preserve backwards compatibility

## Summary
Change `hardcoded-tmp-directory-extend` example to follow the schema:

1e91a09918/ruff.schema.json (L896-L901)
<!-- What's the purpose of the change? What does it do, and why? -->

.github/CODEOWNERS

@@ -12,3 +12,6 @@
 
 # flake8-pyi
 /crates/ruff_linter/src/rules/flake8_pyi/ @AlexWaygood
+
+# Script for fuzzing the parser
+/scripts/fuzz-parser/ @AlexWaygood

.github/workflows/ci.yaml

@@ -23,6 +23,8 @@ jobs:
     name: "Determine changes"
     runs-on: ubuntu-latest
     outputs:
+      # Flag that is raised when any code that affects parser is changed
+      parser: ${{ steps.changed.outputs.parser_any_changed }}
       # Flag that is raised when any code that affects linter is changed
       linter: ${{ steps.changed.outputs.linter_any_changed }}
       # Flag that is raised when any code that affects formatter is changed
@@ -39,6 +41,17 @@ jobs:
         id: changed
         with:
           files_yaml: |
+            parser:
+              - Cargo.toml
+              - Cargo.lock
+              - crates/ruff_python_trivia/**
+              - crates/ruff_source_file/**
+              - crates/ruff_text_size/**
+              - crates/ruff_python_ast/**
+              - crates/ruff_python_parser/**
+              - scripts/fuzz-parser/**
+              - .github/workflows/ci.yaml
+
             linter:
               - Cargo.toml
               - Cargo.lock
@@ -46,7 +59,6 @@ jobs:
               - "!crates/ruff_python_formatter/**"
               - "!crates/ruff_formatter/**"
               - "!crates/ruff_dev/**"
-              - "!crates/ruff_shrinking/**"
               - scripts/*
               - python/**
               - .github/workflows/ci.yaml
@@ -181,6 +193,22 @@ jobs:
           cd crates/ruff_wasm
           wasm-pack test --node
 
+  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' }}
+    timeout-minutes: 20
+    steps:
+      - uses: actions/checkout@v4
+      - name: "Install Rust toolchain"
+        run: rustup show
+      - name: "Install mold"
+        uses: rui314/setup-mold@v1
+      - uses: Swatinem/rust-cache@v2
+      - name: "Build"
+        run: cargo build --release --locked
+
   cargo-fuzz:
     name: "cargo fuzz"
     runs-on: ubuntu-latest
@@ -200,6 +228,38 @@ jobs:
           tool: cargo-fuzz@0.11.2
       - run: cargo fuzz build -s none
 
+  fuzz-parser:
+    name: "Fuzz the parser"
+    runs-on: ubuntu-latest
+    needs:
+      - cargo-test-linux
+      - determine_changes
+    if: ${{ needs.determine_changes.outputs.parser == 'true' }}
+    timeout-minutes: 20
+    env:
+      FORCE_COLOR: 1
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+      - name: Install uv
+        run: curl -LsSf https://astral.sh/uv/install.sh | sh
+      - name: Install Python requirements
+        run: uv pip install -r scripts/fuzz-parser/requirements.txt --system
+      - uses: actions/download-artifact@v4
+        name: Download Ruff binary to test
+        id: download-cached-binary
+        with:
+          name: ruff
+          path: ruff-to-test
+      - name: Fuzz
+        run: |
+          # 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
+
   scripts:
     name: "test scripts"
     runs-on: ubuntu-latest
@@ -228,9 +288,7 @@ jobs:
       - determine_changes
     # Only runs on pull requests, since that is the only we way we can find the base version for comparison.
     # Ecosystem check needs linter and/or formatter changes.
-    if: github.event_name == 'pull_request' && ${{
-      needs.determine_changes.outputs.code == 'true'
-      }}
+    if: ${{ github.event_name == 'pull_request' && needs.determine_changes.outputs.code == 'true' }}
     timeout-minutes: 20
     steps:
       - uses: actions/checkout@v4
@@ -509,7 +567,7 @@ jobs:
   benchmarks:
     runs-on: ubuntu-latest
     needs: determine_changes
-    if: ${{ needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main' }}
+    if: ${{ github.repository == 'astral-sh/ruff' && (needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main') }}
     timeout-minutes: 20
     steps:
       - name: "Checkout Branch"
@@ -525,23 +583,8 @@ jobs:
 
       - uses: Swatinem/rust-cache@v2
 
-      # Codspeed comes with a very ancient cargo version (1.66) that resolves features flags differently than what we use now.
-      # This can result in build failures; see https://github.com/astral-sh/ruff/pull/10700.
-      # There's a pending codspeed PR to upgrade to a newer cargo version, but until that's merged, we need to use the workaround below.
-      # https://github.com/CodSpeedHQ/codspeed-rust/pull/31
-      # What we do is to call cargo build manually with the correct feature flags and RUSTC settings. We'll have to
-      # manually maintain the list of benchmarks to run with codspeed (the benefit is that we could detect which benchmarks to run and build based on the changes).
-      # This is inspired by https://github.com/oxc-project/oxc/blob/a0532adc654039a0c7ead7b35216dfa0b0cb8e8f/.github/workflows/benchmark.yml
       - name: "Build benchmarks"
-        env:
-          RUSTFLAGS: "-C debuginfo=2 -C strip=none -g --cfg codspeed"
-        shell: bash
-        # Build all benchmarks, copy the binary to the codspeed directory, remove any `*.d` files that might have been created.
-        run: |
-          cargo build --release -p ruff_benchmark --bench parser --bench linter --bench formatter --bench lexer --features=codspeed
-          mkdir -p ./target/codspeed/ruff_benchmark
-          cp ./target/release/deps/{lexer,parser,linter,formatter}* target/codspeed/ruff_benchmark/
-          rm -rf ./target/codspeed/ruff_benchmark/*.d
+        run: cargo codspeed build --features codspeed -p ruff_benchmark
 
       - name: "Run benchmarks"
         uses: CodSpeedHQ/action@v2

.github/workflows/daily_fuzz.yaml

@@ -0,0 +1,72 @@
+name: Daily parser fuzz
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: "0 0 * * *"
+  pull_request:
+    paths:
+      - ".github/workflows/daily_fuzz.yaml"
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+env:
+  CARGO_INCREMENTAL: 0
+  CARGO_NET_RETRY: 10
+  CARGO_TERM_COLOR: always
+  RUSTUP_MAX_RETRIES: 10
+  PACKAGE_NAME: ruff
+  FORCE_COLOR: 1
+
+jobs:
+  fuzz:
+    name: Fuzz
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    # Don't run the cron job on forks:
+    if: ${{ github.repository == 'astral-sh/ruff' || github.event_name != 'schedule' }}
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+      - name: Install uv
+        run: curl -LsSf https://astral.sh/uv/install.sh | sh
+      - name: Install Python requirements
+        run: uv pip install -r scripts/fuzz-parser/requirements.txt --system
+      - name: "Install Rust toolchain"
+        run: rustup show
+      - name: "Install mold"
+        uses: rui314/setup-mold@v1
+      - uses: Swatinem/rust-cache@v2
+      - name: Build ruff
+        # A debug build means the script runs slower once it gets started,
+        # 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
+
+  create-issue-on-failure:
+    name: Create an issue if the daily fuzz surfaced any bugs
+    runs-on: ubuntu-latest
+    needs: fuzz
+    if: ${{ github.repository == 'astral-sh/ruff' && always() && github.event_name == 'schedule' && needs.fuzz.result == 'failure' }}
+    permissions:
+      issues: write
+    steps:
+      - uses: actions/github-script@v7
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+          script: |
+            await github.rest.issues.create({
+              owner: "astral-sh",
+              repo: "ruff",
+              title: `Daily parser fuzz failed on ${new Date().toDateString()}`,
+              body: "Runs listed here: https://github.com/astral-sh/ruff/actions/workflows/daily_fuzz.yml",
+              labels: ["bug", "parser", "fuzzer"],
+            })

.github/workflows/release.yaml

@@ -58,7 +58,7 @@ jobs:
           path: dist
 
   macos-x86_64:
-    runs-on: macos-latest
+    runs-on: macos-12
     steps:
       - uses: actions/checkout@v4
         with:
@@ -97,8 +97,8 @@ jobs:
             *.tar.gz
             *.sha256
 
-  macos-universal:
-    runs-on: macos-latest
+  macos-aarch64:
+    runs-on: macos-14
     steps:
       - uses: actions/checkout@v4
         with:
@@ -106,16 +106,17 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: ${{ env.PYTHON_VERSION }}
-          architecture: x64
+          architecture: arm64
       - name: "Prep README.md"
         run: python scripts/transform_readme.py --target pypi
-      - name: "Build wheels - universal2"
+      - name: "Build wheels - aarch64"
         uses: PyO3/maturin-action@v1
         with:
-          args: --release --locked --target universal2-apple-darwin --out dist
-      - name: "Test wheel - universal2"
+          target: aarch64
+          args: --release --locked --out dist
+      - name: "Test wheel - aarch64"
         run: |
-          pip install dist/${{ env.PACKAGE_NAME }}-*universal2.whl --force-reinstall
+          pip install dist/${{ env.PACKAGE_NAME }}-*.whl --force-reinstall
           ruff --help
           python -m ruff --help
       - name: "Upload wheels"
@@ -451,7 +452,7 @@ jobs:
     name: Upload to PyPI
     runs-on: ubuntu-latest
     needs:
-      - macos-universal
+      - macos-aarch64
       - macos-x86_64
       - windows
       - linux

.pre-commit-config.yaml

@@ -41,7 +41,7 @@ repos:
           )$
 
   - repo: https://github.com/crate-ci/typos
-    rev: v1.20.8
+    rev: v1.20.10
     hooks:
       - id: typos
 
@@ -55,7 +55,7 @@ repos:
         pass_filenames: false # This makes it a lot faster
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.3.7
+    rev: v0.4.2
     hooks:
       - id: ruff-format
       - id: ruff

CHANGELOG.md

@@ -1,5 +1,102 @@
 # Changelog
 
+## 0.4.3
+
+### Enhancements
+
+- Add support for PEP 696 syntax ([#11120](https://github.com/astral-sh/ruff/pull/11120))
+
+### Preview features
+
+- \[`refurb`\] Use function range for `reimplemented-operator` diagnostics ([#11271](https://github.com/astral-sh/ruff/pull/11271))
+- \[`refurb`\] Ignore methods in `reimplemented-operator` (`FURB118`) ([#11270](https://github.com/astral-sh/ruff/pull/11270))
+- \[`refurb`\] Implement `fstring-number-format` (`FURB116`) ([#10921](https://github.com/astral-sh/ruff/pull/10921))
+- \[`ruff`\] Implement `redirected-noqa` (`RUF101`) ([#11052](https://github.com/astral-sh/ruff/pull/11052))
+- \[`pyflakes`\] Distinguish between first-party and third-party imports for fix suggestions ([#11168](https://github.com/astral-sh/ruff/pull/11168))
+
+### Rule changes
+
+- \[`flake8-bugbear`\] Ignore non-abstract class attributes when enforcing `B024` ([#11210](https://github.com/astral-sh/ruff/pull/11210))
+- \[`flake8-logging`\] Include inline instantiations when detecting loggers ([#11154](https://github.com/astral-sh/ruff/pull/11154))
+- \[`pylint`\] Also emit `PLR0206` for properties with variadic parameters ([#11200](https://github.com/astral-sh/ruff/pull/11200))
+- \[`ruff`\] Detect duplicate codes as part of `unused-noqa` (`RUF100`) ([#10850](https://github.com/astral-sh/ruff/pull/10850))
+
+### Formatter
+
+- Avoid multiline expression if format specifier is present ([#11123](https://github.com/astral-sh/ruff/pull/11123))
+
+### LSP
+
+- Write `ruff server` setup guide for Helix ([#11183](https://github.com/astral-sh/ruff/pull/11183))
+- `ruff server` no longer hangs after shutdown ([#11222](https://github.com/astral-sh/ruff/pull/11222))
+- `ruff server` reads from a configuration TOML file in the user configuration directory if no local configuration exists ([#11225](https://github.com/astral-sh/ruff/pull/11225))
+- `ruff server` respects `per-file-ignores` configuration ([#11224](https://github.com/astral-sh/ruff/pull/11224))
+- `ruff server`: Support a custom TOML configuration file ([#11140](https://github.com/astral-sh/ruff/pull/11140))
+- `ruff server`: Support setting to prioritize project configuration over editor configuration ([#11086](https://github.com/astral-sh/ruff/pull/11086))
+
+### Bug fixes
+
+- Avoid debug assertion around NFKC renames ([#11249](https://github.com/astral-sh/ruff/pull/11249))
+- \[`pyflakes`\] Prioritize `redefined-while-unused` over `unused-import` ([#11173](https://github.com/astral-sh/ruff/pull/11173))
+- \[`ruff`\] Respect `async` expressions in comprehension bodies ([#11219](https://github.com/astral-sh/ruff/pull/11219))
+- \[`pygrep_hooks`\] Fix `blanket-noqa` panic when last line has noqa with no newline (`PGH004`) ([#11108](https://github.com/astral-sh/ruff/pull/11108))
+- \[`perflint`\] Ignore list-copy recommendations for async `for` loops ([#11250](https://github.com/astral-sh/ruff/pull/11250))
+- \[`pyflakes`\] Improve `invalid-print-syntax` documentation ([#11171](https://github.com/astral-sh/ruff/pull/11171))
+
+### Performance
+
+- Avoid allocations for isort module names ([#11251](https://github.com/astral-sh/ruff/pull/11251))
+- Build a separate ARM wheel for macOS ([#11149](https://github.com/astral-sh/ruff/pull/11149))
+
+## 0.4.2
+
+### Rule changes
+
+- \[`flake8-pyi`\] Allow for overloaded `__exit__` and `__aexit__` definitions (`PYI036`) ([#11057](https://github.com/astral-sh/ruff/pull/11057))
+- \[`pyupgrade`\] Catch usages of `"%s" % var` and provide an unsafe fix (`UP031`) ([#11019](https://github.com/astral-sh/ruff/pull/11019))
+- \[`refurb`\] Implement new rule that suggests min/max over `sorted()` (`FURB192`) ([#10868](https://github.com/astral-sh/ruff/pull/10868))
+
+### Server
+
+- Fix an issue with missing diagnostics for Neovim and Helix ([#11092](https://github.com/astral-sh/ruff/pull/11092))
+- Implement hover documentation for `noqa` codes ([#11096](https://github.com/astral-sh/ruff/pull/11096))
+- Introduce common Ruff configuration options with new server settings ([#11062](https://github.com/astral-sh/ruff/pull/11062))
+
+### Bug fixes
+
+- Use `macos-12` for building release wheels to enable macOS 11 compatibility ([#11146](https://github.com/astral-sh/ruff/pull/11146))
+- \[`flake8-blind-expect`\] Allow raise from in `BLE001` ([#11131](https://github.com/astral-sh/ruff/pull/11131))
+- \[`flake8-pyi`\] Allow simple assignments to `None` in enum class scopes (`PYI026`) ([#11128](https://github.com/astral-sh/ruff/pull/11128))
+- \[`flake8-simplify`\] Avoid raising `SIM911` for non-`zip` attribute calls ([#11126](https://github.com/astral-sh/ruff/pull/11126))
+- \[`refurb`\] Avoid `operator.itemgetter` suggestion for single-item tuple ([#11095](https://github.com/astral-sh/ruff/pull/11095))
+- \[`ruff`\] Respect per-file-ignores for `RUF100` with no other diagnostics ([#11058](https://github.com/astral-sh/ruff/pull/11058))
+- \[`ruff`\] Fix async comprehension false positive (`RUF029`) ([#11070](https://github.com/astral-sh/ruff/pull/11070))
+
+### Documentation
+
+- \[`flake8-bugbear`\] Document explicitly disabling strict zip (`B905`) ([#11040](https://github.com/astral-sh/ruff/pull/11040))
+- \[`flake8-type-checking`\] Mention `lint.typing-modules` in `TCH001`, `TCH002`, and `TCH003` ([#11144](https://github.com/astral-sh/ruff/pull/11144))
+- \[`isort`\] Improve documentation around custom `isort` sections ([#11050](https://github.com/astral-sh/ruff/pull/11050))
+- \[`pylint`\] Fix documentation oversight for `invalid-X-returns` ([#11094](https://github.com/astral-sh/ruff/pull/11094))
+
+### Performance
+
+- Use `matchit` to resolve per-file settings ([#11111](https://github.com/astral-sh/ruff/pull/11111))
+
+## 0.4.1
+
+### Preview features
+
+- \[`pylint`\] Implement `invalid-hash-returned` (`PLE0309`) ([#10961](https://github.com/astral-sh/ruff/pull/10961))
+- \[`pylint`\] Implement `invalid-index-returned` (`PLE0305`) ([#10962](https://github.com/astral-sh/ruff/pull/10962))
+
+### Bug fixes
+
+- \[`pylint`\] Allow `NoReturn`-like functions for `__str__`, `__len__`, etc. (`PLE0307`) ([#11017](https://github.com/astral-sh/ruff/pull/11017))
+- Parser: Use empty range when there's "gap" in token source ([#11032](https://github.com/astral-sh/ruff/pull/11032))
+- \[`ruff`\] Ignore stub functions in `unused-async` (`RUF029`) ([#11026](https://github.com/astral-sh/ruff/pull/11026))
+- Parser: Expect indented case block instead of match stmt ([#11033](https://github.com/astral-sh/ruff/pull/11033))
+
 ## 0.4.0
 
 ### A new, hand-written parser

Cargo.toml

@@ -24,13 +24,14 @@ chrono = { version = "0.4.35", default-features = false, features = ["clock"] }
 clap = { version = "4.5.3", features = ["derive"] }
 clap_complete_command = { version = "0.5.1" }
 clearscreen = { version = "3.0.0" }
-codspeed-criterion-compat = { version = "2.4.0", default-features = false }
+codspeed-criterion-compat = { version = "2.6.0", default-features = false }
 colored = { version = "2.1.0" }
 console_error_panic_hook = { version = "0.1.7" }
 console_log = { version = "1.0.0" }
 countme = { version = "3.0.1" }
 criterion = { version = "0.5.1", default-features = false }
 crossbeam = { version = "0.8.4" }
+dashmap = { version = "5.5.3" }
 dirs = { version = "5.0.0" }
 drop_bomb = { version = "0.1.5" }
 env_logger = { version = "0.11.0" }
@@ -39,10 +40,12 @@ filetime = { version = "0.2.23" }
 fs-err = { version = "2.11.0" }
 glob = { version = "0.3.1" }
 globset = { version = "0.4.14" }
+hashbrown = "0.14.3"
 hexf-parse = { version = "0.2.1" }
 ignore = { version = "0.4.22" }
 imara-diff = { version = "0.1.5" }
 imperative = { version = "1.0.4" }
+indexmap = { version = "2.2.6" }
 indicatif = { version = "0.17.8" }
 indoc = { version = "2.0.4" }
 insta = { version = "1.35.1", feature = ["filters", "glob"] }
@@ -58,19 +61,21 @@ libcst = { version = "1.1.0", default-features = false }
 log = { version = "0.4.17" }
 lsp-server = { version = "0.7.6" }
 lsp-types = { version = "0.95.0", features = ["proposed"] }
+matchit = { version = "0.8.1" }
 memchr = { version = "2.7.1" }
 mimalloc = { version = "0.1.39" }
 natord = { version = "1.0.9" }
 notify = { version = "6.1.1" }
-num_cpus = { version = "1.16.0" }
 once_cell = { version = "1.19.0" }
 path-absolutize = { version = "3.1.1" }
+path-slash = { version = "0.2.1" }
 pathdiff = { version = "0.2.1" }
+parking_lot = "0.12.1"
 pep440_rs = { version = "0.6.0", features = ["serde"] }
 pretty_assertions = "1.3.0"
 proc-macro2 = { version = "1.0.79" }
 pyproject-toml = { version = "0.9.0" }
-quick-junit = { version = "0.3.5" }
+quick-junit = { version = "0.4.0" }
 quote = { version = "1.0.23" }
 rand = { version = "0.8.5" }
 rayon = { version = "1.10.0" }
@@ -85,7 +90,6 @@ serde_json = { version = "1.0.113" }
 serde_test = { version = "1.0.152" }
 serde_with = { version = "3.6.0", default-features = false, features = ["macros"] }
 shellexpand = { version = "3.0.0" }
-shlex = { version = "1.3.0" }
 similar = { version = "2.4.0", features = ["inline"] }
 smallvec = { version = "1.13.2" }
 static_assertions = "1.1.0"

README.md

@@ -4,7 +4,7 @@
 
 [![Ruff](https://img.shields.io/endpoint?url=https://raw.githubusercontent.com/astral-sh/ruff/main/assets/badge/v2.json)](https://github.com/astral-sh/ruff)
 [![image](https://img.shields.io/pypi/v/ruff.svg)](https://pypi.python.org/pypi/ruff)
-[![image](https://img.shields.io/pypi/l/ruff.svg)](https://pypi.python.org/pypi/ruff)
+[![image](https://img.shields.io/pypi/l/ruff.svg)](https://github.com/astral-sh/ruff/blob/main/LICENSE)
 [![image](https://img.shields.io/pypi/pyversions/ruff.svg)](https://pypi.python.org/pypi/ruff)
 [![Actions status](https://github.com/astral-sh/ruff/workflows/CI/badge.svg)](https://github.com/astral-sh/ruff/actions)
 [![Discord](https://img.shields.io/badge/Discord-%235865F2.svg?logo=discord&logoColor=white)](https://discord.com/invite/astral-sh)
@@ -50,6 +50,7 @@ times faster than any individual tool.
 Ruff is extremely actively developed and used in major open-source projects like:
 
 - [Apache Airflow](https://github.com/apache/airflow)
+- [Apache Superset](https://github.com/apache/superset)
 - [FastAPI](https://github.com/tiangolo/fastapi)
 - [Hugging Face](https://github.com/huggingface/transformers)
 - [Pandas](https://github.com/pandas-dev/pandas)
@@ -151,7 +152,7 @@ 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: v0.4.0
+  rev: v0.4.3
   hooks:
     # Run the linter.
     - id: ruff
@@ -498,7 +499,7 @@ If you're using Ruff, consider adding the Ruff badge to your project's `README.m
 
 ## License
 
-MIT
+This repository is licensed under the [MIT License](https://github.com/astral-sh/ruff/blob/main/LICENSE)
 
