type(sys.version_info.serial)  # revealed: @Todo(instance attributes)
 ```
 
 ## Accessing fields by index/slice
@@ -126,14 +125,14 @@ The fields of `sys.version_info` can be accessed by index or by slice:
 import sys
 
 reveal_type(sys.version_info[0] < 3)  # revealed: Literal[False]
-reveal_type(sys.version_info[1] > 8)  # revealed: Literal[False]
+reveal_type(sys.version_info[1] > 9)  # revealed: Literal[False]
 
-# revealed: tuple[Literal[3], Literal[8], int, Literal["alpha", "beta", "candidate", "final"], int]
+# revealed: tuple[Literal[3], Literal[9], int, Literal["alpha", "beta", "candidate", "final"], int]
 reveal_type(sys.version_info[:5])
 
-reveal_type(sys.version_info[:2] >= (3, 8))  # revealed: Literal[True]
-reveal_type(sys.version_info[0:2] >= (3, 9))  # revealed: Literal[False]
-reveal_type(sys.version_info[:3] >= (3, 9, 1))  # revealed: Literal[False]
+reveal_type(sys.version_info[:2] >= (3, 9))  # revealed: Literal[True]
+reveal_type(sys.version_info[0:2] >= (3, 10))  # revealed: Literal[False]
+reveal_type(sys.version_info[:3] >= (3, 10, 1))  # revealed: Literal[False]
 reveal_type(sys.version_info[3] == "final")  # revealed: bool
 reveal_type(sys.version_info[3] == "finalllllll")  # revealed: Literal[False]
 ```

crates/red_knot_python_semantic/resources/mdtest/type_alias.md

@@ -0,0 +1,71 @@
+# Type aliases
+
+## Basic
+
+```py
+type IntOrStr = int | str
+
+reveal_type(IntOrStr)  # revealed: typing.TypeAliasType
+reveal_type(IntOrStr.__name__)  # revealed: Literal["IntOrStr"]
+
+x: IntOrStr = 1
+
+reveal_type(x)  # revealed: Literal[1]
+
+def f() -> None:
+    reveal_type(x)  # revealed: int | str
+```
+
+## `__value__` attribute
+
+```py
+type IntOrStr = int | str
+
+# TODO: This should either fall back to the specified type from typeshed,
+# which is `Any`, or be the actual type of the runtime value expression
+# `int | str`, i.e. `types.UnionType`.
+reveal_type(IntOrStr.__value__)  # revealed: @Todo(instance attributes)
+```
+
+## Invalid assignment
+
+```py
+type OptionalInt = int | None
+
+# error: [invalid-assignment]
+x: OptionalInt = "1"
+```
+
+## Type aliases in type aliases
+
+```py
+type IntOrStr = int | str
+type IntOrStrOrBytes = IntOrStr | bytes
+
+x: IntOrStrOrBytes = 1
+
+def f() -> None:
+    reveal_type(x)  # revealed: int | str | bytes
+```
+
+## Aliased type aliases
+
+```py
+type IntOrStr = int | str
+MyIntOrStr = IntOrStr
+
+x: MyIntOrStr = 1
+
+# error: [invalid-assignment]
+y: MyIntOrStr = None
+```
+
+## Generic type aliases
+
+```py
+type ListOrSet[T] = list[T] | set[T]
+
+# TODO: Should be `tuple[typing.TypeVar | typing.ParamSpec | typing.TypeVarTuple, ...]`,
+# as specified in the `typeshed` stubs.
+reveal_type(ListOrSet.__type_params__)  # revealed: @Todo(instance attributes)
+```

crates/red_knot_python_semantic/resources/mdtest/unpacking.md

@@ -84,7 +84,7 @@ reveal_type(b)  # revealed: Literal[2]
 [a, *b, c, d] = (1, 2)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[Any] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: Literal[2]
 reveal_type(d)  # revealed: Unknown
 ```
@@ -95,7 +95,7 @@ reveal_type(d)  # revealed: Unknown
 [a, *b, c] = (1, 2)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[Any] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: Literal[2]
 ```
 
@@ -105,7 +105,7 @@ reveal_type(c)  # revealed: Literal[2]
 [a, *b, c] = (1, 2, 3)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: Literal[3]
 ```
 
@@ -115,7 +115,7 @@ reveal_type(c)  # revealed: Literal[3]
 [a, *b, c, d] = (1, 2, 3, 4, 5, 6)
 reveal_type(a)  # revealed: Literal[1]
 # TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: Literal[5]
 reveal_type(d)  # revealed: Literal[6]
 ```
@@ -127,7 +127,7 @@ reveal_type(d)  # revealed: Literal[6]
 reveal_type(a)  # revealed: Literal[1]
 reveal_type(b)  # revealed: Literal[2]
 # TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(c)  # revealed: @Todo
+reveal_type(c)  # revealed: @Todo(starred unpacking)
 ```
 
 ### Starred expression (6)
@@ -138,7 +138,7 @@ reveal_type(c)  # revealed: @Todo
 reveal_type(a)  # revealed: Literal[1]
 reveal_type(b)  # revealed: Unknown
 reveal_type(c)  # revealed: Unknown
-reveal_type(d)  # revealed: @Todo
+reveal_type(d)  # revealed: @Todo(starred unpacking)
 reveal_type(e)  # revealed: Unknown
 reveal_type(f)  # revealed: Unknown
 ```
@@ -222,7 +222,7 @@ reveal_type(b)  # revealed: LiteralString
 (a, *b, c, d) = "ab"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: LiteralString
 reveal_type(d)  # revealed: Unknown
 ```
@@ -233,7 +233,7 @@ reveal_type(d)  # revealed: Unknown
 (a, *b, c) = "ab"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[Any] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: LiteralString
 ```
 
@@ -243,7 +243,7 @@ reveal_type(c)  # revealed: LiteralString
 (a, *b, c) = "abc"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: LiteralString
 ```
 
@@ -253,7 +253,7 @@ reveal_type(c)  # revealed: LiteralString
 (a, *b, c, d) = "abcdef"
 reveal_type(a)  # revealed: LiteralString
 # TODO: Should be list[LiteralString] once support for assigning to starred expression is added
-reveal_type(b)  # revealed: @Todo
+reveal_type(b)  # revealed: @Todo(starred unpacking)
 reveal_type(c)  # revealed: LiteralString
 reveal_type(d)  # revealed: LiteralString
 ```
@@ -265,5 +265,5 @@ reveal_type(d)  # revealed: LiteralString
 reveal_type(a)  # revealed: LiteralString
 reveal_type(b)  # revealed: LiteralString
 # TODO: Should be list[int] once support for assigning to starred expression is added
-reveal_type(c)  # revealed: @Todo
+reveal_type(c)  # revealed: @Todo(starred unpacking)
 ```

crates/red_knot_python_semantic/resources/mdtest/with/async.md

@@ -17,5 +17,5 @@ class Manager:
 
 async def test():
     async with Manager() as f:
-        reveal_type(f)  # revealed: @Todo
+        reveal_type(f)  # revealed: @Todo(async with statement)
 ```

crates/red_knot_python_semantic/src/module_resolver/typeshed.rs

@@ -459,11 +459,11 @@ foo: 3.8-   # trailing comment
 ";
         let parsed_versions = TypeshedVersions::from_str(VERSIONS).unwrap();
         assert_eq!(parsed_versions.len(), 3);
-        assert_snapshot!(parsed_versions.to_string(), @r###"
+        assert_snapshot!(parsed_versions.to_string(), @r"
         bar: 2.7-3.10
         bar.baz: 3.1-3.9
         foo: 3.8-
-        "###
+        "
         );
     }
 

crates/red_knot_python_semantic/src/program.rs

@@ -54,6 +54,7 @@ impl Program {
 }
 
 #[derive(Clone, Debug, Eq, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize))]
 pub struct ProgramSettings {
     pub target_version: PythonVersion,
     pub search_paths: SearchPathSettings,
@@ -61,6 +62,7 @@ pub struct ProgramSettings {
 
 /// Configures the search paths for module resolution.
 #[derive(Eq, PartialEq, Debug, Clone)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize))]
 pub struct SearchPathSettings {
     /// List of user-provided paths that should take first priority in the module resolution.
     /// Examples in other type checkers are mypy's MYPYPATH environment variable,
@@ -91,6 +93,7 @@ impl SearchPathSettings {
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize))]
 pub enum SitePackages {
     Derived {
         venv_path: SystemPathBuf,

crates/red_knot_python_semantic/src/python_version.rs

@@ -5,6 +5,7 @@ use std::fmt;
 /// Unlike the `TargetVersion` enums in the CLI crates,
 /// this does not necessarily represent a Python version that we actually support.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[cfg_attr(feature = "serde", derive(serde::Serialize))]
 pub struct PythonVersion {
     pub major: u8,
     pub minor: u8,
@@ -38,7 +39,7 @@ impl PythonVersion {
 
 impl Default for PythonVersion {
     fn default() -> Self {
-        Self::PY38
+        Self::PY39
     }
 }
 

crates/red_knot_python_semantic/src/semantic_index/ast_ids.rs

@@ -49,64 +49,50 @@ fn ast_ids<'db>(db: &'db dyn Db, scope: ScopeId) -> &'db AstIds {
     semantic_index(db, scope.file(db)).ast_ids(scope.file_scope_id(db))
 }
 
-pub trait HasScopedUseId {
-    /// The type of the ID uniquely identifying the use.
-    type Id: Copy;
-
-    /// Returns the ID that uniquely identifies the use in `scope`.
-    fn scoped_use_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id;
-}
-
 /// Uniquely identifies a use of a name in a [`crate::semantic_index::symbol::FileScopeId`].
 #[newtype_index]
 pub struct ScopedUseId;
 
-impl HasScopedUseId for ast::ExprName {
-    type Id = ScopedUseId;
+pub trait HasScopedUseId {
+    /// Returns the ID that uniquely identifies the use in `scope`.
+    fn scoped_use_id(&self, db: &dyn Db, scope: ScopeId) -> ScopedUseId;
+}
 
-    fn scoped_use_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id {
+impl HasScopedUseId for ast::ExprName {
+    fn scoped_use_id(&self, db: &dyn Db, scope: ScopeId) -> ScopedUseId {
         let expression_ref = ExpressionRef::from(self);
         expression_ref.scoped_use_id(db, scope)
     }
 }
 
 impl HasScopedUseId for ast::ExpressionRef<'_> {
-    type Id = ScopedUseId;
-
-    fn scoped_use_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id {
+    fn scoped_use_id(&self, db: &dyn Db, scope: ScopeId) -> ScopedUseId {
         let ast_ids = ast_ids(db, scope);
         ast_ids.use_id(*self)
     }
 }
 
-pub trait HasScopedAstId {
-    /// The type of the ID uniquely identifying the node.
-    type Id: Copy;
-
-    /// Returns the ID that uniquely identifies the node in `scope`.
-    fn scoped_ast_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id;
-}
-
-impl<T: HasScopedAstId> HasScopedAstId for Box<T> {
-    type Id = <T as HasScopedAstId>::Id;
-
-    fn scoped_ast_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id {
-        self.as_ref().scoped_ast_id(db, scope)
-    }
-}
-
 /// Uniquely identifies an [`ast::Expr`] in a [`crate::semantic_index::symbol::FileScopeId`].
 #[newtype_index]
 pub struct ScopedExpressionId;
 
+pub trait HasScopedExpressionId {
+    /// Returns the ID that uniquely identifies the node in `scope`.
+    fn scoped_expression_id(&self, db: &dyn Db, scope: ScopeId) -> ScopedExpressionId;
+}
+
+impl<T: HasScopedExpressionId> HasScopedExpressionId for Box<T> {
+    fn scoped_expression_id(&self, db: &dyn Db, scope: ScopeId) -> ScopedExpressionId {
+        self.as_ref().scoped_expression_id(db, scope)
+    }
+}
+
 macro_rules! impl_has_scoped_expression_id {
     ($ty: ty) => {
-        impl HasScopedAstId for $ty {
-            type Id = ScopedExpressionId;
-
-            fn scoped_ast_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id {
+        impl HasScopedExpressionId for $ty {
+            fn scoped_expression_id(&self, db: &dyn Db, scope: ScopeId) -> ScopedExpressionId {
                 let expression_ref = ExpressionRef::from(self);
-                expression_ref.scoped_ast_id(db, scope)
+                expression_ref.scoped_expression_id(db, scope)
             }
         }
     };
@@ -146,29 +132,20 @@ impl_has_scoped_expression_id!(ast::ExprSlice);
 impl_has_scoped_expression_id!(ast::ExprIpyEscapeCommand);
 impl_has_scoped_expression_id!(ast::Expr);
 
-impl HasScopedAstId for ast::ExpressionRef<'_> {
-    type Id = ScopedExpressionId;
-
-    fn scoped_ast_id(&self, db: &dyn Db, scope: ScopeId) -> Self::Id {
+impl HasScopedExpressionId for ast::ExpressionRef<'_> {
+    fn scoped_expression_id(&self, db: &dyn Db, scope: ScopeId) -> ScopedExpressionId {
         let ast_ids = ast_ids(db, scope);
         ast_ids.expression_id(*self)
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub(super) struct AstIdsBuilder {
     expressions_map: FxHashMap<ExpressionNodeKey, ScopedExpressionId>,
     uses_map: FxHashMap<ExpressionNodeKey, ScopedUseId>,
 }
 
 impl AstIdsBuilder {
-    pub(super) fn new() -> Self {
-        Self {
-            expressions_map: FxHashMap::default(),
-            uses_map: FxHashMap::default(),
-        }
-    }
-
     /// Adds `expr` to the expression ids map and returns its id.
     pub(super) fn record_expression(&mut self, expr: &ast::Expr) -> ScopedExpressionId {
         let expression_id = self.expressions_map.len().into();

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -36,12 +36,25 @@ use super::definition::{
 
 mod except_handlers;
 
+/// Are we in a state where a `break` statement is allowed?
+#[derive(Clone, Copy, Debug)]
+enum LoopState {
+    InLoop,
+    NotInLoop,
+}
+
+impl LoopState {
+    fn is_inside(self) -> bool {
+        matches!(self, LoopState::InLoop)
+    }
+}
+
 pub(super) struct SemanticIndexBuilder<'db> {
     // Builder state
     db: &'db dyn Db,
     file: File,
     module: &'db ParsedModule,
-    scope_stack: Vec<FileScopeId>,
+    scope_stack: Vec<(FileScopeId, LoopState)>,
     /// The assignments we're currently visiting, with
     /// the most recent visit at the end of the Vec
     current_assignments: Vec<CurrentAssignment<'db>>,
@@ -103,9 +116,24 @@ impl<'db> SemanticIndexBuilder<'db> {
         *self
             .scope_stack
             .last()
+            .map(|(scope, _)| scope)
             .expect("Always to have a root scope")
     }
 
+    fn loop_state(&self) -> LoopState {
+        self.scope_stack
+            .last()
+            .expect("Always to have a root scope")
+            .1
+    }
+
+    fn set_inside_loop(&mut self, state: LoopState) {
+        self.scope_stack
+            .last_mut()
+            .expect("Always to have a root scope")
+            .1 = state;
+    }
+
     fn push_scope(&mut self, node: NodeWithScopeRef) {
         let parent = self.current_scope();
         self.push_scope_with_parent(node, Some(parent));
@@ -124,22 +152,23 @@ impl<'db> SemanticIndexBuilder<'db> {
         self.try_node_context_stack_manager.enter_nested_scope();
 
         let file_scope_id = self.scopes.push(scope);
-        self.symbol_tables.push(SymbolTableBuilder::new());
-        self.use_def_maps.push(UseDefMapBuilder::new());
-        let ast_id_scope = self.ast_ids.push(AstIdsBuilder::new());
+        self.symbol_tables.push(SymbolTableBuilder::default());
+        self.use_def_maps.push(UseDefMapBuilder::default());
+        let ast_id_scope = self.ast_ids.push(AstIdsBuilder::default());
 
         let scope_id = ScopeId::new(self.db, self.file, file_scope_id, countme::Count::default());
 
         self.scope_ids_by_scope.push(scope_id);
-        self.scopes_by_node.insert(node.node_key(), file_scope_id);
+        let previous = self.scopes_by_node.insert(node.node_key(), file_scope_id);
+        debug_assert_eq!(previous, None);
 
         debug_assert_eq!(ast_id_scope, file_scope_id);
 
-        self.scope_stack.push(file_scope_id);
+        self.scope_stack.push((file_scope_id, LoopState::NotInLoop));
     }
 
     fn pop_scope(&mut self) -> FileScopeId {
-        let id = self.scope_stack.pop().expect("Root scope to be present");
+        let (id, _) = self.scope_stack.pop().expect("Root scope to be present");
         let children_end = self.scopes.next_index();
         let scope = &mut self.scopes[id];
         scope.descendents = scope.descendents.start..children_end;
@@ -589,6 +618,27 @@ where
                     },
                 );
             }
+            ast::Stmt::TypeAlias(type_alias) => {
+                let symbol = self.add_symbol(
+                    type_alias
+                        .name
+                        .as_name_expr()
+                        .map(|name| name.id.clone())
+                        .unwrap_or("<unknown>".into()),
+                );
+                self.add_definition(symbol, type_alias);
+                self.visit_expr(&type_alias.name);
+
+                self.with_type_params(
+                    NodeWithScopeRef::TypeAliasTypeParameters(type_alias),
+                    type_alias.type_params.as_ref(),
+                    |builder| {
+                        builder.push_scope(NodeWithScopeRef::TypeAlias(type_alias));
+                        builder.visit_expr(&type_alias.value);
+                        builder.pop_scope()
+                    },
+                );
+            }
             ast::Stmt::Import(node) => {
                 for alias in &node.names {
                     let symbol_name = if let Some(asname) = &alias.asname {
@@ -763,7 +813,10 @@ where
 
                 // TODO: definitions created inside the body should be fully visible
                 // to other statements/expressions inside the body --Alex/Carl
+                let outer_loop_state = self.loop_state();
+                self.set_inside_loop(LoopState::InLoop);
                 self.visit_body(body);
+                self.set_inside_loop(outer_loop_state);
 
                 // Get the break states from the body of this loop, and restore the saved outer
                 // ones.
@@ -802,7 +855,9 @@ where
                 self.visit_body(body);
             }
             ast::Stmt::Break(_) => {
-                self.loop_break_states.push(self.flow_snapshot());
+                if self.loop_state().is_inside() {
+                    self.loop_break_states.push(self.flow_snapshot());
+                }
             }
 
             ast::Stmt::For(
@@ -829,7 +884,10 @@ where
                 // TODO: Definitions created by loop variables
                 // (and definitions created inside the body)
                 // are fully visible to other statements/expressions inside the body --Alex/Carl
+                let outer_loop_state = self.loop_state();
+                self.set_inside_loop(LoopState::InLoop);
                 self.visit_body(body);
+                self.set_inside_loop(outer_loop_state);
 
                 let break_states =
                     std::mem::replace(&mut self.loop_break_states, saved_break_states);
@@ -1131,8 +1189,8 @@ where
                 // AST inspection, so we can't simplify here, need to record test expression for
                 // later checking)
                 self.visit_expr(test);
-                let constraint = self.record_expression_constraint(test);
                 let pre_if = self.flow_snapshot();
+                let constraint = self.record_expression_constraint(test);
                 self.visit_expr(body);
                 let post_body = self.flow_snapshot();
                 self.flow_restore(pre_if);

crates/red_knot_python_semantic/src/semantic_index/definition.rs

@@ -83,6 +83,7 @@ pub(crate) enum DefinitionNodeRef<'a> {
     For(ForStmtDefinitionNodeRef<'a>),
     Function(&'a ast::StmtFunctionDef),
     Class(&'a ast::StmtClassDef),
+    TypeAlias(&'a ast::StmtTypeAlias),
     NamedExpression(&'a ast::ExprNamed),
     Assignment(AssignmentDefinitionNodeRef<'a>),
     AnnotatedAssignment(&'a ast::StmtAnnAssign),
@@ -109,6 +110,12 @@ impl<'a> From<&'a ast::StmtClassDef> for DefinitionNodeRef<'a> {
     }
 }
 
+impl<'a> From<&'a ast::StmtTypeAlias> for DefinitionNodeRef<'a> {
+    fn from(node: &'a ast::StmtTypeAlias) -> Self {
+        Self::TypeAlias(node)
+    }
+}
+
 impl<'a> From<&'a ast::ExprNamed> for DefinitionNodeRef<'a> {
     fn from(node: &'a ast::ExprNamed) -> Self {
         Self::NamedExpression(node)
@@ -265,6 +272,9 @@ impl<'db> DefinitionNodeRef<'db> {
             DefinitionNodeRef::Class(class) => {
                 DefinitionKind::Class(AstNodeRef::new(parsed, class))
             }
+            DefinitionNodeRef::TypeAlias(type_alias) => {
+                DefinitionKind::TypeAlias(AstNodeRef::new(parsed, type_alias))
+            }
             DefinitionNodeRef::NamedExpression(named) => {
                 DefinitionKind::NamedExpression(AstNodeRef::new(parsed, named))
             }
@@ -358,6 +368,7 @@ impl<'db> DefinitionNodeRef<'db> {
             }
             Self::Function(node) => node.into(),
             Self::Class(node) => node.into(),
+            Self::TypeAlias(node) => node.into(),
             Self::NamedExpression(node) => node.into(),
             Self::Assignment(AssignmentDefinitionNodeRef {
                 value: _,
@@ -434,6 +445,7 @@ pub enum DefinitionKind<'db> {
     ImportFrom(ImportFromDefinitionKind),
     Function(AstNodeRef<ast::StmtFunctionDef>),
     Class(AstNodeRef<ast::StmtClassDef>),
+    TypeAlias(AstNodeRef<ast::StmtTypeAlias>),
     NamedExpression(AstNodeRef<ast::ExprNamed>),
     Assignment(AssignmentDefinitionKind<'db>),
     AnnotatedAssignment(AstNodeRef<ast::StmtAnnAssign>),
@@ -456,6 +468,7 @@ impl DefinitionKind<'_> {
             // functions, classes, and imports always bind, and we consider them declarations
             DefinitionKind::Function(_)
             | DefinitionKind::Class(_)
+            | DefinitionKind::TypeAlias(_)
             | DefinitionKind::Import(_)
             | DefinitionKind::ImportFrom(_)
             | DefinitionKind::TypeVar(_)
@@ -682,6 +695,12 @@ impl From<&ast::StmtClassDef> for DefinitionNodeKey {
     }
 }
 