 <div align="center">
   <a target="_blank" href="https://astral.sh" style="background:none">

clippy.toml

@@ -3,5 +3,10 @@ doc-valid-idents = [
   "CodeQL",
   "IPython",
   "NumPy",
+  "LibCST",
+  "SCREAMING_SNAKE_CASE",
+  "SQLAlchemy",
+  "McCabe",
+  "FastAPI",
   "..",
 ]

crates/red_knot/Cargo.toml

@@ -0,0 +1,45 @@
+[package]
+name = "red_knot"
+version = "0.1.0"
+edition.workspace = true
+rust-version.workspace = true
+homepage.workspace = true
+documentation.workspace = true
+repository.workspace = true
+authors.workspace = true
+license.workspace = true
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+ruff_python_parser = { path = "../ruff_python_parser" }
+ruff_python_ast = { path = "../ruff_python_ast" }
+ruff_python_trivia = { path = "../ruff_python_trivia" }
+ruff_text_size = { path = "../ruff_text_size" }
+ruff_index = { path = "../ruff_index" }
+ruff_notebook = { path = "../ruff_notebook" }
+
+anyhow = { workspace = true }
+bitflags = { workspace = true }
+ctrlc = "3.4.4"
+crossbeam = { workspace = true }
+dashmap = { workspace = true }
+hashbrown = { workspace = true }
+indexmap = { workspace = true }
+log = { workspace = true }
+notify = { workspace = true }
+parking_lot = { workspace = true }
+rayon = { workspace = true }
+rustc-hash = { workspace = true }
+smallvec = { workspace = true }
+smol_str = "0.2.1"
+tracing = { workspace = true }
+tracing-subscriber = { workspace = true }
+tracing-tree = { workspace = true }
+
+[dev-dependencies]
+textwrap = "0.16.1"
+tempfile = { workspace = true }
+
+[lints]
+workspace = true

crates/red_knot/src/ast_ids.rs

@@ -0,0 +1,415 @@
+use std::any::type_name;
+use std::fmt::{Debug, Formatter};
+use std::hash::{Hash, Hasher};
+use std::marker::PhantomData;
+
+use rustc_hash::FxHashMap;
+
+use ruff_index::{Idx, IndexVec};
+use ruff_python_ast::visitor::preorder;
+use ruff_python_ast::visitor::preorder::{PreorderVisitor, TraversalSignal};
+use ruff_python_ast::{
+    AnyNodeRef, AstNode, ExceptHandler, ExceptHandlerExceptHandler, Expr, MatchCase, ModModule,
+    NodeKind, Parameter, Stmt, StmtAnnAssign, StmtAssign, StmtAugAssign, StmtClassDef,
+    StmtFunctionDef, StmtGlobal, StmtImport, StmtImportFrom, StmtNonlocal, StmtTypeAlias,
+    TypeParam, TypeParamParamSpec, TypeParamTypeVar, TypeParamTypeVarTuple, WithItem,
+};
+use ruff_text_size::{Ranged, TextRange};
+
+/// A type agnostic ID that uniquely identifies an AST node in a file.
+#[ruff_index::newtype_index]
+pub struct AstId;
+
+/// A typed ID that uniquely identifies an AST node in a file.
+///
+/// This is different from [`AstId`] in that it is a combination of ID and the type of the node the ID identifies.
+/// Typing the ID prevents mixing IDs of different node types and allows to restrict the API to only accept
+/// nodes for which an ID has been created (not all AST nodes get an ID).
+pub struct TypedAstId<N: HasAstId> {
+    erased: AstId,
+    _marker: PhantomData<fn() -> N>,
+}
+
+impl<N: HasAstId> TypedAstId<N> {
+    /// Upcasts this ID from a more specific node type to a more general node type.
+    pub fn upcast<M: HasAstId>(self) -> TypedAstId<M>
+    where
+        N: Into<M>,
+    {
+        TypedAstId {
+            erased: self.erased,
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<N: HasAstId> Copy for TypedAstId<N> {}
+impl<N: HasAstId> Clone for TypedAstId<N> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<N: HasAstId> PartialEq for TypedAstId<N> {
+    fn eq(&self, other: &Self) -> bool {
+        self.erased == other.erased
+    }
+}
+
+impl<N: HasAstId> Eq for TypedAstId<N> {}
+impl<N: HasAstId> Hash for TypedAstId<N> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.erased.hash(state);
+    }
+}
+
+impl<N: HasAstId> Debug for TypedAstId<N> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        f.debug_tuple("TypedAstId")
+            .field(&self.erased)
+            .field(&type_name::<N>())
+            .finish()
+    }
+}
+
+pub struct AstIds {
+    ids: IndexVec<AstId, NodeKey>,
+    reverse: FxHashMap<NodeKey, AstId>,
+}
+
+impl AstIds {
+    // TODO rust analyzer doesn't allocate an ID for every node. It only allocates ids for
+    //  nodes with a corresponding HIR element, that is nodes that are definitions.
+    pub fn from_module(module: &ModModule) -> Self {
+        let mut visitor = AstIdsVisitor::default();
+
+        // TODO: visit_module?
+        // Make sure we visit the root
+        visitor.create_id(module);
+        visitor.visit_body(&module.body);
+
+        while let Some(deferred) = visitor.deferred.pop() {
+            match deferred {
+                DeferredNode::FunctionDefinition(def) => {
+                    def.visit_preorder(&mut visitor);
+                }
+                DeferredNode::ClassDefinition(def) => def.visit_preorder(&mut visitor),
+            }
+        }
+
+        AstIds {
+            ids: visitor.ids,
+            reverse: visitor.reverse,
+        }
+    }
+
+    /// Returns the ID to the root node.
+    pub fn root(&self) -> NodeKey {
+        self.ids[AstId::new(0)]
+    }
+
+    /// Returns the [`TypedAstId`] for a node.
+    pub fn ast_id<N: HasAstId>(&self, node: &N) -> TypedAstId<N> {
+        let key = node.syntax_node_key();
+        TypedAstId {
+            erased: self.reverse.get(&key).copied().unwrap(),
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns the [`TypedAstId`] for the node identified with the given [`TypedNodeKey`].
+    pub fn ast_id_for_key<N: HasAstId>(&self, node: &TypedNodeKey<N>) -> TypedAstId<N> {
+        let ast_id = self.ast_id_for_node_key(node.inner);
+
+        TypedAstId {
+            erased: ast_id,
+            _marker: PhantomData,
+        }
+    }
+
+    /// Returns the untyped [`AstId`] for the node identified by the given `node` key.
+    pub fn ast_id_for_node_key(&self, node: NodeKey) -> AstId {
+        self.reverse
+            .get(&node)
+            .copied()
+            .expect("Can't find node in AstIds map.")
+    }
+
+    /// Returns the [`TypedNodeKey`] for the node identified by the given [`TypedAstId`].
+    pub fn key<N: HasAstId>(&self, id: TypedAstId<N>) -> TypedNodeKey<N> {
+        let syntax_key = self.ids[id.erased];
+
+        TypedNodeKey::new(syntax_key).unwrap()
+    }
+
+    pub fn node_key<H: HasAstId>(&self, id: TypedAstId<H>) -> NodeKey {
+        self.ids[id.erased]
+    }
+}
+
+impl std::fmt::Debug for AstIds {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let mut map = f.debug_map();
+        for (key, value) in self.ids.iter_enumerated() {
+            map.entry(&key, &value);
+        }
+
+        map.finish()
+    }
+}
+
+impl PartialEq for AstIds {
+    fn eq(&self, other: &Self) -> bool {
+        self.ids == other.ids
+    }
+}
+
+impl Eq for AstIds {}
+
+#[derive(Default)]
+struct AstIdsVisitor<'a> {
+    ids: IndexVec<AstId, NodeKey>,
+    reverse: FxHashMap<NodeKey, AstId>,
+    deferred: Vec<DeferredNode<'a>>,
+}
+
+impl<'a> AstIdsVisitor<'a> {
+    fn create_id<A: HasAstId>(&mut self, node: &A) {
+        let node_key = node.syntax_node_key();
+
+        let id = self.ids.push(node_key);
+        self.reverse.insert(node_key, id);
+    }
+}
+
+impl<'a> PreorderVisitor<'a> for AstIdsVisitor<'a> {
+    fn visit_stmt(&mut self, stmt: &'a Stmt) {
+        match stmt {
+            Stmt::FunctionDef(def) => {
+                self.create_id(def);
+                self.deferred.push(DeferredNode::FunctionDefinition(def));
+                return;
+            }
+            // TODO defer visiting the assignment body, type alias parameters etc?
+            Stmt::ClassDef(def) => {
+                self.create_id(def);
+                self.deferred.push(DeferredNode::ClassDefinition(def));
+                return;
+            }
+            Stmt::Expr(_) => {
+                // Skip
+                return;
+            }
+            Stmt::Return(_) => {}
+            Stmt::Delete(_) => {}
+            Stmt::Assign(assignment) => self.create_id(assignment),
+            Stmt::AugAssign(assignment) => {
+                self.create_id(assignment);
+            }
+            Stmt::AnnAssign(assignment) => self.create_id(assignment),
+            Stmt::TypeAlias(assignment) => self.create_id(assignment),
+            Stmt::For(_) => {}
+            Stmt::While(_) => {}
+            Stmt::If(_) => {}
+            Stmt::With(_) => {}
+            Stmt::Match(_) => {}
+            Stmt::Raise(_) => {}
+            Stmt::Try(_) => {}
+            Stmt::Assert(_) => {}
+            Stmt::Import(import) => self.create_id(import),
+            Stmt::ImportFrom(import_from) => self.create_id(import_from),
+            Stmt::Global(global) => self.create_id(global),
+            Stmt::Nonlocal(non_local) => self.create_id(non_local),
+            Stmt::Pass(_) => {}
+            Stmt::Break(_) => {}
+            Stmt::Continue(_) => {}
+            Stmt::IpyEscapeCommand(_) => {}
+        }
+
+        preorder::walk_stmt(self, stmt);
+    }
+
+    fn visit_expr(&mut self, _expr: &'a Expr) {}
+
+    fn visit_parameter(&mut self, parameter: &'a Parameter) {
+        self.create_id(parameter);
+        preorder::walk_parameter(self, parameter);
+    }
+
+    fn visit_except_handler(&mut self, except_handler: &'a ExceptHandler) {
+        match except_handler {
+            ExceptHandler::ExceptHandler(except_handler) => {
+                self.create_id(except_handler);
+            }
+        }
+
+        preorder::walk_except_handler(self, except_handler);
+    }
+
+    fn visit_with_item(&mut self, with_item: &'a WithItem) {
+        self.create_id(with_item);
+        preorder::walk_with_item(self, with_item);
+    }
+
+    fn visit_match_case(&mut self, match_case: &'a MatchCase) {
+        self.create_id(match_case);
+        preorder::walk_match_case(self, match_case);
+    }
+
+    fn visit_type_param(&mut self, type_param: &'a TypeParam) {
+        self.create_id(type_param);
+    }
+}
+
+enum DeferredNode<'a> {
+    FunctionDefinition(&'a StmtFunctionDef),
+    ClassDefinition(&'a StmtClassDef),
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
+pub struct TypedNodeKey<N: AstNode> {
+    /// The type erased node key.
+    inner: NodeKey,
+    _marker: PhantomData<fn() -> N>,
+}
+
+impl<N: AstNode> TypedNodeKey<N> {
+    pub fn from_node(node: &N) -> Self {
+        let inner = NodeKey {
+            kind: node.as_any_node_ref().kind(),
+            range: node.range(),
+        };
+        Self {
+            inner,
+            _marker: PhantomData,
+        }
+    }
+
+    pub fn new(node_key: NodeKey) -> Option<Self> {
+        N::can_cast(node_key.kind).then_some(TypedNodeKey {
+            inner: node_key,
+            _marker: PhantomData,
+        })
+    }
+
+    pub fn resolve<'a>(&self, root: AnyNodeRef<'a>) -> Option<N::Ref<'a>> {
+        let node_ref = self.inner.resolve(root)?;
+
+        Some(N::cast_ref(node_ref).unwrap())
+    }
+
+    pub fn resolve_unwrap<'a>(&self, root: AnyNodeRef<'a>) -> N::Ref<'a> {
+        self.resolve(root).expect("node should resolve")
+    }
+
+    pub fn erased(&self) -> &NodeKey {
+        &self.inner
+    }
+}
+
+struct FindNodeKeyVisitor<'a> {
+    key: NodeKey,
+    result: Option<AnyNodeRef<'a>>,
+}
+
+impl<'a> PreorderVisitor<'a> for FindNodeKeyVisitor<'a> {
+    fn enter_node(&mut self, node: AnyNodeRef<'a>) -> TraversalSignal {
+        if self.result.is_some() {
+            return TraversalSignal::Skip;
+        }
+
+        if node.range() == self.key.range && node.kind() == self.key.kind {
+            self.result = Some(node);
+            TraversalSignal::Skip
+        } else if node.range().contains_range(self.key.range) {
+            TraversalSignal::Traverse
+        } else {
+            TraversalSignal::Skip
+        }
+    }
+
+    fn visit_body(&mut self, body: &'a [Stmt]) {
+        // TODO it would be more efficient to use binary search instead of linear
+        for stmt in body {
+            if stmt.range().start() > self.key.range.end() {
+                break;
+            }
+
+            self.visit_stmt(stmt);
+        }
+    }
+}
+
+// TODO an alternative to this is to have a `NodeId` on each node (in increasing order depending on the position).
+//  This would allow to reduce the size of this to a u32.
+//  What would be nice if we could use an `Arc::weak_ref` here but that only works if we use
+//   `Arc` internally
+// TODO: Implement the logic to resolve a node, given a db (and the correct file).
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
+pub struct NodeKey {
+    kind: NodeKind,
+    range: TextRange,
+}
+
+impl NodeKey {
+    pub fn resolve<'a>(&self, root: AnyNodeRef<'a>) -> Option<AnyNodeRef<'a>> {
+        // We need to do a binary search here. Only traverse into a node if the range is withint the node
+        let mut visitor = FindNodeKeyVisitor {
+            key: *self,
+            result: None,
+        };
+
+        if visitor.enter_node(root) == TraversalSignal::Traverse {
+            root.visit_preorder(&mut visitor);
+        }
+
+        visitor.result
+    }
+}
+
+/// Marker trait implemented by AST nodes for which we extract the `AstId`.
+pub trait HasAstId: AstNode {
+    fn node_key(&self) -> TypedNodeKey<Self>
+    where
+        Self: Sized,
+    {
+        TypedNodeKey {
+            inner: self.syntax_node_key(),
+            _marker: PhantomData,
+        }
+    }
+
+    fn syntax_node_key(&self) -> NodeKey {
+        NodeKey {
+            kind: self.as_any_node_ref().kind(),
+            range: self.range(),
+        }
+    }
+}
+
+impl HasAstId for StmtFunctionDef {}
+impl HasAstId for StmtClassDef {}
+impl HasAstId for StmtAnnAssign {}
+impl HasAstId for StmtAugAssign {}
+impl HasAstId for StmtAssign {}
+impl HasAstId for StmtTypeAlias {}
+
+impl HasAstId for ModModule {}
+
+impl HasAstId for StmtImport {}
+
+impl HasAstId for StmtImportFrom {}
+
+impl HasAstId for Parameter {}
+
+impl HasAstId for TypeParam {}
+impl HasAstId for Stmt {}
+impl HasAstId for TypeParamTypeVar {}
+impl HasAstId for TypeParamTypeVarTuple {}
+impl HasAstId for TypeParamParamSpec {}
+impl HasAstId for StmtGlobal {}
+impl HasAstId for StmtNonlocal {}
+
+impl HasAstId for ExceptHandlerExceptHandler {}
+impl HasAstId for WithItem {}
+impl HasAstId for MatchCase {}

crates/red_knot/src/cache.rs

@@ -0,0 +1,165 @@
+use std::fmt::Formatter;
+use std::hash::Hash;
+use std::sync::atomic::{AtomicUsize, Ordering};
+
+use crate::db::QueryResult;
+use dashmap::mapref::entry::Entry;
+
+use crate::FxDashMap;
+
+/// Simple key value cache that locks on a per-key level.
+pub struct KeyValueCache<K, V> {
+    map: FxDashMap<K, V>,
+    statistics: CacheStatistics,
+}
+
+impl<K, V> KeyValueCache<K, V>
+where
+    K: Eq + Hash + Clone,
+    V: Clone,
+{
+    pub fn try_get(&self, key: &K) -> Option<V> {
+        if let Some(existing) = self.map.get(key) {
+            self.statistics.hit();
+            Some(existing.clone())
+        } else {
+            self.statistics.miss();
+            None
+        }
+    }
+
+    pub fn get<F>(&self, key: &K, compute: F) -> QueryResult<V>
+    where
+        F: FnOnce(&K) -> QueryResult<V>,
+    {
+        Ok(match self.map.entry(key.clone()) {
+            Entry::Occupied(cached) => {
+                self.statistics.hit();
+
+                cached.get().clone()
+            }
+            Entry::Vacant(vacant) => {
+                self.statistics.miss();
+
+                let value = compute(key)?;
+                vacant.insert(value.clone());
+                value
+            }
+        })
+    }
+
+    pub fn set(&mut self, key: K, value: V) {
+        self.map.insert(key, value);
+    }
+
+    pub fn remove(&mut self, key: &K) -> Option<V> {
+        self.map.remove(key).map(|(_, value)| value)
+    }
+
+    pub fn clear(&mut self) {
+        self.map.clear();
+        self.map.shrink_to_fit();
+    }
+
+    pub fn statistics(&self) -> Option<Statistics> {
+        self.statistics.to_statistics()
+    }
+}
+
+impl<K, V> Default for KeyValueCache<K, V>
+where
+    K: Eq + Hash,
+    V: Clone,
+{
+    fn default() -> Self {
+        Self {
+            map: FxDashMap::default(),
+            statistics: CacheStatistics::default(),
+        }
+    }
+}
+
+impl<K, V> std::fmt::Debug for KeyValueCache<K, V>
+where
+    K: std::fmt::Debug + Eq + Hash,
+    V: std::fmt::Debug,
+{
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let mut debug = f.debug_map();
+
+        for entry in &self.map {
+            debug.entry(&entry.value(), &entry.key());
+        }
+
+        debug.finish()
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct Statistics {
+    pub hits: usize,
+    pub misses: usize,
+}
+
+impl Statistics {
+    #[allow(clippy::cast_precision_loss)]
+    pub fn hit_rate(&self) -> Option<f64> {
+        if self.hits + self.misses == 0 {
+            return None;
+        }
+
+        Some((self.hits as f64) / (self.hits + self.misses) as f64)
+    }
+}
+
+#[cfg(debug_assertions)]
+pub type CacheStatistics = DebugStatistics;
+
+#[cfg(not(debug_assertions))]
+pub type CacheStatistics = ReleaseStatistics;
+
+pub trait StatisticsRecorder {
+    fn hit(&self);
+    fn miss(&self);
+    fn to_statistics(&self) -> Option<Statistics>;
+}
+
+#[derive(Debug, Default)]
+pub struct DebugStatistics {
+    hits: AtomicUsize,
+    misses: AtomicUsize,
+}
+
+impl StatisticsRecorder for DebugStatistics {
+    // TODO figure out appropriate Ordering
+    fn hit(&self) {
+        self.hits.fetch_add(1, Ordering::SeqCst);
+    }
+
+    fn miss(&self) {
+        self.misses.fetch_add(1, Ordering::SeqCst);
+    }
+
+    fn to_statistics(&self) -> Option<Statistics> {
+        let hits = self.hits.load(Ordering::SeqCst);
+        let misses = self.misses.load(Ordering::SeqCst);
+
+        Some(Statistics { hits, misses })
+    }
+}
+
+#[derive(Debug, Default)]
+pub struct ReleaseStatistics;
+
+impl StatisticsRecorder for ReleaseStatistics {
+    #[inline]
+    fn hit(&self) {}
+
+    #[inline]
+    fn miss(&self) {}
+
+    #[inline]
+    fn to_statistics(&self) -> Option<Statistics> {
+        None
+    }
+}

crates/red_knot/src/cancellation.rs

@@ -0,0 +1,42 @@
+use std::sync::atomic::AtomicBool;
+use std::sync::Arc;
+
+#[derive(Debug, Clone, Default)]
+pub struct CancellationTokenSource {
+    signal: Arc<AtomicBool>,
+}
+
+impl CancellationTokenSource {
+    pub fn new() -> Self {
+        Self {
+            signal: Arc::new(AtomicBool::new(false)),
+        }
+    }
+
+    #[tracing::instrument(level = "trace", skip_all)]
+    pub fn cancel(&self) {
+        self.signal.store(true, std::sync::atomic::Ordering::SeqCst);
+    }
+
+    pub fn is_cancelled(&self) -> bool {
+        self.signal.load(std::sync::atomic::Ordering::SeqCst)
+    }
+
+    pub fn token(&self) -> CancellationToken {
+        CancellationToken {
+            signal: self.signal.clone(),
+        }
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct CancellationToken {
+    signal: Arc<AtomicBool>,
+}
+
+impl CancellationToken {
+    /// Returns `true` if cancellation has been requested.
+    pub fn is_cancelled(&self) -> bool {
+        self.signal.load(std::sync::atomic::Ordering::SeqCst)
+    }
+}

crates/red_knot/src/db.rs

@@ -0,0 +1,297 @@
+use std::path::Path;
+use std::sync::Arc;
+
+pub use jars::{HasJar, HasJars};
+pub use query::{QueryError, QueryResult};
+pub use runtime::DbRuntime;
+pub use storage::JarsStorage;
+
+use crate::files::FileId;
+use crate::lint::{Diagnostics, LintSemanticStorage, LintSyntaxStorage};
+use crate::module::{Module, ModuleData, ModuleName, ModuleResolver, ModuleSearchPath};
+use crate::parse::{Parsed, ParsedStorage};
+use crate::source::{Source, SourceStorage};
+use crate::symbols::{SymbolId, SymbolTable, SymbolTablesStorage};
+use crate::types::{Type, TypeStore};
+
+mod jars;
+mod query;
+mod runtime;
+mod storage;
+
+pub trait Database {
+    /// Returns a reference to the runtime of the current worker.
+    fn runtime(&self) -> &DbRuntime;
+
+    /// Returns a mutable reference to the runtime. Only one worker can hold a mutable reference to the runtime.
+    fn runtime_mut(&mut self) -> &mut DbRuntime;
+
+    /// Returns `Ok` if the queries have not been cancelled and `Err(QueryError::Cancelled)` otherwise.
+    fn cancelled(&self) -> QueryResult<()> {
+        self.runtime().cancelled()
+    }
+
+    /// Returns `true` if the queries have been cancelled.
+    fn is_cancelled(&self) -> bool {
+        self.runtime().is_cancelled()
+    }
+}
+
+/// Database that supports running queries from multiple threads.
+pub trait ParallelDatabase: Database + Send {
+    /// Creates a snapshot of the database state that can be used to query the database in another thread.
+    ///
+    /// The snapshot is a read-only view of the database but query results are shared between threads.
+    /// All queries will be automatically cancelled when applying any mutations (calling [`HasJars::jars_mut`])
+    /// to the database (not the snapshot, because they're readonly).
+    ///
+    /// ## Creating a snapshot
+    ///
+    /// Creating a snapshot of the database's jars is cheap but creating a snapshot of
+    /// other state stored on the database might require deep-cloning data. That's why you should
+    /// avoid creating snapshots in a hot function (e.g. don't create a snapshot for each file, instead
+    /// create a snapshot when scheduling the check of an entire program).
+    ///
+    /// ## Salsa compatibility
+    /// Salsa prohibits creating a snapshot while running a local query (it's fine if other workers run a query) [[source](https://github.com/salsa-rs/salsa/issues/80)].
+    /// We should avoid creating snapshots while running a query because we might want to adopt Salsa in the future (if we can figure out persistent caching).
+    /// Unfortunately, the infrastructure doesn't provide an automated way of knowing when a query is run, that's
+    /// why we have to "enforce" this constraint manually.
+    #[must_use]
+    fn snapshot(&self) -> Snapshot<Self>;
+}
+
+/// Readonly snapshot of a database.
+///
+/// ## Dead locks
+/// A snapshot should always be dropped as soon as it is no longer necessary to run queries.
+/// Storing the snapshot without running a query or periodically checking if cancellation was requested
+/// can lead to deadlocks because mutating the [`Database`] requires cancels all pending queries
+/// and waiting for all [`Snapshot`]s to be dropped.
+#[derive(Debug)]
+pub struct Snapshot<DB: ?Sized>
+where
+    DB: ParallelDatabase,
+{
+    db: DB,
+}
+
+impl<DB> Snapshot<DB>
+where
+    DB: ParallelDatabase,
+{
+    pub fn new(db: DB) -> Self {
+        Snapshot { db }
+    }
+}
+
+impl<DB> std::ops::Deref for Snapshot<DB>
+where
+    DB: ParallelDatabase,
+{
+    type Target = DB;
+
+    fn deref(&self) -> &DB {
+        &self.db
+    }
+}
+
+// Red knot specific databases code.
+
+pub trait SourceDb: Database {
+    // queries
+    fn file_id(&self, path: &std::path::Path) -> FileId;
+
+    fn file_path(&self, file_id: FileId) -> Arc<std::path::Path>;
+
+    fn source(&self, file_id: FileId) -> QueryResult<Source>;
+
+    fn parse(&self, file_id: FileId) -> QueryResult<Parsed>;
+}
+
+pub trait SemanticDb: SourceDb {
+    // queries
+    fn resolve_module(&self, name: ModuleName) -> QueryResult<Option<Module>>;
+
+    fn file_to_module(&self, file_id: FileId) -> QueryResult<Option<Module>>;
+
+    fn path_to_module(&self, path: &Path) -> QueryResult<Option<Module>>;
+
+    fn symbol_table(&self, file_id: FileId) -> QueryResult<Arc<SymbolTable>>;
+
+    fn infer_symbol_type(&self, file_id: FileId, symbol_id: SymbolId) -> QueryResult<Type>;
+
+    // mutations
+
+    fn add_module(&mut self, path: &Path) -> Option<(Module, Vec<Arc<ModuleData>>)>;
+
+    fn set_module_search_paths(&mut self, paths: Vec<ModuleSearchPath>);
+}
+
+pub trait LintDb: SemanticDb {
+    fn lint_syntax(&self, file_id: FileId) -> QueryResult<Diagnostics>;
+
+    fn lint_semantic(&self, file_id: FileId) -> QueryResult<Diagnostics>;
+}
+
+pub trait Db: LintDb {}
+
+#[derive(Debug, Default)]
+pub struct SourceJar {
+    pub sources: SourceStorage,
+    pub parsed: ParsedStorage,
+}
+
+#[derive(Debug, Default)]
+pub struct SemanticJar {
+    pub module_resolver: ModuleResolver,
+    pub symbol_tables: SymbolTablesStorage,
+    pub type_store: TypeStore,
+}
+
+#[derive(Debug, Default)]
+pub struct LintJar {
+    pub lint_syntax: LintSyntaxStorage,
+    pub lint_semantic: LintSemanticStorage,
+}
+
+#[cfg(test)]
+pub(crate) mod tests {
+    use std::path::Path;
+    use std::sync::Arc;
+
+    use crate::db::{
+        Database, DbRuntime, HasJar, HasJars, JarsStorage, LintDb, LintJar, QueryResult, SourceDb,
+        SourceJar,
+    };
+    use crate::files::{FileId, Files};
+    use crate::lint::{lint_semantic, lint_syntax, Diagnostics};
+    use crate::module::{
+        add_module, file_to_module, path_to_module, resolve_module, set_module_search_paths,
+        Module, ModuleData, ModuleName, ModuleSearchPath,
+    };
+    use crate::parse::{parse, Parsed};
+    use crate::source::{source_text, Source};
+    use crate::symbols::{symbol_table, SymbolId, SymbolTable};
+    use crate::types::{infer_symbol_type, Type};
+
+    use super::{SemanticDb, SemanticJar};
+
+    // This can be a partial database used in a single crate for testing.
+    // It would hold fewer data than the full database.
+    #[derive(Debug, Default)]
+    pub(crate) struct TestDb {
+        files: Files,
+        jars: JarsStorage<Self>,
+    }
+
+    impl HasJar<SourceJar> for TestDb {
+        fn jar(&self) -> QueryResult<&SourceJar> {
+            Ok(&self.jars()?.0)
+        }
+
+        fn jar_mut(&mut self) -> &mut SourceJar {
+            &mut self.jars_mut().0
+        }
+    }
+
+    impl HasJar<SemanticJar> for TestDb {
+        fn jar(&self) -> QueryResult<&SemanticJar> {
+            Ok(&self.jars()?.1)
+        }
+
+        fn jar_mut(&mut self) -> &mut SemanticJar {
+            &mut self.jars_mut().1
+        }
+    }
+
+    impl HasJar<LintJar> for TestDb {
+        fn jar(&self) -> QueryResult<&LintJar> {
+            Ok(&self.jars()?.2)
+        }
+
+        fn jar_mut(&mut self) -> &mut LintJar {
+            &mut self.jars_mut().2
+        }
+    }
+
+    impl SourceDb for TestDb {
+        fn file_id(&self, path: &Path) -> FileId {
+            self.files.intern(path)
+        }
+
+        fn file_path(&self, file_id: FileId) -> Arc<Path> {
+            self.files.path(file_id)
+        }
+
+        fn source(&self, file_id: FileId) -> QueryResult<Source> {
+            source_text(self, file_id)
+        }
+
+        fn parse(&self, file_id: FileId) -> QueryResult<Parsed> {
+            parse(self, file_id)
+        }
+    }
+
+    impl SemanticDb for TestDb {
+        fn resolve_module(&self, name: ModuleName) -> QueryResult<Option<Module>> {
+            resolve_module(self, name)
+        }
+
+        fn file_to_module(&self, file_id: FileId) -> QueryResult<Option<Module>> {
+            file_to_module(self, file_id)
+        }
+
+        fn path_to_module(&self, path: &Path) -> QueryResult<Option<Module>> {
+            path_to_module(self, path)
+        }
+
+        fn symbol_table(&self, file_id: FileId) -> QueryResult<Arc<SymbolTable>> {
+            symbol_table(self, file_id)
+        }
+
+        fn infer_symbol_type(&self, file_id: FileId, symbol_id: SymbolId) -> QueryResult<Type> {
+            infer_symbol_type(self, file_id, symbol_id)
+        }
+
+        fn add_module(&mut self, path: &Path) -> Option<(Module, Vec<Arc<ModuleData>>)> {
+            add_module(self, path)
+        }
+
+        fn set_module_search_paths(&mut self, paths: Vec<ModuleSearchPath>) {
+            set_module_search_paths(self, paths);
+        }
+    }
+
+    impl LintDb for TestDb {
+        fn lint_syntax(&self, file_id: FileId) -> QueryResult<Diagnostics> {
+            lint_syntax(self, file_id)
+        }
+
+        fn lint_semantic(&self, file_id: FileId) -> QueryResult<Diagnostics> {
+            lint_semantic(self, file_id)
+        }
+    }
+
+    impl HasJars for TestDb {
+        type Jars = (SourceJar, SemanticJar, LintJar);
+
+        fn jars(&self) -> QueryResult<&Self::Jars> {
+            self.jars.jars()
+        }
+
+        fn jars_mut(&mut self) -> &mut Self::Jars {
+            self.jars.jars_mut()
+        }
+    }
+
+    impl Database for TestDb {
+        fn runtime(&self) -> &DbRuntime {
+            self.jars.runtime()
+        }
+
+        fn runtime_mut(&mut self) -> &mut DbRuntime {
+            self.jars.runtime_mut()
+        }
+    }
+}

crates/red_knot/src/db/jars.rs

@@ -0,0 +1,37 @@
+use crate::db::query::QueryResult;
+
+/// Gives access to a specific jar in the database.
+///
+/// Nope, the terminology isn't borrowed from Java but from Salsa <https://salsa-rs.github.io/salsa/>,
+/// which is an analogy to storing the salsa in different jars.
+///
+/// The basic idea is that each crate can define its own jar and the jars can be combined to a single
+/// database in the top level crate. Each crate also defines its own `Database` trait. The combination of
+/// `Database` trait and the jar allows to write queries in isolation without having to know how they get composed at the upper levels.
+///
+/// Salsa further defines a `HasIngredient` trait which slices the jar to a specific storage (e.g. a specific cache).
+/// We don't need this just jet because we write our queries by hand. We may want a similar trait if we decide
+/// to use a macro to generate the queries.
+pub trait HasJar<T> {
+    /// Gives a read-only reference to the jar.
+    fn jar(&self) -> QueryResult<&T>;
+
+    /// Gives a mutable reference to the jar.
+    fn jar_mut(&mut self) -> &mut T;
+}
+
+/// Gives access to the jars in a database.
+pub trait HasJars {
+    /// A type storing the jars.
+    ///
+    /// Most commonly, this is a tuple where each jar is a tuple element.
+    type Jars: Default;
+
+    /// Gives access to the underlying jars but tests if the queries have been cancelled.
+    ///
+    /// Returns `Err(QueryError::Cancelled)` if the queries have been cancelled.
+    fn jars(&self) -> QueryResult<&Self::Jars>;
+
+    /// Gives mutable access to the underlying jars.
+    fn jars_mut(&mut self) -> &mut Self::Jars;
+}

crates/red_knot/src/db/query.rs

@@ -0,0 +1,20 @@
+use std::fmt::{Display, Formatter};
+
+/// Reason why a db query operation failed.
+#[derive(Debug, Clone, Copy)]
+pub enum QueryError {
+    /// The query was cancelled because the DB was mutated or the query was cancelled by the host (e.g. on a file change or when pressing CTRL+C).
+    Cancelled,
+}
+
+impl Display for QueryError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        match self {
+            QueryError::Cancelled => f.write_str("query was cancelled"),
+        }
+    }
+}
+
+impl std::error::Error for QueryError {}
+
+pub type QueryResult<T> = Result<T, QueryError>;

crates/red_knot/src/db/runtime.rs

@@ -0,0 +1,41 @@
+use crate::cancellation::CancellationTokenSource;
+use crate::db::{QueryError, QueryResult};
+
+/// Holds the jar agnostic state of the database.
+#[derive(Debug, Default)]
+pub struct DbRuntime {
+    /// The cancellation token source used to signal other works that the queries should be aborted and
+    /// exit at the next possible point.
+    cancellation_token: CancellationTokenSource,
+}
+
+impl DbRuntime {
+    pub(super) fn snapshot(&self) -> Self {
+        Self {
+            cancellation_token: self.cancellation_token.clone(),
+        }
+    }
+
+    /// Cancels the pending queries of other workers. The current worker cannot have any pending
+    /// queries because we're holding a mutable reference to the runtime.
+    pub(super) fn cancel_other_workers(&mut self) {
+        self.cancellation_token.cancel();
+        // Set a new cancellation token so that we're in a non-cancelled state again when running the next
+        // query.
+        self.cancellation_token = CancellationTokenSource::default();
+    }
+
+    /// Returns `Ok` if the queries have not been cancelled and `Err(QueryError::Cancelled)` otherwise.
+    pub(super) fn cancelled(&self) -> QueryResult<()> {
+        if self.cancellation_token.is_cancelled() {
+            Err(QueryError::Cancelled)
+        } else {
+            Ok(())
+        }
+    }
+
+    /// Returns `true` if the queries have been cancelled.
+    pub(super) fn is_cancelled(&self) -> bool {
+        self.cancellation_token.is_cancelled()
+    }
+}

crates/red_knot/src/db/storage.rs

@@ -0,0 +1,117 @@
+use std::fmt::Formatter;
+use std::sync::Arc;
+
+use crossbeam::sync::WaitGroup;
+
+use crate::db::query::QueryResult;
+use crate::db::runtime::DbRuntime;
+use crate::db::{HasJars, ParallelDatabase};
+
+/// Stores the jars of a database and the state for each worker.
+///
+/// Today, all state is shared across all workers, but it may be desired to store data per worker in the future.
+pub struct JarsStorage<T>
+where
+    T: HasJars + Sized,
+{
+    // It's important that `jars_wait_group` is declared after `jars` to ensure that `jars` is dropped first.
+    // See https://doc.rust-lang.org/reference/destructors.html
+    /// Stores the jars of the database.
+    jars: Arc<T::Jars>,
+
+    /// Used to count the references to `jars`. Allows implementing `jars_mut` without requiring to clone `jars`.
+    jars_wait_group: WaitGroup,
+
+    /// The data agnostic state.
+    runtime: DbRuntime,
+}
+
+impl<Db> JarsStorage<Db>
+where
+    Db: HasJars,
+{
+    pub(super) fn new() -> Self {
+        Self {
+            jars: Arc::new(Db::Jars::default()),
+            jars_wait_group: WaitGroup::default(),
+            runtime: DbRuntime::default(),
+        }
+    }
+
+    /// Creates a snapshot of the jars.
+    ///
+    /// Creating the snapshot is cheap because it doesn't clone the jars, it only increments a ref counter.
+    #[must_use]
+    pub fn snapshot(&self) -> JarsStorage<Db>
+    where
+        Db: ParallelDatabase,
+    {
+        Self {
+            jars: self.jars.clone(),
+            jars_wait_group: self.jars_wait_group.clone(),
+            runtime: self.runtime.snapshot(),
+        }
+    }
+
+    pub(crate) fn jars(&self) -> QueryResult<&Db::Jars> {
+        self.runtime.cancelled()?;
+        Ok(&self.jars)
+    }
+
+    /// Returns a mutable reference to the jars without cloning their content.
+    ///
+    /// The method cancels any pending queries of other works and waits for them to complete so that
+    /// this instance is the only instance holding a reference to the jars.
+    pub(crate) fn jars_mut(&mut self) -> &mut Db::Jars {
+        // We have a mutable ref here, so no more workers can be spawned between calling this function and taking the mut ref below.
+        self.cancel_other_workers();
+
+        // Now all other references to `self.jars` should have been released. We can now safely return a mutable reference
+        // to the Arc's content.
+        let jars =
+            Arc::get_mut(&mut self.jars).expect("All references to jars should have been released");
+
+        jars
+    }
+
+    pub(crate) fn runtime(&self) -> &DbRuntime {
+        &self.runtime
+    }
+
+    pub(crate) fn runtime_mut(&mut self) -> &mut DbRuntime {
+        // Note: This method may need to use a similar trick to `jars_mut` if `DbRuntime` is ever to store data that is shared between workers.
+        &mut self.runtime
+    }
+
+    #[tracing::instrument(level = "trace", skip(self))]
+    fn cancel_other_workers(&mut self) {
+        self.runtime.cancel_other_workers();
+
+        // Wait for all other works to complete.
+        let existing_wait = std::mem::take(&mut self.jars_wait_group);
+        existing_wait.wait();
+    }
+}
+
+impl<Db> Default for JarsStorage<Db>
+where
+    Db: HasJars,
+{
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T> std::fmt::Debug for JarsStorage<T>
+where
+    T: HasJars,
+    <T as HasJars>::Jars: std::fmt::Debug,
+{
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("SharedStorage")
+            .field("jars", &self.jars)
+            .field("jars_wait_group", &self.jars_wait_group)
+            .field("runtime", &self.runtime)
+            .finish()
+    }
+}

crates/red_knot/src/files.rs

@@ -0,0 +1,158 @@
+use std::fmt::{Debug, Formatter};
+use std::hash::{Hash, Hasher};
+use std::path::Path;
+use std::sync::Arc;
+
+use hashbrown::hash_map::RawEntryMut;
+use parking_lot::RwLock;
+use rustc_hash::FxHasher;
+
+use ruff_index::{newtype_index, IndexVec};
+
+type Map<K, V> = hashbrown::HashMap<K, V, ()>;
+
+#[newtype_index]
+pub struct FileId;
+
+// TODO we'll need a higher level virtual file system abstraction that allows testing if a file exists
+//  or retrieving its content (ideally lazily and in a way that the memory can be retained later)
+//  I suspect that we'll end up with a FileSystem trait and our own Path abstraction.
+#[derive(Default)]
+pub struct Files {
+    inner: Arc<RwLock<FilesInner>>,
+}
+
+impl Files {
+    #[tracing::instrument(level = "debug", skip(self))]
+    pub fn intern(&self, path: &Path) -> FileId {
+        self.inner.write().intern(path)
+    }
+
+    pub fn try_get(&self, path: &Path) -> Option<FileId> {
+        self.inner.read().try_get(path)
+    }
+
+    #[tracing::instrument(level = "debug", skip(self))]
+    pub fn path(&self, id: FileId) -> Arc<Path> {
+        self.inner.read().path(id)
+    }
+
+    /// Snapshots files for a new database snapshot.
+    ///
+    /// This method should not be used outside a database snapshot.
+    #[must_use]
+    pub fn snapshot(&self) -> Files {
+        Files {
+            inner: self.inner.clone(),
+        }
+    }
+}
+
+impl Debug for Files {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        let files = self.inner.read();
+        let mut debug = f.debug_map();
+        for item in files.iter() {
+            debug.entry(&item.0, &item.1);
+        }
+
+        debug.finish()
+    }
+}
+
+impl PartialEq for Files {
+    fn eq(&self, other: &Self) -> bool {
+        self.inner.read().eq(&other.inner.read())
+    }
+}
+
+impl Eq for Files {}
+
+#[derive(Default)]
+struct FilesInner {
+    by_path: Map<FileId, ()>,
+    // TODO should we use a map here to reclaim the space for removed files?
+    // TODO I think we should use our own path abstraction here to avoid having to normalize paths
+    //  and dealing with non-utf paths everywhere.
+    by_id: IndexVec<FileId, Arc<Path>>,
+}
+
+impl FilesInner {
+    /// Inserts the path and returns a new id for it or returns the id if it is an existing path.
+    // TODO should this accept Path or PathBuf?
+    pub(crate) fn intern(&mut self, path: &Path) -> FileId {
+        let mut hasher = FxHasher::default();
+        path.hash(&mut hasher);
+        let hash = hasher.finish();
+
+        let entry = self
+            .by_path
+            .raw_entry_mut()
+            .from_hash(hash, |existing_file| &*self.by_id[*existing_file] == path);
+
+        match entry {
+            RawEntryMut::Occupied(entry) => *entry.key(),
+            RawEntryMut::Vacant(entry) => {
+                let id = self.by_id.push(Arc::from(path));
+                entry.insert_with_hasher(hash, id, (), |_| hash);
+                id
+            }
+        }
+    }
+
+    pub(crate) fn try_get(&self, path: &Path) -> Option<FileId> {
+        let mut hasher = FxHasher::default();
+        path.hash(&mut hasher);
+        let hash = hasher.finish();
+
+        Some(
+            *self
+                .by_path
+                .raw_entry()
+                .from_hash(hash, |existing_file| &*self.by_id[*existing_file] == path)?
+                .0,
+        )
+    }
+
+    /// Returns the path for the file with the given id.
+    pub(crate) fn path(&self, id: FileId) -> Arc<Path> {
+        self.by_id[id].clone()
+    }
+
+    pub(crate) fn iter(&self) -> impl Iterator<Item = (FileId, Arc<Path>)> + '_ {
+        self.by_path.keys().map(|id| (*id, self.by_id[*id].clone()))
+    }
+}
+
+impl PartialEq for FilesInner {
+    fn eq(&self, other: &Self) -> bool {
+        self.by_id == other.by_id
+    }
+}
+
+impl Eq for FilesInner {}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::path::PathBuf;
+
+    #[test]
+    fn insert_path_twice_same_id() {
+        let files = Files::default();
+        let path = PathBuf::from("foo/bar");
+        let id1 = files.intern(&path);
+        let id2 = files.intern(&path);
+        assert_eq!(id1, id2);
+    }
+
+    #[test]
+    fn insert_different_paths_different_ids() {
+        let files = Files::default();
+        let path1 = PathBuf::from("foo/bar");
+        let path2 = PathBuf::from("foo/bar/baz");
+        let id1 = files.intern(&path1);
+        let id2 = files.intern(&path2);
+        assert_ne!(id1, id2);
+    }
+}

crates/red_knot/src/hir.rs

@@ -0,0 +1,67 @@
+//! Key observations
+//!
+//! The HIR avoids allocations to large extends by:
+//! * Using an arena per node type
+//! * using ids and id ranges to reference items.
+//!
+//! Using separate arena per node type has the advantage that the IDs are relatively stable, because
+//! they only change when a node of the same kind has been added or removed. (What's unclear is if that matters or if
+//! it still triggers a re-compute because the AST-id in the node has changed).
+//!
+//! The HIR does not store all details. It mainly stores the *public* interface. There's a reference
+//! back to the AST node to get more details.
+//!
+//!
+
+use crate::ast_ids::{HasAstId, TypedAstId};
+use crate::files::FileId;
+use std::fmt::Formatter;
+use std::hash::{Hash, Hasher};
+
+pub struct HirAstId<N: HasAstId> {
+    file_id: FileId,
+    node_id: TypedAstId<N>,
+}
+
+impl<N: HasAstId> Copy for HirAstId<N> {}
+impl<N: HasAstId> Clone for HirAstId<N> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<N: HasAstId> PartialEq for HirAstId<N> {
+    fn eq(&self, other: &Self) -> bool {
+        self.file_id == other.file_id && self.node_id == other.node_id
+    }
+}
+
+impl<N: HasAstId> Eq for HirAstId<N> {}
+
+impl<N: HasAstId> std::fmt::Debug for HirAstId<N> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("HirAstId")
+            .field("file_id", &self.file_id)
+            .field("node_id", &self.node_id)
+            .finish()
+    }
+}
+
+impl<N: HasAstId> Hash for HirAstId<N> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.file_id.hash(state);
+        self.node_id.hash(state);
+    }
+}
+
+impl<N: HasAstId> HirAstId<N> {
+    pub fn upcast<M: HasAstId>(self) -> HirAstId<M>
+    where
+        N: Into<M>,
+    {
+        HirAstId {
+            file_id: self.file_id,
+            node_id: self.node_id.upcast(),
+        }
+    }
+}