+impl From<&ast::StmtTypeAlias> for DefinitionNodeKey {
+    fn from(node: &ast::StmtTypeAlias) -> Self {
+        Self(NodeKey::from_node(node))
+    }
+}
+
 impl From<&ast::ExprName> for DefinitionNodeKey {
     fn from(node: &ast::ExprName) -> Self {
         Self(NodeKey::from_node(node))

crates/red_knot_python_semantic/src/semantic_index/symbol.rs

@@ -116,14 +116,11 @@ impl<'db> ScopeId<'db> {
         // Type parameter scopes behave like function scopes in terms of name resolution; CPython
         // symbol table also uses the term "function-like" for these scopes.
         matches!(
-            self.node(db),
-            NodeWithScopeKind::ClassTypeParameters(_)
-                | NodeWithScopeKind::FunctionTypeParameters(_)
-                | NodeWithScopeKind::Function(_)
-                | NodeWithScopeKind::ListComprehension(_)
-                | NodeWithScopeKind::SetComprehension(_)
-                | NodeWithScopeKind::DictComprehension(_)
-                | NodeWithScopeKind::GeneratorExpression(_)
+            self.node(db).scope_kind(),
+            ScopeKind::Annotation
+                | ScopeKind::Function
+                | ScopeKind::TypeAlias
+                | ScopeKind::Comprehension
         )
     }
 
@@ -144,6 +141,12 @@ impl<'db> ScopeId<'db> {
             }
             NodeWithScopeKind::Function(function)
             | NodeWithScopeKind::FunctionTypeParameters(function) => function.name.as_str(),
+            NodeWithScopeKind::TypeAlias(type_alias)
+            | NodeWithScopeKind::TypeAliasTypeParameters(type_alias) => type_alias
+                .name
+                .as_name_expr()
+                .map(|name| name.id.as_str())
+                .unwrap_or("<type alias>"),
             NodeWithScopeKind::Lambda(_) => "<lambda>",
             NodeWithScopeKind::ListComprehension(_) => "<listcomp>",
             NodeWithScopeKind::SetComprehension(_) => "<setcomp>",
@@ -201,6 +204,7 @@ pub enum ScopeKind {
     Class,
     Function,
     Comprehension,
+    TypeAlias,
 }
 
 impl ScopeKind {
@@ -210,7 +214,7 @@ impl ScopeKind {
 }
 
 /// Symbol table for a specific [`Scope`].
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct SymbolTable {
     /// The symbols in this scope.
     symbols: IndexVec<ScopedSymbolId, Symbol>,
@@ -220,13 +224,6 @@ pub struct SymbolTable {
 }
 
 impl SymbolTable {
-    fn new() -> Self {
-        Self {
-            symbols: IndexVec::new(),
-            symbols_by_name: SymbolMap::default(),
-        }
-    }
-
     fn shrink_to_fit(&mut self) {
         self.symbols.shrink_to_fit();
     }
@@ -278,18 +275,12 @@ impl PartialEq for SymbolTable {
 
 impl Eq for SymbolTable {}
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub(super) struct SymbolTableBuilder {
     table: SymbolTable,
 }
 
 impl SymbolTableBuilder {
-    pub(super) fn new() -> Self {
-        Self {
-            table: SymbolTable::new(),
-        }
-    }
-
     pub(super) fn add_symbol(&mut self, name: Name) -> (ScopedSymbolId, bool) {
         let hash = SymbolTable::hash_name(&name);
         let entry = self
@@ -339,6 +330,8 @@ pub(crate) enum NodeWithScopeRef<'a> {
     Lambda(&'a ast::ExprLambda),
     FunctionTypeParameters(&'a ast::StmtFunctionDef),
     ClassTypeParameters(&'a ast::StmtClassDef),
+    TypeAlias(&'a ast::StmtTypeAlias),
+    TypeAliasTypeParameters(&'a ast::StmtTypeAlias),
     ListComprehension(&'a ast::ExprListComp),
     SetComprehension(&'a ast::ExprSetComp),
     DictComprehension(&'a ast::ExprDictComp),
@@ -360,6 +353,12 @@ impl NodeWithScopeRef<'_> {
             NodeWithScopeRef::Function(function) => {
                 NodeWithScopeKind::Function(AstNodeRef::new(module, function))
             }
+            NodeWithScopeRef::TypeAlias(type_alias) => {
+                NodeWithScopeKind::TypeAlias(AstNodeRef::new(module, type_alias))
+            }
+            NodeWithScopeRef::TypeAliasTypeParameters(type_alias) => {
+                NodeWithScopeKind::TypeAliasTypeParameters(AstNodeRef::new(module, type_alias))
+            }
             NodeWithScopeRef::Lambda(lambda) => {
                 NodeWithScopeKind::Lambda(AstNodeRef::new(module, lambda))
             }
@@ -400,6 +399,12 @@ impl NodeWithScopeRef<'_> {
             NodeWithScopeRef::ClassTypeParameters(class) => {
                 NodeWithScopeKey::ClassTypeParameters(NodeKey::from_node(class))
             }
+            NodeWithScopeRef::TypeAlias(type_alias) => {
+                NodeWithScopeKey::TypeAlias(NodeKey::from_node(type_alias))
+            }
+            NodeWithScopeRef::TypeAliasTypeParameters(type_alias) => {
+                NodeWithScopeKey::TypeAliasTypeParameters(NodeKey::from_node(type_alias))
+            }
             NodeWithScopeRef::ListComprehension(comprehension) => {
                 NodeWithScopeKey::ListComprehension(NodeKey::from_node(comprehension))
             }
@@ -424,6 +429,8 @@ pub enum NodeWithScopeKind {
     ClassTypeParameters(AstNodeRef<ast::StmtClassDef>),
     Function(AstNodeRef<ast::StmtFunctionDef>),
     FunctionTypeParameters(AstNodeRef<ast::StmtFunctionDef>),
+    TypeAliasTypeParameters(AstNodeRef<ast::StmtTypeAlias>),
+    TypeAlias(AstNodeRef<ast::StmtTypeAlias>),
     Lambda(AstNodeRef<ast::ExprLambda>),
     ListComprehension(AstNodeRef<ast::ExprListComp>),
     SetComprehension(AstNodeRef<ast::ExprSetComp>),
@@ -436,9 +443,11 @@ impl NodeWithScopeKind {
         match self {
             Self::Module => ScopeKind::Module,
             Self::Class(_) => ScopeKind::Class,
-            Self::Function(_) => ScopeKind::Function,
-            Self::Lambda(_) => ScopeKind::Function,
-            Self::FunctionTypeParameters(_) | Self::ClassTypeParameters(_) => ScopeKind::Annotation,
+            Self::Function(_) | Self::Lambda(_) => ScopeKind::Function,
+            Self::FunctionTypeParameters(_)
+            | Self::ClassTypeParameters(_)
+            | Self::TypeAliasTypeParameters(_) => ScopeKind::Annotation,
+            Self::TypeAlias(_) => ScopeKind::TypeAlias,
             Self::ListComprehension(_)
             | Self::SetComprehension(_)
             | Self::DictComprehension(_)
@@ -459,6 +468,13 @@ impl NodeWithScopeKind {
             _ => panic!("expected function"),
         }
     }
+
+    pub fn expect_type_alias(&self) -> &ast::StmtTypeAlias {
+        match self {
+            Self::TypeAlias(type_alias) => type_alias.node(),
+            _ => panic!("expected type alias"),
+        }
+    }
 }
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
@@ -468,6 +484,8 @@ pub(crate) enum NodeWithScopeKey {
     ClassTypeParameters(NodeKey),
     Function(NodeKey),
     FunctionTypeParameters(NodeKey),
+    TypeAlias(NodeKey),
+    TypeAliasTypeParameters(NodeKey),
     Lambda(NodeKey),
     ListComprehension(NodeKey),
     SetComprehension(NodeKey),

crates/red_knot_python_semantic/src/semantic_index/use_def.rs

@@ -459,10 +459,6 @@ pub(super) struct UseDefMapBuilder<'db> {
 }
 
 impl<'db> UseDefMapBuilder<'db> {
-    pub(super) fn new() -> Self {
-        Self::default()
-    }
-
     pub(super) fn add_symbol(&mut self, symbol: ScopedSymbolId) {
         let new_symbol = self.symbol_states.push(SymbolState::undefined());
         debug_assert_eq!(symbol, new_symbol);

crates/red_knot_python_semantic/src/semantic_model.rs

@@ -6,7 +6,7 @@ use ruff_source_file::LineIndex;
 
 use crate::module_name::ModuleName;
 use crate::module_resolver::{resolve_module, Module};
-use crate::semantic_index::ast_ids::HasScopedAstId;
+use crate::semantic_index::ast_ids::HasScopedExpressionId;
 use crate::semantic_index::semantic_index;
 use crate::types::{binding_ty, infer_scope_types, Type};
 use crate::Db;
@@ -54,7 +54,7 @@ impl HasTy for ast::ExpressionRef<'_> {
         let file_scope = index.expression_scope_id(*self);
         let scope = file_scope.to_scope_id(model.db, model.file);
 
-        let expression_id = self.scoped_ast_id(model.db, scope);
+        let expression_id = self.scoped_expression_id(model.db, scope);
         infer_scope_types(model.db, scope).expression_ty(expression_id)
     }
 }

crates/red_knot_python_semantic/src/site_packages.rs

@@ -732,7 +732,20 @@ mod tests {
         let system = TestSystem::default();
         assert!(matches!(
             VirtualEnvironment::new("/.venv", &system),
-            Err(SitePackagesDiscoveryError::VenvDirIsNotADirectory(_))
+            Err(SitePackagesDiscoveryError::VenvDirCanonicalizationError(..))
+        ));
+    }
+
+    #[test]
+    fn reject_venv_that_is_not_a_directory() {
+        let system = TestSystem::default();
+        system
+            .memory_file_system()
+            .write_file("/.venv", "")
+            .unwrap();
+        assert!(matches!(
+            VirtualEnvironment::new("/.venv", &system),
+            Err(SitePackagesDiscoveryError::VenvDirIsNotADirectory(..))
         ));
     }
 

crates/red_knot_python_semantic/src/types.rs

@@ -14,7 +14,7 @@ pub(crate) use self::infer::{
 };
 pub(crate) use self::signatures::Signature;
 use crate::module_resolver::file_to_module;
-use crate::semantic_index::ast_ids::HasScopedAstId;
+use crate::semantic_index::ast_ids::HasScopedExpressionId;
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::symbol::{self as symbol, ScopeId, ScopedSymbolId};
 use crate::semantic_index::{
@@ -47,7 +47,7 @@ pub fn check_types(db: &dyn Db, file: File) -> TypeCheckDiagnostics {
     tracing::debug!("Checking file '{path}'", path = file.path(db));
 
     let index = semantic_index(db, file);
-    let mut diagnostics = TypeCheckDiagnostics::new();
+    let mut diagnostics = TypeCheckDiagnostics::default();
 
     for scope_id in index.scope_ids() {
         let result = infer_scope_types(db, scope_id);
@@ -207,7 +207,7 @@ fn definition_expression_ty<'db>(
     let index = semantic_index(db, file);
     let file_scope = index.expression_scope_id(expression);
     let scope = file_scope.to_scope_id(db, file);
-    let expr_id = expression.scoped_ast_id(db, scope);
+    let expr_id = expression.scoped_expression_id(db, scope);
     if scope == definition.scope(db) {
         // expression is in the definition scope
         let inference = infer_definition_types(db, definition);
@@ -324,6 +324,61 @@ fn declarations_ty<'db>(
     }
 }
 
+/// Meta data for `Type::Todo`, which represents a known limitation in red-knot.
+#[cfg(debug_assertions)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum TodoType {
+    FileAndLine(&'static str, u32),
+    Message(&'static str),
+}
+
+#[cfg(debug_assertions)]
+impl std::fmt::Display for TodoType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            TodoType::FileAndLine(file, line) => write!(f, "[{file}:{line}]"),
+            TodoType::Message(msg) => write!(f, "({msg})"),
+        }
+    }
+}
+
+#[cfg(not(debug_assertions))]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct TodoType;
+
+#[cfg(not(debug_assertions))]
+impl std::fmt::Display for TodoType {
+    fn fmt(&self, _: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        Ok(())
+    }
+}
+
+/// Create a `Type::Todo` variant to represent a known limitation in the type system.
+///
+/// It can be used with a custom message (preferred): `todo_type!("PEP 604 not supported")`,
+/// or simply using `todo_type!()`, which will include information about the file and line.
+#[cfg(debug_assertions)]
+macro_rules! todo_type {
+    () => {
+        Type::Todo(crate::types::TodoType::FileAndLine(file!(), line!()))
+    };
+    ($message:literal) => {
+        Type::Todo(crate::types::TodoType::Message($message))
+    };
+}
+
+#[cfg(not(debug_assertions))]
+macro_rules! todo_type {
+    () => {
+        Type::Todo(crate::types::TodoType)
+    };
+    ($message:literal) => {
+        Type::Todo(crate::types::TodoType)
+    };
+}
+
+pub(crate) use todo_type;
+
 /// Representation of a type: a set of possible values at runtime.
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, salsa::Update)]
 pub enum Type<'db> {
@@ -340,7 +395,9 @@ pub enum Type<'db> {
     /// General rule: `Todo` should only propagate when the presence of the input `Todo` caused the
     /// output to be unknown. An output should only be `Todo` if fixing all `Todo` inputs to be not
     /// `Todo` would change the output type.
-    Todo,
+    ///
+    /// This variant should be created with the `todo_type!` macro.
+    Todo(TodoType),
     /// The empty set of values
     Never,
     /// A specific function object
@@ -384,7 +441,7 @@ impl<'db> Type<'db> {
     }
 
     pub const fn is_todo(&self) -> bool {
-        matches!(self, Type::Todo)
+        matches!(self, Type::Todo(_))
     }
 
     pub const fn class_literal(class: Class<'db>) -> Self {
@@ -480,6 +537,19 @@ impl<'db> Type<'db> {
             .expect("Expected a Type::IntLiteral variant")
     }
 
+    pub const fn into_known_instance(self) -> Option<KnownInstanceType<'db>> {
+        match self {
+            Type::KnownInstance(known_instance) => Some(known_instance),
+            _ => None,
+        }
+    }
+
+    #[track_caller]
+    pub fn expect_known_instance(self) -> KnownInstanceType<'db> {
+        self.into_known_instance()
+            .expect("Expected a Type::KnownInstance variant")
+    }
+
     pub const fn is_boolean_literal(&self) -> bool {
         matches!(self, Type::BooleanLiteral(..))
     }
@@ -530,8 +600,8 @@ impl<'db> Type<'db> {
             return true;
         }
         match (self, target) {
-            (Type::Unknown | Type::Any | Type::Todo, _) => false,
-            (_, Type::Unknown | Type::Any | Type::Todo) => false,
+            (Type::Unknown | Type::Any | Type::Todo(_), _) => false,
+            (_, Type::Unknown | Type::Any | Type::Todo(_)) => false,
             (Type::Never, _) => true,
             (_, Type::Never) => false,
             (_, Type::Instance(InstanceType { class }))
@@ -666,8 +736,8 @@ impl<'db> Type<'db> {
             return true;
         }
         match (self, target) {
-            (Type::Unknown | Type::Any | Type::Todo, _) => true,
-            (_, Type::Unknown | Type::Any | Type::Todo) => true,
+            (Type::Unknown | Type::Any | Type::Todo(_), _) => true,
+            (_, Type::Unknown | Type::Any | Type::Todo(_)) => true,
             (Type::Union(union), ty) => union
                 .elements(db)
                 .iter()
@@ -703,6 +773,7 @@ impl<'db> Type<'db> {
         // of `NoneType` and `NoDefaultType` in typeshed. This should not be required anymore once
         // we understand `sys.version_info` branches.
         self == other
+            || matches!((self, other), (Type::Todo(_), Type::Todo(_)))
             || matches!((self, other),
                 (
                     Type::Instance(InstanceType { class: self_class }),
@@ -726,7 +797,7 @@ impl<'db> Type<'db> {
 
             (Type::Any, _) | (_, Type::Any) => false,
             (Type::Unknown, _) | (_, Type::Unknown) => false,
-            (Type::Todo, _) | (_, Type::Todo) => false,
+            (Type::Todo(_), _) | (_, Type::Todo(_)) => false,
 
             (Type::Union(union), other) | (other, Type::Union(union)) => union
                 .elements(db)
@@ -931,7 +1002,7 @@ impl<'db> Type<'db> {
             Type::Any
             | Type::Never
             | Type::Unknown
-            | Type::Todo
+            | Type::Todo(_)
             | Type::IntLiteral(..)
             | Type::StringLiteral(..)
             | Type::BytesLiteral(..)
@@ -1007,7 +1078,10 @@ impl<'db> Type<'db> {
 
             Type::Instance(InstanceType { class }) => match class.known(db) {
                 Some(
-                    KnownClass::NoneType | KnownClass::NoDefaultType | KnownClass::VersionInfo,
+                    KnownClass::NoneType
+                    | KnownClass::NoDefaultType
+                    | KnownClass::VersionInfo
+                    | KnownClass::TypeAliasType,
                 ) => true,
                 Some(
                     KnownClass::Bool
@@ -1034,7 +1108,7 @@ impl<'db> Type<'db> {
             Type::Any
             | Type::Never
             | Type::Unknown
-            | Type::Todo
+            | Type::Todo(_)
             | Type::Union(..)
             | Type::Intersection(..)
             | Type::LiteralString => false,
@@ -1052,12 +1126,12 @@ impl<'db> Type<'db> {
             Type::Any => Type::Any.into(),
             Type::Never => {
                 // TODO: attribute lookup on Never type
-                Type::Todo.into()
+                todo_type!().into()
             }
             Type::Unknown => Type::Unknown.into(),
             Type::FunctionLiteral(_) => {
                 // TODO: attribute lookup on function type
-                Type::Todo.into()
+                todo_type!().into()
             }
             Type::ModuleLiteral(file) => {
                 // `__dict__` is a very special member that is never overridden by module globals;
@@ -1107,7 +1181,7 @@ impl<'db> Type<'db> {
                         Type::IntLiteral(Program::get(db).target_version(db).minor.into())
                     }
                     // TODO MRO? get_own_instance_member, get_instance_member
-                    _ => Type::Todo,
+                    _ => todo_type!("instance attributes"),
                 };
                 ty.into()
             }
@@ -1149,36 +1223,36 @@ impl<'db> Type<'db> {
             Type::Intersection(_) => {
                 // TODO perform the get_member on each type in the intersection
                 // TODO return the intersection of those results
-                Type::Todo.into()
+                todo_type!().into()
             }
             Type::IntLiteral(_) => {
                 // TODO raise error
-                Type::Todo.into()
+                todo_type!().into()
             }
-            Type::BooleanLiteral(_) => Type::Todo.into(),
+            Type::BooleanLiteral(_) => todo_type!().into(),
             Type::StringLiteral(_) => {
                 // TODO defer to `typing.LiteralString`/`builtins.str` methods
                 // from typeshed's stubs
-                Type::Todo.into()
+                todo_type!().into()
             }
             Type::LiteralString => {
                 // TODO defer to `typing.LiteralString`/`builtins.str` methods
                 // from typeshed's stubs
-                Type::Todo.into()
+                todo_type!().into()
             }
             Type::BytesLiteral(_) => {
                 // TODO defer to Type::Instance(<bytes from typeshed>).member
-                Type::Todo.into()
+                todo_type!().into()
             }
             Type::SliceLiteral(_) => {
                 // TODO defer to `builtins.slice` methods
-                Type::Todo.into()
+                todo_type!().into()
             }
             Type::Tuple(_) => {
                 // TODO: implement tuple methods
-                Type::Todo.into()
+                todo_type!().into()
             }
-            Type::Todo => Type::Todo.into(),
+            &todo @ Type::Todo(_) => todo.into(),
         }
     }
 
@@ -1188,7 +1262,7 @@ impl<'db> Type<'db> {
     /// when `bool(x)` is called on an object `x`.
     fn bool(&self, db: &'db dyn Db) -> Truthiness {
         match self {
-            Type::Any | Type::Todo | Type::Never | Type::Unknown => Truthiness::Ambiguous,
+            Type::Any | Type::Todo(_) | Type::Never | Type::Unknown => Truthiness::Ambiguous,
             Type::FunctionLiteral(_) => Truthiness::AlwaysTrue,
             Type::ModuleLiteral(_) => Truthiness::AlwaysTrue,
             Type::ClassLiteral(_) => {
@@ -1329,7 +1403,7 @@ impl<'db> Type<'db> {
             // `Any` is callable, and its return type is also `Any`.
             Type::Any => CallOutcome::callable(Type::Any),
 
-            Type::Todo => CallOutcome::callable(Type::Todo),
+            Type::Todo(_) => CallOutcome::callable(todo_type!()),
 
             Type::Unknown => CallOutcome::callable(Type::Unknown),
 
@@ -1342,7 +1416,7 @@ impl<'db> Type<'db> {
             ),
 
             // TODO: intersection types
-            Type::Intersection(_) => CallOutcome::callable(Type::Todo),
+            Type::Intersection(_) => CallOutcome::callable(todo_type!()),
 