crates/red_knot/src/hir/definition.rs

@@ -0,0 +1,556 @@
+use std::ops::{Index, Range};
+
+use ruff_index::{newtype_index, IndexVec};
+use ruff_python_ast::visitor::preorder;
+use ruff_python_ast::visitor::preorder::PreorderVisitor;
+use ruff_python_ast::{
+    Decorator, ExceptHandler, ExceptHandlerExceptHandler, Expr, MatchCase, ModModule, Stmt,
+    StmtAnnAssign, StmtAssign, StmtClassDef, StmtFunctionDef, StmtGlobal, StmtImport,
+    StmtImportFrom, StmtNonlocal, StmtTypeAlias, TypeParam, TypeParamParamSpec, TypeParamTypeVar,
+    TypeParamTypeVarTuple, WithItem,
+};
+
+use crate::ast_ids::{AstIds, HasAstId};
+use crate::files::FileId;
+use crate::hir::HirAstId;
+use crate::Name;
+
+#[newtype_index]
+pub struct FunctionId;
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct Function {
+    ast_id: HirAstId<StmtFunctionDef>,
+    name: Name,
+    parameters: Range<ParameterId>,
+    type_parameters: Range<TypeParameterId>, // TODO: type_parameters, return expression, decorators
+}
+
+#[newtype_index]
+pub struct ParameterId;
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct Parameter {
+    kind: ParameterKind,
+    name: Name,
+    default: Option<()>, // TODO use expression HIR
+    ast_id: HirAstId<ruff_python_ast::Parameter>,
+}
+
+// TODO or should `Parameter` be an enum?
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
+pub enum ParameterKind {
+    PositionalOnly,
+    Arguments,
+    Vararg,
+    KeywordOnly,
+    Kwarg,
+}
+
+#[newtype_index]
+pub struct ClassId;
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct Class {
+    name: Name,
+    ast_id: HirAstId<StmtClassDef>,
+    // TODO type parameters, inheritance, decorators, members
+}
+
+#[newtype_index]
+pub struct AssignmentId;
+
+// This can have more than one name...
+// but that means we can't implement `name()` on `ModuleItem`.
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct Assignment {
+    // TODO: Handle multiple names  / targets
+    name: Name,
+    ast_id: HirAstId<StmtAssign>,
+}
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct AnnotatedAssignment {
+    name: Name,
+    ast_id: HirAstId<StmtAnnAssign>,
+}
+
+#[newtype_index]
+pub struct AnnotatedAssignmentId;
+
+#[newtype_index]
+pub struct TypeAliasId;
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct TypeAlias {
+    name: Name,
+    ast_id: HirAstId<StmtTypeAlias>,
+    parameters: Range<TypeParameterId>,
+}
+
+#[newtype_index]
+pub struct TypeParameterId;
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub enum TypeParameter {
+    TypeVar(TypeParameterTypeVar),
+    ParamSpec(TypeParameterParamSpec),
+    TypeVarTuple(TypeParameterTypeVarTuple),
+}
+
+impl TypeParameter {
+    pub fn ast_id(&self) -> HirAstId<TypeParam> {
+        match self {
+            TypeParameter::TypeVar(type_var) => type_var.ast_id.upcast(),
+            TypeParameter::ParamSpec(param_spec) => param_spec.ast_id.upcast(),
+            TypeParameter::TypeVarTuple(type_var_tuple) => type_var_tuple.ast_id.upcast(),
+        }
+    }
+}
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct TypeParameterTypeVar {
+    name: Name,
+    ast_id: HirAstId<TypeParamTypeVar>,
+}
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct TypeParameterParamSpec {
+    name: Name,
+    ast_id: HirAstId<TypeParamParamSpec>,
+}
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct TypeParameterTypeVarTuple {
+    name: Name,
+    ast_id: HirAstId<TypeParamTypeVarTuple>,
+}
+
+#[newtype_index]
+pub struct GlobalId;
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct Global {
+    // TODO track names
+    ast_id: HirAstId<StmtGlobal>,
+}
+
+#[newtype_index]
+pub struct NonLocalId;
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub struct NonLocal {
+    // TODO track names
+    ast_id: HirAstId<StmtNonlocal>,
+}
+
+pub enum DefinitionId {
+    Function(FunctionId),
+    Parameter(ParameterId),
+    Class(ClassId),
+    Assignment(AssignmentId),
+    AnnotatedAssignment(AnnotatedAssignmentId),
+    Global(GlobalId),
+    NonLocal(NonLocalId),
+    TypeParameter(TypeParameterId),
+    TypeAlias(TypeAlias),
+}
+
+pub enum DefinitionItem {
+    Function(Function),
+    Parameter(Parameter),
+    Class(Class),
+    Assignment(Assignment),
+    AnnotatedAssignment(AnnotatedAssignment),
+    Global(Global),
+    NonLocal(NonLocal),
+    TypeParameter(TypeParameter),
+    TypeAlias(TypeAlias),
+}
+
+// The closest is rust-analyzers item-tree. It only represents "Items" which make the public interface of a module
+// (it excludes any other statement or expressions). rust-analyzer uses it as the main input to the name resolution
+// algorithm
+// > It is the input to the name resolution algorithm, as well as to the queries defined in `adt.rs`,
+// > `data.rs`, and most things in `attr.rs`.
+//
+// > One important purpose of this layer is to provide an "invalidation barrier" for incremental
+// > computations: when typing inside an item body, the `ItemTree` of the modified file is typically
+// > unaffected, so we don't have to recompute name resolution results or item data (see `data.rs`).
+//
+// I haven't fully figured this out but I think that this composes the "public" interface of a module?
+// But maybe that's too optimistic.
+//
+//
+#[derive(Debug, Clone, Default, Eq, PartialEq)]
+pub struct Definitions {
+    functions: IndexVec<FunctionId, Function>,
+    parameters: IndexVec<ParameterId, Parameter>,
+    classes: IndexVec<ClassId, Class>,
+    assignments: IndexVec<AssignmentId, Assignment>,
+    annotated_assignments: IndexVec<AnnotatedAssignmentId, AnnotatedAssignment>,
+    type_aliases: IndexVec<TypeAliasId, TypeAlias>,
+    type_parameters: IndexVec<TypeParameterId, TypeParameter>,
+    globals: IndexVec<GlobalId, Global>,
+    non_locals: IndexVec<NonLocalId, NonLocal>,
+}
+
+impl Definitions {
+    pub fn from_module(module: &ModModule, ast_ids: &AstIds, file_id: FileId) -> Self {
+        let mut visitor = DefinitionsVisitor {
+            definitions: Definitions::default(),
+            ast_ids,
+            file_id,
+        };
+
+        visitor.visit_body(&module.body);
+
+        visitor.definitions
+    }
+}
+
+impl Index<FunctionId> for Definitions {
+    type Output = Function;
+
+    fn index(&self, index: FunctionId) -> &Self::Output {
+        &self.functions[index]
+    }
+}
+
+impl Index<ParameterId> for Definitions {
+    type Output = Parameter;
+
+    fn index(&self, index: ParameterId) -> &Self::Output {
+        &self.parameters[index]
+    }
+}
+
+impl Index<ClassId> for Definitions {
+    type Output = Class;
+
+    fn index(&self, index: ClassId) -> &Self::Output {
+        &self.classes[index]
+    }
+}
+
+impl Index<AssignmentId> for Definitions {
+    type Output = Assignment;
+
+    fn index(&self, index: AssignmentId) -> &Self::Output {
+        &self.assignments[index]
+    }
+}
+
+impl Index<AnnotatedAssignmentId> for Definitions {
+    type Output = AnnotatedAssignment;
+
+    fn index(&self, index: AnnotatedAssignmentId) -> &Self::Output {
+        &self.annotated_assignments[index]
+    }
+}
+
+impl Index<TypeAliasId> for Definitions {
+    type Output = TypeAlias;
+
+    fn index(&self, index: TypeAliasId) -> &Self::Output {
+        &self.type_aliases[index]
+    }
+}
+
+impl Index<GlobalId> for Definitions {
+    type Output = Global;
+
+    fn index(&self, index: GlobalId) -> &Self::Output {
+        &self.globals[index]
+    }
+}
+
+impl Index<NonLocalId> for Definitions {
+    type Output = NonLocal;
+
+    fn index(&self, index: NonLocalId) -> &Self::Output {
+        &self.non_locals[index]
+    }
+}
+
+impl Index<TypeParameterId> for Definitions {
+    type Output = TypeParameter;
+
+    fn index(&self, index: TypeParameterId) -> &Self::Output {
+        &self.type_parameters[index]
+    }
+}
+
+struct DefinitionsVisitor<'a> {
+    definitions: Definitions,
+    ast_ids: &'a AstIds,
+    file_id: FileId,
+}
+
+impl DefinitionsVisitor<'_> {
+    fn ast_id<N: HasAstId>(&self, node: &N) -> HirAstId<N> {
+        HirAstId {
+            file_id: self.file_id,
+            node_id: self.ast_ids.ast_id(node),
+        }
+    }
+
+    fn lower_function_def(&mut self, function: &StmtFunctionDef) -> FunctionId {
+        let name = Name::new(&function.name);
+
+        let first_type_parameter_id = self.definitions.type_parameters.next_index();
+        let mut last_type_parameter_id = first_type_parameter_id;
+
+        if let Some(type_params) = &function.type_params {
+            for parameter in &type_params.type_params {
+                let id = self.lower_type_parameter(parameter);
+                last_type_parameter_id = id;
+            }
+        }
+
+        let parameters = self.lower_parameters(&function.parameters);
+
+        self.definitions.functions.push(Function {
+            name,
+            ast_id: self.ast_id(function),
+            parameters,
+            type_parameters: first_type_parameter_id..last_type_parameter_id,
+        })
+    }
+
+    fn lower_parameters(&mut self, parameters: &ruff_python_ast::Parameters) -> Range<ParameterId> {
+        let first_parameter_id = self.definitions.parameters.next_index();
+        let mut last_parameter_id = first_parameter_id;
+
+        for parameter in &parameters.posonlyargs {
+            last_parameter_id = self.definitions.parameters.push(Parameter {
+                kind: ParameterKind::PositionalOnly,
+                name: Name::new(&parameter.parameter.name),
+                default: None,
+                ast_id: self.ast_id(&parameter.parameter),
+            });
+        }
+
+        if let Some(vararg) = &parameters.vararg {
+            last_parameter_id = self.definitions.parameters.push(Parameter {
+                kind: ParameterKind::Vararg,
+                name: Name::new(&vararg.name),
+                default: None,
+                ast_id: self.ast_id(vararg),
+            });
+        }
+
+        for parameter in &parameters.kwonlyargs {
+            last_parameter_id = self.definitions.parameters.push(Parameter {
+                kind: ParameterKind::KeywordOnly,
+                name: Name::new(&parameter.parameter.name),
+                default: None,
+                ast_id: self.ast_id(&parameter.parameter),
+            });
+        }
+
+        if let Some(kwarg) = &parameters.kwarg {
+            last_parameter_id = self.definitions.parameters.push(Parameter {
+                kind: ParameterKind::KeywordOnly,
+                name: Name::new(&kwarg.name),
+                default: None,
+                ast_id: self.ast_id(kwarg),
+            });
+        }
+
+        first_parameter_id..last_parameter_id
+    }
+
+    fn lower_class_def(&mut self, class: &StmtClassDef) -> ClassId {
+        let name = Name::new(&class.name);
+
+        self.definitions.classes.push(Class {
+            name,
+            ast_id: self.ast_id(class),
+        })
+    }
+
+    fn lower_assignment(&mut self, assignment: &StmtAssign) {
+        // FIXME handle multiple names
+        if let Some(Expr::Name(name)) = assignment.targets.first() {
+            self.definitions.assignments.push(Assignment {
+                name: Name::new(&name.id),
+                ast_id: self.ast_id(assignment),
+            });
+        }
+    }
+
+    fn lower_annotated_assignment(&mut self, annotated_assignment: &StmtAnnAssign) {
+        if let Expr::Name(name) = &*annotated_assignment.target {
+            self.definitions
+                .annotated_assignments
+                .push(AnnotatedAssignment {
+                    name: Name::new(&name.id),
+                    ast_id: self.ast_id(annotated_assignment),
+                });
+        }
+    }
+
+    fn lower_type_alias(&mut self, type_alias: &StmtTypeAlias) {
+        if let Expr::Name(name) = &*type_alias.name {
+            let name = Name::new(&name.id);
+
+            let lower_parameters_id = self.definitions.type_parameters.next_index();
+            let mut last_parameter_id = lower_parameters_id;
+
+            if let Some(type_params) = &type_alias.type_params {
+                for type_parameter in &type_params.type_params {
+                    let id = self.lower_type_parameter(type_parameter);
+                    last_parameter_id = id;
+                }
+            }
+
+            self.definitions.type_aliases.push(TypeAlias {
+                name,
+                ast_id: self.ast_id(type_alias),
+                parameters: lower_parameters_id..last_parameter_id,
+            });
+        }
+    }
+
+    fn lower_type_parameter(&mut self, type_parameter: &TypeParam) -> TypeParameterId {
+        match type_parameter {
+            TypeParam::TypeVar(type_var) => {
+                self.definitions
+                    .type_parameters
+                    .push(TypeParameter::TypeVar(TypeParameterTypeVar {
+                        name: Name::new(&type_var.name),
+                        ast_id: self.ast_id(type_var),
+                    }))
+            }
+            TypeParam::ParamSpec(param_spec) => {
+                self.definitions
+                    .type_parameters
+                    .push(TypeParameter::ParamSpec(TypeParameterParamSpec {
+                        name: Name::new(&param_spec.name),
+                        ast_id: self.ast_id(param_spec),
+                    }))
+            }
+            TypeParam::TypeVarTuple(type_var_tuple) => {
+                self.definitions
+                    .type_parameters
+                    .push(TypeParameter::TypeVarTuple(TypeParameterTypeVarTuple {
+                        name: Name::new(&type_var_tuple.name),
+                        ast_id: self.ast_id(type_var_tuple),
+                    }))
+            }
+        }
+    }
+
+    fn lower_import(&mut self, _import: &StmtImport) {
+        // TODO
+    }
+
+    fn lower_import_from(&mut self, _import_from: &StmtImportFrom) {
+        // TODO
+    }
+
+    fn lower_global(&mut self, global: &StmtGlobal) -> GlobalId {
+        self.definitions.globals.push(Global {
+            ast_id: self.ast_id(global),
+        })
+    }
+
+    fn lower_non_local(&mut self, non_local: &StmtNonlocal) -> NonLocalId {
+        self.definitions.non_locals.push(NonLocal {
+            ast_id: self.ast_id(non_local),
+        })
+    }
+
+    fn lower_except_handler(&mut self, _except_handler: &ExceptHandlerExceptHandler) {
+        // TODO
+    }
+
+    fn lower_with_item(&mut self, _with_item: &WithItem) {
+        // TODO
+    }
+
+    fn lower_match_case(&mut self, _match_case: &MatchCase) {
+        // TODO
+    }
+}
+
+impl PreorderVisitor<'_> for DefinitionsVisitor<'_> {
+    fn visit_stmt(&mut self, stmt: &Stmt) {
+        match stmt {
+            // Definition statements
+            Stmt::FunctionDef(definition) => {
+                self.lower_function_def(definition);
+                self.visit_body(&definition.body);
+            }
+            Stmt::ClassDef(definition) => {
+                self.lower_class_def(definition);
+                self.visit_body(&definition.body);
+            }
+            Stmt::Assign(assignment) => {
+                self.lower_assignment(assignment);
+            }
+            Stmt::AnnAssign(annotated_assignment) => {
+                self.lower_annotated_assignment(annotated_assignment);
+            }
+            Stmt::TypeAlias(type_alias) => {
+                self.lower_type_alias(type_alias);
+            }
+
+            Stmt::Import(import) => self.lower_import(import),
+            Stmt::ImportFrom(import_from) => self.lower_import_from(import_from),
+            Stmt::Global(global) => {
+                self.lower_global(global);
+            }
+            Stmt::Nonlocal(non_local) => {
+                self.lower_non_local(non_local);
+            }
+
+            // Visit the compound statement bodies because they can contain other definitions.
+            Stmt::For(_)
+            | Stmt::While(_)
+            | Stmt::If(_)
+            | Stmt::With(_)
+            | Stmt::Match(_)
+            | Stmt::Try(_) => {
+                preorder::walk_stmt(self, stmt);
+            }
+
+            // Skip over simple statements because they can't contain any other definitions.
+            Stmt::Return(_)
+            | Stmt::Delete(_)
+            | Stmt::AugAssign(_)
+            | Stmt::Raise(_)
+            | Stmt::Assert(_)
+            | Stmt::Expr(_)
+            | Stmt::Pass(_)
+            | Stmt::Break(_)
+            | Stmt::Continue(_)
+            | Stmt::IpyEscapeCommand(_) => {
+                // No op
+            }
+        }
+    }
+
+    fn visit_expr(&mut self, _: &'_ Expr) {}
+
+    fn visit_decorator(&mut self, _decorator: &'_ Decorator) {}
+
+    fn visit_except_handler(&mut self, except_handler: &'_ ExceptHandler) {
+        match except_handler {
+            ExceptHandler::ExceptHandler(except_handler) => {
+                self.lower_except_handler(except_handler);
+            }
+        }
+    }
+
+    fn visit_with_item(&mut self, with_item: &'_ WithItem) {
+        self.lower_with_item(with_item);
+    }
+
+    fn visit_match_case(&mut self, match_case: &'_ MatchCase) {
+        self.lower_match_case(match_case);
+        self.visit_body(&match_case.body);
+    }
+}

crates/red_knot/src/lib.rs

@@ -0,0 +1,109 @@
+use std::fmt::Formatter;
+use std::hash::BuildHasherDefault;
+use std::ops::Deref;
+use std::path::{Path, PathBuf};
+
+use rustc_hash::{FxHashSet, FxHasher};
+
+use crate::files::FileId;
+
+pub mod ast_ids;
+pub mod cache;
+pub mod cancellation;
+pub mod db;
+pub mod files;
+pub mod hir;
+pub mod lint;
+pub mod module;
+mod parse;
+pub mod program;
+pub mod source;
+mod symbols;
+mod types;
+pub mod watch;
+
+pub(crate) type FxDashMap<K, V> = dashmap::DashMap<K, V, BuildHasherDefault<FxHasher>>;
+#[allow(unused)]
+pub(crate) type FxDashSet<V> = dashmap::DashSet<V, BuildHasherDefault<FxHasher>>;
+pub(crate) type FxIndexSet<V> = indexmap::set::IndexSet<V, BuildHasherDefault<FxHasher>>;
+
+#[derive(Debug, Clone)]
+pub struct Workspace {
+    /// TODO this should be a resolved path. We should probably use a newtype wrapper that guarantees that
+    /// PATH is a UTF-8 path and is normalized.
+    root: PathBuf,
+    /// The files that are open in the workspace.
+    ///
+    /// * Editor: The files that are actively being edited in the editor (the user has a tab open with the file).
+    /// * CLI: The resolved files passed as arguments to the CLI.
+    open_files: FxHashSet<FileId>,
+}
+
+impl Workspace {
+    pub fn new(root: PathBuf) -> Self {
+        Self {
+            root,
+            open_files: FxHashSet::default(),
+        }
+    }
+
+    pub fn root(&self) -> &Path {
+        self.root.as_path()
+    }
+
+    // TODO having the content in workspace feels wrong.
+    pub fn open_file(&mut self, file_id: FileId) {
+        self.open_files.insert(file_id);
+    }
+
+    pub fn close_file(&mut self, file_id: FileId) {
+        self.open_files.remove(&file_id);
+    }
+
+    // TODO introduce an `OpenFile` type instead of using an anonymous tuple.
+    pub fn open_files(&self) -> impl Iterator<Item = FileId> + '_ {
+        self.open_files.iter().copied()
+    }
+
+    pub fn is_file_open(&self, file_id: FileId) -> bool {
+        self.open_files.contains(&file_id)
+    }
+}
+
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub struct Name(smol_str::SmolStr);
+
+impl Name {
+    #[inline]
+    pub fn new(name: &str) -> Self {
+        Self(smol_str::SmolStr::new(name))
+    }
+
+    pub fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
+impl Deref for Name {
+    type Target = str;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        self.as_str()
+    }
+}
+
+impl<T> From<T> for Name
+where
+    T: Into<smol_str::SmolStr>,
+{
+    fn from(value: T) -> Self {
+        Self(value.into())
+    }
+}
+
+impl std::fmt::Display for Name {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.as_str())
+    }
+}

crates/red_knot/src/lint.rs

@@ -0,0 +1,264 @@
+use std::cell::RefCell;
+use std::ops::{Deref, DerefMut};
+use std::sync::Arc;
+use std::time::Duration;
+
+use ruff_python_ast::visitor::Visitor;
+use ruff_python_ast::{ModModule, StringLiteral};
+
+use crate::cache::KeyValueCache;
+use crate::db::{HasJar, LintDb, LintJar, QueryResult, SemanticDb};
+use crate::files::FileId;
+use crate::parse::Parsed;
+use crate::source::Source;
+use crate::symbols::{Definition, SymbolId, SymbolTable};
+use crate::types::Type;
+
+#[tracing::instrument(level = "debug", skip(db))]
+pub(crate) fn lint_syntax<Db>(db: &Db, file_id: FileId) -> QueryResult<Diagnostics>
+where
+    Db: LintDb + HasJar<LintJar>,
+{
+    let storage = &db.jar()?.lint_syntax;
+
+    #[allow(clippy::print_stdout)]
+    if std::env::var("RED_KNOT_SLOW_LINT").is_ok() {
+        for i in 0..10 {
+            db.cancelled()?;
+            println!("RED_KNOT_SLOW_LINT is set, sleeping for {i}/10 seconds");
+            std::thread::sleep(Duration::from_secs(1));
+        }
+    }
+
+    storage.get(&file_id, |file_id| {
+        let mut diagnostics = Vec::new();
+
+        let source = db.source(*file_id)?;
+        lint_lines(source.text(), &mut diagnostics);
+
+        let parsed = db.parse(*file_id)?;
+
+        if parsed.errors().is_empty() {
+            let ast = parsed.ast();
+
+            let mut visitor = SyntaxLintVisitor {
+                diagnostics,
+                source: source.text(),
+            };
+            visitor.visit_body(&ast.body);
+            diagnostics = visitor.diagnostics;
+        } else {
+            diagnostics.extend(parsed.errors().iter().map(std::string::ToString::to_string));
+        }
+
+        Ok(Diagnostics::from(diagnostics))
+    })
+}
+
+fn lint_lines(source: &str, diagnostics: &mut Vec<String>) {
+    for (line_number, line) in source.lines().enumerate() {
+        if line.len() < 88 {
+            continue;
+        }
+
+        let char_count = line.chars().count();
+        if char_count > 88 {
+            diagnostics.push(format!(
+                "Line {} is too long ({} characters)",
+                line_number + 1,
+                char_count
+            ));
+        }
+    }
+}
+
+#[tracing::instrument(level = "debug", skip(db))]
+pub(crate) fn lint_semantic<Db>(db: &Db, file_id: FileId) -> QueryResult<Diagnostics>
+where
+    Db: LintDb + HasJar<LintJar>,
+{
+    let storage = &db.jar()?.lint_semantic;
+
+    storage.get(&file_id, |file_id| {
+        let source = db.source(*file_id)?;
+        let parsed = db.parse(*file_id)?;
+        let symbols = db.symbol_table(*file_id)?;
+
+        let context = SemanticLintContext {
+            file_id: *file_id,
+            source,
+            parsed,
+            symbols,
+            db,
+            diagnostics: RefCell::new(Vec::new()),
+        };
+
+        lint_unresolved_imports(&context)?;
+
+        Ok(Diagnostics::from(context.diagnostics.take()))
+    })
+}
+
+fn lint_unresolved_imports(context: &SemanticLintContext) -> QueryResult<()> {
+    // TODO: Consider iterating over the dependencies (imports) only instead of all definitions.
+    for (symbol, definition) in context.symbols().all_definitions() {
+        match definition {
+            Definition::Import(import) => {
+                let ty = context.infer_symbol_type(symbol)?;
+
+                if ty.is_unknown() {
+                    context.push_diagnostic(format!("Unresolved module {}", import.module));
+                }
+            }
+            Definition::ImportFrom(import) => {
+                let ty = context.infer_symbol_type(symbol)?;
+
+                if ty.is_unknown() {
+                    let module_name = import.module().map(Deref::deref).unwrap_or_default();
+                    let message = if import.level() > 0 {
+                        format!(
+                            "Unresolved relative import '{}' from {}{}",
+                            import.name(),
+                            ".".repeat(import.level() as usize),
+                            module_name
+                        )
+                    } else {
+                        format!(
+                            "Unresolved import '{}' from '{}'",
+                            import.name(),
+                            module_name
+                        )
+                    };
+
+                    context.push_diagnostic(message);
+                }
+            }
+            _ => {}
+        }
+    }
+
+    Ok(())
+}
+
+pub struct SemanticLintContext<'a> {
+    file_id: FileId,
+    source: Source,
+    parsed: Parsed,
+    symbols: Arc<SymbolTable>,
+    db: &'a dyn SemanticDb,
+    diagnostics: RefCell<Vec<String>>,
+}
+
+impl<'a> SemanticLintContext<'a> {
+    pub fn source_text(&self) -> &str {
+        self.source.text()
+    }
+
+    pub fn file_id(&self) -> FileId {
+        self.file_id
+    }
+
+    pub fn ast(&self) -> &ModModule {
+        self.parsed.ast()
+    }
+
+    pub fn symbols(&self) -> &SymbolTable {
+        &self.symbols
+    }
+
+    pub fn infer_symbol_type(&self, symbol_id: SymbolId) -> QueryResult<Type> {
+        self.db.infer_symbol_type(self.file_id, symbol_id)
+    }
+
+    pub fn push_diagnostic(&self, diagnostic: String) {
+        self.diagnostics.borrow_mut().push(diagnostic);
+    }
+
+    pub fn extend_diagnostics(&mut self, diagnostics: impl IntoIterator<Item = String>) {
+        self.diagnostics.get_mut().extend(diagnostics);
+    }
+}
+
+#[derive(Debug)]
+struct SyntaxLintVisitor<'a> {
+    diagnostics: Vec<String>,
+    source: &'a str,
+}
+
+impl Visitor<'_> for SyntaxLintVisitor<'_> {
+    fn visit_string_literal(&mut self, string_literal: &'_ StringLiteral) {
+        // A very naive implementation of use double quotes
+        let text = &self.source[string_literal.range];
+
+        if text.starts_with('\'') {
+            self.diagnostics
+                .push("Use double quotes for strings".to_string());
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub enum Diagnostics {
+    Empty,
+    List(Arc<Vec<String>>),
+}
+
+impl Diagnostics {
+    pub fn as_slice(&self) -> &[String] {
+        match self {
+            Diagnostics::Empty => &[],
+            Diagnostics::List(list) => list.as_slice(),
+        }
+    }
+}
+
+impl Deref for Diagnostics {
+    type Target = [String];
+    fn deref(&self) -> &Self::Target {
+        self.as_slice()
+    }
+}
+
+impl From<Vec<String>> for Diagnostics {
+    fn from(value: Vec<String>) -> Self {
+        if value.is_empty() {
+            Diagnostics::Empty
+        } else {
+            Diagnostics::List(Arc::new(value))
+        }
+    }
+}
+
+#[derive(Default, Debug)]
+pub struct LintSyntaxStorage(KeyValueCache<FileId, Diagnostics>);
+
+impl Deref for LintSyntaxStorage {
+    type Target = KeyValueCache<FileId, Diagnostics>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for LintSyntaxStorage {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+#[derive(Default, Debug)]
+pub struct LintSemanticStorage(KeyValueCache<FileId, Diagnostics>);
+
+impl Deref for LintSemanticStorage {
+    type Target = KeyValueCache<FileId, Diagnostics>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for LintSemanticStorage {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}

crates/red_knot/src/main.rs

@@ -0,0 +1,359 @@
+#![allow(clippy::dbg_macro)]
+
+use std::path::Path;
+use std::sync::Mutex;
+
+use crossbeam::channel as crossbeam_channel;
+use tracing::subscriber::Interest;
+use tracing::{Level, Metadata};
+use tracing_subscriber::filter::LevelFilter;
+use tracing_subscriber::layer::{Context, Filter, SubscriberExt};
+use tracing_subscriber::{Layer, Registry};
+use tracing_tree::time::Uptime;
+
+use red_knot::db::{HasJar, ParallelDatabase, QueryError, SemanticDb, SourceDb, SourceJar};
+use red_knot::module::{ModuleSearchPath, ModuleSearchPathKind};
+use red_knot::program::check::ExecutionMode;
+use red_knot::program::{FileWatcherChange, Program};
+use red_knot::watch::FileWatcher;
+use red_knot::Workspace;
+
+#[allow(clippy::print_stdout, clippy::unnecessary_wraps, clippy::print_stderr)]
+fn main() -> anyhow::Result<()> {
+    setup_tracing();
+
+    let arguments: Vec<_> = std::env::args().collect();
+
+    if arguments.len() < 2 {
+        eprintln!("Usage: red_knot <path>");
+        return Err(anyhow::anyhow!("Invalid arguments"));
+    }
+
+    let entry_point = Path::new(&arguments[1]);
+
+    if !entry_point.exists() {
+        eprintln!("The entry point does not exist.");
+        return Err(anyhow::anyhow!("Invalid arguments"));
+    }
+
+    if !entry_point.is_file() {
+        eprintln!("The entry point is not a file.");
+        return Err(anyhow::anyhow!("Invalid arguments"));
+    }
+
+    let workspace_folder = entry_point.parent().unwrap();
+    let workspace = Workspace::new(workspace_folder.to_path_buf());
+
+    let workspace_search_path = ModuleSearchPath::new(
+        workspace.root().to_path_buf(),
+        ModuleSearchPathKind::FirstParty,
+    );
+    let mut program = Program::new(workspace);
+    program.set_module_search_paths(vec![workspace_search_path]);
+
+    let entry_id = program.file_id(entry_point);
+    program.workspace_mut().open_file(entry_id);
+
+    let (main_loop, main_loop_cancellation_token) = MainLoop::new();
+
+    // Listen to Ctrl+C and abort the watch mode.
+    let main_loop_cancellation_token = Mutex::new(Some(main_loop_cancellation_token));
+    ctrlc::set_handler(move || {
+        let mut lock = main_loop_cancellation_token.lock().unwrap();
+
+        if let Some(token) = lock.take() {
+            token.stop();
+        }
+    })?;
+
+    let file_changes_notifier = main_loop.file_changes_notifier();
+
+    // Watch for file changes and re-trigger the analysis.
+    let mut file_watcher = FileWatcher::new(move |changes| {
+        file_changes_notifier.notify(changes);
+    })?;
+
+    file_watcher.watch_folder(workspace_folder)?;
+
+    main_loop.run(&mut program);
+
+    let source_jar: &SourceJar = program.jar().unwrap();
+
+    dbg!(source_jar.parsed.statistics());
+    dbg!(source_jar.sources.statistics());
+
+    Ok(())
+}
+
+struct MainLoop {
+    orchestrator_sender: crossbeam_channel::Sender<OrchestratorMessage>,
+    main_loop_receiver: crossbeam_channel::Receiver<MainLoopMessage>,
+}
+
+impl MainLoop {
+    fn new() -> (Self, MainLoopCancellationToken) {
+        let (orchestrator_sender, orchestrator_receiver) = crossbeam_channel::bounded(1);
+        let (main_loop_sender, main_loop_receiver) = crossbeam_channel::bounded(1);
+
+        let mut orchestrator = Orchestrator {
+            receiver: orchestrator_receiver,
+            sender: main_loop_sender.clone(),
+            revision: 0,
+        };
+
+        std::thread::spawn(move || {
+            orchestrator.run();
+        });
+
+        (
+            Self {
+                orchestrator_sender,
+                main_loop_receiver,
+            },
+            MainLoopCancellationToken {
+                sender: main_loop_sender,
+            },
+        )
+    }
+
+    fn file_changes_notifier(&self) -> FileChangesNotifier {
+        FileChangesNotifier {
+            sender: self.orchestrator_sender.clone(),
+        }
+    }
+
+    fn run(self, program: &mut Program) {
+        self.orchestrator_sender
+            .send(OrchestratorMessage::Run)
+            .unwrap();
+
+        for message in &self.main_loop_receiver {
+            tracing::trace!("Main Loop: Tick");
+
+            match message {
+                MainLoopMessage::CheckProgram { revision } => {
+                    let program = program.snapshot();
+                    let sender = self.orchestrator_sender.clone();
+
+                    // Spawn a new task that checks the program. This needs to be done in a separate thread
+                    // to prevent blocking the main loop here.
+                    rayon::spawn(move || match program.check(ExecutionMode::ThreadPool) {
+                        Ok(result) => {
+                            sender
+                                .send(OrchestratorMessage::CheckProgramCompleted {
+                                    diagnostics: result,
+                                    revision,
+                                })
+                                .unwrap();
+                        }
+                        Err(QueryError::Cancelled) => {}
+                    });
+                }
+                MainLoopMessage::ApplyChanges(changes) => {
+                    // Automatically cancels any pending queries and waits for them to complete.
+                    program.apply_changes(changes);
+                }
+                MainLoopMessage::CheckCompleted(diagnostics) => {
+                    dbg!(diagnostics);
+                }
+                MainLoopMessage::Exit => {
+                    return;
+                }
+            }
+        }
+    }
+}
+
+impl Drop for MainLoop {
+    fn drop(&mut self) {
+        self.orchestrator_sender
+            .send(OrchestratorMessage::Shutdown)
+            .unwrap();
+    }
+}
+
+#[derive(Debug, Clone)]
+struct FileChangesNotifier {
+    sender: crossbeam_channel::Sender<OrchestratorMessage>,
+}
+
+impl FileChangesNotifier {
+    fn notify(&self, changes: Vec<FileWatcherChange>) {
+        self.sender
+            .send(OrchestratorMessage::FileChanges(changes))
+            .unwrap();
+    }
+}
+
+#[derive(Debug)]
+struct MainLoopCancellationToken {
+    sender: crossbeam_channel::Sender<MainLoopMessage>,
+}
+
+impl MainLoopCancellationToken {
+    fn stop(self) {
+        self.sender.send(MainLoopMessage::Exit).unwrap();
+    }
+}
+
+struct Orchestrator {
+    /// Sends messages to the main loop.
+    sender: crossbeam_channel::Sender<MainLoopMessage>,
+    /// Receives messages from the main loop.
+    receiver: crossbeam_channel::Receiver<OrchestratorMessage>,
+    revision: usize,
+}
+
+impl Orchestrator {
+    fn run(&mut self) {
+        while let Ok(message) = self.receiver.recv() {
+            match message {
+                OrchestratorMessage::Run => {
+                    self.sender
+                        .send(MainLoopMessage::CheckProgram {
+                            revision: self.revision,
+                        })
+                        .unwrap();
+                }
+
+                OrchestratorMessage::CheckProgramCompleted {
+                    diagnostics,
+                    revision,
+                } => {
+                    // Only take the diagnostics if they are for the latest revision.
+                    if self.revision == revision {
+                        self.sender
+                            .send(MainLoopMessage::CheckCompleted(diagnostics))
+                            .unwrap();
+                    } else {
+                        tracing::debug!("Discarding diagnostics for outdated revision {revision} (current: {}).", self.revision);
+                    }
+                }
+
+                OrchestratorMessage::FileChanges(changes) => {
+                    // Request cancellation, but wait until all analysis tasks have completed to
+                    // avoid stale messages in the next main loop.
+
+                    self.revision += 1;
+                    self.debounce_changes(changes);
+                }
+                OrchestratorMessage::Shutdown => {
+                    return self.shutdown();
+                }
+            }
+        }
+    }
+
+    fn debounce_changes(&self, mut changes: Vec<FileWatcherChange>) {
+        loop {
+            // Consume possibly incoming file change messages before running a new analysis, but don't wait for more than 100ms.
+            crossbeam_channel::select! {
+                recv(self.receiver) -> message => {
+                    match message {
+                        Ok(OrchestratorMessage::Shutdown) => {
+                            return self.shutdown();
+                        }
+                        Ok(OrchestratorMessage::FileChanges(file_changes)) => {
+                            changes.extend(file_changes);
+                        }
+
+                        Ok(OrchestratorMessage::CheckProgramCompleted { .. })=> {
+                            // disregard any outdated completion message.
+                        }
+                        Ok(OrchestratorMessage::Run) => unreachable!("The orchestrator is already running."),
+
+                        Err(_) => {
+                            // There are no more senders, no point in waiting for more messages
+                            return;
+                        }
+                    }
+                },
+                default(std::time::Duration::from_millis(10)) => {
+                    // No more file changes after 10 ms, send the changes and schedule a new analysis
+                    self.sender.send(MainLoopMessage::ApplyChanges(changes)).unwrap();
+                    self.sender.send(MainLoopMessage::CheckProgram { revision: self.revision}).unwrap();
+                    return;
+                }
+            }
+        }
+    }
+
+    #[allow(clippy::unused_self)]
+    fn shutdown(&self) {
+        tracing::trace!("Shutting down orchestrator.");
+    }
+}
+
+/// Message sent from the orchestrator to the main loop.
+#[derive(Debug)]
+enum MainLoopMessage {
+    CheckProgram { revision: usize },
+    CheckCompleted(Vec<String>),
+    ApplyChanges(Vec<FileWatcherChange>),
+    Exit,
+}
+
+#[derive(Debug)]
+enum OrchestratorMessage {
+    Run,
+    Shutdown,
+
+    CheckProgramCompleted {
+        diagnostics: Vec<String>,
+        revision: usize,
+    },
+
+    FileChanges(Vec<FileWatcherChange>),
+}
+
+fn setup_tracing() {
+    let subscriber = Registry::default().with(
+        tracing_tree::HierarchicalLayer::default()
+            .with_indent_lines(true)
+            .with_indent_amount(2)
+            .with_bracketed_fields(true)
+            .with_thread_ids(true)
+            .with_targets(true)
+            .with_writer(|| Box::new(std::io::stderr()))
+            .with_timer(Uptime::default())
+            .with_filter(LoggingFilter {
+                trace_level: Level::TRACE,
+            }),
+    );
+
+    tracing::subscriber::set_global_default(subscriber).unwrap();
+}
+
+struct LoggingFilter {
+    trace_level: Level,
+}
+
+impl LoggingFilter {
+    fn is_enabled(&self, meta: &Metadata<'_>) -> bool {
+        let filter = if meta.target().starts_with("red_knot") || meta.target().starts_with("ruff") {
+            self.trace_level
+        } else {
+            Level::INFO
+        };
+
+        meta.level() <= &filter
+    }
+}
+
+impl<S> Filter<S> for LoggingFilter {
+    fn enabled(&self, meta: &Metadata<'_>, _cx: &Context<'_, S>) -> bool {
+        self.is_enabled(meta)
+    }
+
+    fn callsite_enabled(&self, meta: &'static Metadata<'static>) -> Interest {
+        if self.is_enabled(meta) {
+            Interest::always()
+        } else {
+            Interest::never()
+        }
+    }
+
+    fn max_level_hint(&self) -> Option<LevelFilter> {
+        Some(LevelFilter::from_level(self.trace_level))
+    }
+}

crates/red_knot/src/parse.rs

@@ -0,0 +1,95 @@
+use std::ops::{Deref, DerefMut};
+use std::sync::Arc;
+
+use ruff_python_ast as ast;
+use ruff_python_parser::{Mode, ParseError};
+use ruff_text_size::{Ranged, TextRange};
+
+use crate::cache::KeyValueCache;
+use crate::db::{HasJar, QueryResult, SourceDb, SourceJar};
+use crate::files::FileId;
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct Parsed {
+    inner: Arc<ParsedInner>,
+}
+
+#[derive(Debug, PartialEq)]
+struct ParsedInner {
+    ast: ast::ModModule,
+    errors: Vec<ParseError>,
+}
+
+impl Parsed {
+    fn new(ast: ast::ModModule, errors: Vec<ParseError>) -> Self {
+        Self {
+            inner: Arc::new(ParsedInner { ast, errors }),
+        }
+    }
+
+    pub(crate) fn from_text(text: &str) -> Self {
+        let result = ruff_python_parser::parse(text, Mode::Module);
+
+        let (module, errors) = match result {
+            Ok(ast::Mod::Module(module)) => (module, vec![]),
+            Ok(ast::Mod::Expression(expression)) => (
+                ast::ModModule {
+                    range: expression.range(),
+                    body: vec![ast::Stmt::Expr(ast::StmtExpr {
+                        range: expression.range(),
+                        value: expression.body,
+                    })],
+                },
+                vec![],
+            ),
+            Err(errors) => (
+                ast::ModModule {
+                    range: TextRange::default(),
+                    body: Vec::new(),
+                },
+                vec![errors],
+            ),
+        };
+
+        Parsed::new(module, errors)
+    }
+
+    pub fn ast(&self) -> &ast::ModModule {
+        &self.inner.ast
+    }
+
+    pub fn errors(&self) -> &[ParseError] {
+        &self.inner.errors
+    }
+}
+
+#[tracing::instrument(level = "debug", skip(db))]
+pub(crate) fn parse<Db>(db: &Db, file_id: FileId) -> QueryResult<Parsed>
+where
+    Db: SourceDb + HasJar<SourceJar>,
+{
+    let parsed = db.jar()?;
+
+    parsed.parsed.get(&file_id, |file_id| {
+        let source = db.source(*file_id)?;
+
+        Ok(Parsed::from_text(source.text()))
+    })
+}
+
+#[derive(Debug, Default)]
+pub struct ParsedStorage(KeyValueCache<FileId, Parsed>);
+
+impl Deref for ParsedStorage {
+    type Target = KeyValueCache<FileId, Parsed>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for ParsedStorage {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}

crates/red_knot/src/program/check.rs

@@ -0,0 +1,412 @@
+use rayon::{current_num_threads, yield_local};
+use rustc_hash::FxHashSet;
+
+use crate::db::{Database, LintDb, QueryError, QueryResult, SemanticDb};
+use crate::files::FileId;
+use crate::lint::Diagnostics;
+use crate::program::Program;
+use crate::symbols::Dependency;
+
+impl Program {
+    /// Checks all open files in the workspace and its dependencies.
+    #[tracing::instrument(level = "debug", skip_all)]
+    pub fn check(&self, mode: ExecutionMode) -> QueryResult<Vec<String>> {
+        self.cancelled()?;
+
+        let mut context = CheckContext::new(self);
+
+        match mode {
+            ExecutionMode::SingleThreaded => SingleThreadedExecutor.run(&mut context)?,
+            ExecutionMode::ThreadPool => ThreadPoolExecutor.run(&mut context)?,
+        };
+
+        Ok(context.finish())
+    }
+
+    #[tracing::instrument(level = "debug", skip(self, context))]
+    fn check_file(&self, file: FileId, context: &CheckFileContext) -> QueryResult<Diagnostics> {
+        self.cancelled()?;
+
+        let symbol_table = self.symbol_table(file)?;
+        let dependencies = symbol_table.dependencies();
+
+        if !dependencies.is_empty() {
+            let module = self.file_to_module(file)?;
+
+            // TODO scheduling all dependencies here is wasteful if we don't infer any types on them
+            //  but I think that's unlikely, so it is okay?
+            //  Anyway, we need to figure out a way to retrieve the dependencies of a module
+            //  from the persistent cache. So maybe it should be a separate query after all.
+            for dependency in dependencies {
+                let dependency_name = match dependency {
+                    Dependency::Module(name) => Some(name.clone()),
+                    Dependency::Relative { .. } => match &module {
+                        Some(module) => module.resolve_dependency(self, dependency)?,
+                        None => None,
+                    },
+                };
+
+                if let Some(dependency_name) = dependency_name {
+                    // TODO We may want to have a different check functions for non-first-party
+                    //   files because we only need to index them and not check them.
+                    //   Supporting non-first-party code also requires supporting typing stubs.
+                    if let Some(dependency) = self.resolve_module(dependency_name)? {
+                        if dependency.path(self)?.root().kind().is_first_party() {
+                            context.schedule_dependency(dependency.path(self)?.file());
+                        }
+                    }
+                }
+            }
+        }
+
+        let mut diagnostics = Vec::new();
+
+        if self.workspace().is_file_open(file) {
+            diagnostics.extend_from_slice(&self.lint_syntax(file)?);
+            diagnostics.extend_from_slice(&self.lint_semantic(file)?);
+        }
+
+        Ok(Diagnostics::from(diagnostics))
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum ExecutionMode {
+    SingleThreaded,
+    ThreadPool,
+}
+
+/// Context that stores state information about the entire check operation.
+struct CheckContext<'a> {
+    /// IDs of the files that have been queued for checking.
+    ///
+    /// Used to avoid queuing the same file twice.
+    scheduled_files: FxHashSet<FileId>,
+
+    /// Reference to the program that is checked.
+    program: &'a Program,
+
+    /// The aggregated diagnostics
+    diagnostics: Vec<String>,
+}
+
+impl<'a> CheckContext<'a> {
+    fn new(program: &'a Program) -> Self {
+        Self {
+            scheduled_files: FxHashSet::default(),
+            program,
+            diagnostics: Vec::new(),
+        }
+    }
+
+    /// Returns the tasks to check all open files in the workspace.
+    fn check_open_files(&mut self) -> Vec<CheckOpenFileTask> {
+        self.scheduled_files
+            .extend(self.program.workspace().open_files());
+
+        self.program
+            .workspace()
+            .open_files()
+            .map(|file_id| CheckOpenFileTask { file_id })
+            .collect()
+    }
+
+    /// Returns the task to check a dependency.
+    fn check_dependency(&mut self, file_id: FileId) -> Option<CheckDependencyTask> {
+        if self.scheduled_files.insert(file_id) {
+            Some(CheckDependencyTask { file_id })
+        } else {
+            None
+        }
+    }
+
+    /// Pushes the result for a single file check operation
+    fn push_diagnostics(&mut self, diagnostics: &Diagnostics) {
+        self.diagnostics.extend_from_slice(diagnostics);
+    }
+
+    /// Returns a reference to the program that is being checked.
+    fn program(&self) -> &'a Program {
+        self.program
+    }
+
+    /// Creates a task context that is used to check a single file.
+    fn task_context<'b, S>(&self, dependency_scheduler: &'b S) -> CheckTaskContext<'a, 'b, S>
+    where
+        S: ScheduleDependency,
+    {
+        CheckTaskContext {
+            program: self.program,
+            dependency_scheduler,
+        }
+    }
+
+    fn finish(self) -> Vec<String> {
+        self.diagnostics
+    }
+}
+
+/// Trait that abstracts away how a dependency of a file gets scheduled for checking.
+trait ScheduleDependency {
+    /// Schedules the file with the given ID for checking.
+    fn schedule(&self, file_id: FileId);
+}
+
+impl<T> ScheduleDependency for T
+where
+    T: Fn(FileId),
+{
+    fn schedule(&self, file_id: FileId) {
+        let f = self;
+        f(file_id);
+    }
+}
+
+/// Context that is used to run a single file check task.
+///
+/// The task is generic over `S` because it is passed across thread boundaries and
+/// we don't want to add the requirement that [`ScheduleDependency`] must be [`Send`].
+struct CheckTaskContext<'a, 'scheduler, S>
+where
+    S: ScheduleDependency,
+{
+    dependency_scheduler: &'scheduler S,
+    program: &'a Program,
+}
+
+impl<'a, 'scheduler, S> CheckTaskContext<'a, 'scheduler, S>
+where
+    S: ScheduleDependency,
+{
+    fn as_file_context(&self) -> CheckFileContext<'scheduler> {
+        CheckFileContext {
+            dependency_scheduler: self.dependency_scheduler,
+        }
+    }
+}
+
+/// Context passed when checking a single file.
+///
+/// This is a trimmed down version of [`CheckTaskContext`] with the type parameter `S` erased
+/// to avoid monomorphization of [`Program:check_file`].
+struct CheckFileContext<'a> {
+    dependency_scheduler: &'a dyn ScheduleDependency,
+}
+
+impl<'a> CheckFileContext<'a> {
+    fn schedule_dependency(&self, file_id: FileId) {
+        self.dependency_scheduler.schedule(file_id);
+    }
+}
+
+#[derive(Debug)]
+enum CheckFileTask {
+    OpenFile(CheckOpenFileTask),
+    Dependency(CheckDependencyTask),
+}
+
+impl CheckFileTask {
+    /// Runs the task and returns the results for checking this file.
+    fn run<S>(&self, context: &CheckTaskContext<S>) -> QueryResult<Diagnostics>
+    where
+        S: ScheduleDependency,
+    {
+        match self {
+            Self::OpenFile(task) => task.run(context),
+            Self::Dependency(task) => task.run(context),
+        }
+    }
+
+    fn file_id(&self) -> FileId {
+        match self {
+            CheckFileTask::OpenFile(task) => task.file_id,
+            CheckFileTask::Dependency(task) => task.file_id,
+        }
+    }
+}
+
+/// Task to check an open file.
+
+#[derive(Debug)]
+struct CheckOpenFileTask {
+    file_id: FileId,
+}
+
+impl CheckOpenFileTask {
+    fn run<S>(&self, context: &CheckTaskContext<S>) -> QueryResult<Diagnostics>
+    where
+        S: ScheduleDependency,
+    {
+        context
+            .program
+            .check_file(self.file_id, &context.as_file_context())
+    }
+}
+
+/// Task to check a dependency file.
+#[derive(Debug)]
+struct CheckDependencyTask {
+    file_id: FileId,
+}
+
+impl CheckDependencyTask {
+    fn run<S>(&self, context: &CheckTaskContext<S>) -> QueryResult<Diagnostics>
+    where
+        S: ScheduleDependency,
+    {
+        context
+            .program
+            .check_file(self.file_id, &context.as_file_context())
+    }
+}
+
+/// Executor that schedules the checking of individual program files.
+trait CheckExecutor {
+    fn run(self, context: &mut CheckContext) -> QueryResult<()>;
+}
+
+/// Executor that runs all check operations on the current thread.
+///
+/// The executor does not schedule dependencies for checking.
+/// The main motivation for scheduling dependencies
+/// in a multithreaded environment is to parse and index the dependencies concurrently.
+/// However, that doesn't make sense in a single threaded environment, because the dependencies then compute
+/// with checking the open files. Checking dependencies in a single threaded environment is more likely
+/// to hurt performance because we end up analyzing files in their entirety, even if we only need to type check parts of them.
+#[derive(Debug, Default)]
+struct SingleThreadedExecutor;
+
+impl CheckExecutor for SingleThreadedExecutor {
+    fn run(self, context: &mut CheckContext) -> QueryResult<()> {
+        let mut queue = context.check_open_files();
+
+        let noop_schedule_dependency = |_| {};
+
+        while let Some(file) = queue.pop() {
+            context.program().cancelled()?;
+
+            let task_context = context.task_context(&noop_schedule_dependency);
+            context.push_diagnostics(&file.run(&task_context)?);
+        }
+
+        Ok(())
+    }
+}
+
+/// Executor that runs the check operations on a thread pool.
+///
+/// The executor runs each check operation as its own task using a thread pool.
+///
+/// Other than [`SingleThreadedExecutor`], this executor schedules dependencies for checking. It
+/// even schedules dependencies for checking when the thread pool size is 1 for a better debugging experience.
+#[derive(Debug, Default)]
+struct ThreadPoolExecutor;
+
+impl CheckExecutor for ThreadPoolExecutor {
+    fn run(self, context: &mut CheckContext) -> QueryResult<()> {
+        let num_threads = current_num_threads();
+        let single_threaded = num_threads == 1;
+        let span = tracing::trace_span!("ThreadPoolExecutor::run", num_threads);
+        let _ = span.enter();
+
+        let mut queue: Vec<_> = context
+            .check_open_files()
+            .into_iter()
+            .map(CheckFileTask::OpenFile)
+            .collect();
+
+        let (sender, receiver) = if single_threaded {
+            // Use an unbounded queue for single threaded execution to prevent deadlocks
+            // when a single file schedules multiple dependencies.
+            crossbeam::channel::unbounded()
+        } else {
+            // Use a bounded queue to apply backpressure when the orchestration thread isn't able to keep
+            // up processing messages from the worker threads.
+            crossbeam::channel::bounded(num_threads)
+        };
+
+        let schedule_sender = sender.clone();
+        let schedule_dependency = move |file_id| {
+            schedule_sender
+                .send(ThreadPoolMessage::ScheduleDependency(file_id))
+                .unwrap();
+        };
+
+        let result = rayon::in_place_scope(|scope| {
+            let mut pending = 0usize;
+
+            loop {
+                context.program().cancelled()?;
+
+                // 1. Try to get a queued message to ensure that we have always remaining space in the channel to prevent blocking the worker threads.
+                // 2. Try to process a queued file
+                // 3. If there's no queued file wait for the next incoming message.
+                // 4. Exit if there are no more messages and no senders.
+                let message = if let Ok(message) = receiver.try_recv() {
+                    message
+                } else if let Some(task) = queue.pop() {
+                    pending += 1;
+
+                    let task_context = context.task_context(&schedule_dependency);
+                    let sender = sender.clone();
+                    let task_span = tracing::trace_span!(
+                        parent: &span,
+                        "CheckFileTask::run",
+                        file_id = task.file_id().as_u32(),
+                    );
+
+                    scope.spawn(move |_| {
+                        task_span.in_scope(|| match task.run(&task_context) {
+                            Ok(result) => {
+                                sender.send(ThreadPoolMessage::Completed(result)).unwrap();
+                            }
+                            Err(err) => sender.send(ThreadPoolMessage::Errored(err)).unwrap(),
+                        });
+                    });
+
+                    // If this is a single threaded rayon thread pool, yield the current thread
+                    // or we never start processing the work items.
+                    if single_threaded {
+                        yield_local();
+                    }
+
+                    continue;
+                } else if let Ok(message) = receiver.recv() {
+                    message
+                } else {
+                    break;
+                };
+
+                match message {
+                    ThreadPoolMessage::ScheduleDependency(dependency) => {
+                        if let Some(task) = context.check_dependency(dependency) {
+                            queue.push(CheckFileTask::Dependency(task));
+                        }
+                    }
+                    ThreadPoolMessage::Completed(diagnostics) => {
+                        context.push_diagnostics(&diagnostics);
+                        pending -= 1;
+
+                        if pending == 0 && queue.is_empty() {
+                            break;
+                        }
+                    }
+                    ThreadPoolMessage::Errored(err) => {
+                        return Err(err);
+                    }
+                }
+            }
+
+            Ok(())
+        });
+
+        result
+    }
+}
+
+#[derive(Debug)]
+enum ThreadPoolMessage {
+    ScheduleDependency(FileId),
+    Completed(Diagnostics),
+    Errored(QueryError),
+}