             _ => CallOutcome::not_callable(self),
         }
@@ -1381,7 +1455,7 @@ impl<'db> Type<'db> {
             };
         }
 
-        if matches!(self, Type::Unknown | Type::Any | Type::Todo) {
+        if matches!(self, Type::Unknown | Type::Any | Type::Todo(_)) {
             // Explicit handling of `Unknown` and `Any` necessary until `type[Unknown]` and
             // `type[Any]` are not defined as `Todo` anymore.
             return IterationOutcome::Iterable { element_ty: self };
@@ -1440,14 +1514,14 @@ impl<'db> Type<'db> {
     pub fn to_instance(&self, db: &'db dyn Db) -> Type<'db> {
         match self {
             Type::Any => Type::Any,
-            Type::Todo => Type::Todo,
+            todo @ Type::Todo(_) => *todo,
             Type::Unknown => Type::Unknown,
             Type::Never => Type::Never,
             Type::ClassLiteral(ClassLiteralType { class }) => Type::instance(*class),
             Type::SubclassOf(SubclassOfType { class }) => Type::instance(*class),
             Type::Union(union) => union.map(db, |element| element.to_instance(db)),
             // TODO: we can probably do better here: --Alex
-            Type::Intersection(_) => Type::Todo,
+            Type::Intersection(_) => todo_type!(),
             // TODO: calling `.to_instance()` on any of these should result in a diagnostic,
             // since they already indicate that the object is an instance of some kind:
             Type::BooleanLiteral(_)
@@ -1478,7 +1552,11 @@ impl<'db> Type<'db> {
             Type::Unknown => Type::Unknown,
             // TODO map this to a new `Type::TypeVar` variant
             Type::KnownInstance(KnownInstanceType::TypeVar(_)) => *self,
-            _ => Type::Todo,
+            Type::KnownInstance(KnownInstanceType::TypeAliasType(alias)) => alias.value_ty(db),
+            Type::KnownInstance(KnownInstanceType::Never | KnownInstanceType::NoReturn) => {
+                Type::Never
+            }
+            _ => todo_type!(),
         }
     }
 
@@ -1553,8 +1631,8 @@ impl<'db> Type<'db> {
             // TODO: `type[Unknown]`?
             Type::Unknown => Type::Unknown,
             // TODO intersections
-            Type::Intersection(_) => Type::Todo,
-            Type::Todo => Type::Todo,
+            Type::Intersection(_) => todo_type!(),
+            todo @ Type::Todo(_) => *todo,
         }
     }
 
@@ -1642,6 +1720,7 @@ pub enum KnownClass {
     // Typing
     SpecialForm,
     TypeVar,
+    TypeAliasType,
     NoDefaultType,
     // sys
     VersionInfo,
@@ -1668,6 +1747,7 @@ impl<'db> KnownClass {
             Self::NoneType => "NoneType",
             Self::SpecialForm => "_SpecialForm",
             Self::TypeVar => "TypeVar",
+            Self::TypeAliasType => "TypeAliasType",
             Self::NoDefaultType => "_NoDefaultType",
             // This is the name the type of `sys.version_info` has in typeshed,
             // which is different to what `type(sys.version_info).__name__` is at runtime.
@@ -1706,7 +1786,7 @@ impl<'db> KnownClass {
             Self::VersionInfo => CoreStdlibModule::Sys,
             Self::GenericAlias | Self::ModuleType | Self::FunctionType => CoreStdlibModule::Types,
             Self::NoneType => CoreStdlibModule::Typeshed,
-            Self::SpecialForm | Self::TypeVar => CoreStdlibModule::Typing,
+            Self::SpecialForm | Self::TypeVar | Self::TypeAliasType => CoreStdlibModule::Typing,
             // TODO when we understand sys.version_info, we will need an explicit fallback here,
             // because typing_extensions has a 3.13+ re-export for the `typing.NoDefault`
             // singleton, but not for `typing._NoDefaultType`
@@ -1720,7 +1800,7 @@ impl<'db> KnownClass {
     const fn is_singleton(self) -> bool {
         // TODO there are other singleton types (EllipsisType, NotImplementedType)
         match self {
-            Self::NoneType | Self::NoDefaultType | Self::VersionInfo => true,
+            Self::NoneType | Self::NoDefaultType | Self::VersionInfo | Self::TypeAliasType => true,
             Self::Bool
             | Self::Object
             | Self::Bytes
@@ -1762,6 +1842,7 @@ impl<'db> KnownClass {
             "NoneType" => Self::NoneType,
             "ModuleType" => Self::ModuleType,
             "FunctionType" => Self::FunctionType,
+            "TypeAliasType" => Self::TypeAliasType,
             "_SpecialForm" => Self::SpecialForm,
             "_NoDefaultType" => Self::NoDefaultType,
             "_version_info" => Self::VersionInfo,
@@ -1795,7 +1876,7 @@ impl<'db> KnownClass {
             | Self::VersionInfo
             | Self::FunctionType => module.name() == self.canonical_module().as_str(),
             Self::NoneType => matches!(module.name().as_str(), "_typeshed" | "types"),
-            Self::SpecialForm | Self::TypeVar | Self::NoDefaultType => {
+            Self::SpecialForm | Self::TypeVar | Self::TypeAliasType | Self::NoDefaultType => {
                 matches!(module.name().as_str(), "typing" | "typing_extensions")
             }
         }
@@ -1807,24 +1888,44 @@ impl<'db> KnownClass {
 pub enum KnownInstanceType<'db> {
     /// The symbol `typing.Literal` (which can also be found as `typing_extensions.Literal`)
     Literal,
+    /// The symbol `typing.Optional` (which can also be found as `typing_extensions.Optional`)
+    Optional,
+    /// The symbol `typing.Union` (which can also be found as `typing_extensions.Union`)
+    Union,
+    /// The symbol `typing.NoReturn` (which can also be found as `typing_extensions.NoReturn`)
+    NoReturn,
+    /// The symbol `typing.Never` available since 3.11 (which can also be found as `typing_extensions.Never`)
+    Never,
     /// A single instance of `typing.TypeVar`
     TypeVar(TypeVarInstance<'db>),
+    /// A single instance of `typing.TypeAliasType` (PEP 695 type alias)
+    TypeAliasType(TypeAliasType<'db>),
     // TODO: fill this enum out with more special forms, etc.
 }
 
 impl<'db> KnownInstanceType<'db> {
     pub const fn as_str(self) -> &'static str {
         match self {
-            KnownInstanceType::Literal => "Literal",
-            KnownInstanceType::TypeVar(_) => "TypeVar",
+            Self::Literal => "Literal",
+            Self::Optional => "Optional",
+            Self::Union => "Union",
+            Self::TypeVar(_) => "TypeVar",
+            Self::NoReturn => "NoReturn",
+            Self::Never => "Never",
+            Self::TypeAliasType(_) => "TypeAliasType",
         }
     }
 
     /// Evaluate the known instance in boolean context
     pub const fn bool(self) -> Truthiness {
         match self {
-            Self::Literal => Truthiness::AlwaysTrue,
-            Self::TypeVar(_) => Truthiness::AlwaysTrue,
+            Self::Literal
+            | Self::Optional
+            | Self::TypeVar(_)
+            | Self::Union
+            | Self::NoReturn
+            | Self::Never
+            | Self::TypeAliasType(_) => Truthiness::AlwaysTrue,
         }
     }
 
@@ -1832,7 +1933,12 @@ impl<'db> KnownInstanceType<'db> {
     pub fn repr(self, db: &'db dyn Db) -> &'db str {
         match self {
             Self::Literal => "typing.Literal",
+            Self::Optional => "typing.Optional",
+            Self::Union => "typing.Union",
+            Self::NoReturn => "typing.NoReturn",
+            Self::Never => "typing.Never",
             Self::TypeVar(typevar) => typevar.name(db),
+            Self::TypeAliasType(_) => "typing.TypeAliasType",
         }
     }
 
@@ -1840,7 +1946,12 @@ impl<'db> KnownInstanceType<'db> {
     pub const fn class(self) -> KnownClass {
         match self {
             Self::Literal => KnownClass::SpecialForm,
+            Self::Optional => KnownClass::SpecialForm,
+            Self::Union => KnownClass::SpecialForm,
+            Self::NoReturn => KnownClass::SpecialForm,
+            Self::Never => KnownClass::SpecialForm,
             Self::TypeVar(_) => KnownClass::TypeVar,
+            Self::TypeAliasType(_) => KnownClass::TypeAliasType,
         }
     }
 
@@ -1859,6 +1970,10 @@ impl<'db> KnownInstanceType<'db> {
         }
         match (module.name().as_str(), instance_name) {
             ("typing" | "typing_extensions", "Literal") => Some(Self::Literal),
+            ("typing" | "typing_extensions", "Optional") => Some(Self::Optional),
+            ("typing" | "typing_extensions", "Union") => Some(Self::Union),
+            ("typing" | "typing_extensions", "NoReturn") => Some(Self::NoReturn),
+            ("typing" | "typing_extensions", "Never") => Some(Self::Never),
             _ => None,
         }
     }
@@ -1866,23 +1981,7 @@ impl<'db> KnownInstanceType<'db> {
     fn member(self, db: &'db dyn Db, name: &str) -> Symbol<'db> {
         let ty = match (self, name) {
             (Self::TypeVar(typevar), "__name__") => Type::string_literal(db, typevar.name(db)),
-            (Self::TypeVar(typevar), "__bound__") => typevar
-                .upper_bound(db)
-                .map(|ty| ty.to_meta_type(db))
-                .unwrap_or_else(|| KnownClass::NoneType.to_instance(db)),
-            (Self::TypeVar(typevar), "__constraints__") => {
-                let tuple_elements: Vec<Type<'db>> = typevar
-                    .constraints(db)
-                    .unwrap_or_default()
-                    .iter()
-                    .map(|ty| ty.to_meta_type(db))
-                    .collect();
-                Type::tuple(db, &tuple_elements)
-            }
-            (Self::TypeVar(typevar), "__default__") => typevar
-                .default_ty(db)
-                .map(|ty| ty.to_meta_type(db))
-                .unwrap_or_else(|| KnownClass::NoDefaultType.to_instance(db)),
+            (Self::TypeAliasType(alias), "__name__") => Type::string_literal(db, alias.name(db)),
             _ => return self.instance_fallback(db).member(db, name),
         };
         ty.into()
@@ -1914,6 +2013,7 @@ pub struct TypeVarInstance<'db> {
 }
 
 impl<'db> TypeVarInstance<'db> {
+    #[allow(unused)]
     pub(crate) fn upper_bound(self, db: &'db dyn Db) -> Option<Type<'db>> {
         if let Some(TypeVarBoundOrConstraints::UpperBound(ty)) = self.bound_or_constraints(db) {
             Some(ty)
@@ -1922,6 +2022,7 @@ impl<'db> TypeVarInstance<'db> {
         }
     }
 
+    #[allow(unused)]
     pub(crate) fn constraints(self, db: &'db dyn Db) -> Option<&[Type<'db>]> {
         if let Some(TypeVarBoundOrConstraints::Constraints(tuple)) = self.bound_or_constraints(db) {
             Some(tuple.elements(db))
@@ -2594,7 +2695,7 @@ impl<'db> Class<'db> {
             // TODO: If the metaclass is not a class, we should verify that it's a callable
             // which accepts the same arguments as `type.__new__` (otherwise error), and return
             // the meta-type of its return type. (And validate that is a class type?)
-            return Ok(Type::Todo);
+            return Ok(todo_type!("metaclass not a class"));
         };
 
         // Reconcile all base classes' metaclasses with the candidate metaclass.
@@ -2708,6 +2809,27 @@ impl<'db> Class<'db> {
     }
 }
 
+#[salsa::interned]
+pub struct TypeAliasType<'db> {
+    #[return_ref]
+    pub name: ast::name::Name,
+
+    rhs_scope: ScopeId<'db>,
+}
+
+#[salsa::tracked]
+impl<'db> TypeAliasType<'db> {
+    #[salsa::tracked]
+    pub fn value_ty(self, db: &'db dyn Db) -> Type<'db> {
+        let scope = self.rhs_scope(db);
+
+        let type_alias_stmt_node = scope.node(db).expect_type_alias();
+        let definition = semantic_index(db, scope.file(db)).definition(type_alias_stmt_node);
+
+        definition_expression_ty(db, definition, &type_alias_stmt_node.value)
+    }
+}
+
 /// Either the explicit `metaclass=` keyword of the class, or the inferred metaclass of one of its base classes.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(super) struct MetaclassCandidate<'db> {
@@ -2894,6 +3016,11 @@ impl<'db> TupleType<'db> {
     }
 }
 
+// Make sure that the `Type` enum does not grow unexpectedly.
+#[cfg(not(debug_assertions))]
+#[cfg(target_pointer_width = "64")]
+static_assertions::assert_eq_size!(Type, [u8; 16]);
+
 #[cfg(test)]
 pub(crate) mod tests {
     use super::*;
@@ -2909,13 +3036,6 @@ pub(crate) mod tests {
     use ruff_python_ast as ast;
     use test_case::test_case;
 
-    #[cfg(target_pointer_width = "64")]
-    #[test]
-    fn no_bloat_enum_sizes() {
-        use std::mem::size_of;
-        assert_eq!(size_of::<Type>(), 16);
-    }
-
     pub(crate) fn setup_db() -> TestDb {
         let db = TestDb::new();
 
@@ -2969,7 +3089,7 @@ pub(crate) mod tests {
                 Ty::Unknown => Type::Unknown,
                 Ty::None => Type::none(db),
                 Ty::Any => Type::Any,
-                Ty::Todo => Type::Todo,
+                Ty::Todo => todo_type!("Ty::Todo"),
                 Ty::IntLiteral(n) => Type::IntLiteral(n),
                 Ty::StringLiteral(s) => Type::string_literal(db, s),
                 Ty::BooleanLiteral(b) => Type::BooleanLiteral(b),
@@ -3560,4 +3680,95 @@ pub(crate) mod tests {
 
         Ok(())
     }
+
+    #[test]
+    fn type_alias_types() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_dedented(
+            "src/mod.py",
+            r#"
+            type Alias1 = int
+            type Alias2 = int
+        "#,
+        )?;
+
+        let mod_py = system_path_to_file(&db, "src/mod.py")?;
+        let ty_alias1 = global_symbol(&db, mod_py, "Alias1").expect_type();
+        let ty_alias2 = global_symbol(&db, mod_py, "Alias2").expect_type();
+
+        let Type::KnownInstance(KnownInstanceType::TypeAliasType(alias1)) = ty_alias1 else {
+            panic!("Expected TypeAliasType, got {ty_alias1:?}");
+        };
+        assert_eq!(alias1.name(&db), "Alias1");
+        assert_eq!(alias1.value_ty(&db), KnownClass::Int.to_instance(&db));
+
+        // Two type aliases are distinct and disjoint, even if they refer to the same type
+        assert!(!ty_alias1.is_equivalent_to(&db, ty_alias2));
+        assert!(ty_alias1.is_disjoint_from(&db, ty_alias2));
+
+        Ok(())
+    }
+
+    /// All other tests also make sure that `Type::Todo` works as expected. This particular
+    /// test makes sure that we handle `Todo` types correctly, even if they originate from
+    /// different sources.
+    #[test]
+    fn todo_types() {
+        let db = setup_db();
+
+        let todo1 = todo_type!("1");
+        let todo2 = todo_type!("2");
+        let todo3 = todo_type!();
+        let todo4 = todo_type!();
+
+        assert!(todo1.is_equivalent_to(&db, todo2));
+        assert!(todo3.is_equivalent_to(&db, todo4));
+        assert!(todo1.is_equivalent_to(&db, todo3));
+
+        assert!(todo1.is_subtype_of(&db, todo2));
+        assert!(todo2.is_subtype_of(&db, todo1));
+
+        assert!(todo3.is_subtype_of(&db, todo4));
+        assert!(todo4.is_subtype_of(&db, todo3));
+
+        assert!(todo1.is_subtype_of(&db, todo3));
+        assert!(todo3.is_subtype_of(&db, todo1));
+
+        let int = KnownClass::Int.to_instance(&db);
+
+        assert!(int.is_assignable_to(&db, todo1));
+        assert!(int.is_assignable_to(&db, todo3));
+
+        assert!(todo1.is_assignable_to(&db, int));
+        assert!(todo3.is_assignable_to(&db, int));
+
+        // We lose information when combining several `Todo` types. This is an
+        // acknowledged limitation of the current implementation. We can not
+        // easily store the meta information of several `Todo`s in a single
+        // variant, as `TodoType` needs to implement `Copy`, meaning it can't
+        // contain `Vec`/`Box`/etc., and can't be boxed itself.
+        //
+        // Lifting this restriction would require us to intern `TodoType` in
+        // salsa, but that would mean we would have to pass in `db` everywhere.
+
+        // A union of several `Todo` types collapses to a single `Todo` type:
+        assert!(UnionType::from_elements(&db, vec![todo1, todo2, todo3, todo4]).is_todo());
+
+        // And similar for intersection types:
+        assert!(IntersectionBuilder::new(&db)
+            .add_positive(todo1)
+            .add_positive(todo2)
+            .add_positive(todo3)
+            .add_positive(todo4)
+            .build()
+            .is_todo());
+        assert!(IntersectionBuilder::new(&db)
+            .add_positive(todo1)
+            .add_negative(todo2)
+            .add_positive(todo3)
+            .add_negative(todo4)
+            .build()
+            .is_todo());
+    }
 }

crates/red_knot_python_semantic/src/types/builder.rs

@@ -128,7 +128,7 @@ impl<'db> IntersectionBuilder<'db> {
     pub(crate) fn new(db: &'db dyn Db) -> Self {
         Self {
             db,
-            intersections: vec![InnerIntersectionBuilder::new()],
+            intersections: vec![InnerIntersectionBuilder::default()],
         }
     }
 
@@ -231,10 +231,6 @@ struct InnerIntersectionBuilder<'db> {
 }
 
 impl<'db> InnerIntersectionBuilder<'db> {
-    fn new() -> Self {
-        Self::default()
-    }
-
     /// Adds a positive type to this intersection.
     fn add_positive(&mut self, db: &'db dyn Db, new_positive: Type<'db>) {
         if let Type::Intersection(other) = new_positive {
@@ -253,7 +249,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                         .iter()
                         .find(|element| element.is_boolean_literal())
                     {
-                        *self = Self::new();
+                        *self = Self::default();
                         self.positive.insert(Type::BooleanLiteral(!value));
                         return;
                     }
@@ -272,7 +268,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 }
                 // A & B = Never    if A and B are disjoint
                 if new_positive.is_disjoint_from(db, *existing_positive) {
-                    *self = Self::new();
+                    *self = Self::default();
                     self.positive.insert(Type::Never);
                     return;
                 }
@@ -285,7 +281,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
             for (index, existing_negative) in self.negative.iter().enumerate() {
                 // S & ~T = Never    if S <: T
                 if new_positive.is_subtype_of(db, *existing_negative) {
-                    *self = Self::new();
+                    *self = Self::default();
                     self.positive.insert(Type::Never);
                     return;
                 }
@@ -313,7 +309,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                     self.add_positive(db, *neg);
                 }
             }
-            ty @ (Type::Any | Type::Unknown | Type::Todo) => {
+            ty @ (Type::Any | Type::Unknown | Type::Todo(_)) => {
                 // Adding any of these types to the negative side of an intersection
                 // is equivalent to adding it to the positive side. We do this to
                 // simplify the representation.
@@ -326,7 +322,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                     .iter()
                     .any(|pos| *pos == KnownClass::Bool.to_instance(db)) =>
             {
-                *self = Self::new();
+                *self = Self::default();
                 self.positive.insert(Type::BooleanLiteral(!bool));
             }
             _ => {
@@ -348,7 +344,7 @@ impl<'db> InnerIntersectionBuilder<'db> {
                 for existing_positive in &self.positive {
                     // S & ~T = Never    if S <: T
                     if existing_positive.is_subtype_of(db, new_negative) {
-                        *self = Self::new();
+                        *self = Self::default();
                         self.positive.insert(Type::Never);
                         return;
                     }
@@ -383,7 +379,7 @@ mod tests {
     use crate::program::{Program, SearchPathSettings};
     use crate::python_version::PythonVersion;
     use crate::stdlib::typing_symbol;
-    use crate::types::{global_symbol, KnownClass, UnionBuilder};
+    use crate::types::{global_symbol, todo_type, KnownClass, UnionBuilder};
     use crate::ProgramSettings;
     use ruff_db::files::system_path_to_file;
     use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
@@ -991,7 +987,7 @@ mod tests {
 
     #[test_case(Type::Any)]
     #[test_case(Type::Unknown)]
-    #[test_case(Type::Todo)]
+    #[test_case(todo_type!())]
     fn build_intersection_t_and_negative_t_does_not_simplify(ty: Type) {
         let db = setup_db();
 

crates/red_knot_python_semantic/src/types/diagnostic.rs

@@ -73,10 +73,6 @@ pub struct TypeCheckDiagnostics {
 }
 
 impl TypeCheckDiagnostics {
-    pub fn new() -> Self {
-        Self { inner: Vec::new() }
-    }
-
     pub(super) fn push(&mut self, diagnostic: TypeCheckDiagnostic) {
         self.inner.push(Arc::new(diagnostic));
     }
@@ -148,7 +144,7 @@ impl<'db> TypeCheckDiagnosticsBuilder<'db> {
         Self {
             db,
             file,
-            diagnostics: TypeCheckDiagnostics::new(),
+            diagnostics: TypeCheckDiagnostics::default(),
         }
     }
 

crates/red_knot_python_semantic/src/types/display.rs

@@ -77,7 +77,7 @@ impl Display for DisplayRepresentation<'_> {
             }
             // `[Type::Todo]`'s display should be explicit that is not a valid display of
             // any other type
-            Type::Todo => f.write_str("@Todo"),
+            Type::Todo(todo) => write!(f, "@Todo{todo}"),
             Type::ModuleLiteral(file) => {
                 write!(f, "<module '{:?}'>", file.path(self.db))
             }

crates/red_knot_python_semantic/src/types/mro.rs

@@ -5,7 +5,7 @@ use itertools::Either;
 use rustc_hash::FxHashSet;
 
 use super::{Class, ClassLiteralType, KnownClass, KnownInstanceType, Type};
-use crate::Db;
+use crate::{types::todo_type, Db};
 