crates/red_knot/src/program/mod.rs

@@ -0,0 +1,305 @@
+use std::collections::hash_map::Entry;
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+
+use rustc_hash::FxHashMap;
+
+use crate::db::{
+    Database, Db, DbRuntime, HasJar, HasJars, JarsStorage, LintDb, LintJar, ParallelDatabase,
+    QueryResult, SemanticDb, SemanticJar, Snapshot, SourceDb, SourceJar,
+};
+use crate::files::{FileId, Files};
+use crate::lint::{lint_semantic, lint_syntax, Diagnostics};
+use crate::module::{
+    add_module, file_to_module, path_to_module, resolve_module, set_module_search_paths, Module,
+    ModuleData, ModuleName, ModuleSearchPath,
+};
+use crate::parse::{parse, Parsed};
+use crate::source::{source_text, Source};
+use crate::symbols::{symbol_table, SymbolId, SymbolTable};
+use crate::types::{infer_symbol_type, Type};
+use crate::Workspace;
+
+pub mod check;
+
+#[derive(Debug)]
+pub struct Program {
+    jars: JarsStorage<Program>,
+    files: Files,
+    workspace: Workspace,
+}
+
+impl Program {
+    pub fn new(workspace: Workspace) -> Self {
+        Self {
+            jars: JarsStorage::default(),
+            files: Files::default(),
+            workspace,
+        }
+    }
+
+    pub fn apply_changes<I>(&mut self, changes: I)
+    where
+        I: IntoIterator<Item = FileWatcherChange>,
+    {
+        let mut aggregated_changes = AggregatedChanges::default();
+
+        aggregated_changes.extend(changes.into_iter().map(|change| FileChange {
+            id: self.files.intern(&change.path),
+            kind: change.kind,
+        }));
+
+        let (source, semantic, lint) = self.jars_mut();
+        for change in aggregated_changes.iter() {
+            semantic.module_resolver.remove_module(change.id);
+            semantic.symbol_tables.remove(&change.id);
+            source.sources.remove(&change.id);
+            source.parsed.remove(&change.id);
+            // TODO: remove all dependent modules as well
+            semantic.type_store.remove_module(change.id);
+            lint.lint_syntax.remove(&change.id);
+            lint.lint_semantic.remove(&change.id);
+        }
+    }
+
+    pub fn files(&self) -> &Files {
+        &self.files
+    }
+
+    pub fn workspace(&self) -> &Workspace {
+        &self.workspace
+    }
+
+    pub fn workspace_mut(&mut self) -> &mut Workspace {
+        &mut self.workspace
+    }
+}
+
+impl SourceDb for Program {
+    fn file_id(&self, path: &Path) -> FileId {
+        self.files.intern(path)
+    }
+
+    fn file_path(&self, file_id: FileId) -> Arc<Path> {
+        self.files.path(file_id)
+    }
+
+    fn source(&self, file_id: FileId) -> QueryResult<Source> {
+        source_text(self, file_id)
+    }
+
+    fn parse(&self, file_id: FileId) -> QueryResult<Parsed> {
+        parse(self, file_id)
+    }
+}
+
+impl SemanticDb for Program {
+    fn resolve_module(&self, name: ModuleName) -> QueryResult<Option<Module>> {
+        resolve_module(self, name)
+    }
+
+    fn file_to_module(&self, file_id: FileId) -> QueryResult<Option<Module>> {
+        file_to_module(self, file_id)
+    }
+
+    fn path_to_module(&self, path: &Path) -> QueryResult<Option<Module>> {
+        path_to_module(self, path)
+    }
+
+    fn symbol_table(&self, file_id: FileId) -> QueryResult<Arc<SymbolTable>> {
+        symbol_table(self, file_id)
+    }
+
+    fn infer_symbol_type(&self, file_id: FileId, symbol_id: SymbolId) -> QueryResult<Type> {
+        infer_symbol_type(self, file_id, symbol_id)
+    }
+
+    // Mutations
+    fn add_module(&mut self, path: &Path) -> Option<(Module, Vec<Arc<ModuleData>>)> {
+        add_module(self, path)
+    }
+
+    fn set_module_search_paths(&mut self, paths: Vec<ModuleSearchPath>) {
+        set_module_search_paths(self, paths);
+    }
+}
+
+impl LintDb for Program {
+    fn lint_syntax(&self, file_id: FileId) -> QueryResult<Diagnostics> {
+        lint_syntax(self, file_id)
+    }
+
+    fn lint_semantic(&self, file_id: FileId) -> QueryResult<Diagnostics> {
+        lint_semantic(self, file_id)
+    }
+}
+
+impl Db for Program {}
+
+impl Database for Program {
+    fn runtime(&self) -> &DbRuntime {
+        self.jars.runtime()
+    }
+
+    fn runtime_mut(&mut self) -> &mut DbRuntime {
+        self.jars.runtime_mut()
+    }
+}
+
+impl ParallelDatabase for Program {
+    fn snapshot(&self) -> Snapshot<Self> {
+        Snapshot::new(Self {
+            jars: self.jars.snapshot(),
+            files: self.files.snapshot(),
+            workspace: self.workspace.clone(),
+        })
+    }
+}
+
+impl HasJars for Program {
+    type Jars = (SourceJar, SemanticJar, LintJar);
+
+    fn jars(&self) -> QueryResult<&Self::Jars> {
+        self.jars.jars()
+    }
+
+    fn jars_mut(&mut self) -> &mut Self::Jars {
+        self.jars.jars_mut()
+    }
+}
+
+impl HasJar<SourceJar> for Program {
+    fn jar(&self) -> QueryResult<&SourceJar> {
+        Ok(&self.jars()?.0)
+    }
+
+    fn jar_mut(&mut self) -> &mut SourceJar {
+        &mut self.jars_mut().0
+    }
+}
+
+impl HasJar<SemanticJar> for Program {
+    fn jar(&self) -> QueryResult<&SemanticJar> {
+        Ok(&self.jars()?.1)
+    }
+
+    fn jar_mut(&mut self) -> &mut SemanticJar {
+        &mut self.jars_mut().1
+    }
+}
+
+impl HasJar<LintJar> for Program {
+    fn jar(&self) -> QueryResult<&LintJar> {
+        Ok(&self.jars()?.2)
+    }
+
+    fn jar_mut(&mut self) -> &mut LintJar {
+        &mut self.jars_mut().2
+    }
+}
+
+#[derive(Clone, Debug)]
+pub struct FileWatcherChange {
+    path: PathBuf,
+    kind: FileChangeKind,
+}
+
+impl FileWatcherChange {
+    pub fn new(path: PathBuf, kind: FileChangeKind) -> Self {
+        Self { path, kind }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+struct FileChange {
+    id: FileId,
+    kind: FileChangeKind,
+}
+
+impl FileChange {
+    fn file_id(self) -> FileId {
+        self.id
+    }
+
+    fn kind(self) -> FileChangeKind {
+        self.kind
+    }
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub enum FileChangeKind {
+    Created,
+    Modified,
+    Deleted,
+}
+
+#[derive(Default, Debug)]
+struct AggregatedChanges {
+    changes: FxHashMap<FileId, FileChangeKind>,
+}
+
+impl AggregatedChanges {
+    fn add(&mut self, change: FileChange) {
+        match self.changes.entry(change.file_id()) {
+            Entry::Occupied(mut entry) => {
+                let merged = entry.get_mut();
+
+                match (merged, change.kind()) {
+                    (FileChangeKind::Created, FileChangeKind::Deleted) => {
+                        // Deletion after creations means that ruff never saw the file.
+                        entry.remove();
+                    }
+                    (FileChangeKind::Created, FileChangeKind::Modified) => {
+                        // No-op, for ruff, modifying a file that it doesn't yet know that it exists is still considered a creation.
+                    }
+
+                    (FileChangeKind::Modified, FileChangeKind::Created) => {
+                        // Uhh, that should probably not happen. Continue considering it a modification.
+                    }
+
+                    (FileChangeKind::Modified, FileChangeKind::Deleted) => {
+                        *entry.get_mut() = FileChangeKind::Deleted;
+                    }
+
+                    (FileChangeKind::Deleted, FileChangeKind::Created) => {
+                        *entry.get_mut() = FileChangeKind::Modified;
+                    }
+
+                    (FileChangeKind::Deleted, FileChangeKind::Modified) => {
+                        // That's weird, but let's consider it a modification.
+                        *entry.get_mut() = FileChangeKind::Modified;
+                    }
+
+                    (FileChangeKind::Created, FileChangeKind::Created)
+                    | (FileChangeKind::Modified, FileChangeKind::Modified)
+                    | (FileChangeKind::Deleted, FileChangeKind::Deleted) => {
+                        // No-op transitions. Some of them should be impossible but we handle them anyway.
+                    }
+                }
+            }
+            Entry::Vacant(entry) => {
+                entry.insert(change.kind());
+            }
+        }
+    }
+
+    fn extend<I>(&mut self, changes: I)
+    where
+        I: IntoIterator<Item = FileChange>,
+    {
+        let iter = changes.into_iter();
+        let (lower, _) = iter.size_hint();
+        self.changes.reserve(lower);
+
+        for change in iter {
+            self.add(change);
+        }
+    }
+
+    fn iter(&self) -> impl Iterator<Item = FileChange> + '_ {
+        self.changes.iter().map(|(id, kind)| FileChange {
+            id: *id,
+            kind: *kind,
+        })
+    }
+}

crates/red_knot/src/source.rs

@@ -0,0 +1,96 @@
+use crate::cache::KeyValueCache;
+use crate::db::{HasJar, QueryResult, SourceDb, SourceJar};
+use ruff_notebook::Notebook;
+use ruff_python_ast::PySourceType;
+use std::ops::{Deref, DerefMut};
+use std::sync::Arc;
+
+use crate::files::FileId;
+
+#[tracing::instrument(level = "debug", skip(db))]
+pub(crate) fn source_text<Db>(db: &Db, file_id: FileId) -> QueryResult<Source>
+where
+    Db: SourceDb + HasJar<SourceJar>,
+{
+    let sources = &db.jar()?.sources;
+
+    sources.get(&file_id, |file_id| {
+        let path = db.file_path(*file_id);
+
+        let source_text = std::fs::read_to_string(&path).unwrap_or_else(|err| {
+            tracing::error!("Failed to read file '{path:?}: {err}'. Falling back to empty text");
+            String::new()
+        });
+
+        let python_ty = PySourceType::from(&path);
+
+        let kind = match python_ty {
+            PySourceType::Python => {
+                SourceKind::Python(Arc::from(source_text))
+            }
+            PySourceType::Stub => SourceKind::Stub(Arc::from(source_text)),
+            PySourceType::Ipynb => {
+                let notebook = Notebook::from_source_code(&source_text).unwrap_or_else(|err| {
+                    // TODO should this be changed to never fail?
+                    // or should we instead add a diagnostic somewhere? But what would we return in this case?
+                    tracing::error!(
+                        "Failed to parse notebook '{path:?}: {err}'. Falling back to an empty notebook"
+                    );
+                    Notebook::from_source_code("").unwrap()
+                });
+
+                SourceKind::IpyNotebook(Arc::new(notebook))
+            }
+        };
+
+        Ok(Source { kind })
+    })
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum SourceKind {
+    Python(Arc<str>),
+    Stub(Arc<str>),
+    IpyNotebook(Arc<Notebook>),
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub struct Source {
+    kind: SourceKind,
+}
+
+impl Source {
+    pub fn python<T: Into<Arc<str>>>(source: T) -> Self {
+        Self {
+            kind: SourceKind::Python(source.into()),
+        }
+    }
+    pub fn kind(&self) -> &SourceKind {
+        &self.kind
+    }
+
+    pub fn text(&self) -> &str {
+        match &self.kind {
+            SourceKind::Python(text) => text,
+            SourceKind::Stub(text) => text,
+            SourceKind::IpyNotebook(notebook) => notebook.source_code(),
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+pub struct SourceStorage(pub(crate) KeyValueCache<FileId, Source>);
+
+impl Deref for SourceStorage {
+    type Target = KeyValueCache<FileId, Source>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for SourceStorage {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}

crates/red_knot/src/symbols.rs

@@ -0,0 +1,956 @@
+#![allow(dead_code)]
+
+use std::hash::{Hash, Hasher};
+use std::iter::{Copied, DoubleEndedIterator, FusedIterator};
+use std::num::NonZeroU32;
+use std::ops::{Deref, DerefMut};
+use std::sync::Arc;
+
+use bitflags::bitflags;
+use hashbrown::hash_map::{Keys, RawEntryMut};
+use rustc_hash::{FxHashMap, FxHasher};
+
+use ruff_index::{newtype_index, IndexVec};
+use ruff_python_ast as ast;
+use ruff_python_ast::visitor::preorder::PreorderVisitor;
+
+use crate::ast_ids::TypedNodeKey;
+use crate::cache::KeyValueCache;
+use crate::db::{HasJar, QueryResult, SemanticDb, SemanticJar};
+use crate::files::FileId;
+use crate::module::ModuleName;
+use crate::Name;
+
+#[allow(unreachable_pub)]
+#[tracing::instrument(level = "debug", skip(db))]
+pub fn symbol_table<Db>(db: &Db, file_id: FileId) -> QueryResult<Arc<SymbolTable>>
+where
+    Db: SemanticDb + HasJar<SemanticJar>,
+{
+    let jar = db.jar()?;
+
+    jar.symbol_tables.get(&file_id, |_| {
+        let parsed = db.parse(file_id)?;
+        Ok(Arc::from(SymbolTable::from_ast(parsed.ast())))
+    })
+}
+
+type Map<K, V> = hashbrown::HashMap<K, V, ()>;
+
+#[newtype_index]
+pub(crate) struct ScopeId;
+
+impl ScopeId {
+    pub(crate) fn scope(self, table: &SymbolTable) -> &Scope {
+        &table.scopes_by_id[self]
+    }
+}
+
+#[newtype_index]
+pub struct SymbolId;
+
+impl SymbolId {
+    pub(crate) fn symbol(self, table: &SymbolTable) -> &Symbol {
+        &table.symbols_by_id[self]
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub(crate) enum ScopeKind {
+    Module,
+    Annotation,
+    Class,
+    Function,
+}
+
+#[derive(Debug)]
+pub(crate) struct Scope {
+    name: Name,
+    kind: ScopeKind,
+    child_scopes: Vec<ScopeId>,
+    /// symbol IDs, hashed by symbol name
+    symbols_by_name: Map<SymbolId, ()>,
+}
+
+impl Scope {
+    pub(crate) fn name(&self) -> &str {
+        self.name.as_str()
+    }
+
+    pub(crate) fn kind(&self) -> ScopeKind {
+        self.kind
+    }
+}
+
+#[derive(Debug)]
+pub(crate) enum Kind {
+    FreeVar,
+    CellVar,
+    CellVarAssigned,
+    ExplicitGlobal,
+    ImplicitGlobal,
+}
+
+bitflags! {
+    #[derive(Copy,Clone,Debug)]
+    pub(crate) struct SymbolFlags: u8 {
+        const IS_USED         = 1 << 0;
+        const IS_DEFINED      = 1 << 1;
+        /// TODO: This flag is not yet set by anything
+        const MARKED_GLOBAL   = 1 << 2;
+        /// TODO: This flag is not yet set by anything
+        const MARKED_NONLOCAL = 1 << 3;
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Symbol {
+    name: Name,
+    flags: SymbolFlags,
+    scope_id: ScopeId,
+    // kind: Kind,
+}
+
+impl Symbol {
+    pub(crate) fn name(&self) -> &str {
+        self.name.as_str()
+    }
+
+    /// Is the symbol used in its containing scope?
+    pub(crate) fn is_used(&self) -> bool {
+        self.flags.contains(SymbolFlags::IS_USED)
+    }
+
+    /// Is the symbol defined in its containing scope?
+    pub(crate) fn is_defined(&self) -> bool {
+        self.flags.contains(SymbolFlags::IS_DEFINED)
+    }
+
+    // TODO: implement Symbol.kind 2-pass analysis to categorize as: free-var, cell-var,
+    // explicit-global, implicit-global and implement Symbol.kind by modifying the preorder
+    // traversal code
+}
+
+// TODO storing TypedNodeKey for definitions means we have to search to find them again in the AST;
+// this is at best O(log n). If looking up definitions is a bottleneck we should look for
+// alternatives here.
+#[derive(Clone, Debug)]
+pub(crate) enum Definition {
+    // For the import cases, we don't need reference to any arbitrary AST subtrees (annotations,
+    // RHS), and referencing just the import statement node is imprecise (a single import statement
+    // can assign many symbols, we'd have to re-search for the one we care about), so we just copy
+    // the small amount of information we need from the AST.
+    Import(ImportDefinition),
+    ImportFrom(ImportFromDefinition),
+    ClassDef(ClassDefinition),
+    FunctionDef(TypedNodeKey<ast::StmtFunctionDef>),
+    Assignment(TypedNodeKey<ast::StmtAssign>),
+    AnnotatedAssignment(TypedNodeKey<ast::StmtAnnAssign>),
+    // TODO with statements, except handlers, function args...
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct ImportDefinition {
+    pub(crate) module: ModuleName,
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct ImportFromDefinition {
+    pub(crate) module: Option<ModuleName>,
+    pub(crate) name: Name,
+    pub(crate) level: u32,
+}
+
+impl ImportFromDefinition {
+    pub(crate) fn module(&self) -> Option<&ModuleName> {
+        self.module.as_ref()
+    }
+
+    pub(crate) fn name(&self) -> &Name {
+        &self.name
+    }
+
+    pub(crate) fn level(&self) -> u32 {
+        self.level
+    }
+}
+
+#[derive(Clone, Debug)]
+pub(crate) struct ClassDefinition {
+    pub(crate) node_key: TypedNodeKey<ast::StmtClassDef>,
+    pub(crate) scope_id: ScopeId,
+}
+
+#[derive(Debug, Clone)]
+pub enum Dependency {
+    Module(ModuleName),
+    Relative {
+        level: NonZeroU32,
+        module: Option<ModuleName>,
+    },
+}
+
+/// Table of all symbols in all scopes for a module.
+#[derive(Debug)]
+pub struct SymbolTable {
+    scopes_by_id: IndexVec<ScopeId, Scope>,
+    symbols_by_id: IndexVec<SymbolId, Symbol>,
+    defs: FxHashMap<SymbolId, Vec<Definition>>,
+    dependencies: Vec<Dependency>,
+}
+
+impl SymbolTable {
+    pub(crate) fn from_ast(module: &ast::ModModule) -> Self {
+        let root_scope_id = SymbolTable::root_scope_id();
+        let mut builder = SymbolTableBuilder {
+            table: SymbolTable::new(),
+            scopes: vec![root_scope_id],
+            current_definition: None,
+        };
+        builder.visit_body(&module.body);
+        builder.table
+    }
+
+    pub(crate) fn new() -> Self {
+        let mut table = SymbolTable {
+            scopes_by_id: IndexVec::new(),
+            symbols_by_id: IndexVec::new(),
+            defs: FxHashMap::default(),
+            dependencies: Vec::new(),
+        };
+        table.scopes_by_id.push(Scope {
+            name: Name::new("<module>"),
+            kind: ScopeKind::Module,
+            child_scopes: Vec::new(),
+            symbols_by_name: Map::default(),
+        });
+        table
+    }
+
+    pub(crate) fn dependencies(&self) -> &[Dependency] {
+        &self.dependencies
+    }
+
+    pub(crate) const fn root_scope_id() -> ScopeId {
+        ScopeId::from_usize(0)
+    }
+
+    pub(crate) fn root_scope(&self) -> &Scope {
+        &self.scopes_by_id[SymbolTable::root_scope_id()]
+    }
+
+    pub(crate) fn symbol_ids_for_scope(&self, scope_id: ScopeId) -> Copied<Keys<SymbolId, ()>> {
+        self.scopes_by_id[scope_id].symbols_by_name.keys().copied()
+    }
+
+    pub(crate) fn symbols_for_scope(
+        &self,
+        scope_id: ScopeId,
+    ) -> SymbolIterator<Copied<Keys<SymbolId, ()>>> {
+        SymbolIterator {
+            table: self,
+            ids: self.symbol_ids_for_scope(scope_id),
+        }
+    }
+
+    pub(crate) fn root_symbol_ids(&self) -> Copied<Keys<SymbolId, ()>> {
+        self.symbol_ids_for_scope(SymbolTable::root_scope_id())
+    }
+
+    pub(crate) fn root_symbols(&self) -> SymbolIterator<Copied<Keys<SymbolId, ()>>> {
+        self.symbols_for_scope(SymbolTable::root_scope_id())
+    }
+
+    pub(crate) fn child_scope_ids_of(&self, scope_id: ScopeId) -> &[ScopeId] {
+        &self.scopes_by_id[scope_id].child_scopes
+    }
+
+    pub(crate) fn child_scopes_of(&self, scope_id: ScopeId) -> ScopeIterator<&[ScopeId]> {
+        ScopeIterator {
+            table: self,
+            ids: self.child_scope_ids_of(scope_id),
+        }
+    }
+
+    pub(crate) fn root_child_scope_ids(&self) -> &[ScopeId] {
+        self.child_scope_ids_of(SymbolTable::root_scope_id())
+    }
+
+    pub(crate) fn root_child_scopes(&self) -> ScopeIterator<&[ScopeId]> {
+        self.child_scopes_of(SymbolTable::root_scope_id())
+    }
+
+    pub(crate) fn symbol_id_by_name(&self, scope_id: ScopeId, name: &str) -> Option<SymbolId> {
+        let scope = &self.scopes_by_id[scope_id];
+        let hash = SymbolTable::hash_name(name);
+        let name = Name::new(name);
+        scope
+            .symbols_by_name
+            .raw_entry()
+            .from_hash(hash, |symid| self.symbols_by_id[*symid].name == name)
+            .map(|(symbol_id, ())| *symbol_id)
+    }
+
+    pub(crate) fn symbol_by_name(&self, scope_id: ScopeId, name: &str) -> Option<&Symbol> {
+        Some(&self.symbols_by_id[self.symbol_id_by_name(scope_id, name)?])
+    }
+
+    pub(crate) fn root_symbol_id_by_name(&self, name: &str) -> Option<SymbolId> {
+        self.symbol_id_by_name(SymbolTable::root_scope_id(), name)
+    }
+
+    pub(crate) fn root_symbol_by_name(&self, name: &str) -> Option<&Symbol> {
+        self.symbol_by_name(SymbolTable::root_scope_id(), name)
+    }
+
+    pub(crate) fn definitions(&self, symbol_id: SymbolId) -> &[Definition] {
+        self.defs
+            .get(&symbol_id)
+            .map(std::vec::Vec::as_slice)
+            .unwrap_or_default()
+    }
+
+    pub(crate) fn all_definitions(&self) -> impl Iterator<Item = (SymbolId, &Definition)> + '_ {
+        self.defs
+            .iter()
+            .flat_map(|(sym_id, defs)| defs.iter().map(move |def| (*sym_id, def)))
+    }
+
+    fn add_or_update_symbol(
+        &mut self,
+        scope_id: ScopeId,
+        name: &str,
+        flags: SymbolFlags,
+    ) -> SymbolId {
+        let hash = SymbolTable::hash_name(name);
+        let scope = &mut self.scopes_by_id[scope_id];
+        let name = Name::new(name);
+
+        let entry = scope
+            .symbols_by_name
+            .raw_entry_mut()
+            .from_hash(hash, |existing| self.symbols_by_id[*existing].name == name);
+
+        match entry {
+            RawEntryMut::Occupied(entry) => {
+                if let Some(symbol) = self.symbols_by_id.get_mut(*entry.key()) {
+                    symbol.flags.insert(flags);
+                };
+                *entry.key()
+            }
+            RawEntryMut::Vacant(entry) => {
+                let id = self.symbols_by_id.push(Symbol {
+                    name,
+                    flags,
+                    scope_id,
+                });
+                entry.insert_with_hasher(hash, id, (), |_| hash);
+                id
+            }
+        }
+    }
+
+    fn add_child_scope(
+        &mut self,
+        parent_scope_id: ScopeId,
+        name: &str,
+        kind: ScopeKind,
+    ) -> ScopeId {
+        let new_scope_id = self.scopes_by_id.push(Scope {
+            name: Name::new(name),
+            kind,
+            child_scopes: Vec::new(),
+            symbols_by_name: Map::default(),
+        });
+        let parent_scope = &mut self.scopes_by_id[parent_scope_id];
+        parent_scope.child_scopes.push(new_scope_id);
+        new_scope_id
+    }
+
+    fn hash_name(name: &str) -> u64 {
+        let mut hasher = FxHasher::default();
+        name.hash(&mut hasher);
+        hasher.finish()
+    }
+}
+
+pub(crate) struct SymbolIterator<'a, I> {
+    table: &'a SymbolTable,
+    ids: I,
+}
+
+impl<'a, I> Iterator for SymbolIterator<'a, I>
+where
+    I: Iterator<Item = SymbolId>,
+{
+    type Item = &'a Symbol;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let id = self.ids.next()?;
+        Some(&self.table.symbols_by_id[id])
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.ids.size_hint()
+    }
+}
+
+impl<'a, I> FusedIterator for SymbolIterator<'a, I> where
+    I: Iterator<Item = SymbolId> + FusedIterator
+{
+}
+
+impl<'a, I> DoubleEndedIterator for SymbolIterator<'a, I>
+where
+    I: Iterator<Item = SymbolId> + DoubleEndedIterator,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        let id = self.ids.next_back()?;
+        Some(&self.table.symbols_by_id[id])
+    }
+}
+
+pub(crate) struct ScopeIterator<'a, I> {
+    table: &'a SymbolTable,
+    ids: I,
+}
+
+impl<'a, I> Iterator for ScopeIterator<'a, I>
+where
+    I: Iterator<Item = ScopeId>,
+{
+    type Item = &'a Scope;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let id = self.ids.next()?;
+        Some(&self.table.scopes_by_id[id])
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.ids.size_hint()
+    }
+}
+
+impl<'a, I> FusedIterator for ScopeIterator<'a, I> where I: Iterator<Item = ScopeId> + FusedIterator {}
+
+impl<'a, I> DoubleEndedIterator for ScopeIterator<'a, I>
+where
+    I: Iterator<Item = ScopeId> + DoubleEndedIterator,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        let id = self.ids.next_back()?;
+        Some(&self.table.scopes_by_id[id])
+    }
+}
+
+struct SymbolTableBuilder {
+    table: SymbolTable,
+    scopes: Vec<ScopeId>,
+    /// the definition whose target(s) we are currently walking
+    current_definition: Option<Definition>,
+}
+
+impl SymbolTableBuilder {
+    fn add_or_update_symbol(&mut self, identifier: &str, flags: SymbolFlags) -> SymbolId {
+        self.table
+            .add_or_update_symbol(self.cur_scope(), identifier, flags)
+    }
+
+    fn add_or_update_symbol_with_def(
+        &mut self,
+        identifier: &str,
+        definition: Definition,
+    ) -> SymbolId {
+        let symbol_id = self.add_or_update_symbol(identifier, SymbolFlags::IS_DEFINED);
+        self.table
+            .defs
+            .entry(symbol_id)
+            .or_default()
+            .push(definition);
+        symbol_id
+    }
+
+    fn push_scope(&mut self, name: &str, kind: ScopeKind) -> ScopeId {
+        let scope_id = self.table.add_child_scope(self.cur_scope(), name, kind);
+        self.scopes.push(scope_id);
+        scope_id
+    }
+
+    fn pop_scope(&mut self) -> ScopeId {
+        self.scopes
+            .pop()
+            .expect("Scope stack should never be empty")
+    }
+
+    fn cur_scope(&self) -> ScopeId {
+        *self
+            .scopes
+            .last()
+            .expect("Scope stack should never be empty")
+    }
+
+    fn with_type_params(
+        &mut self,
+        name: &str,
+        params: &Option<Box<ast::TypeParams>>,
+        nested: impl FnOnce(&mut Self) -> ScopeId,
+    ) -> ScopeId {
+        if let Some(type_params) = params {
+            self.push_scope(name, ScopeKind::Annotation);
+            for type_param in &type_params.type_params {
+                let name = match type_param {
+                    ast::TypeParam::TypeVar(ast::TypeParamTypeVar { name, .. }) => name,
+                    ast::TypeParam::ParamSpec(ast::TypeParamParamSpec { name, .. }) => name,
+                    ast::TypeParam::TypeVarTuple(ast::TypeParamTypeVarTuple { name, .. }) => name,
+                };
+                self.add_or_update_symbol(name, SymbolFlags::IS_DEFINED);
+            }
+        }
+        let scope_id = nested(self);
+        if params.is_some() {
+            self.pop_scope();
+        }
+        scope_id
+    }
+}
+
+impl PreorderVisitor<'_> for SymbolTableBuilder {
+    fn visit_expr(&mut self, expr: &ast::Expr) {
+        if let ast::Expr::Name(ast::ExprName { id, ctx, .. }) = expr {
+            let flags = match ctx {
+                ast::ExprContext::Load => SymbolFlags::IS_USED,
+                ast::ExprContext::Store => SymbolFlags::IS_DEFINED,
+                ast::ExprContext::Del => SymbolFlags::IS_DEFINED,
+                ast::ExprContext::Invalid => SymbolFlags::empty(),
+            };
+            self.add_or_update_symbol(id, flags);
+            if flags.contains(SymbolFlags::IS_DEFINED) {
+                if let Some(curdef) = self.current_definition.clone() {
+                    self.add_or_update_symbol_with_def(id, curdef);
+                }
+            }
+        }
+        ast::visitor::preorder::walk_expr(self, expr);
+    }
+
+    fn visit_stmt(&mut self, stmt: &ast::Stmt) {
+        // TODO need to capture more definition statements here
+        match stmt {
+            ast::Stmt::ClassDef(node) => {
+                let scope_id = self.with_type_params(&node.name, &node.type_params, |builder| {
+                    let scope_id = builder.push_scope(&node.name, ScopeKind::Class);
+                    ast::visitor::preorder::walk_stmt(builder, stmt);
+                    builder.pop_scope();
+                    scope_id
+                });
+                let def = Definition::ClassDef(ClassDefinition {
+                    node_key: TypedNodeKey::from_node(node),
+                    scope_id,
+                });
+                self.add_or_update_symbol_with_def(&node.name, def);
+            }
+            ast::Stmt::FunctionDef(node) => {
+                let def = Definition::FunctionDef(TypedNodeKey::from_node(node));
+                self.add_or_update_symbol_with_def(&node.name, def);
+                self.with_type_params(&node.name, &node.type_params, |builder| {
+                    let scope_id = builder.push_scope(&node.name, ScopeKind::Function);
+                    ast::visitor::preorder::walk_stmt(builder, stmt);
+                    builder.pop_scope();
+                    scope_id
+                });
+            }
+            ast::Stmt::Import(ast::StmtImport { names, .. }) => {
+                for alias in names {
+                    let symbol_name = if let Some(asname) = &alias.asname {
+                        asname.id.as_str()
+                    } else {
+                        alias.name.id.split('.').next().unwrap()
+                    };
+
+                    let module = ModuleName::new(&alias.name.id);
+
+                    let def = Definition::Import(ImportDefinition {
+                        module: module.clone(),
+                    });
+                    self.add_or_update_symbol_with_def(symbol_name, def);
+                    self.table.dependencies.push(Dependency::Module(module));
+                }
+            }
+            ast::Stmt::ImportFrom(ast::StmtImportFrom {
+                module,
+                names,
+                level,
+                ..
+            }) => {
+                let module = module.as_ref().map(|m| ModuleName::new(&m.id));
+
+                for alias in names {
+                    let symbol_name = if let Some(asname) = &alias.asname {
+                        asname.id.as_str()
+                    } else {
+                        alias.name.id.as_str()
+                    };
+                    let def = Definition::ImportFrom(ImportFromDefinition {
+                        module: module.clone(),
+                        name: Name::new(&alias.name.id),
+                        level: *level,
+                    });
+                    self.add_or_update_symbol_with_def(symbol_name, def);
+                }
+
+                let dependency = if let Some(module) = module {
+                    match NonZeroU32::new(*level) {
+                        Some(level) => Dependency::Relative {
+                            level,
+                            module: Some(module),
+                        },
+                        None => Dependency::Module(module),
+                    }
+                } else {
+                    Dependency::Relative {
+                        level: NonZeroU32::new(*level)
+                            .expect("Import without a module to have a level > 0"),
+                        module,
+                    }
+                };
+
+                self.table.dependencies.push(dependency);
+            }
+            ast::Stmt::Assign(node) => {
+                debug_assert!(self.current_definition.is_none());
+                self.current_definition =
+                    Some(Definition::Assignment(TypedNodeKey::from_node(node)));
+                ast::visitor::preorder::walk_stmt(self, stmt);
+                self.current_definition = None;
+            }
+            _ => {
+                ast::visitor::preorder::walk_stmt(self, stmt);
+            }
+        }
+    }
+}
+
+#[derive(Debug, Default)]
+pub struct SymbolTablesStorage(KeyValueCache<FileId, Arc<SymbolTable>>);
+
+impl Deref for SymbolTablesStorage {
+    type Target = KeyValueCache<FileId, Arc<SymbolTable>>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl DerefMut for SymbolTablesStorage {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use textwrap::dedent;
+
+    use crate::parse::Parsed;
+    use crate::symbols::ScopeKind;
+
+    use super::{SymbolFlags, SymbolId, SymbolIterator, SymbolTable};
+
+    mod from_ast {
+        use super::*;
+
+        fn parse(code: &str) -> Parsed {
+            Parsed::from_text(&dedent(code))
+        }
+
+        fn names<I>(it: SymbolIterator<I>) -> Vec<&str>
+        where
+            I: Iterator<Item = SymbolId>,
+        {
+            let mut symbols: Vec<_> = it.map(|sym| sym.name.as_str()).collect();
+            symbols.sort_unstable();
+            symbols
+        }
+
+        #[test]
+        fn empty() {
+            let parsed = parse("");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()).len(), 0);
+        }
+
+        #[test]
+        fn simple() {
+            let parsed = parse("x");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["x"]);
+            assert_eq!(
+                table
+                    .definitions(table.root_symbol_id_by_name("x").unwrap())
+                    .len(),
+                0
+            );
+        }
+
+        #[test]
+        fn annotation_only() {
+            let parsed = parse("x: int");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["int", "x"]);
+            // TODO record definition
+        }
+
+        #[test]
+        fn import() {
+            let parsed = parse("import foo");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["foo"]);
+            assert_eq!(
+                table
+                    .definitions(table.root_symbol_id_by_name("foo").unwrap())
+                    .len(),
+                1
+            );
+        }
+
+        #[test]
+        fn import_sub() {
+            let parsed = parse("import foo.bar");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["foo"]);
+        }
+
+        #[test]
+        fn import_as() {
+            let parsed = parse("import foo.bar as baz");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["baz"]);
+        }
+
+        #[test]
+        fn import_from() {
+            let parsed = parse("from bar import foo");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["foo"]);
+            assert_eq!(
+                table
+                    .definitions(table.root_symbol_id_by_name("foo").unwrap())
+                    .len(),
+                1
+            );
+            assert!(
+                table.root_symbol_id_by_name("foo").is_some_and(|sid| {
+                    let s = sid.symbol(&table);
+                    s.is_defined() || !s.is_used()
+                }),
+                "symbols that are defined get the defined flag"
+            );
+        }
+
+        #[test]
+        fn assign() {
+            let parsed = parse("x = foo");
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["foo", "x"]);
+            assert_eq!(
+                table
+                    .definitions(table.root_symbol_id_by_name("x").unwrap())
+                    .len(),
+                1
+            );
+            assert!(
+                table.root_symbol_id_by_name("foo").is_some_and(|sid| {
+                    let s = sid.symbol(&table);
+                    !s.is_defined() && s.is_used()
+                }),
+                "a symbol used but not defined in a scope should have only the used flag"
+            );
+        }
+
+        #[test]
+        fn class_scope() {
+            let parsed = parse(
+                "
+                class C:
+                    x = 1
+                y = 2
+                ",
+            );
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["C", "y"]);
+            let scopes = table.root_child_scope_ids();
+            assert_eq!(scopes.len(), 1);
+            let c_scope = scopes[0].scope(&table);
+            assert_eq!(c_scope.kind(), ScopeKind::Class);
+            assert_eq!(c_scope.name(), "C");
+            assert_eq!(names(table.symbols_for_scope(scopes[0])), vec!["x"]);
+            assert_eq!(
+                table
+                    .definitions(table.root_symbol_id_by_name("C").unwrap())
+                    .len(),
+                1
+            );
+        }
+
+        #[test]
+        fn func_scope() {
+            let parsed = parse(
+                "
+                def func():
+                    x = 1
+                y = 2
+                ",
+            );
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["func", "y"]);
+            let scopes = table.root_child_scope_ids();
+            assert_eq!(scopes.len(), 1);
+            let func_scope = scopes[0].scope(&table);
+            assert_eq!(func_scope.kind(), ScopeKind::Function);
+            assert_eq!(func_scope.name(), "func");
+            assert_eq!(names(table.symbols_for_scope(scopes[0])), vec!["x"]);
+            assert_eq!(
+                table
+                    .definitions(table.root_symbol_id_by_name("func").unwrap())
+                    .len(),
+                1
+            );
+        }
+
+        #[test]
+        fn dupes() {
+            let parsed = parse(
+                "
+                def func():
+                    x = 1
+                def func():
+                    y = 2
+                ",
+            );
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["func"]);
+            let scopes = table.root_child_scope_ids();
+            assert_eq!(scopes.len(), 2);
+            let func_scope_1 = scopes[0].scope(&table);
+            let func_scope_2 = scopes[1].scope(&table);
+            assert_eq!(func_scope_1.kind(), ScopeKind::Function);
+            assert_eq!(func_scope_1.name(), "func");
+            assert_eq!(func_scope_2.kind(), ScopeKind::Function);
+            assert_eq!(func_scope_2.name(), "func");
+            assert_eq!(names(table.symbols_for_scope(scopes[0])), vec!["x"]);
+            assert_eq!(names(table.symbols_for_scope(scopes[1])), vec!["y"]);
+            assert_eq!(
+                table
+                    .definitions(table.root_symbol_id_by_name("func").unwrap())
+                    .len(),
+                2
+            );
+        }
+
+        #[test]
+        fn generic_func() {
+            let parsed = parse(
+                "
+                def func[T]():
+                    x = 1
+                ",
+            );
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["func"]);
+            let scopes = table.root_child_scope_ids();
+            assert_eq!(scopes.len(), 1);
+            let ann_scope_id = scopes[0];
+            let ann_scope = ann_scope_id.scope(&table);
+            assert_eq!(ann_scope.kind(), ScopeKind::Annotation);
+            assert_eq!(ann_scope.name(), "func");
+            assert_eq!(names(table.symbols_for_scope(ann_scope_id)), vec!["T"]);
+            let scopes = table.child_scope_ids_of(ann_scope_id);
+            assert_eq!(scopes.len(), 1);
+            let func_scope_id = scopes[0];
+            let func_scope = func_scope_id.scope(&table);
+            assert_eq!(func_scope.kind(), ScopeKind::Function);
+            assert_eq!(func_scope.name(), "func");
+            assert_eq!(names(table.symbols_for_scope(func_scope_id)), vec!["x"]);
+        }
+
+        #[test]
+        fn generic_class() {
+            let parsed = parse(
+                "
+                class C[T]:
+                    x = 1
+                ",
+            );
+            let table = SymbolTable::from_ast(parsed.ast());
+            assert_eq!(names(table.root_symbols()), vec!["C"]);
+            let scopes = table.root_child_scope_ids();
+            assert_eq!(scopes.len(), 1);
+            let ann_scope_id = scopes[0];
+            let ann_scope = ann_scope_id.scope(&table);
+            assert_eq!(ann_scope.kind(), ScopeKind::Annotation);
+            assert_eq!(ann_scope.name(), "C");
+            assert_eq!(names(table.symbols_for_scope(ann_scope_id)), vec!["T"]);
+            assert!(
+                table
+                    .symbol_by_name(ann_scope_id, "T")
+                    .is_some_and(|s| s.is_defined() && !s.is_used()),
+                "type parameters are defined by the scope that introduces them"
+            );
+            let scopes = table.child_scope_ids_of(ann_scope_id);
+            assert_eq!(scopes.len(), 1);
+            let func_scope_id = scopes[0];
+            let func_scope = func_scope_id.scope(&table);
+            assert_eq!(func_scope.kind(), ScopeKind::Class);
+            assert_eq!(func_scope.name(), "C");
+            assert_eq!(names(table.symbols_for_scope(func_scope_id)), vec!["x"]);
+        }
+    }
+
+    #[test]
+    fn insert_same_name_symbol_twice() {
+        let mut table = SymbolTable::new();
+        let root_scope_id = SymbolTable::root_scope_id();
+        let symbol_id_1 = table.add_or_update_symbol(root_scope_id, "foo", SymbolFlags::IS_DEFINED);
+        let symbol_id_2 = table.add_or_update_symbol(root_scope_id, "foo", SymbolFlags::IS_USED);
+        assert_eq!(symbol_id_1, symbol_id_2);
+        assert!(symbol_id_1.symbol(&table).is_used(), "flags must merge");
+        assert!(symbol_id_1.symbol(&table).is_defined(), "flags must merge");
+    }
+
+    #[test]
+    fn insert_different_named_symbols() {
+        let mut table = SymbolTable::new();
+        let root_scope_id = SymbolTable::root_scope_id();
+        let symbol_id_1 = table.add_or_update_symbol(root_scope_id, "foo", SymbolFlags::empty());
+        let symbol_id_2 = table.add_or_update_symbol(root_scope_id, "bar", SymbolFlags::empty());
+        assert_ne!(symbol_id_1, symbol_id_2);
+    }
+
+    #[test]
+    fn add_child_scope_with_symbol() {
+        let mut table = SymbolTable::new();
+        let root_scope_id = SymbolTable::root_scope_id();
+        let foo_symbol_top = table.add_or_update_symbol(root_scope_id, "foo", SymbolFlags::empty());
+        let c_scope = table.add_child_scope(root_scope_id, "C", ScopeKind::Class);
+        let foo_symbol_inner = table.add_or_update_symbol(c_scope, "foo", SymbolFlags::empty());
+        assert_ne!(foo_symbol_top, foo_symbol_inner);
+    }
+
+    #[test]
+    fn scope_from_id() {
+        let table = SymbolTable::new();
+        let root_scope_id = SymbolTable::root_scope_id();
+        let scope = root_scope_id.scope(&table);
+        assert_eq!(scope.name.as_str(), "<module>");
+        assert_eq!(scope.kind, ScopeKind::Module);
+    }
+
+    #[test]
+    fn symbol_from_id() {
+        let mut table = SymbolTable::new();
+        let root_scope_id = SymbolTable::root_scope_id();
+        let foo_symbol_id = table.add_or_update_symbol(root_scope_id, "foo", SymbolFlags::empty());
+        let symbol = foo_symbol_id.symbol(&table);
+        assert_eq!(symbol.name.as_str(), "foo");
+    }
+}