 /// The inferred method resolution order of a given class.
 ///
@@ -354,7 +354,7 @@ impl<'db> ClassBase<'db> {
         match ty {
             Type::Any => Some(Self::Any),
             Type::Unknown => Some(Self::Unknown),
-            Type::Todo => Some(Self::Todo),
+            Type::Todo(_) => Some(Self::Todo),
             Type::ClassLiteral(ClassLiteralType { class }) => Some(Self::Class(class)),
             Type::Union(_) => None, // TODO -- forces consideration of multiple possible MROs?
             Type::Intersection(_) => None, // TODO -- probably incorrect?
@@ -371,8 +371,13 @@ impl<'db> ClassBase<'db> {
             | Type::ModuleLiteral(_)
             | Type::SubclassOf(_) => None,
             Type::KnownInstance(known_instance) => match known_instance {
-                KnownInstanceType::Literal => None,
-                KnownInstanceType::TypeVar(_) => None,
+                KnownInstanceType::TypeVar(_)
+                | KnownInstanceType::TypeAliasType(_)
+                | KnownInstanceType::Literal
+                | KnownInstanceType::Union
+                | KnownInstanceType::NoReturn
+                | KnownInstanceType::Never
+                | KnownInstanceType::Optional => None,
             },
         }
     }
@@ -404,7 +409,7 @@ impl<'db> From<ClassBase<'db>> for Type<'db> {
     fn from(value: ClassBase<'db>) -> Self {
         match value {
             ClassBase::Any => Type::Any,
-            ClassBase::Todo => Type::Todo,
+            ClassBase::Todo => todo_type!(),
             ClassBase::Unknown => Type::Unknown,
             ClassBase::Class(class) => Type::class_literal(class),
         }

crates/red_knot_python_semantic/src/types/narrow.rs

@@ -1,4 +1,4 @@
-use crate::semantic_index::ast_ids::HasScopedAstId;
+use crate::semantic_index::ast_ids::HasScopedExpressionId;
 use crate::semantic_index::constraint::{Constraint, ConstraintNode, PatternConstraint};
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::expression::Expression;
@@ -257,17 +257,26 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
         expression: Expression<'db>,
         is_positive: bool,
     ) -> Option<NarrowingConstraints<'db>> {
+        fn is_narrowing_target_candidate(expr: &ast::Expr) -> bool {
+            matches!(expr, ast::Expr::Name(_) | ast::Expr::Call(_))
+        }
+
         let ast::ExprCompare {
             range: _,
             left,
             ops,
             comparators,
         } = expr_compare;
-        if !left.is_name_expr() && comparators.iter().all(|c| !c.is_name_expr()) {
-            // If none of the comparators are name expressions,
-            // we have no symbol to narrow down the type of.
+
+        // Performance optimization: early return if there are no potential narrowing targets.
+        if !is_narrowing_target_candidate(left)
+            && comparators
+                .iter()
+                .all(|c| !is_narrowing_target_candidate(c))
+        {
             return None;
         }
+
         if !is_positive && comparators.len() > 1 {
             // We can't negate a constraint made by a multi-comparator expression, since we can't
             // know which comparison part is the one being negated.
@@ -283,42 +292,85 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
             .tuple_windows::<(&ruff_python_ast::Expr, &ruff_python_ast::Expr)>();
         let mut constraints = NarrowingConstraints::default();
         for (op, (left, right)) in std::iter::zip(&**ops, comparator_tuples) {
-            if let ast::Expr::Name(ast::ExprName {
-                range: _,
-                id,
-                ctx: _,
-            }) = left
-            {
-                // SAFETY: we should always have a symbol for every Name node.
-                let symbol = self.symbols().symbol_id_by_name(id).unwrap();
-                let rhs_ty = inference.expression_ty(right.scoped_ast_id(self.db, scope));
+            let rhs_ty = inference.expression_ty(right.scoped_expression_id(self.db, scope));
 
-                match if is_positive { *op } else { op.negate() } {
-                    ast::CmpOp::IsNot => {
-                        if rhs_ty.is_singleton(self.db) {
-                            let ty = IntersectionBuilder::new(self.db)
-                                .add_negative(rhs_ty)
-                                .build();
-                            constraints.insert(symbol, ty);
-                        } else {
-                            // Non-singletons cannot be safely narrowed using `is not`
+            match left {
+                ast::Expr::Name(ast::ExprName {
+                    range: _,
+                    id,
+                    ctx: _,
+                }) => {
+                    let symbol = self
+                        .symbols()
+                        .symbol_id_by_name(id)
+                        .expect("Should always have a symbol for every Name node");
+
+                    match if is_positive { *op } else { op.negate() } {
+                        ast::CmpOp::IsNot => {
+                            if rhs_ty.is_singleton(self.db) {
+                                let ty = IntersectionBuilder::new(self.db)
+                                    .add_negative(rhs_ty)
+                                    .build();
+                                constraints.insert(symbol, ty);
+                            } else {
+                                // Non-singletons cannot be safely narrowed using `is not`
+                            }
                         }
-                    }
-                    ast::CmpOp::Is => {
-                        constraints.insert(symbol, rhs_ty);
-                    }
-                    ast::CmpOp::NotEq => {
-                        if rhs_ty.is_single_valued(self.db) {
-                            let ty = IntersectionBuilder::new(self.db)
-                                .add_negative(rhs_ty)
-                                .build();
-                            constraints.insert(symbol, ty);
+                        ast::CmpOp::Is => {
+                            constraints.insert(symbol, rhs_ty);
+                        }
+                        ast::CmpOp::NotEq => {
+                            if rhs_ty.is_single_valued(self.db) {
+                                let ty = IntersectionBuilder::new(self.db)
+                                    .add_negative(rhs_ty)
+                                    .build();
+                                constraints.insert(symbol, ty);
+                            }
+                        }
+                        _ => {
+                            // TODO other comparison types
                         }
-                    }
-                    _ => {
-                        // TODO other comparison types
                     }
                 }
+                ast::Expr::Call(ast::ExprCall {
+                    range: _,
+                    func: callable,
+                    arguments:
+                        ast::Arguments {
+                            args,
+                            keywords,
+                            range: _,
+                        },
+                }) if rhs_ty.is_class_literal() && keywords.is_empty() => {
+                    let [ast::Expr::Name(ast::ExprName { id, .. })] = &**args else {
+                        continue;
+                    };
+
+                    let is_valid_constraint = if is_positive {
+                        op == &ast::CmpOp::Is
+                    } else {
+                        op == &ast::CmpOp::IsNot
+                    };
+
+                    if !is_valid_constraint {
+                        continue;
+                    }
+
+                    let callable_ty =
+                        inference.expression_ty(callable.scoped_expression_id(self.db, scope));
+
+                    if callable_ty
+                        .into_class_literal()
+                        .is_some_and(|c| c.class.is_known(self.db, KnownClass::Type))
+                    {
+                        let symbol = self
+                            .symbols()
+                            .symbol_id_by_name(id)
+                            .expect("Should always have a symbol for every Name node");
+                        constraints.insert(symbol, rhs_ty.to_instance(self.db));
+                    }
+                }
+                _ => {}
             }
         }
         Some(constraints)
@@ -336,7 +388,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
         // TODO: add support for PEP 604 union types on the right hand side of `isinstance`
         // and `issubclass`, for example `isinstance(x, str | (int | float))`.
         match inference
-            .expression_ty(expr_call.func.scoped_ast_id(self.db, scope))
+            .expression_ty(expr_call.func.scoped_expression_id(self.db, scope))
             .into_function_literal()
             .and_then(|f| f.known(self.db))
             .and_then(KnownFunction::constraint_function)
@@ -348,7 +400,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
                     let symbol = self.symbols().symbol_id_by_name(id).unwrap();
 
                     let class_info_ty =
-                        inference.expression_ty(class_info.scoped_ast_id(self.db, scope));
+                        inference.expression_ty(class_info.scoped_expression_id(self.db, scope));
 
                     let to_constraint = match function {
                         KnownConstraintFunction::IsInstance => {
@@ -414,7 +466,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
             // filter our arms with statically known truthiness
             .filter(|expr| {
                 inference
-                    .expression_ty(expr.scoped_ast_id(self.db, scope))
+                    .expression_ty(expr.scoped_expression_id(self.db, scope))
                     .bool(self.db)
                     != match expr_bool_op.op {
                         BoolOp::And => Truthiness::AlwaysTrue,

crates/red_knot_python_semantic/src/types/signatures.rs

@@ -1,7 +1,7 @@
 #![allow(dead_code)]
 use super::{definition_expression_ty, Type};
-use crate::semantic_index::definition::Definition;
 use crate::Db;
+use crate::{semantic_index::definition::Definition, types::todo_type};
 use ruff_python_ast::{self as ast, name::Name};
 
 /// A typed callable signature.
@@ -18,7 +18,7 @@ impl<'db> Signature<'db> {
     pub(crate) fn todo() -> Self {
         Self {
             parameters: Parameters::todo(),
-            return_ty: Type::Todo,
+            return_ty: todo_type!("return type"),
         }
     }
 
@@ -33,8 +33,7 @@ impl<'db> Signature<'db> {
             .as_ref()
             .map(|returns| {
                 if function_node.is_async {
-                    // TODO: generic `types.CoroutineType`!
-                    Type::Todo
+                    todo_type!("generic types.CoroutineType")
                 } else {
                     definition_expression_ty(db, definition, returns.as_ref())
                 }
@@ -81,11 +80,11 @@ impl<'db> Parameters<'db> {
         Self {
             variadic: Some(Parameter {
                 name: Some(Name::new_static("args")),
-                annotated_ty: Type::Todo,
+                annotated_ty: todo_type!(),
             }),
             keywords: Some(Parameter {
                 name: Some(Name::new_static("kwargs")),
-                annotated_ty: Type::Todo,
+                annotated_ty: todo_type!(),
             }),
             ..Default::default()
         }

crates/red_knot_python_semantic/src/types/unpacker.rs

@@ -4,9 +4,9 @@ use ruff_db::files::File;
 use ruff_python_ast::{self as ast, AnyNodeRef};
 use rustc_hash::FxHashMap;
 
-use crate::semantic_index::ast_ids::{HasScopedAstId, ScopedExpressionId};
+use crate::semantic_index::ast_ids::{HasScopedExpressionId, ScopedExpressionId};
 use crate::semantic_index::symbol::ScopeId;
-use crate::types::{Type, TypeCheckDiagnostics, TypeCheckDiagnosticsBuilder};
+use crate::types::{todo_type, Type, TypeCheckDiagnostics, TypeCheckDiagnosticsBuilder};
 use crate::Db;
 
 /// Unpacks the value expression type to their respective targets.
@@ -29,7 +29,7 @@ impl<'db> Unpacker<'db> {
         match target {
             ast::Expr::Name(target_name) => {
                 self.targets
-                    .insert(target_name.scoped_ast_id(self.db, scope), value_ty);
+                    .insert(target_name.scoped_expression_id(self.db, scope), value_ty);
             }
             ast::Expr::Starred(ast::ExprStarred { value, .. }) => {
                 self.unpack(value, value_ty, scope);
@@ -59,7 +59,7 @@ impl<'db> Unpacker<'db> {
                             // TODO: Combine the types into a list type. If the
                             // starred_element_types is empty, then it should be `List[Any]`.
                             // combine_types(starred_element_types);
-                            element_types.push(Type::Todo);
+                            element_types.push(todo_type!("starred unpacking"));
 
                             element_types.extend_from_slice(
                                 // SAFETY: Safe because of the length check above.
@@ -72,7 +72,7 @@ impl<'db> Unpacker<'db> {
                             // index.
                             element_types.resize(elts.len() - 1, Type::Unknown);
                             // TODO: This should be `list[Unknown]`
-                            element_types.insert(starred_index, Type::Todo);
+                            element_types.insert(starred_index, todo_type!("starred unpacking"));
                             Cow::Owned(element_types)
                         }
                     } else {

crates/red_knot_server/src/session.rs

@@ -68,7 +68,7 @@ impl Session {
             let system = LSPSystem::new(index.clone());
 
             // TODO(dhruvmanila): Get the values from the client settings
-            let metadata = WorkspaceMetadata::from_path(system_path, &system, None)?;
+            let metadata = WorkspaceMetadata::discover(system_path, &system, None)?;
             // TODO(micha): Handle the case where the program settings are incorrect more gracefully.
             workspaces.insert(path, RootDatabase::new(metadata, system)?);
         }

crates/red_knot_server/src/system.rs

@@ -7,8 +7,8 @@ use lsp_types::Url;
 use ruff_db::file_revision::FileRevision;
 use ruff_db::system::walk_directory::WalkDirectoryBuilder;
 use ruff_db::system::{
-    DirectoryEntry, FileType, Metadata, OsSystem, Result, System, SystemPath, SystemPathBuf,
-    SystemVirtualPath, SystemVirtualPathBuf,
+    DirectoryEntry, FileType, GlobError, Metadata, OsSystem, PatternError, Result, System,
+    SystemPath, SystemPathBuf, SystemVirtualPath, SystemVirtualPathBuf,
 };
 use ruff_notebook::{Notebook, NotebookError};
 
@@ -198,6 +198,16 @@ impl System for LSPSystem {
         self.os_system.walk_directory(path)
     }
 
+    fn glob(
+        &self,
+        pattern: &str,
+    ) -> std::result::Result<
+        Box<dyn Iterator<Item = std::result::Result<SystemPathBuf, GlobError>>>,
+        PatternError,
+    > {
+        self.os_system.glob(pattern)
+    }
+
     fn as_any(&self) -> &dyn Any {
         self
     }

crates/red_knot_test/src/matcher.rs

@@ -180,6 +180,16 @@ where
     }
 }
 
+/// Discard `@Todo`-type metadata from expected types, which is not available
+/// when running in release mode.
+#[cfg(not(debug_assertions))]
+fn discard_todo_metadata(ty: &str) -> std::borrow::Cow<'_, str> {
+    static TODO_METADATA_REGEX: std::sync::LazyLock<regex::Regex> =
+        std::sync::LazyLock::new(|| regex::Regex::new(r"@Todo\([^)]*\)").unwrap());
+
+    TODO_METADATA_REGEX.replace_all(ty, "@Todo")
+}
+
 struct Matcher {
     line_index: LineIndex,
     source: SourceText,
@@ -276,6 +286,9 @@ impl Matcher {
                 }
             }
             Assertion::Revealed(expected_type) => {
+                #[cfg(not(debug_assertions))]
+                let expected_type = discard_todo_metadata(&expected_type);
+
                 let mut matched_revealed_type = None;
                 let mut matched_undefined_reveal = None;
                 let expected_reveal_type_message = format!("Revealed type is `{expected_type}`");

crates/red_knot_wasm/src/lib.rs

@@ -3,15 +3,15 @@ use std::any::Any;
 use js_sys::Error;
 use wasm_bindgen::prelude::*;
 
-use red_knot_workspace::db::RootDatabase;
+use red_knot_workspace::db::{Db, RootDatabase};
 use red_knot_workspace::workspace::settings::Configuration;
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_db::diagnostic::Diagnostic;
 use ruff_db::files::{system_path_to_file, File};
 use ruff_db::system::walk_directory::WalkDirectoryBuilder;
 use ruff_db::system::{
-    DirectoryEntry, MemoryFileSystem, Metadata, System, SystemPath, SystemPathBuf,
-    SystemVirtualPath,
+    DirectoryEntry, GlobError, MemoryFileSystem, Metadata, PatternError, System, SystemPath,
+    SystemPathBuf, SystemVirtualPath,
 };
 use ruff_notebook::Notebook;
 
@@ -42,10 +42,10 @@ impl Workspace {
     #[wasm_bindgen(constructor)]
     pub fn new(root: &str, settings: &Settings) -> Result<Workspace, Error> {
         let system = WasmSystem::new(SystemPath::new(root));
-        let workspace = WorkspaceMetadata::from_path(
+        let workspace = WorkspaceMetadata::discover(
             SystemPath::new(root),
             &system,
-            Some(Configuration {
+            Some(&Configuration {
                 target_version: Some(settings.target_version.into()),
                 ..Configuration::default()
             }),
@@ -184,8 +184,8 @@ impl Settings {
 #[derive(Copy, Clone, Hash, PartialEq, Eq, PartialOrd, Ord, Default)]
 pub enum TargetVersion {
     Py37,
-    #[default]
     Py38,
+    #[default]
     Py39,
     Py310,
     Py311,
@@ -226,7 +226,7 @@ impl System for WasmSystem {
     }
 
     fn canonicalize_path(&self, path: &SystemPath) -> ruff_db::system::Result<SystemPathBuf> {
-        Ok(self.fs.canonicalize(path))
+        self.fs.canonicalize(path)
     }
 
     fn read_to_string(&self, path: &SystemPath) -> ruff_db::system::Result<String> {
@@ -272,6 +272,13 @@ impl System for WasmSystem {
         self.fs.walk_directory(path)
     }
 
+    fn glob(
+        &self,
+        pattern: &str,
+    ) -> Result<Box<dyn Iterator<Item = Result<SystemPathBuf, GlobError>>>, PatternError> {
+        Ok(Box::new(self.fs.glob(pattern)?))
+    }
+
     fn as_any(&self) -> &dyn Any {
         self
     }
@@ -284,3 +291,17 @@ impl System for WasmSystem {
 fn not_found() -> std::io::Error {
     std::io::Error::new(std::io::ErrorKind::NotFound, "No such file or directory")
 }
+
+#[cfg(test)]
+mod tests {
+    use crate::TargetVersion;
+    use red_knot_python_semantic::PythonVersion;
+
+    #[test]
+    fn same_default_as_python_version() {
+        assert_eq!(
+            PythonVersion::from(TargetVersion::default()),
+            PythonVersion::default()
+        );
+    }
+}

crates/red_knot_workspace/Cargo.toml

@@ -15,22 +15,29 @@ license.workspace = true
 red_knot_python_semantic = { workspace = true }
 
 ruff_cache = { workspace = true }
-ruff_db = { workspace = true, features = ["os", "cache"] }
-ruff_python_ast = { workspace = true }
+ruff_db = { workspace = true, features = ["os", "cache", "serde"] }
+ruff_python_ast = { workspace = true, features = ["serde"] }
 ruff_text_size = { workspace = true }
 red_knot_vendored = { workspace = true }
 
 anyhow = { workspace = true }
 crossbeam = { workspace = true }
+glob = { workspace = true }
 notify = { workspace = true }
+pep440_rs = { workspace = true }
 rayon = { workspace = true }
 rustc-hash = { workspace = true }
 salsa = { workspace = true }
+serde = { workspace = true }
+thiserror = { workspace = true }
+toml = { workspace = true }
 tracing = { workspace = true }
 
 [dev-dependencies]
+red_knot_python_semantic = { workspace = true, features = ["serde"] }
 ruff_db = { workspace = true, features = ["testing"] }
-tempfile = { workspace = true }
+glob = { workspace = true }
+insta = { workspace = true, features = ["redactions", "ron"] }
 
 [features]
 default = ["zstd"]

crates/red_knot_workspace/resources/test/corpus/15_while_break_invalid_in_class.py

@@ -0,0 +1,6 @@
+while True:
+
+    class A:
+        x: int
+
+        break

crates/red_knot_workspace/resources/test/corpus/15_while_break_invalid_in_func.py

@@ -0,0 +1,6 @@
+while True:
+
+    def b():
+        x: int
+
+        break

crates/red_knot_workspace/resources/test/corpus/16_for_break_invalid_in_class.py

@@ -0,0 +1,6 @@
+for _ in range(1):
+
+    class A:
+        x: int
+
+        break

crates/red_knot_workspace/resources/test/corpus/16_for_break_invalid_in_func.py

@@ -0,0 +1,6 @@
+for _ in range(1):
+
+    def b():
+        x: int
+
+        break

crates/red_knot_workspace/resources/test/corpus/89_type_alias_invalid_bound.py

@@ -0,0 +1 @@
+../../../../ruff_python_parser/resources/inline/err/type_param_invalid_bound_expr.py

crates/red_knot_workspace/resources/test/corpus/94_strformat_conversion.py

@@ -0,0 +1,3 @@
+msg = "hello"
+
+f"{msg!r:>{10+10}}"

crates/red_knot_workspace/resources/test/corpus/95_annotation_fstring_invalid.py

@@ -0,0 +1 @@
+x: f"Literal[{1 + 2}]" = 3

crates/red_knot_workspace/resources/test/corpus/95_annotation_union.py

@@ -0,0 +1,3 @@
+from typing import Union
+
+x: Union[int, str] = 1

crates/red_knot_workspace/src/db.rs

@@ -15,7 +15,9 @@ use ruff_db::{Db as SourceDb, Upcast};
 mod changes;
 
 #[salsa::db]
-pub trait Db: SemanticDb + Upcast<dyn SemanticDb> {}
+pub trait Db: SemanticDb + Upcast<dyn SemanticDb> {
+    fn workspace(&self) -> Workspace;
+}
 
 #[salsa::db]
 pub struct RootDatabase {
@@ -45,11 +47,6 @@ impl RootDatabase {
         Ok(db)
     }
 
-    pub fn workspace(&self) -> Workspace {
-        // SAFETY: The workspace is always initialized in `new`.
-        self.workspace.unwrap()
-    }
-
     /// Checks all open files in the workspace and its dependencies.
     pub fn check(&self) -> Result<Vec<Box<dyn Diagnostic>>, Cancelled> {
         self.with_db(|db| db.workspace().check(db))
@@ -153,7 +150,11 @@ impl salsa::Database for RootDatabase {
 }
 
 #[salsa::db]
-impl Db for RootDatabase {}
+impl Db for RootDatabase {
+    fn workspace(&self) -> Workspace {
+        self.workspace.unwrap()
+    }
+}
 
 #[cfg(test)]
 pub(crate) mod tests {
@@ -168,6 +169,7 @@ pub(crate) mod tests {
     use ruff_db::{Db as SourceDb, Upcast};
 
     use crate::db::Db;
+    use crate::workspace::{Workspace, WorkspaceMetadata};
 
     #[salsa::db]
     pub(crate) struct TestDb {
@@ -176,17 +178,23 @@ pub(crate) mod tests {
         files: Files,
         system: TestSystem,
         vendored: VendoredFileSystem,
+        workspace: Option<Workspace>,
     }
 
     impl TestDb {
-        pub(crate) fn new() -> Self {
-            Self {
+        pub(crate) fn new(workspace: WorkspaceMetadata) -> Self {
+            let mut db = Self {
                 storage: salsa::Storage::default(),
                 system: TestSystem::default(),
                 vendored: red_knot_vendored::file_system().clone(),
                 files: Files::default(),
                 events: Arc::default(),
-            }
+                workspace: None,
+            };
+
+            let workspace = Workspace::from_metadata(&db, workspace);
+            db.workspace = Some(workspace);
+            db
         }
     }
 
@@ -254,7 +262,11 @@ pub(crate) mod tests {
     }
 
     #[salsa::db]
-    impl Db for TestDb {}
+    impl Db for TestDb {
+        fn workspace(&self) -> Workspace {
+            self.workspace.unwrap()
+        }
+    }
 
     #[salsa::db]
     impl salsa::Database for TestDb {

crates/red_knot_workspace/src/db/changes.rs

@@ -1,16 +1,15 @@
+use crate::db::{Db, RootDatabase};
+use crate::watch;
+use crate::watch::{ChangeEvent, CreatedKind, DeletedKind};
+use crate::workspace::settings::Configuration;
+use crate::workspace::{Workspace, WorkspaceMetadata};
 use red_knot_python_semantic::Program;
 use ruff_db::files::{system_path_to_file, File, Files};
 use ruff_db::system::walk_directory::WalkState;
 use ruff_db::system::SystemPath;
-use ruff_db::Db;
+use ruff_db::Db as _;
 use rustc_hash::FxHashSet;
 
-use crate::db::RootDatabase;
-use crate::watch;
-use crate::watch::{CreatedKind, DeletedKind};
-use crate::workspace::settings::Configuration;
-use crate::workspace::WorkspaceMetadata;
-
 impl RootDatabase {
     #[tracing::instrument(level = "debug", skip(self, changes, base_configuration))]
     pub fn apply_changes(
@@ -18,7 +17,7 @@ impl RootDatabase {
         changes: Vec<watch::ChangeEvent>,
         base_configuration: Option<&Configuration>,
     ) {
-        let workspace = self.workspace();
+        let mut workspace = self.workspace();
         let workspace_path = workspace.root(self).to_path_buf();
         let program = Program::get(self);
         let custom_stdlib_versions_path = program
@@ -58,6 +57,12 @@ impl RootDatabase {
                     // Changes to ignore files or settings can change the workspace structure or add/remove files
                     // from packages.
                     if let Some(package) = workspace.package(self, path) {
+                        if package.root(self) == workspace.root(self)
+                            || matches!(change, ChangeEvent::Deleted { .. })
+                        {
+                            workspace_change = true;
+                        }
+
                         changed_packages.insert(package);
                     } else {
                         workspace_change = true;
@@ -151,18 +156,22 @@ impl RootDatabase {
         }
 
         if workspace_change {
-            match WorkspaceMetadata::from_path(
-                &workspace_path,
-                self.system(),
-                base_configuration.cloned(),
-            ) {
+            match WorkspaceMetadata::discover(&workspace_path, self.system(), base_configuration) {
                 Ok(metadata) => {
-                    tracing::debug!("Reloading workspace after structural change");
-                    // TODO: Handle changes in the program settings.
-                    workspace.reload(self, metadata);
+                    if metadata.root() == workspace.root(self) {
+                        tracing::debug!("Reloading workspace after structural change");
+                        // TODO: Handle changes in the program settings.
+                        workspace.reload(self, metadata);
+                    } else {
+                        tracing::debug!("Replace workspace after structural change");
+                        workspace = Workspace::from_metadata(self, metadata);
+                        self.workspace = Some(workspace);
+                    }
                 }
                 Err(error) => {
-                    tracing::error!("Failed to load workspace, keep old workspace: {error}");
+                    tracing::error!(
+                        "Failed to load workspace, keeping old workspace configuration: {error}"
+                    );
                 }
             }
 
@@ -227,6 +236,3 @@ impl RootDatabase {
         }
     }
 }
-
-#[cfg(test)]
-mod tests {}

crates/red_knot_workspace/src/watch/watcher.rs

@@ -210,7 +210,15 @@ impl Debouncer {
         }
 
         let kind = event.kind;
-        let path = match SystemPathBuf::from_path_buf(event.paths.into_iter().next().unwrap()) {
+
+        // There are cases where paths can be empty.
+        // https://github.com/astral-sh/ruff/issues/14222
+        let Some(path) = event.paths.into_iter().next() else {
+            tracing::debug!("Ignoring change event with kind '{kind:?}' without a path",);
+            return;
+        };
+
+        let path = match SystemPathBuf::from_path_buf(path) {
             Ok(path) => path,
             Err(path) => {
                 tracing::debug!(

crates/red_knot_workspace/src/watch/workspace_watcher.rs

@@ -6,9 +6,9 @@ use tracing::info;
 use red_knot_python_semantic::system_module_search_paths;
 use ruff_cache::{CacheKey, CacheKeyHasher};
 use ruff_db::system::{SystemPath, SystemPathBuf};
-use ruff_db::Upcast;
+use ruff_db::{Db as _, Upcast};
 
-use crate::db::RootDatabase;
+use crate::db::{Db, RootDatabase};
 use crate::watch::Watcher;
 
 /// Wrapper around a [`Watcher`] that watches the relevant paths of a workspace.
@@ -68,10 +68,9 @@ impl WorkspaceWatcher {
 
         self.has_errored_paths = false;
 
-        let workspace_path = workspace_path
-            .as_utf8_path()
-            .canonicalize_utf8()
-            .map(SystemPathBuf::from_utf8_path_buf)
+        let workspace_path = db
+            .system()
+            .canonicalize_path(&workspace_path)
             .unwrap_or(workspace_path);
 
         // Find the non-overlapping module search paths and filter out paths that are already covered by the workspace.

crates/red_knot_workspace/src/workspace.rs

@@ -1,26 +1,28 @@
-use rustc_hash::{FxBuildHasher, FxHashSet};
-use salsa::{Durability, Setter as _};
-use std::borrow::Cow;
-use std::{collections::BTreeMap, sync::Arc};
-
 use crate::db::Db;
 use crate::db::RootDatabase;
 use crate::workspace::files::{Index, Indexed, IndexedIter, PackageFiles};
-pub use metadata::{PackageMetadata, WorkspaceMetadata};
+pub use metadata::{PackageMetadata, WorkspaceDiscoveryError, WorkspaceMetadata};
 use red_knot_python_semantic::types::check_types;
 use red_knot_python_semantic::SearchPathSettings;
 use ruff_db::diagnostic::{Diagnostic, ParseDiagnostic, Severity};
 use ruff_db::parsed::parsed_module;
 use ruff_db::source::{source_text, SourceTextError};
+use ruff_db::system::FileType;
 use ruff_db::{
     files::{system_path_to_file, File},
     system::{walk_directory::WalkState, SystemPath, SystemPathBuf},
 };
 use ruff_python_ast::{name::Name, PySourceType};
 use ruff_text_size::TextRange;
+use rustc_hash::{FxBuildHasher, FxHashSet};
+use salsa::{Durability, Setter as _};
+use std::borrow::Cow;
+use std::iter::FusedIterator;
+use std::{collections::BTreeMap, sync::Arc};
 
 mod files;
 mod metadata;
+mod pyproject;
 pub mod settings;
 
 /// The project workspace as a Salsa ingredient.
@@ -81,7 +83,7 @@ pub struct Workspace {
 
     /// The (first-party) packages in this workspace.
     #[return_ref]
-    package_tree: BTreeMap<SystemPathBuf, Package>,
+    package_tree: PackageTree,
 
     /// The unresolved search path configuration.
     #[return_ref]
@@ -106,7 +108,6 @@ pub struct Package {
 }
 
 impl Workspace {
-    /// Discovers the closest workspace at `path` and returns its metadata.
     pub fn from_metadata(db: &dyn Db, metadata: WorkspaceMetadata) -> Self {
         let mut packages = BTreeMap::new();
 
@@ -114,10 +115,12 @@ impl Workspace {
             packages.insert(package.root.clone(), Package::from_metadata(db, package));
         }
 
+        let program_settings = metadata.settings.program;
+
         Workspace::builder(
             metadata.root,
-            packages,
-            metadata.settings.program.search_paths,
+            PackageTree(packages),
+            program_settings.search_paths,
         )
         .durability(Durability::MEDIUM)
         .open_fileset_durability(Durability::LOW)
@@ -128,15 +131,11 @@ impl Workspace {
         self.root_buf(db)
     }
 
-    pub fn packages(self, db: &dyn Db) -> impl Iterator<Item = Package> + '_ {
-        self.package_tree(db).values().copied()
-    }
-
     pub fn reload(self, db: &mut dyn Db, metadata: WorkspaceMetadata) {
         tracing::debug!("Reloading workspace");
         assert_eq!(self.root(db), metadata.root());
 
-        let mut old_packages = self.package_tree(db).clone();
+        let mut old_packages = self.package_tree(db).0.clone();
         let mut new_packages = BTreeMap::new();
 
         for package_metadata in metadata.packages {
@@ -157,13 +156,13 @@ impl Workspace {
                 .to(metadata.settings.program.search_paths);
         }
 
-        self.set_package_tree(db).to(new_packages);
+        self.set_package_tree(db).to(PackageTree(new_packages));
     }
 
     pub fn update_package(self, db: &mut dyn Db, metadata: PackageMetadata) -> anyhow::Result<()> {
         let path = metadata.root().to_path_buf();
 
-        if let Some(package) = self.package_tree(db).get(&path).copied() {
+        if let Some(package) = self.package_tree(db).get(&path) {
             package.update(db, metadata);
             Ok(())
         } else {
@@ -171,20 +170,17 @@ impl Workspace {
         }
     }
 
+    pub fn packages(self, db: &dyn Db) -> &PackageTree {
+        self.package_tree(db)
+    }
+
     /// Returns the closest package to which the first-party `path` belongs.
     ///
     /// Returns `None` if the `path` is outside of any package or if `file` isn't a first-party file
     /// (e.g. third-party dependencies or `excluded`).
-    pub fn package(self, db: &dyn Db, path: &SystemPath) -> Option<Package> {
+    pub fn package(self, db: &dyn Db, path: impl AsRef<SystemPath>) -> Option<Package> {
         let packages = self.package_tree(db);
-
-        let (package_path, package) = packages.range(..=path.to_path_buf()).next_back()?;
-
-        if path.starts_with(package_path) {
-            Some(*package)
-        } else {
-            None
-        }
+        packages.get(path.as_ref())
     }
 
     /// Checks all open files in the workspace and its dependencies.
@@ -342,7 +338,7 @@ impl Package {
                 let _entered =
                     tracing::debug_span!("index_package_files", package = %self.name(db)).entered();
 
-                let files = discover_package_files(db, self.root(db));
+                let files = discover_package_files(db, self);
                 tracing::info!("Found {} files in package `{}`", files.len(), self.name(db));
                 vacant.set(files)
             }
@@ -407,23 +403,33 @@ pub(super) fn check_file(db: &dyn Db, file: File) -> Vec<Box<dyn Diagnostic>> {
     diagnostics
 }
 
-fn discover_package_files(db: &dyn Db, path: &SystemPath) -> FxHashSet<File> {
+fn discover_package_files(db: &dyn Db, package: Package) -> FxHashSet<File> {
     let paths = std::sync::Mutex::new(Vec::new());
+    let packages = db.workspace().packages(db);
 
-    db.system().walk_directory(path).run(|| {
+    db.system().walk_directory(package.root(db)).run(|| {
         Box::new(|entry| {
             match entry {
                 Ok(entry) => {
                     // Skip over any non python files to avoid creating too many entries in `Files`.
-                    if entry.file_type().is_file()
-                        && entry
-                            .path()
-                            .extension()
-                            .and_then(PySourceType::try_from_extension)
-                            .is_some()
-                    {
-                        let mut paths = paths.lock().unwrap();
-                        paths.push(entry.into_path());
+                    match entry.file_type() {
+                        FileType::File => {
+                            if entry
+                                .path()
+                                .extension()
+                                .and_then(PySourceType::try_from_extension)
+                                .is_some()
+                            {
+                                let mut paths = paths.lock().unwrap();
+                                paths.push(entry.into_path());
+                            }
+                        }
+                        FileType::Directory | FileType::Symlink => {
+                            // Don't traverse into nested packages (the workspace-package is an ancestor of all other packages)
+                            if packages.get(entry.path()) != Some(package) {
+                                return WalkState::Skip;
+                            }
+                        }
                     }
                 }
                 Err(error) => {
@@ -464,6 +470,7 @@ impl<'a> WorkspaceFiles<'a> {
             WorkspaceFiles::PackageFiles(
                 workspace
                     .packages(db)
+                    .iter()
                     .map(|package| package.files(db))
                     .collect(),
             )
@@ -545,20 +552,78 @@ impl Diagnostic for IOErrorDiagnostic {
     }
 }
 
+#[derive(Debug, Eq, PartialEq, Clone)]
+pub struct PackageTree(BTreeMap<SystemPathBuf, Package>);
+
+impl PackageTree {
+    pub fn get(&self, path: &SystemPath) -> Option<Package> {
+        let (package_path, package) = self.0.range(..=path.to_path_buf()).next_back()?;
+
+        if path.starts_with(package_path) {
+            Some(*package)
+        } else {
+            None
+        }
+    }
+
+    // The package table should never be empty, that's why `is_empty` makes little sense
+    #[allow(clippy::len_without_is_empty)]
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    pub fn iter(&self) -> PackageTreeIter {
+        PackageTreeIter(self.0.values())
+    }
+}
+
+impl<'a> IntoIterator for &'a PackageTree {
+    type Item = Package;
+    type IntoIter = PackageTreeIter<'a>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.iter()
+    }
+}
+
+pub struct PackageTreeIter<'a>(std::collections::btree_map::Values<'a, SystemPathBuf, Package>);
+
+impl Iterator for PackageTreeIter<'_> {
+    type Item = Package;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.next().copied()
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
+
+    fn last(mut self) -> Option<Self::Item> {
+        self.0.next_back().copied()
+    }
+}
+
+impl ExactSizeIterator for PackageTreeIter<'_> {}
+impl FusedIterator for PackageTreeIter<'_> {}
+
 #[cfg(test)]
 mod tests {
     use crate::db::tests::TestDb;
-    use crate::workspace::check_file;
+    use crate::workspace::{check_file, WorkspaceMetadata};
     use red_knot_python_semantic::types::check_types;
     use ruff_db::diagnostic::Diagnostic;
     use ruff_db::files::system_path_to_file;
     use ruff_db::source::source_text;
-    use ruff_db::system::{DbWithTestSystem, SystemPath};
+    use ruff_db::system::{DbWithTestSystem, SystemPath, SystemPathBuf};
     use ruff_db::testing::assert_function_query_was_not_run;
+    use ruff_python_ast::name::Name;
 
     #[test]
     fn check_file_skips_type_checking_when_file_cant_be_read() -> ruff_db::system::Result<()> {
-        let mut db = TestDb::new();
+        let workspace =
+            WorkspaceMetadata::single_package(Name::new_static("test"), SystemPathBuf::from("/"));
+        let mut db = TestDb::new(workspace);
         let path = SystemPath::new("test.py");
 
         db.write_file(path, "x = 10")?;

crates/red_knot_workspace/src/workspace/files.rs

@@ -232,21 +232,28 @@ impl Drop for IndexedMut<'_> {
 mod tests {
     use rustc_hash::FxHashSet;
 
+    use crate::db::tests::TestDb;
+    use crate::db::Db;
+    use crate::workspace::files::Index;
+    use crate::workspace::WorkspaceMetadata;
     use ruff_db::files::system_path_to_file;
     use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
     use ruff_python_ast::name::Name;
 
-    use crate::db::tests::TestDb;
-    use crate::workspace::files::Index;
-    use crate::workspace::Package;
-
     #[test]
     fn re_entrance() -> anyhow::Result<()> {
-        let mut db = TestDb::new();
+        let metadata = WorkspaceMetadata::single_package(
+            Name::new_static("test"),
+            SystemPathBuf::from("/test"),
+        );
+        let mut db = TestDb::new(metadata);
 
         db.write_file("test.py", "")?;
 
-        let package = Package::new(&db, Name::new("test"), SystemPathBuf::from("/test"));
+        let package = db
+            .workspace()
+            .package(&db, "/test")
+            .expect("test package to exist");
 
         let file = system_path_to_file(&db, "test.py").unwrap();
 

crates/red_knot_workspace/src/workspace/metadata.rs

@@ -1,67 +1,191 @@
-use crate::workspace::settings::{Configuration, WorkspaceSettings};
-use ruff_db::system::{System, SystemPath, SystemPathBuf};
+use ruff_db::system::{GlobError, System, SystemPath, SystemPathBuf};
 use ruff_python_ast::name::Name;
+use rustc_hash::{FxBuildHasher, FxHashMap, FxHashSet};
+use thiserror::Error;
 
-#[derive(Debug)]
+use crate::workspace::pyproject::{PyProject, PyProjectError, Workspace};
+use crate::workspace::settings::{Configuration, WorkspaceSettings};
+
+#[derive(Debug, PartialEq, Eq)]
+#[cfg_attr(test, derive(serde::Serialize))]
 pub struct WorkspaceMetadata {
     pub(super) root: SystemPathBuf,
 
     /// The (first-party) packages in this workspace.
     pub(super) packages: Vec<PackageMetadata>,
 
+    /// The resolved settings for this workspace.
     pub(super) settings: WorkspaceSettings,
 }
 
 /// A first-party package in a workspace.
-#[derive(Debug)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(test, derive(serde::Serialize))]
 pub struct PackageMetadata {
     pub(super) name: Name,
 
     /// The path to the root directory of the package.
     pub(super) root: SystemPathBuf,
-    // TODO: Add the loaded package configuration (not the nested ruff settings)
+
+    pub(super) configuration: Configuration,
 }
 
 impl WorkspaceMetadata {
+    /// Creates a workspace that consists of a single package located at `root`.
+    pub fn single_package(name: Name, root: SystemPathBuf) -> Self {
+        let package = PackageMetadata {
+            name,
+            root: root.clone(),
+            configuration: Configuration::default(),
+        };
+
+        let packages = vec![package];
+        let settings = packages[0]
+            .configuration
+            .to_workspace_settings(&root, &packages);
+
+        Self {
+            root,
+            packages,
+            settings,
+        }
+    }
+
     /// Discovers the closest workspace at `path` and returns its metadata.
-    pub fn from_path(
+    ///
+    /// 1. Traverse upwards in the `path`'s ancestor chain and find the first `pyproject.toml`.
+    /// 1. If the `pyproject.toml` contains no `knot.workspace` table, then keep traversing the `path`'s ancestor
+    ///    chain until we find one or reach the root.
+    /// 1. If we've found a workspace, then resolve the workspace's members and assert that the closest
+    ///    package (the first found package without a `knot.workspace` table is a member. If not, create
+    ///    a single package workspace for the closest package.
+    /// 1. If there's no `pyrpoject.toml` with a `knot.workspace` table, then create a single-package workspace.
+    /// 1. If no ancestor directory contains any `pyproject.toml`, create an ad-hoc workspace for `path`
+    ///    that consists of a single package and uses the default settings.
+    pub fn discover(
         path: &SystemPath,
         system: &dyn System,
-        base_configuration: Option<Configuration>,
-    ) -> anyhow::Result<WorkspaceMetadata> {
-        assert!(
-            system.is_directory(path),
-            "Workspace root path must be a directory"
-        );
-        tracing::debug!("Searching for workspace in '{path}'");
+        base_configuration: Option<&Configuration>,
+    ) -> Result<WorkspaceMetadata, WorkspaceDiscoveryError> {
+        tracing::debug!("Searching for a workspace in '{path}'");
 
-        let root = path.to_path_buf();
-
-        // TODO: Discover package name from `pyproject.toml`.
-        let package_name: Name = path.file_name().unwrap_or("<root>").into();
-
-        let package = PackageMetadata {
-            name: package_name,
-            root: root.clone(),
-        };
-
-        // TODO: Load the configuration from disk.
-        let mut configuration = Configuration::default();
-
-        if let Some(base_configuration) = base_configuration {
-            configuration.extend(base_configuration);
+        if !system.is_directory(path) {
+            return Err(WorkspaceDiscoveryError::NotADirectory(path.to_path_buf()));
         }
 
-        // TODO: Respect the package configurations when resolving settings (e.g. for the target version).
-        let settings = configuration.into_workspace_settings(&root);
+        let mut closest_package: Option<PackageMetadata> = None;
 
-        let workspace = WorkspaceMetadata {
-            root,
-            packages: vec![package],
-            settings,
+        for ancestor in path.ancestors() {
+            let pyproject_path = ancestor.join("pyproject.toml");
+            if let Ok(pyproject_str) = system.read_to_string(&pyproject_path) {
+                let pyproject = PyProject::from_str(&pyproject_str).map_err(|error| {
+                    WorkspaceDiscoveryError::InvalidPyProject {
+                        path: pyproject_path,
+                        source: Box::new(error),
+                    }
+                })?;
+
+                let workspace_table = pyproject.workspace().cloned();
+                let package = PackageMetadata::from_pyproject(
+                    pyproject,
+                    ancestor.to_path_buf(),
+                    base_configuration,
+                );
+
+                if let Some(workspace_table) = workspace_table {
+                    let workspace_root = ancestor;
+                    tracing::debug!("Found workspace at '{}'", workspace_root);
+
+                    match collect_packages(
+                        package,
+                        &workspace_table,
+                        closest_package,
+                        base_configuration,
+                        system,
+                    )? {
+                        CollectedPackagesOrStandalone::Packages(mut packages) => {
+                            let mut by_name =
+                                FxHashMap::with_capacity_and_hasher(packages.len(), FxBuildHasher);
+
+                            let mut workspace_package = None;
+
+                            for package in &packages {
+                                if let Some(conflicting) = by_name.insert(package.name(), package) {
+                                    return Err(WorkspaceDiscoveryError::DuplicatePackageNames {
+                                        name: package.name().clone(),
+                                        first: conflicting.root().to_path_buf(),
+                                        second: package.root().to_path_buf(),
+                                    });
+                                }
+
+                                if package.root() == workspace_root {
+                                    workspace_package = Some(package);
+                                } else if !package.root().starts_with(workspace_root) {
+                                    return Err(WorkspaceDiscoveryError::PackageOutsideWorkspace {
+                                        package_name: package.name().clone(),
+                                        package_root: package.root().to_path_buf(),
+                                        workspace_root: workspace_root.to_path_buf(),
+                                    });
+                                }
+                            }
+
+                            let workspace_package = workspace_package
+                                .expect("workspace package to be part of the workspace's packages");
+
+                            let settings = workspace_package
+                                .configuration
+                                .to_workspace_settings(workspace_root, &packages);
+
+                            packages.sort_unstable_by(|a, b| a.root().cmp(b.root()));
+
+                            return Ok(Self {
+                                root: workspace_root.to_path_buf(),
+                                packages,
+                                settings,
+                            });
+                        }
+                        CollectedPackagesOrStandalone::Standalone(package) => {
+                            closest_package = Some(package);
+                            break;
+                        }
+                    }
+                }
+
+                // Not a workspace itself, keep looking for an enclosing workspace.
+                if closest_package.is_none() {
+                    closest_package = Some(package);
+                }
+            }
+        }
+
+        // No workspace found, but maybe a pyproject.toml was found.
+        let package = if let Some(enclosing_package) = closest_package {
+            tracing::debug!("Single package workspace at '{}'", enclosing_package.root());
+
+            enclosing_package
+        } else {
+            tracing::debug!("The ancestor directories contain no `pyproject.toml`. Falling back to a virtual project.");
+
+            // Create a package with a default configuration
+            PackageMetadata {
+                name: path.file_name().unwrap_or("root").into(),
+                root: path.to_path_buf(),
+                // TODO create the configuration from the pyproject toml
+                configuration: base_configuration.cloned().unwrap_or_default(),
+            }
         };
 
-        Ok(workspace)
+        let root = package.root().to_path_buf();
+        let packages = vec![package];
+        let settings = packages[0]
+            .configuration
+            .to_workspace_settings(&root, &packages);
+
+        Ok(Self {
+            root,
+            packages,
+            settings,
+        })
     }
 
     pub fn root(&self) -> &SystemPath {
@@ -78,6 +202,30 @@ impl WorkspaceMetadata {
 }
 
 impl PackageMetadata {
+    pub(crate) fn from_pyproject(
+        pyproject: PyProject,
+        root: SystemPathBuf,
+        base_configuration: Option<&Configuration>,
+    ) -> Self {
+        let name = pyproject.project.and_then(|project| project.name);
+        let name = name
+            .map(|name| Name::new(&*name))
+            .unwrap_or_else(|| Name::new(root.file_name().unwrap_or("root")));
+
+        // TODO: load configuration from pyrpoject.toml
+        let mut configuration = Configuration::default();
+
+        if let Some(base_configuration) = base_configuration {
+            configuration.extend(base_configuration.clone());
+        }
+
+        PackageMetadata {
+            name,
+            root,
+            configuration,
+        }
+    }
+
     pub fn name(&self) -> &Name {
         &self.name
     }
@@ -86,3 +234,577 @@ impl PackageMetadata {
         &self.root
     }
 }
+
+fn collect_packages(
+    workspace_package: PackageMetadata,
+    workspace_table: &Workspace,
+    closest_package: Option<PackageMetadata>,
+    base_configuration: Option<&Configuration>,
+    system: &dyn System,
+) -> Result<CollectedPackagesOrStandalone, WorkspaceDiscoveryError> {
+    let workspace_root = workspace_package.root().to_path_buf();
+    let mut member_paths = FxHashSet::default();
+
+    for glob in workspace_table.members() {
+        let full_glob = workspace_package.root().join(glob);
+
+        let matches = system.glob(full_glob.as_str()).map_err(|error| {
+            WorkspaceDiscoveryError::InvalidMembersPattern {
+                raw_glob: glob.clone(),
+                source: error,
+            }
+        })?;
+
+        for result in matches {
+            let path = result?;
+            let normalized = SystemPath::absolute(path, &workspace_root);
+
+            // Skip over non-directory entry. E.g.finder might end up creating a `.DS_STORE` file
+            // that ends up matching `/projects/*`.
+            if system.is_directory(&normalized) {
+                member_paths.insert(normalized);
+            } else {
+                tracing::debug!("Ignoring non-directory workspace member '{normalized}'");
+            }
+        }
+    }
+
+    // The workspace root is always a member. Don't re-add it
+    let mut packages = vec![workspace_package];
+    member_paths.remove(&workspace_root);
+
+    // Add the package that is closest to the current working directory except
+    // if that package isn't a workspace member, then fallback to creating a single
+    // package workspace.
+    if let Some(closest_package) = closest_package {
+        // the closest `pyproject.toml` isn't a member of this workspace because it is
+        // explicitly included or simply not listed.
+        // Create a standalone workspace.
+        if !member_paths.remove(closest_package.root())
+            || workspace_table.is_excluded(closest_package.root(), &workspace_root)?
+        {
+            tracing::debug!(
+                "Ignoring workspace '{workspace_root}' because package '{package}' is not a member",
+                package = closest_package.name()
+            );
+            return Ok(CollectedPackagesOrStandalone::Standalone(closest_package));
+        }
+
+        tracing::debug!("adding package '{}'", closest_package.name());
+        packages.push(closest_package);
+    }
+
+    // Add all remaining member paths
+    for member_path in member_paths {
+        if workspace_table.is_excluded(&member_path, workspace_root.as_path())? {
+            tracing::debug!("Ignoring excluded member '{member_path}'");
+            continue;
+        }
+
+        let pyproject_path = member_path.join("pyproject.toml");
+
+        let pyproject_str = match system.read_to_string(&pyproject_path) {
+            Ok(pyproject_str) => pyproject_str,
+
+            Err(error) => {
+                if error.kind() == std::io::ErrorKind::NotFound
+                    && member_path
+                        .file_name()
+                        .is_some_and(|name| name.starts_with('.'))
+                {
+                    tracing::debug!(
+                        "Ignore member '{member_path}' because it has no pyproject.toml and is hidden",
+                    );
+                    continue;
+                }
+
+                return Err(WorkspaceDiscoveryError::MemberFailedToReadPyProject {
+                    package_root: member_path,
+                    source: error,
+                });
+            }
+        };
+
+        let pyproject = PyProject::from_str(&pyproject_str).map_err(|error| {
+            WorkspaceDiscoveryError::InvalidPyProject {
+                source: Box::new(error),
+                path: pyproject_path,
+            }
+        })?;
+
+        if pyproject.workspace().is_some() {
+            return Err(WorkspaceDiscoveryError::NestedWorkspaces {
+                package_root: member_path,
+            });
+        }
+
+        let package = PackageMetadata::from_pyproject(pyproject, member_path, base_configuration);
+
+        tracing::debug!(
+            "Adding package '{}' at '{}'",
+            package.name(),
+            package.root()
+        );
+
+        packages.push(package);
+    }
+
+    Ok(CollectedPackagesOrStandalone::Packages(packages))
+}
+
+enum CollectedPackagesOrStandalone {
+    Packages(Vec<PackageMetadata>),
+    Standalone(PackageMetadata),
+}
+
+#[derive(Debug, Error)]
+pub enum WorkspaceDiscoveryError {
+    #[error("workspace path '{0}' is not a directory")]
+    NotADirectory(SystemPathBuf),
+
+    #[error("nested workspaces aren't supported but the package located at '{package_root}' defines a `knot.workspace` table")]
+    NestedWorkspaces { package_root: SystemPathBuf },
+
+    #[error("the workspace contains two packages named '{name}': '{first}' and '{second}'")]
+    DuplicatePackageNames {
+        name: Name,
+        first: SystemPathBuf,
+        second: SystemPathBuf,
+    },
+
+    #[error("the package '{package_name}' located at '{package_root}' is outside the workspace's root directory '{workspace_root}'")]
+    PackageOutsideWorkspace {
+        workspace_root: SystemPathBuf,
+        package_name: Name,
+        package_root: SystemPathBuf,
+    },
+
+    #[error(
+        "failed to read the `pyproject.toml` for the package located at '{package_root}': {source}"
+    )]
+    MemberFailedToReadPyProject {
+        package_root: SystemPathBuf,
+        source: std::io::Error,
+    },
+
+    #[error("{path} is not a valid `pyproject.toml`: {source}")]
+    InvalidPyProject {
+        source: Box<PyProjectError>,
+        path: SystemPathBuf,
+    },
+
+    #[error("invalid glob '{raw_glob}' in `tool.knot.workspace.members`: {source}")]
+    InvalidMembersPattern {
+        source: glob::PatternError,
+        raw_glob: String,
+    },
+
+    #[error("failed to match member glob: {error}")]
+    FailedToMatchGlob {
+        #[from]
+        error: GlobError,
+    },
+}
+
+#[cfg(test)]
+mod tests {
+    //! Integration tests for workspace discovery
+
+    use crate::snapshot_workspace;
+    use anyhow::Context;
+    use insta::assert_ron_snapshot;
+    use ruff_db::system::{SystemPathBuf, TestSystem};
+
+    use crate::workspace::{WorkspaceDiscoveryError, WorkspaceMetadata};
+
+    #[test]
+    fn package_without_pyproject() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([(root.join("foo.py"), ""), (root.join("bar.py"), "")])
+            .context("Failed to write files")?;
+
+        let workspace = WorkspaceMetadata::discover(&root, &system, None)
+            .context("Failed to discover workspace")?;
+
+        assert_eq!(workspace.root(), &*root);
+
+        snapshot_workspace!(workspace);
+
+        Ok(())
+    }
+
+    #[test]
+    fn single_package() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "backend"
+                    "#,
+                ),
+                (root.join("db/__init__.py"), ""),
+            ])
+            .context("Failed to write files")?;
+
+        let workspace = WorkspaceMetadata::discover(&root, &system, None)
+            .context("Failed to discover workspace")?;
+
+        assert_eq!(workspace.root(), &*root);
+        snapshot_workspace!(workspace);
+
+        // Discovering the same package from a subdirectory should give the same result
+        let from_src = WorkspaceMetadata::discover(&root.join("db"), &system, None)
+            .context("Failed to discover workspace from src sub-directory")?;
+
+        assert_eq!(from_src, workspace);
+
+        Ok(())
+    }
+
+    #[test]
+    fn workspace_members() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    exclude = ["packages/excluded"]
+                    "#,
+                ),
+                (
+                    root.join("packages/a/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "member-a"
+                    "#,
+                ),
+                (
+                    root.join("packages/x/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "member-x"
+                    "#,
+                ),
+            ])
+            .context("Failed to write files")?;
+
+        let workspace = WorkspaceMetadata::discover(&root, &system, None)
+            .context("Failed to discover workspace")?;
+
+        assert_eq!(workspace.root(), &*root);
+
+        snapshot_workspace!(workspace);
+
+        // Discovering the same package from a member should give the same result
+        let from_src = WorkspaceMetadata::discover(&root.join("packages/a"), &system, None)
+            .context("Failed to discover workspace from src sub-directory")?;
+
+        assert_eq!(from_src, workspace);
+
+        Ok(())
+    }
+
+    #[test]
+    fn workspace_excluded() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    exclude = ["packages/excluded"]
+                    "#,
+                ),
+                (
+                    root.join("packages/a/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "member-a"
+                    "#,
+                ),
+                (
+                    root.join("packages/excluded/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "member-x"
+                    "#,
+                ),
+            ])
+            .context("Failed to write files")?;
+
+        let workspace = WorkspaceMetadata::discover(&root, &system, None)
+            .context("Failed to discover workspace")?;
+
+        assert_eq!(workspace.root(), &*root);
+        snapshot_workspace!(workspace);
+
+        // Discovering the `workspace` for `excluded` should discover a single-package workspace
+        let excluded_workspace =
+            WorkspaceMetadata::discover(&root.join("packages/excluded"), &system, None)
+                .context("Failed to discover workspace from src sub-directory")?;
+
+        assert_ne!(excluded_workspace, workspace);
+
+        Ok(())
+    }
+
+    #[test]
+    fn workspace_non_unique_member_names() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    "#,
+                ),
+                (
+                    root.join("packages/a/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "a"
+                    "#,
+                ),
+                (
+                    root.join("packages/b/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "a"
+                    "#,
+                ),
+            ])
+            .context("Failed to write files")?;
+
+        let error = WorkspaceMetadata::discover(&root, &system, None).expect_err(
+            "Discovery should error because the workspace contains two packages with the same names.",
+        );
+
+        assert_error_eq(&error, "the workspace contains two packages named 'a': '/app/packages/a' and '/app/packages/b'");
+
+        Ok(())
+    }
+
+    #[test]
+    fn nested_workspaces() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    "#,
+                ),
+                (
+                    root.join("packages/a/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "nested-workspace"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    "#,
+                ),
+            ])
+            .context("Failed to write files")?;
+
+        let error = WorkspaceMetadata::discover(&root, &system, None).expect_err(
+            "Discovery should error because the workspace has a package that itself is a workspace",
+        );
+
+        assert_error_eq(&error, "nested workspaces aren't supported but the package located at '/app/packages/a' defines a `knot.workspace` table");
+
+        Ok(())
+    }
+
+    #[test]
+    fn member_missing_pyproject_toml() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    "#,
+                ),
+                (root.join("packages/a/test.py"), ""),
+            ])
+            .context("Failed to write files")?;
+
+        let error = WorkspaceMetadata::discover(&root, &system, None)
+            .expect_err("Discovery should error because member `a` has no `pypyroject.toml`");
+
+        assert_error_eq(&error, "failed to read the `pyproject.toml` for the package located at '/app/packages/a': No such file or directory");
+
+        Ok(())
+    }
+
+    /// Folders that match the members pattern but don't have a pyproject.toml
+    /// aren't valid members and discovery fails. However, don't fail
+    /// if the folder name indicates that it is a hidden folder that might
+    /// have been crated by another tool
+    #[test]
+    fn member_pattern_matching_hidden_folder() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    "#,
+                ),
+                (root.join("packages/.hidden/a.py"), ""),
+            ])
+            .context("Failed to write files")?;
+
+        let workspace = WorkspaceMetadata::discover(&root, &system, None)?;
+
+        snapshot_workspace!(workspace);
+
+        Ok(())
+    }
+
+    #[test]
+    fn member_pattern_matching_file() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["packages/*"]
+                    "#,
+                ),
+                (root.join("packages/.DS_STORE"), ""),
+            ])
+            .context("Failed to write files")?;
+
+        let workspace = WorkspaceMetadata::discover(&root, &system, None)?;
+
+        snapshot_workspace!(&workspace);
+
+        Ok(())
+    }
+
+    #[test]
+    fn workspace_root_not_an_ancestor_of_member() -> anyhow::Result<()> {
+        let system = TestSystem::default();
+        let root = SystemPathBuf::from("/app");
+
+        system
+            .memory_file_system()
+            .write_files([
+                (
+                    root.join("pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "workspace-root"
+
+                    [tool.knot.workspace]
+                    members = ["../packages/*"]
+                    "#,
+                ),
+                (
+                    root.join("../packages/a/pyproject.toml"),
+                    r#"
+                    [project]
+                    name = "a"
+                    "#,
+                ),
+            ])
+            .context("Failed to write files")?;
+
+        let error = WorkspaceMetadata::discover(&root, &system, None).expect_err(
+            "Discovery should error because member `a` is outside the workspace's directory`",
+        );
+
+        assert_error_eq(&error, "the package 'a' located at '/packages/a' is outside the workspace's root directory '/app'");
+
+        Ok(())
+    }
+
+    #[track_caller]
+    fn assert_error_eq(error: &WorkspaceDiscoveryError, message: &str) {
+        assert_eq!(error.to_string().replace('\\', "/"), message);
+    }
+
+    /// Snapshots a workspace but with all paths using unix separators.
+    #[macro_export]
+    macro_rules! snapshot_workspace {
+    ($workspace:expr) => {{
+        assert_ron_snapshot!($workspace,{
+            ".root" => insta::dynamic_redaction(|content, _content_path| {
+                content.as_str().unwrap().replace("\\", "/")
+            }),
+            ".packages[].root" => insta::dynamic_redaction(|content, _content_path| {
+                content.as_str().unwrap().replace("\\", "/")
+            }),
+        });
+    }};
+}
+}

crates/red_knot_workspace/src/workspace/pyproject.rs

@@ -0,0 +1,108 @@
+mod package_name;
+
+use pep440_rs::{Version, VersionSpecifiers};
+use serde::Deserialize;
+use thiserror::Error;
+
+use crate::workspace::metadata::WorkspaceDiscoveryError;
+pub(crate) use package_name::PackageName;
+use ruff_db::system::SystemPath;
+
+/// A `pyproject.toml` as specified in PEP 517.
+#[derive(Deserialize, Debug, Default, Clone)]
+#[serde(rename_all = "kebab-case")]
+pub(crate) struct PyProject {
+    /// PEP 621-compliant project metadata.
+    pub project: Option<Project>,
+    /// Tool-specific metadata.
+    pub tool: Option<Tool>,
+}
+
+impl PyProject {
+    pub(crate) fn workspace(&self) -> Option<&Workspace> {
+        self.tool
+            .as_ref()
+            .and_then(|tool| tool.knot.as_ref())
+            .and_then(|knot| knot.workspace.as_ref())
+    }
+}
+
+#[derive(Error, Debug)]
+pub enum PyProjectError {
+    #[error(transparent)]
+    TomlSyntax(#[from] toml::de::Error),
+}
+
+impl PyProject {
+    pub(crate) fn from_str(content: &str) -> Result<Self, PyProjectError> {
+        toml::from_str(content).map_err(PyProjectError::TomlSyntax)
+    }
+}
+
+/// PEP 621 project metadata (`project`).
+///
+/// See <https://packaging.python.org/en/latest/specifications/pyproject-toml>.
+#[derive(Deserialize, Debug, Clone, PartialEq)]
+#[cfg_attr(test, derive(serde::Serialize))]
+#[serde(rename_all = "kebab-case")]
+pub(crate) struct Project {
+    /// The name of the project
+    ///
+    /// Note: Intentionally option to be more permissive during deserialization.
+    /// `PackageMetadata::from_pyproject` reports missing names.
+    pub name: Option<PackageName>,
+    /// The version of the project
+    pub version: Option<Version>,
+    /// The Python versions this project is compatible with.
+    pub requires_python: Option<VersionSpecifiers>,
+}
+
+#[derive(Deserialize, Debug, Clone, PartialEq, Eq)]
+pub(crate) struct Tool {
+    pub knot: Option<Knot>,
+}
+
+#[derive(Deserialize, Debug, Clone, PartialEq, Eq)]
+#[serde(rename_all = "kebab-case", deny_unknown_fields)]
+pub(crate) struct Knot {
+    pub(crate) workspace: Option<Workspace>,
+}
+
+#[derive(Deserialize, Debug, Clone, PartialEq, Eq)]
+#[serde(rename_all = "kebab-case", deny_unknown_fields)]
+pub(crate) struct Workspace {
+    pub(crate) members: Option<Vec<String>>,
+    pub(crate) exclude: Option<Vec<String>>,
+}
+
+impl Workspace {
+    pub(crate) fn members(&self) -> &[String] {
+        self.members.as_deref().unwrap_or_default()
+    }
+
+    pub(crate) fn exclude(&self) -> &[String] {
+        self.exclude.as_deref().unwrap_or_default()
+    }
+
+    pub(crate) fn is_excluded(
+        &self,
+        path: &SystemPath,
+        workspace_root: &SystemPath,
+    ) -> Result<bool, WorkspaceDiscoveryError> {
+        for exclude in self.exclude() {
+            let full_glob =
+                glob::Pattern::new(workspace_root.join(exclude).as_str()).map_err(|error| {
+                    WorkspaceDiscoveryError::InvalidMembersPattern {
+                        raw_glob: exclude.clone(),
+                        source: error,
+                    }
+                })?;
+
+            if full_glob.matches_path(path.as_std_path()) {
+                return Ok(true);
+            }
+        }
+
+        Ok(false)
+    }
+}

crates/red_knot_workspace/src/workspace/pyproject/package_name.rs

@@ -0,0 +1,140 @@
+use serde::{Deserialize, Deserializer, Serialize};
+use std::ops::Deref;
+use thiserror::Error;
+
+/// The normalized name of a package.
+///
+/// Converts the name to lowercase and collapses runs of `-`, `_`, and `.` down to a single `-`.
+/// For example, `---`, `.`, and `__` are all converted to a single `-`.
+///
+/// See: <https://packaging.python.org/en/latest/specifications/name-normalization/>
+#[derive(Debug, Default, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize)]
+pub(crate) struct PackageName(String);
+
+impl PackageName {
+    /// Create a validated, normalized package name.
+    pub(crate) fn new(name: String) -> Result<Self, InvalidPackageNameError> {
+        if name.is_empty() {
+            return Err(InvalidPackageNameError::Empty);
+        }
+
+        if name.starts_with(['-', '_', '.']) {
+            return Err(InvalidPackageNameError::NonAlphanumericStart(
+                name.chars().next().unwrap(),
+            ));
+        }
+
+        if name.ends_with(['-', '_', '.']) {
+            return Err(InvalidPackageNameError::NonAlphanumericEnd(
+                name.chars().last().unwrap(),
+            ));
+        }
+
+        let Some(start) = name.find(|c: char| {
+            !c.is_ascii() || c.is_ascii_uppercase() || matches!(c, '-' | '_' | '.')
+        }) else {
+            return Ok(Self(name));
+        };
+
+        let (already_normalized, maybe_normalized) = name.split_at(start);
+
+        let mut normalized = String::with_capacity(name.len());
+        normalized.push_str(already_normalized);
+        let mut last = None;
+
+        for c in maybe_normalized.chars() {
+            if !c.is_ascii() {
+                return Err(InvalidPackageNameError::InvalidCharacter(c));
+            }
+
+            if c.is_ascii_uppercase() {
+                normalized.push(c.to_ascii_lowercase());
+            } else if matches!(c, '-' | '_' | '.') {
+                if matches!(last, Some('-' | '_' | '.')) {
+                    // Only keep a single instance of `-`, `_` and `.`
+                } else {
+                    normalized.push('-');
+                }
+            } else {
+                normalized.push(c);
+            }
+
+            last = Some(c);
+        }
+
+        Ok(Self(normalized))
+    }
+
+    /// Returns the underlying package name.
+    pub(crate) fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl From<PackageName> for String {
+    fn from(value: PackageName) -> Self {
+        value.0
+    }
+}
+
+impl<'de> Deserialize<'de> for PackageName {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let s = String::deserialize(deserializer)?;
+        Self::new(s).map_err(serde::de::Error::custom)
+    }
+}
+
+impl std::fmt::Display for PackageName {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl Deref for PackageName {
+    type Target = str;
+    fn deref(&self) -> &Self::Target {
+        self.as_str()
+    }
+}
+
+#[derive(Error, Debug)]
+pub(crate) enum InvalidPackageNameError {
+    #[error("name must start with letter or number but it starts with '{0}'")]
+    NonAlphanumericStart(char),
+    #[error("name must end with letter or number but it ends with '{0}'")]
+    NonAlphanumericEnd(char),
+    #[error("valid name consists only of ASCII letters and numbers, period, underscore and hyphen but name contains '{0}'"
+    )]
+    InvalidCharacter(char),
+    #[error("name must not be empty")]
+    Empty,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::PackageName;
+
+    #[test]
+    fn normalize() {
+        let inputs = [
+            "friendly-bard",
+            "Friendly-Bard",
+            "FRIENDLY-BARD",
+            "friendly.bard",
+            "friendly_bard",
+            "friendly--bard",
+            "friendly-.bard",
+            "FrIeNdLy-._.-bArD",
+        ];
+
+        for input in inputs {
+            assert_eq!(
+                PackageName::new(input.to_string()).unwrap(),
+                PackageName::new("friendly-bard".to_string()).unwrap(),
+            );
+        }
+    }
+}

crates/red_knot_workspace/src/workspace/settings.rs

@@ -1,10 +1,12 @@
+use crate::workspace::PackageMetadata;
 use red_knot_python_semantic::{ProgramSettings, PythonVersion, SearchPathSettings, SitePackages};
 use ruff_db::system::{SystemPath, SystemPathBuf};
 
 /// The resolved configurations.
 ///
 /// The main difference to [`Configuration`] is that default values are filled in.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[cfg_attr(test, derive(serde::Serialize))]
 pub struct WorkspaceSettings {
     pub(super) program: ProgramSettings,
 }
@@ -16,7 +18,8 @@ impl WorkspaceSettings {
 }
 
 /// The configuration for the workspace or a package.
-#[derive(Debug, Default, Clone)]
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
+#[cfg_attr(test, derive(serde::Serialize))]
 pub struct Configuration {
     pub target_version: Option<PythonVersion>,
     pub search_paths: SearchPathConfiguration,
@@ -29,17 +32,22 @@ impl Configuration {
         self.search_paths.extend(with.search_paths);
     }
 
-    pub fn into_workspace_settings(self, workspace_root: &SystemPath) -> WorkspaceSettings {
+    pub fn to_workspace_settings(
+        &self,
+        workspace_root: &SystemPath,
+        _packages: &[PackageMetadata],
+    ) -> WorkspaceSettings {
         WorkspaceSettings {
             program: ProgramSettings {
                 target_version: self.target_version.unwrap_or_default(),
-                search_paths: self.search_paths.into_settings(workspace_root),
+                search_paths: self.search_paths.to_settings(workspace_root),
             },
         }
     }
 }
 
 #[derive(Debug, Default, Clone, Eq, PartialEq)]
+#[cfg_attr(test, derive(serde::Serialize))]
 pub struct SearchPathConfiguration {
     /// List of user-provided paths that should take first priority in the module resolution.
     /// Examples in other type checkers are mypy's MYPYPATH environment variable,
@@ -59,15 +67,19 @@ pub struct SearchPathConfiguration {
 }
 
 impl SearchPathConfiguration {
-    pub fn into_settings(self, workspace_root: &SystemPath) -> SearchPathSettings {
-        let site_packages = self.site_packages.unwrap_or(SitePackages::Known(vec![]));
+    pub fn to_settings(&self, workspace_root: &SystemPath) -> SearchPathSettings {
+        let site_packages = self
+            .site_packages
+            .clone()
+            .unwrap_or(SitePackages::Known(vec![]));
 
         SearchPathSettings {
-            extra_paths: self.extra_paths.unwrap_or_default(),
+            extra_paths: self.extra_paths.clone().unwrap_or_default(),
             src_root: self
+                .clone()
                 .src_root
                 .unwrap_or_else(|| workspace_root.to_path_buf()),
-            custom_typeshed: self.custom_typeshed,
+            custom_typeshed: self.custom_typeshed.clone(),
             site_packages,
         }
     }

crates/red_knot_workspace/src/workspace/snapshots/red_knot_workspace__workspace__metadata__tests__member_pattern_matching_file.snap

@@ -0,0 +1,37 @@
+---
+source: crates/red_knot_workspace/src/workspace/metadata.rs
+expression: "&workspace"
+snapshot_kind: text
+---
+WorkspaceMetadata(
+  root: "/app",
+  packages: [
+    PackageMetadata(
+      name: Name("workspace-root"),
+      root: "/app",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+  ],
+  settings: WorkspaceSettings(
+    program: ProgramSettings(
+      target_version: PythonVersion(
+        major: 3,
+        minor: 9,
+      ),
+      search_paths: SearchPathSettings(
+        extra_paths: [],
+        src_root: "/app",
+        custom_typeshed: None,
+        site_packages: Known([]),
+      ),
+    ),
+  ),
+)

crates/red_knot_workspace/src/workspace/snapshots/red_knot_workspace__workspace__metadata__tests__member_pattern_matching_hidden_folder.snap

@@ -0,0 +1,37 @@
+---
+source: crates/red_knot_workspace/src/workspace/metadata.rs
+expression: workspace
+snapshot_kind: text
+---
+WorkspaceMetadata(
+  root: "/app",
+  packages: [
+    PackageMetadata(
+      name: Name("workspace-root"),
+      root: "/app",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+  ],
+  settings: WorkspaceSettings(
+    program: ProgramSettings(
+      target_version: PythonVersion(
+        major: 3,
+        minor: 9,
+      ),
+      search_paths: SearchPathSettings(
+        extra_paths: [],
+        src_root: "/app",
+        custom_typeshed: None,
+        site_packages: Known([]),
+      ),
+    ),
+  ),
+)

crates/red_knot_workspace/src/workspace/snapshots/red_knot_workspace__workspace__metadata__tests__package_without_pyproject.snap

@@ -0,0 +1,37 @@
+---
+source: crates/red_knot_workspace/src/workspace/metadata.rs
+expression: workspace
+snapshot_kind: text
+---
+WorkspaceMetadata(
+  root: "/app",
+  packages: [
+    PackageMetadata(
+      name: Name("app"),
+      root: "/app",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+  ],
+  settings: WorkspaceSettings(
+    program: ProgramSettings(
+      target_version: PythonVersion(
+        major: 3,
+        minor: 9,
+      ),
+      search_paths: SearchPathSettings(
+        extra_paths: [],
+        src_root: "/app",
+        custom_typeshed: None,
+        site_packages: Known([]),
+      ),
+    ),
+  ),
+)

crates/red_knot_workspace/src/workspace/snapshots/red_knot_workspace__workspace__metadata__tests__single_package.snap

@@ -0,0 +1,37 @@
+---
+source: crates/red_knot_workspace/src/workspace/metadata.rs
+expression: workspace
+snapshot_kind: text
+---
+WorkspaceMetadata(
+  root: "/app",
+  packages: [
+    PackageMetadata(
+      name: Name("backend"),
+      root: "/app",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+  ],
+  settings: WorkspaceSettings(
+    program: ProgramSettings(
+      target_version: PythonVersion(
+        major: 3,
+        minor: 9,
+      ),
+      search_paths: SearchPathSettings(
+        extra_paths: [],
+        src_root: "/app",
+        custom_typeshed: None,
+        site_packages: Known([]),
+      ),
+    ),
+  ),
+)

crates/red_knot_workspace/src/workspace/snapshots/red_knot_workspace__workspace__metadata__tests__workspace_excluded.snap

@@ -0,0 +1,50 @@
+---
+source: crates/red_knot_workspace/src/workspace/metadata.rs
+expression: workspace
+snapshot_kind: text
+---
+WorkspaceMetadata(
+  root: "/app",
+  packages: [
+    PackageMetadata(
+      name: Name("workspace-root"),
+      root: "/app",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+    PackageMetadata(
+      name: Name("member-a"),
+      root: "/app/packages/a",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+  ],
+  settings: WorkspaceSettings(
+    program: ProgramSettings(
+      target_version: PythonVersion(
+        major: 3,
+        minor: 9,
+      ),
+      search_paths: SearchPathSettings(
+        extra_paths: [],
+        src_root: "/app",
+        custom_typeshed: None,
+        site_packages: Known([]),
+      ),
+    ),
+  ),
+)

crates/red_knot_workspace/src/workspace/snapshots/red_knot_workspace__workspace__metadata__tests__workspace_members.snap

@@ -0,0 +1,63 @@
+---
+source: crates/red_knot_workspace/src/workspace/metadata.rs
+expression: workspace
+snapshot_kind: text
+---
+WorkspaceMetadata(
+  root: "/app",
+  packages: [
+    PackageMetadata(
+      name: Name("workspace-root"),
+      root: "/app",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+    PackageMetadata(
+      name: Name("member-a"),
+      root: "/app/packages/a",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+    PackageMetadata(
+      name: Name("member-x"),
+      root: "/app/packages/x",
+      configuration: Configuration(
+        target_version: None,
+        search_paths: SearchPathConfiguration(
+          extra_paths: None,
+          src_root: None,
+          custom_typeshed: None,
+          site_packages: None,
+        ),
+      ),
+    ),
+  ],
+  settings: WorkspaceSettings(
+    program: ProgramSettings(
+      target_version: PythonVersion(
+        major: 3,
+        minor: 9,
+      ),
+      search_paths: SearchPathSettings(
+        extra_paths: [],
+        src_root: "/app",
+        custom_typeshed: None,
+        site_packages: Known([]),
+      ),
+    ),
+  ),
+)

crates/red_knot_workspace/tests/check.rs

@@ -1,49 +1,167 @@
-use std::fs;
-use std::path::PathBuf;
-
+use anyhow::{anyhow, Context};
 use red_knot_python_semantic::{HasTy, SemanticModel};
 use red_knot_workspace::db::RootDatabase;
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_db::files::{system_path_to_file, File};
 use ruff_db::parsed::parsed_module;
-use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf};
+use ruff_db::system::{SystemPath, SystemPathBuf, TestSystem};
 use ruff_python_ast::visitor::source_order;
 use ruff_python_ast::visitor::source_order::SourceOrderVisitor;
 use ruff_python_ast::{self as ast, Alias, Expr, Parameter, ParameterWithDefault, Stmt};
 
-fn setup_db(workspace_root: &SystemPath) -> anyhow::Result<RootDatabase> {
-    let system = OsSystem::new(workspace_root);
-    let workspace = WorkspaceMetadata::from_path(workspace_root, &system, None)?;
+fn setup_db(workspace_root: &SystemPath, system: TestSystem) -> anyhow::Result<RootDatabase> {
+    let workspace = WorkspaceMetadata::discover(workspace_root, &system, None)?;
     RootDatabase::new(workspace, system)
 }
 
-/// Test that all snippets in testcorpus can be checked without panic
+fn get_workspace_root() -> anyhow::Result<SystemPathBuf> {
+    Ok(SystemPathBuf::from(String::from_utf8(
+        std::process::Command::new("cargo")
+            .args(["locate-project", "--workspace", "--message-format", "plain"])
+            .output()?
+            .stdout,
+    )?)
+    .parent()
+    .unwrap()
+    .to_owned())
+}
+
+/// Test that all snippets in testcorpus can be checked without panic (except for [`KNOWN_FAILURES`])
 #[test]
-#[allow(clippy::print_stdout)]
 fn corpus_no_panic() -> anyhow::Result<()> {
-    let root = SystemPathBuf::from_path_buf(tempfile::TempDir::new()?.into_path()).unwrap();
-    let db = setup_db(&root)?;
+    let crate_root = String::from(env!("CARGO_MANIFEST_DIR"));
+    run_corpus_tests(&format!("{crate_root}/resources/test/corpus/**/*.py"))
+}
 
-    let corpus = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("resources/test/corpus");
+#[test]
+fn parser_no_panic() -> anyhow::Result<()> {
+    let workspace_root = get_workspace_root()?;
+    run_corpus_tests(&format!(
+        "{workspace_root}/crates/ruff_python_parser/resources/**/*.py"
+    ))
+}
 
-    for path in fs::read_dir(&corpus)? {
-        let source = path?.path();
-        println!("checking {source:?}");
-        let source_fn = source.file_name().unwrap().to_str().unwrap();
-        let py_dest = root.join(source_fn);
-        fs::copy(&source, py_dest.as_std_path())?;
-        // this test is only asserting that we can pull every expression type without a panic
-        // (and some non-expressions that clearly define a single type)
-        let file = system_path_to_file(&db, py_dest).unwrap();
-        pull_types(&db, file);
+#[test]
+#[ignore = "Enable running once there are fewer failures"]
+fn linter_af_no_panic() -> anyhow::Result<()> {
+    let workspace_root = get_workspace_root()?;
+    run_corpus_tests(&format!(
+        "{workspace_root}/crates/ruff_linter/resources/test/fixtures/[a-f]*/**/*.py"
+    ))
+}
 
-        // try the file as a stub also
-        println!("re-checking as .pyi");
-        let pyi_dest = root.join(format!("{source_fn}i"));
-        std::fs::copy(source, pyi_dest.as_std_path())?;
-        let file = system_path_to_file(&db, pyi_dest).unwrap();
-        pull_types(&db, file);
+#[test]
+#[ignore = "Enable running once there are fewer failures"]
+fn linter_gz_no_panic() -> anyhow::Result<()> {
+    let workspace_root = get_workspace_root()?;
+    run_corpus_tests(&format!(
+        "{workspace_root}/crates/ruff_linter/resources/test/fixtures/[g-z]*/**/*.py"
+    ))
+}
+
+#[test]
+#[ignore = "Enable running once there are fewer failures"]
+fn linter_stubs_no_panic() -> anyhow::Result<()> {
+    let workspace_root = get_workspace_root()?;
+    run_corpus_tests(&format!(
+        "{workspace_root}/crates/ruff_linter/resources/test/fixtures/**/*.pyi"
+    ))
+}
+
+#[test]
+#[ignore = "Enable running over typeshed stubs once there are fewer failures"]
+fn typeshed_no_panic() -> anyhow::Result<()> {
+    let workspace_root = get_workspace_root()?;
+    run_corpus_tests(&format!(
+        "{workspace_root}/crates/red_knot_vendored/vendor/typeshed/**/*.pyi"
+    ))
+}
+
+#[allow(clippy::print_stdout)]
+fn run_corpus_tests(pattern: &str) -> anyhow::Result<()> {
+    let root = SystemPathBuf::from("/src");
+
+    let system = TestSystem::default();
+    let memory_fs = system.memory_file_system();
+    memory_fs.create_directory_all(root.as_ref())?;
+
+    let mut db = setup_db(&root, system.clone())?;
+
+    let workspace_root = get_workspace_root()?;
+    let workspace_root = workspace_root.to_string();
+
+    let corpus = glob::glob(pattern).context("Failed to compile pattern")?;
+
+    for path in corpus {
+        let path = path.context("Failed to glob path")?;
+        let path = SystemPathBuf::from_path_buf(path).map_err(|path| {
+            anyhow!(
+                "Failed to convert path '{path}' to system path",
+                path = path.display()
+            )
+        })?;
+
+        let relative_path = path.strip_prefix(&workspace_root)?;
+
+        let (py_expected_to_fail, pyi_expected_to_fail) = KNOWN_FAILURES
+            .iter()
+            .find_map(|(path, py_fail, pyi_fail)| {
+                if *path == relative_path.as_str().replace('\\', "/") {
+                    Some((*py_fail, *pyi_fail))
+                } else {
+                    None
+                }
+            })
+            .unwrap_or((false, false));
+
+        let source = path.as_path();
+        let source_filename = source.file_name().unwrap();
+
+        let code = std::fs::read_to_string(source)?;
+
+        let mut check_with_file_name = |path: &SystemPath| {
+            memory_fs.write_file(path, &code).unwrap();
+            File::sync_path(&mut db, path);
+
+            // this test is only asserting that we can pull every expression type without a panic
+            // (and some non-expressions that clearly define a single type)
+            let file = system_path_to_file(&db, path).unwrap();
+
+            let result = std::panic::catch_unwind(|| pull_types(&db, file));
+
+            let expected_to_fail = if path.extension().map(|e| e == "pyi").unwrap_or(false) {
+                pyi_expected_to_fail
+            } else {
+                py_expected_to_fail
+            };
+            if let Err(err) = result {
+                if !expected_to_fail {
+                    println!("Check failed for {relative_path:?}. Consider fixing it or adding it to KNOWN_FAILURES");
+                    std::panic::resume_unwind(err);
+                }
+            } else {
+                assert!(!expected_to_fail, "Expected to panic, but did not. Consider removing this path from KNOWN_FAILURES");
+            }
+
+            memory_fs.remove_file(path).unwrap();
+            file.sync(&mut db);
+        };
+
+        if source.extension() == Some("pyi") {
+            println!("checking {relative_path}");
+            let pyi_dest = root.join(source_filename);
+            check_with_file_name(&pyi_dest);
+        } else {
+            println!("checking {relative_path}");
+            let py_dest = root.join(source_filename);
+            check_with_file_name(&py_dest);
+
+            let pyi_dest = root.join(format!("{source_filename}i"));
+            println!("re-checking as stub file: {pyi_dest}");
+            check_with_file_name(&pyi_dest);
+        }
     }
+
     Ok(())
 }
 
@@ -144,3 +262,14 @@ impl SourceOrderVisitor<'_> for PullTypesVisitor<'_> {
         source_order::walk_alias(self, alias);
     }
 }
+
+/// Whether or not the .py/.pyi version of this file is expected to fail
+#[rustfmt::skip]
+const KNOWN_FAILURES: &[(&str, bool, bool)] = &[
+    // related to circular references in class definitions
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_26.py", true, false),
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F811_19.py", true, false),
+    ("crates/ruff_linter/resources/test/fixtures/pyupgrade/UP039.py", true, false),
+    // related to circular references in type aliases (salsa cycle panic):
+    ("crates/ruff_python_parser/resources/inline/err/type_alias_invalid_value_expr.py", true, true),
+];

crates/ruff/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ruff"
-version = "0.7.4"
+version = "0.8.0"
 publish = true
 authors = { workspace = true }
 edition = { workspace = true }

crates/ruff/src/snapshots/ruff__version__tests__version_formatting.snap

@@ -1,5 +1,6 @@
 ---
 source: crates/ruff/src/version.rs
 expression: version
+snapshot_kind: text
 ---
 0.0.0

crates/ruff/src/snapshots/ruff__version__tests__version_formatting_with_commit_info.snap

@@ -1,5 +1,6 @@
 ---
 source: crates/ruff/src/version.rs
 expression: version
+snapshot_kind: text
 ---
 0.0.0 (53b0f5d92 2023-10-19)

crates/ruff/src/snapshots/ruff__version__tests__version_formatting_with_commits_since_last_tag.snap

@@ -1,5 +1,6 @@
 ---
 source: crates/ruff/src/version.rs
 expression: version
+snapshot_kind: text
 ---
 0.0.0+24 (53b0f5d92 2023-10-19)

crates/ruff/src/snapshots/ruff__version__tests__version_serializable.snap

@@ -1,6 +1,7 @@
 ---
 source: crates/ruff/src/version.rs
 expression: version
+snapshot_kind: text
 ---
 {
   "version": "0.0.0",

crates/ruff/tests/config.rs

@@ -8,7 +8,7 @@ const BIN_NAME: &str = "ruff";
 #[test]
 fn lint_select() {
     assert_cmd_snapshot!(
-        Command::new(get_cargo_bin(BIN_NAME)).arg("config").arg("lint.select"), @r###"
+        Command::new(get_cargo_bin(BIN_NAME)).arg("config").arg("lint.select"), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -29,14 +29,14 @@ fn lint_select() {
     ```
 
     ----- stderr -----
-    "###
+    "#
     );
 }
 
 #[test]
 fn lint_select_json() {
     assert_cmd_snapshot!(
-        Command::new(get_cargo_bin(BIN_NAME)).arg("config").arg("lint.select").arg("--output-format").arg("json"), @r###"
+        Command::new(get_cargo_bin(BIN_NAME)).arg("config").arg("lint.select").arg("--output-format").arg("json"), @r##"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -50,6 +50,6 @@ fn lint_select_json() {
     }
 
     ----- stderr -----
-    "###
+    "##
     );
 }

crates/ruff/tests/format.rs

@@ -30,7 +30,7 @@ if condition:
 
     print('Hy "Micha"') # Should not change quotes
 
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -45,7 +45,7 @@ if condition:
         print('Hy "Micha"')  # Should not change quotes
 
     ----- stderr -----
-    "###);
+    "#);
 }
 
 #[test]
@@ -65,7 +65,7 @@ bar =     "needs formatting"
     )?;
 
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
-        .args(["format", "--isolated", "--no-cache", "--check"]).current_dir(tempdir.path()), @r###"
+        .args(["format", "--isolated", "--no-cache", "--check"]).current_dir(tempdir.path()), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -74,7 +74,7 @@ bar =     "needs formatting"
     2 files would be reformatted
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -84,7 +84,7 @@ fn format_warn_stdin_filename_with_files() {
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .args(["format", "--isolated", "--stdin-filename", "foo.py"])
         .arg("foo.py")
-        .pass_stdin("foo =     1"), @r###"
+        .pass_stdin("foo =     1"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -92,13 +92,13 @@ fn format_warn_stdin_filename_with_files() {
 
     ----- stderr -----
     warning: Ignoring file foo.py in favor of standard input.
-    "###);
+    ");
 }
 
 #[test]
 fn nonexistent_config_file() {
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
-        .args(["format", "--config", "foo.toml", "."]), @r###"
+        .args(["format", "--config", "foo.toml", "."]), @r"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -114,13 +114,13 @@ fn nonexistent_config_file() {
     The path `foo.toml` does not point to a configuration file
 
     For more information, try '--help'.
-    "###);
+    ");
 }
 
 #[test]
 fn config_override_rejected_if_invalid_toml() {
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
-        .args(["format", "--config", "foo = bar", "."]), @r###"
+        .args(["format", "--config", "foo = bar", "."]), @r#"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -142,7 +142,7 @@ fn config_override_rejected_if_invalid_toml() {
     expected `"`, `'`
 
     For more information, try '--help'.
-    "###);
+    "#);
 }
 
 #[test]
@@ -161,19 +161,18 @@ fn too_many_config_files() -> Result<()> {
         .arg(&ruff_dot_toml)
         .arg("--config")
         .arg(&ruff2_dot_toml)
-        .arg("."), @r###"
-            success: false
-            exit_code: 2
-            ----- stdout -----
+        .arg("."), @r"
+        success: false
+        exit_code: 2
+        ----- stdout -----
 
-            ----- stderr -----
-            ruff failed
-              Cause: You cannot specify more than one configuration file on the command line.
+        ----- stderr -----
+        ruff failed
+          Cause: You cannot specify more than one configuration file on the command line.
 
-              tip: remove either `--config=[TMP]/ruff.toml` or `--config=[TMP]/ruff2.toml`.
-                   For more information, try `--help`.
-
-            "###);
+          tip: remove either `--config=[TMP]/ruff.toml` or `--config=[TMP]/ruff2.toml`.
+               For more information, try `--help`.
+        ");
     });
     Ok(())
 }
@@ -191,7 +190,7 @@ fn config_file_and_isolated() -> Result<()> {
         .arg("--config")
         .arg(&ruff_dot_toml)
         .arg("--isolated")
-        .arg("."), @r###"
+        .arg("."), @r"
         success: false
         exit_code: 2
         ----- stdout -----
@@ -203,8 +202,7 @@ fn config_file_and_isolated() -> Result<()> {
           tip: You cannot specify a configuration file and also specify `--isolated`,
                as `--isolated` causes ruff to ignore all configuration files.
                For more information, try `--help`.
-
-        "###);
+        ");
     });
     Ok(())
 }
@@ -226,7 +224,7 @@ def foo():
         // This overrides the long line length set in the config file
         .args(["--config", "line-length=80"])
         .arg("-")
-        .pass_stdin(fixture), @r###"
+        .pass_stdin(fixture), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -236,7 +234,7 @@ def foo():
         )
 
     ----- stderr -----
-    "###);
+    "#);
     Ok(())
 }
 
@@ -259,7 +257,7 @@ def foo():
         // ...but this overrides them both:
         .args(["--line-length", "100"])
         .arg("-")
-        .pass_stdin(fixture), @r###"
+        .pass_stdin(fixture), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -267,7 +265,7 @@ def foo():
         print("looooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong string")
 
     ----- stderr -----
-    "###);
+    "#);
     Ok(())
 }
 
@@ -302,7 +300,7 @@ if condition:
 
     print("Should change quotes")
 
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -316,7 +314,7 @@ if condition:
     	print('Should change quotes')
 
     ----- stderr -----
-    "###);
+    "#);
     Ok(())
 }
 
@@ -357,7 +355,7 @@ def f(x):
     >>> foo, bar, quux = this_is_a_long_line(lion, hippo, lemur, bear)
     '''
     pass
-"), @r###"
+"), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -403,7 +401,7 @@ def f(x):
         pass
 
     ----- stderr -----
-    "###);
+    "#);
     Ok(())
 }
 
@@ -424,14 +422,14 @@ fn mixed_line_endings() -> Result<()> {
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .current_dir(tempdir.path())
         .args(["format", "--no-cache", "--diff", "--isolated"])
-        .arg("."), @r###"
+        .arg("."), @r"
     success: true
     exit_code: 0
     ----- stdout -----
 
     ----- stderr -----
     2 files already formatted
-    "###);
+    ");
     Ok(())
 }
 
@@ -490,7 +488,7 @@ OTHER = "OTHER"
         // Explicitly pass test.py, should be formatted regardless of it being excluded by format.exclude
         .arg(test_path.file_name().unwrap())
         // Format all other files in the directory, should respect the `exclude` and `format.exclude` options
-        .arg("."), @r###"
+        .arg("."), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -499,7 +497,7 @@ OTHER = "OTHER"
     2 files would be reformatted
 
     ----- stderr -----
-    "###);
+    ");
     Ok(())
 }
 
@@ -517,14 +515,14 @@ from module import =
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .current_dir(tempdir.path())
         .args(["format", "--no-cache", "--isolated", "--check"])
-        .arg("main.py"), @r###"
+        .arg("main.py"), @r"
     success: false
     exit_code: 2
     ----- stdout -----
 
     ----- stderr -----
     error: Failed to parse main.py:2:20: Expected an import name
-    "###);
+    ");
 
     Ok(())
 }
@@ -546,7 +544,7 @@ if __name__ == "__main__":
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .current_dir(tempdir.path())
         .args(["format", "--no-cache", "--isolated", "--check"])
-        .arg("main.py"), @r###"
+        .arg("main.py"), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -554,31 +552,31 @@ if __name__ == "__main__":
     1 file would be reformatted
 
     ----- stderr -----
-    "###);
+    ");
 
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .current_dir(tempdir.path())
         .args(["format", "--no-cache", "--isolated"])
-        .arg("main.py"), @r###"
+        .arg("main.py"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
     1 file reformatted
 
     ----- stderr -----
-    "###);
+    ");
 
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .current_dir(tempdir.path())
         .args(["format", "--no-cache", "--isolated"])
-        .arg("main.py"), @r###"
+        .arg("main.py"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
     1 file left unchanged
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -638,7 +636,7 @@ OTHER = "OTHER"
         // Explicitly pass test.py, should be respect the `format.exclude` when `--force-exclude` is present
         .arg(test_path.file_name().unwrap())
         // Format all other files in the directory, should respect the `exclude` and `format.exclude` options
-        .arg("."), @r###"
+        .arg("."), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -646,7 +644,7 @@ OTHER = "OTHER"
     1 file would be reformatted
 
     ----- stderr -----
-    "###);
+    ");
     Ok(())
 }
 
@@ -673,7 +671,7 @@ from test import say_hy
 
 if __name__ == '__main__':
     say_hy("dear Ruff contributor")
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -686,7 +684,7 @@ if __name__ == '__main__':
     warning: The top-level linter settings are deprecated in favour of their counterparts in the `lint` section. Please update the following options in `ruff.toml`:
       - 'extend-select' -> 'lint.extend-select'
       - 'ignore' -> 'lint.ignore'
-    "###);
+    "#);
     Ok(())
 }
 
@@ -713,7 +711,7 @@ from test import say_hy
 
 if __name__ == '__main__':
     say_hy("dear Ruff contributor")
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -727,7 +725,7 @@ if __name__ == '__main__':
     warning: The top-level linter settings are deprecated in favour of their counterparts in the `lint` section. Please update the following options in `ruff.toml`:
       - 'extend-select' -> 'lint.extend-select'
       - 'ignore' -> 'lint.ignore'
-    "###);
+    "#);
     Ok(())
 }
 
@@ -770,7 +768,7 @@ if condition:
 
     print("Should change quotes")
 
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -786,7 +784,7 @@ if condition:
 
     ----- stderr -----
     warning: The following rule may cause conflicts when used with the formatter: `COM812`. To avoid unexpected behavior, we recommend disabling this rule, either by removing it from the `select` or `extend-select` configuration, or adding it to the `ignore` configuration.
-    "###);
+    "#);
     Ok(())
 }
 
@@ -811,7 +809,7 @@ tab-size = 2
             .pass_stdin(r"
 if True:
     pass
-    "), @r###"
+    "), @r"
         success: false
         exit_code: 2
         ----- stdout -----
@@ -824,8 +822,7 @@ if True:
         1 | 
           | ^
         unknown field `tab-size`
-
-        "###);
+        ");
     });
     Ok(())
 }
@@ -851,7 +848,7 @@ format = "json"
             .arg("-")
             .pass_stdin(r"
     import os
-    "), @r###"
+    "), @r#"
         success: false
         exit_code: 2
         ----- stdout -----
@@ -864,8 +861,7 @@ format = "json"
         2 | format = "json"
           |          ^^^^^^
         invalid type: string "json", expected struct FormatOptions
-
-        "###);
+        "#);
     });
     Ok(())
 }
@@ -912,7 +908,7 @@ def say_hy(name: str):
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .args(["format", "--no-cache", "--config"])
         .arg(&ruff_toml)
-        .arg(test_path), @r###"
+        .arg(test_path), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -929,7 +925,7 @@ def say_hy(name: str):
     warning: The isort option `isort.lines-between-types` with a value greater than 1 is incompatible with the formatter. To avoid unexpected behavior, we recommend setting the option to one of: `1` or `0` (default).
     warning: The isort option `isort.force-wrap-aliases` is incompatible with the formatter `format.skip-magic-trailing-comma=true` option. To avoid unexpected behavior, we recommend either setting `isort.force-wrap-aliases=false` or `format.skip-magic-trailing-comma=false`.
     warning: The isort option `isort.split-on-trailing-comma` is incompatible with the formatter `format.skip-magic-trailing-comma=true` option. To avoid unexpected behavior, we recommend either setting `isort.split-on-trailing-comma=false` or `format.skip-magic-trailing-comma=false`.
-    "###);
+    "#);
     Ok(())
 }
 
@@ -970,7 +966,7 @@ indent-style = "tab"
         .arg("-")
         .pass_stdin(r#"
 def say_hy(name: str):
-        print(f"Hy {name}")"#), @r###"
+        print(f"Hy {name}")"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -988,7 +984,7 @@ def say_hy(name: str):
     warning: The isort option `isort.lines-between-types` with a value greater than 1 is incompatible with the formatter. To avoid unexpected behavior, we recommend setting the option to one of: `1` or `0` (default).
     warning: The isort option `isort.force-wrap-aliases` is incompatible with the formatter `format.skip-magic-trailing-comma=true` option. To avoid unexpected behavior, we recommend either setting `isort.force-wrap-aliases=false` or `format.skip-magic-trailing-comma=false`.
     warning: The isort option `isort.split-on-trailing-comma` is incompatible with the formatter `format.skip-magic-trailing-comma=true` option. To avoid unexpected behavior, we recommend either setting `isort.split-on-trailing-comma=false` or `format.skip-magic-trailing-comma=false`.
-    "###);
+    "#);
     Ok(())
 }
 
@@ -1032,14 +1028,14 @@ def say_hy(name: str):
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .args(["format", "--no-cache", "--config"])
         .arg(&ruff_toml)
-        .arg(test_path), @r###"
+        .arg(test_path), @r"
     success: true
     exit_code: 0
     ----- stdout -----
     1 file reformatted
 
     ----- stderr -----
-    "###);
+    ");
     Ok(())
 }
 
@@ -1074,14 +1070,14 @@ def say_hy(name: str):
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .args(["format", "--no-cache", "--config"])
         .arg(&ruff_toml)
-        .arg(test_path), @r###"
+        .arg(test_path), @r"
     success: true
     exit_code: 0
     ----- stdout -----
     1 file reformatted
 
     ----- stderr -----
-    "###);
+    ");
     Ok(())
 }
 
@@ -1109,7 +1105,7 @@ def say_hy(name: str):
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .args(["format", "--no-cache", "--config"])
         .arg(&ruff_toml)
-        .arg(test_path), @r###"
+        .arg(test_path), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -1119,7 +1115,7 @@ def say_hy(name: str):
     warning: `one-blank-line-before-class` (D203) and `no-blank-line-before-class` (D211) are incompatible. Ignoring `one-blank-line-before-class`.
     warning: `multi-line-summary-first-line` (D212) and `multi-line-summary-second-line` (D213) are incompatible. Ignoring `multi-line-summary-second-line`.
     warning: The following rules may cause conflicts when used with the formatter: `COM812`, `ISC001`. To avoid unexpected behavior, we recommend disabling these rules, either by removing them from the `select` or `extend-select` configuration, or adding them to the `ignore` configuration.
-    "###);
+    ");
     Ok(())
 }
 
@@ -1138,7 +1134,7 @@ fn test_diff() {
     ]}, {
         assert_cmd_snapshot!(
             Command::new(get_cargo_bin(BIN_NAME)).args(args).args(paths),
-            @r###"
+            @r"
         success: false
         exit_code: 1
         ----- stdout -----
@@ -1186,7 +1182,7 @@ fn test_diff() {
 
         ----- stderr -----
         2 files would be reformatted, 1 file already formatted
-        "###);
+        ");
     });
 }
 
@@ -1201,7 +1197,7 @@ fn test_diff_no_change() {
     ]}, {
         assert_cmd_snapshot!(
             Command::new(get_cargo_bin(BIN_NAME)).args(args).args(paths),
-            @r###"
+            @r"
         success: false
         exit_code: 1
         ----- stdout -----
@@ -1216,7 +1212,7 @@ fn test_diff_no_change() {
 
         ----- stderr -----
         1 file would be reformatted
-        "###
+        "
         );
     });
 }
@@ -1235,7 +1231,7 @@ fn test_diff_stdin_unformatted() {
     let unformatted = fs::read(fixtures.join("unformatted.py")).unwrap();
     assert_cmd_snapshot!(
         Command::new(get_cargo_bin(BIN_NAME)).args(args).pass_stdin(unformatted),
-        @r###"
+        @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -1249,7 +1245,7 @@ fn test_diff_stdin_unformatted() {
 
 
     ----- stderr -----
-    "###);
+    ");
 }
 
 #[test]
@@ -1259,13 +1255,13 @@ fn test_diff_stdin_formatted() {
     let unformatted = fs::read(fixtures.join("formatted.py")).unwrap();
     assert_cmd_snapshot!(
         Command::new(get_cargo_bin(BIN_NAME)).args(args).pass_stdin(unformatted),
-        @r###"
+        @r"
     success: true
     exit_code: 0
     ----- stdout -----
 
     ----- stderr -----
-    "###);
+    ");
 }
 
 #[test]
@@ -1275,7 +1271,7 @@ fn test_notebook_trailing_semicolon() {
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .args(["format", "--isolated", "--stdin-filename", "test.ipynb"])
         .arg("-")
-        .pass_stdin(unformatted), @r###"
+        .pass_stdin(unformatted), @r##"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -1694,7 +1690,7 @@ fn test_notebook_trailing_semicolon() {
     }
 
     ----- stderr -----
-    "###);
+    "##);
 }
 
 #[test]
@@ -1756,14 +1752,14 @@ include = ["*.ipy"]
         .arg("--no-cache")
         .args(["--config", &ruff_toml.file_name().unwrap().to_string_lossy()])
         .args(["--extension", "ipy:ipynb"])
-        .arg("."), @r###"
+        .arg("."), @r"
     success: true
     exit_code: 0
     ----- stdout -----
     1 file reformatted
 
     ----- stderr -----
-    "###);
+    ");
     Ok(())
 }
 
@@ -1776,7 +1772,7 @@ fn range_formatting() {
 def foo(arg1, arg2,):
     print("Shouldn't format this" )
 
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -1789,7 +1785,7 @@ def foo(arg1, arg2,):
 
 
     ----- stderr -----
-    "###);
+    "#);
 }
 
 #[test]
@@ -1799,7 +1795,7 @@ fn range_formatting_unicode() {
         .arg("-")
         .pass_stdin(r#"
 def foo(arg1="👋🏽" ): print("Format this" )
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -1808,7 +1804,7 @@ def foo(arg1="👋🏽" ): print("Format this" )
         print("Format this")
 
     ----- stderr -----
-    "###);
+    "#);
 }
 
 #[test]
@@ -1839,7 +1835,7 @@ def file2(arg1, arg2,):
     assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
         .args(["format", "--isolated", "--range=1:8-1:15"])
         .arg(file1)
-        .arg(file2),  @r###"
+        .arg(file2),  @r"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -1847,7 +1843,7 @@ def file2(arg1, arg2,):
     ----- stderr -----
     ruff failed
       Cause: The `--range` option is only supported when formatting a single file but the specified paths resolve to 2 files.
-    "###);
+    ");
 
     Ok(())
 }
@@ -1861,7 +1857,7 @@ fn range_formatting_out_of_bounds() {
 def foo(arg1, arg2,):
     print("Shouldn't format this" )
 
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -1871,7 +1867,7 @@ def foo(arg1, arg2,):
 
 
     ----- stderr -----
-    "###);
+    "#);
 }
 
 #[test]
@@ -1883,7 +1879,7 @@ fn range_start_larger_than_end() {
 def foo(arg1, arg2,):
     print("Shouldn't format this" )
 
-"#), @r###"
+"#), @r"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -1893,7 +1889,7 @@ def foo(arg1, arg2,):
         tip: Try switching start and end: '50:1-90:1'
 
     For more information, try '--help'.
-    "###);
+    ");
 }
 
 #[test]
@@ -1905,7 +1901,7 @@ fn range_line_numbers_only() {
 def foo(arg1, arg2,):
     print("Shouldn't format this" )
 
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -1918,7 +1914,7 @@ def foo(arg1, arg2,):
 
 
     ----- stderr -----
-    "###);
+    "#);
 }
 
 #[test]
@@ -1930,7 +1926,7 @@ fn range_start_only() {
 def foo(arg1, arg2,):
     print("Should format this" )
 
-"#), @r###"
+"#), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -1939,7 +1935,7 @@ def foo(arg1, arg2,):
         print("Should format this")
 
     ----- stderr -----
-    "###);
+    "#);
 }
 
 #[test]
@@ -1975,7 +1971,7 @@ fn range_missing_line() {
 def foo(arg1, arg2,):
     print("Should format this" )
 
-"#), @r###"
+"#), @r"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -1985,7 +1981,7 @@ def foo(arg1, arg2,):
       tip: The format is 'line:column'.
 
     For more information, try '--help'.
-    "###);
+    ");
 }
 
 #[test]
@@ -1997,7 +1993,7 @@ fn zero_line_number() {
 def foo(arg1, arg2,):
     print("Should format this" )
 
-"#), @r###"
+"#), @r"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -2008,7 +2004,7 @@ def foo(arg1, arg2,):
       tip: Try 1:2 instead.
 
     For more information, try '--help'.
-    "###);
+    ");
 }
 
 #[test]
@@ -2020,7 +2016,7 @@ fn column_and_line_zero() {
 def foo(arg1, arg2,):
     print("Should format this" )
 
-"#), @r###"
+"#), @r"
     success: false
     exit_code: 2
     ----- stdout -----
@@ -2031,7 +2027,7 @@ def foo(arg1, arg2,):
       tip: Try 1:1 instead.
 
     For more information, try '--help'.
-    "###);
+    ");
 }
 
 #[test]
@@ -2075,12 +2071,12 @@ fn range_formatting_notebook() {
  "nbformat": 4,
  "nbformat_minor": 5
 }
-"#), @r###"
+"#), @r"
     success: false
     exit_code: 2
     ----- stdout -----
 
     ----- stderr -----
     error: Failed to format main.ipynb: Range formatting isn't supported for notebooks.
-    "###);
+    ");
 }