crates/red_knot/src/types.rs

@@ -0,0 +1,600 @@
+#![allow(dead_code)]
+use crate::ast_ids::NodeKey;
+use crate::db::{HasJar, QueryResult, SemanticDb, SemanticJar};
+use crate::files::FileId;
+use crate::symbols::{ScopeId, SymbolId};
+use crate::{FxDashMap, FxIndexSet, Name};
+use ruff_index::{newtype_index, IndexVec};
+use rustc_hash::FxHashMap;
+
+pub(crate) mod infer;
+
+pub(crate) use infer::infer_symbol_type;
+
+/// unique ID for a type
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub enum Type {
+    /// the dynamic or gradual type: a statically-unknown set of values
+    Any,
+    /// the empty set of values
+    Never,
+    /// unknown type (no annotation)
+    /// equivalent to Any, or to object in strict mode
+    Unknown,
+    /// name is not bound to any value
+    Unbound,
+    /// a specific function object
+    Function(FunctionTypeId),
+    /// a specific class object
+    Class(ClassTypeId),
+    /// the set of Python objects with the given class in their __class__'s method resolution order
+    Instance(ClassTypeId),
+    Union(UnionTypeId),
+    Intersection(IntersectionTypeId),
+    // TODO protocols, callable types, overloads, generics, type vars
+}
+
+impl Type {
+    fn display<'a>(&'a self, store: &'a TypeStore) -> DisplayType<'a> {
+        DisplayType { ty: self, store }
+    }
+
+    pub const fn is_unbound(&self) -> bool {
+        matches!(self, Type::Unbound)
+    }
+
+    pub const fn is_unknown(&self) -> bool {
+        matches!(self, Type::Unknown)
+    }
+}
+
+impl From<FunctionTypeId> for Type {
+    fn from(id: FunctionTypeId) -> Self {
+        Type::Function(id)
+    }
+}
+
+impl From<UnionTypeId> for Type {
+    fn from(id: UnionTypeId) -> Self {
+        Type::Union(id)
+    }
+}
+
+impl From<IntersectionTypeId> for Type {
+    fn from(id: IntersectionTypeId) -> Self {
+        Type::Intersection(id)
+    }
+}
+
+// TODO: currently calling `get_function` et al and holding on to the `FunctionTypeRef` will lock a
+// shard of this dashmap, for as long as you hold the reference. This may be a problem. We could
+// switch to having all the arenas hold Arc, or we could see if we can split up ModuleTypeStore,
+// and/or give it inner mutability and finer-grained internal locking.
+#[derive(Debug, Default)]
+pub struct TypeStore {
+    modules: FxDashMap<FileId, ModuleTypeStore>,
+}
+
+impl TypeStore {
+    pub fn remove_module(&mut self, file_id: FileId) {
+        self.modules.remove(&file_id);
+    }
+
+    pub fn cache_symbol_type(&self, file_id: FileId, symbol_id: SymbolId, ty: Type) {
+        self.add_or_get_module(file_id)
+            .symbol_types
+            .insert(symbol_id, ty);
+    }
+
+    pub fn cache_node_type(&self, file_id: FileId, node_key: NodeKey, ty: Type) {
+        self.add_or_get_module(file_id)
+            .node_types
+            .insert(node_key, ty);
+    }
+
+    pub fn get_cached_symbol_type(&self, file_id: FileId, symbol_id: SymbolId) -> Option<Type> {
+        self.try_get_module(file_id)?
+            .symbol_types
+            .get(&symbol_id)
+            .copied()
+    }
+
+    pub fn get_cached_node_type(&self, file_id: FileId, node_key: &NodeKey) -> Option<Type> {
+        self.try_get_module(file_id)?
+            .node_types
+            .get(node_key)
+            .copied()
+    }
+
+    fn add_or_get_module(&self, file_id: FileId) -> ModuleStoreRefMut {
+        self.modules
+            .entry(file_id)
+            .or_insert_with(|| ModuleTypeStore::new(file_id))
+    }
+
+    fn get_module(&self, file_id: FileId) -> ModuleStoreRef {
+        self.try_get_module(file_id).expect("module should exist")
+    }
+
+    fn try_get_module(&self, file_id: FileId) -> Option<ModuleStoreRef> {
+        self.modules.get(&file_id)
+    }
+
+    fn add_function(&self, file_id: FileId, name: &str, decorators: Vec<Type>) -> FunctionTypeId {
+        self.add_or_get_module(file_id)
+            .add_function(name, decorators)
+    }
+
+    fn add_class(
+        &self,
+        file_id: FileId,
+        name: &str,
+        scope_id: ScopeId,
+        bases: Vec<Type>,
+    ) -> ClassTypeId {
+        self.add_or_get_module(file_id)
+            .add_class(name, scope_id, bases)
+    }
+
+    fn add_union(&mut self, file_id: FileId, elems: &[Type]) -> UnionTypeId {
+        self.add_or_get_module(file_id).add_union(elems)
+    }
+
+    fn add_intersection(
+        &mut self,
+        file_id: FileId,
+        positive: &[Type],
+        negative: &[Type],
+    ) -> IntersectionTypeId {
+        self.add_or_get_module(file_id)
+            .add_intersection(positive, negative)
+    }
+
+    fn get_function(&self, id: FunctionTypeId) -> FunctionTypeRef {
+        FunctionTypeRef {
+            module_store: self.get_module(id.file_id),
+            function_id: id.func_id,
+        }
+    }
+
+    fn get_class(&self, id: ClassTypeId) -> ClassTypeRef {
+        ClassTypeRef {
+            module_store: self.get_module(id.file_id),
+            class_id: id.class_id,
+        }
+    }
+
+    fn get_union(&self, id: UnionTypeId) -> UnionTypeRef {
+        UnionTypeRef {
+            module_store: self.get_module(id.file_id),
+            union_id: id.union_id,
+        }
+    }
+
+    fn get_intersection(&self, id: IntersectionTypeId) -> IntersectionTypeRef {
+        IntersectionTypeRef {
+            module_store: self.get_module(id.file_id),
+            intersection_id: id.intersection_id,
+        }
+    }
+}
+
+type ModuleStoreRef<'a> = dashmap::mapref::one::Ref<
+    'a,
+    FileId,
+    ModuleTypeStore,
+    std::hash::BuildHasherDefault<rustc_hash::FxHasher>,
+>;
+
+type ModuleStoreRefMut<'a> = dashmap::mapref::one::RefMut<
+    'a,
+    FileId,
+    ModuleTypeStore,
+    std::hash::BuildHasherDefault<rustc_hash::FxHasher>,
+>;
+
+#[derive(Debug)]
+pub(crate) struct FunctionTypeRef<'a> {
+    module_store: ModuleStoreRef<'a>,
+    function_id: ModuleFunctionTypeId,
+}
+
+impl<'a> std::ops::Deref for FunctionTypeRef<'a> {
+    type Target = FunctionType;
+
+    fn deref(&self) -> &Self::Target {
+        self.module_store.get_function(self.function_id)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct ClassTypeRef<'a> {
+    module_store: ModuleStoreRef<'a>,
+    class_id: ModuleClassTypeId,
+}
+
+impl<'a> std::ops::Deref for ClassTypeRef<'a> {
+    type Target = ClassType;
+
+    fn deref(&self) -> &Self::Target {
+        self.module_store.get_class(self.class_id)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct UnionTypeRef<'a> {
+    module_store: ModuleStoreRef<'a>,
+    union_id: ModuleUnionTypeId,
+}
+
+impl<'a> std::ops::Deref for UnionTypeRef<'a> {
+    type Target = UnionType;
+
+    fn deref(&self) -> &Self::Target {
+        self.module_store.get_union(self.union_id)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct IntersectionTypeRef<'a> {
+    module_store: ModuleStoreRef<'a>,
+    intersection_id: ModuleIntersectionTypeId,
+}
+
+impl<'a> std::ops::Deref for IntersectionTypeRef<'a> {
+    type Target = IntersectionType;
+
+    fn deref(&self) -> &Self::Target {
+        self.module_store.get_intersection(self.intersection_id)
+    }
+}
+
+#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
+pub struct FunctionTypeId {
+    file_id: FileId,
+    func_id: ModuleFunctionTypeId,
+}
+
+#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
+pub struct ClassTypeId {
+    file_id: FileId,
+    class_id: ModuleClassTypeId,
+}
+
+impl ClassTypeId {
+    fn get_own_class_member<Db>(self, db: &Db, name: &Name) -> QueryResult<Option<Type>>
+    where
+        Db: SemanticDb + HasJar<SemanticJar>,
+    {
+        // TODO: this should distinguish instance-only members (e.g. `x: int`) and not return them
+        let ClassType { scope_id, .. } = *db.jar()?.type_store.get_class(self);
+        let table = db.symbol_table(self.file_id)?;
+        if let Some(symbol_id) = table.symbol_id_by_name(scope_id, name) {
+            Ok(Some(db.infer_symbol_type(self.file_id, symbol_id)?))
+        } else {
+            Ok(None)
+        }
+    }
+
+    // TODO: get_own_instance_member, get_class_member, get_instance_member
+}
+
+#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
+pub struct UnionTypeId {
+    file_id: FileId,
+    union_id: ModuleUnionTypeId,
+}
+
+#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq)]
+pub struct IntersectionTypeId {
+    file_id: FileId,
+    intersection_id: ModuleIntersectionTypeId,
+}
+
+#[newtype_index]
+struct ModuleFunctionTypeId;
+
+#[newtype_index]
+struct ModuleClassTypeId;
+
+#[newtype_index]
+struct ModuleUnionTypeId;
+
+#[newtype_index]
+struct ModuleIntersectionTypeId;
+
+#[derive(Debug)]
+struct ModuleTypeStore {
+    file_id: FileId,
+    /// arena of all function types defined in this module
+    functions: IndexVec<ModuleFunctionTypeId, FunctionType>,
+    /// arena of all class types defined in this module
+    classes: IndexVec<ModuleClassTypeId, ClassType>,
+    /// arenda of all union types created in this module
+    unions: IndexVec<ModuleUnionTypeId, UnionType>,
+    /// arena of all intersection types created in this module
+    intersections: IndexVec<ModuleIntersectionTypeId, IntersectionType>,
+    /// cached types of symbols in this module
+    symbol_types: FxHashMap<SymbolId, Type>,
+    /// cached types of AST nodes in this module
+    node_types: FxHashMap<NodeKey, Type>,
+}
+
+impl ModuleTypeStore {
+    fn new(file_id: FileId) -> Self {
+        Self {
+            file_id,
+            functions: IndexVec::default(),
+            classes: IndexVec::default(),
+            unions: IndexVec::default(),
+            intersections: IndexVec::default(),
+            symbol_types: FxHashMap::default(),
+            node_types: FxHashMap::default(),
+        }
+    }
+
+    fn add_function(&mut self, name: &str, decorators: Vec<Type>) -> FunctionTypeId {
+        let func_id = self.functions.push(FunctionType {
+            name: Name::new(name),
+            decorators,
+        });
+        FunctionTypeId {
+            file_id: self.file_id,
+            func_id,
+        }
+    }
+
+    fn add_class(&mut self, name: &str, scope_id: ScopeId, bases: Vec<Type>) -> ClassTypeId {
+        let class_id = self.classes.push(ClassType {
+            name: Name::new(name),
+            scope_id,
+            // TODO: if no bases are given, that should imply [object]
+            bases,
+        });
+        ClassTypeId {
+            file_id: self.file_id,
+            class_id,
+        }
+    }
+
+    fn add_union(&mut self, elems: &[Type]) -> UnionTypeId {
+        let union_id = self.unions.push(UnionType {
+            elements: elems.iter().copied().collect(),
+        });
+        UnionTypeId {
+            file_id: self.file_id,
+            union_id,
+        }
+    }
+
+    fn add_intersection(&mut self, positive: &[Type], negative: &[Type]) -> IntersectionTypeId {
+        let intersection_id = self.intersections.push(IntersectionType {
+            positive: positive.iter().copied().collect(),
+            negative: negative.iter().copied().collect(),
+        });
+        IntersectionTypeId {
+            file_id: self.file_id,
+            intersection_id,
+        }
+    }
+
+    fn get_function(&self, func_id: ModuleFunctionTypeId) -> &FunctionType {
+        &self.functions[func_id]
+    }
+
+    fn get_class(&self, class_id: ModuleClassTypeId) -> &ClassType {
+        &self.classes[class_id]
+    }
+
+    fn get_union(&self, union_id: ModuleUnionTypeId) -> &UnionType {
+        &self.unions[union_id]
+    }
+
+    fn get_intersection(&self, intersection_id: ModuleIntersectionTypeId) -> &IntersectionType {
+        &self.intersections[intersection_id]
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+struct DisplayType<'a> {
+    ty: &'a Type,
+    store: &'a TypeStore,
+}
+
+impl std::fmt::Display for DisplayType<'_> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self.ty {
+            Type::Any => f.write_str("Any"),
+            Type::Never => f.write_str("Never"),
+            Type::Unknown => f.write_str("Unknown"),
+            Type::Unbound => f.write_str("Unbound"),
+            // TODO functions and classes should display using a fully qualified name
+            Type::Class(class_id) => {
+                f.write_str("Literal[")?;
+                f.write_str(self.store.get_class(*class_id).name())?;
+                f.write_str("]")
+            }
+            Type::Instance(class_id) => f.write_str(self.store.get_class(*class_id).name()),
+            Type::Function(func_id) => f.write_str(self.store.get_function(*func_id).name()),
+            Type::Union(union_id) => self
+                .store
+                .get_module(union_id.file_id)
+                .get_union(union_id.union_id)
+                .display(f, self.store),
+            Type::Intersection(int_id) => self
+                .store
+                .get_module(int_id.file_id)
+                .get_intersection(int_id.intersection_id)
+                .display(f, self.store),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct ClassType {
+    /// Name of the class at definition
+    name: Name,
+    /// `ScopeId` of the class body
+    pub(crate) scope_id: ScopeId,
+    /// Types of all class bases
+    bases: Vec<Type>,
+}
+
+impl ClassType {
+    fn name(&self) -> &str {
+        self.name.as_str()
+    }
+
+    fn bases(&self) -> &[Type] {
+        self.bases.as_slice()
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct FunctionType {
+    name: Name,
+    decorators: Vec<Type>,
+}
+
+impl FunctionType {
+    fn name(&self) -> &str {
+        self.name.as_str()
+    }
+
+    fn decorators(&self) -> &[Type] {
+        self.decorators.as_slice()
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct UnionType {
+    // the union type includes values in any of these types
+    elements: FxIndexSet<Type>,
+}
+
+impl UnionType {
+    fn display(&self, f: &mut std::fmt::Formatter<'_>, store: &TypeStore) -> std::fmt::Result {
+        f.write_str("(")?;
+        let mut first = true;
+        for ty in &self.elements {
+            if !first {
+                f.write_str(" | ")?;
+            };
+            first = false;
+            write!(f, "{}", ty.display(store))?;
+        }
+        f.write_str(")")
+    }
+}
+
+// Negation types aren't expressible in annotations, and are most likely to arise from type
+// narrowing along with intersections (e.g. `if not isinstance(...)`), so we represent them
+// directly in intersections rather than as a separate type. This sacrifices some efficiency in the
+// case where a Not appears outside an intersection (unclear when that could even happen, but we'd
+// have to represent it as a single-element intersection if it did) in exchange for better
+// efficiency in the not-within-intersection case.
+#[derive(Debug)]
+pub(crate) struct IntersectionType {
+    // the intersection type includes only values in all of these types
+    positive: FxIndexSet<Type>,
+    // negated elements of the intersection, e.g.
+    negative: FxIndexSet<Type>,
+}
+
+impl IntersectionType {
+    fn display(&self, f: &mut std::fmt::Formatter<'_>, store: &TypeStore) -> std::fmt::Result {
+        f.write_str("(")?;
+        let mut first = true;
+        for (neg, ty) in self
+            .positive
+            .iter()
+            .map(|ty| (false, ty))
+            .chain(self.negative.iter().map(|ty| (true, ty)))
+        {
+            if !first {
+                f.write_str(" & ")?;
+            };
+            first = false;
+            if neg {
+                f.write_str("~")?;
+            };
+            write!(f, "{}", ty.display(store))?;
+        }
+        f.write_str(")")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::files::Files;
+    use crate::symbols::SymbolTable;
+    use crate::types::{Type, TypeStore};
+    use crate::FxIndexSet;
+    use std::path::Path;
+
+    #[test]
+    fn add_class() {
+        let store = TypeStore::default();
+        let files = Files::default();
+        let file_id = files.intern(Path::new("/foo"));
+        let id = store.add_class(file_id, "C", SymbolTable::root_scope_id(), Vec::new());
+        assert_eq!(store.get_class(id).name(), "C");
+        let inst = Type::Instance(id);
+        assert_eq!(format!("{}", inst.display(&store)), "C");
+    }
+
+    #[test]
+    fn add_function() {
+        let store = TypeStore::default();
+        let files = Files::default();
+        let file_id = files.intern(Path::new("/foo"));
+        let id = store.add_function(file_id, "func", vec![Type::Unknown]);
+        assert_eq!(store.get_function(id).name(), "func");
+        assert_eq!(store.get_function(id).decorators(), vec![Type::Unknown]);
+        let func = Type::Function(id);
+        assert_eq!(format!("{}", func.display(&store)), "func");
+    }
+
+    #[test]
+    fn add_union() {
+        let mut store = TypeStore::default();
+        let files = Files::default();
+        let file_id = files.intern(Path::new("/foo"));
+        let c1 = store.add_class(file_id, "C1", SymbolTable::root_scope_id(), Vec::new());
+        let c2 = store.add_class(file_id, "C2", SymbolTable::root_scope_id(), Vec::new());
+        let elems = vec![Type::Instance(c1), Type::Instance(c2)];
+        let id = store.add_union(file_id, &elems);
+        assert_eq!(
+            store.get_union(id).elements,
+            elems.into_iter().collect::<FxIndexSet<_>>()
+        );
+        let union = Type::Union(id);
+        assert_eq!(format!("{}", union.display(&store)), "(C1 | C2)");
+    }
+
+    #[test]
+    fn add_intersection() {
+        let mut store = TypeStore::default();
+        let files = Files::default();
+        let file_id = files.intern(Path::new("/foo"));
+        let c1 = store.add_class(file_id, "C1", SymbolTable::root_scope_id(), Vec::new());
+        let c2 = store.add_class(file_id, "C2", SymbolTable::root_scope_id(), Vec::new());
+        let c3 = store.add_class(file_id, "C3", SymbolTable::root_scope_id(), Vec::new());
+        let pos = vec![Type::Instance(c1), Type::Instance(c2)];
+        let neg = vec![Type::Instance(c3)];
+        let id = store.add_intersection(file_id, &pos, &neg);
+        assert_eq!(
+            store.get_intersection(id).positive,
+            pos.into_iter().collect::<FxIndexSet<_>>()
+        );
+        assert_eq!(
+            store.get_intersection(id).negative,
+            neg.into_iter().collect::<FxIndexSet<_>>()
+        );
+        let intersection = Type::Intersection(id);
+        assert_eq!(
+            format!("{}", intersection.display(&store)),
+            "(C1 & C2 & ~C3)"
+        );
+    }
+}

crates/red_knot/src/types/infer.rs

@@ -0,0 +1,265 @@
+#![allow(dead_code)]
+
+use ruff_python_ast::AstNode;
+
+use crate::db::{HasJar, QueryResult, SemanticDb, SemanticJar};
+use crate::module::ModuleName;
+use crate::symbols::{ClassDefinition, Definition, ImportFromDefinition, SymbolId};
+use crate::types::Type;
+use crate::FileId;
+use ruff_python_ast as ast;
+
+// FIXME: Figure out proper dead-lock free synchronisation now that this takes `&db` instead of `&mut db`.
+#[tracing::instrument(level = "trace", skip(db))]
+pub fn infer_symbol_type<Db>(db: &Db, file_id: FileId, symbol_id: SymbolId) -> QueryResult<Type>
+where
+    Db: SemanticDb + HasJar<SemanticJar>,
+{
+    let symbols = db.symbol_table(file_id)?;
+    let defs = symbols.definitions(symbol_id);
+
+    if let Some(ty) = db
+        .jar()?
+        .type_store
+        .get_cached_symbol_type(file_id, symbol_id)
+    {
+        return Ok(ty);
+    }
+
+    // TODO handle multiple defs, conditional defs...
+    assert_eq!(defs.len(), 1);
+    let type_store = &db.jar()?.type_store;
+
+    let ty = match &defs[0] {
+        Definition::ImportFrom(ImportFromDefinition {
+            module,
+            name,
+            level,
+        }) => {
+            // TODO relative imports
+            assert!(matches!(level, 0));
+            let module_name = ModuleName::new(module.as_ref().expect("TODO relative imports"));
+            if let Some(module) = db.resolve_module(module_name)? {
+                let remote_file_id = module.path(db)?.file();
+                let remote_symbols = db.symbol_table(remote_file_id)?;
+                if let Some(remote_symbol_id) = remote_symbols.root_symbol_id_by_name(name) {
+                    db.infer_symbol_type(remote_file_id, remote_symbol_id)?
+                } else {
+                    Type::Unknown
+                }
+            } else {
+                Type::Unknown
+            }
+        }
+        Definition::ClassDef(ClassDefinition { node_key, scope_id }) => {
+            if let Some(ty) = type_store.get_cached_node_type(file_id, node_key.erased()) {
+                ty
+            } else {
+                let parsed = db.parse(file_id)?;
+                let ast = parsed.ast();
+                let node = node_key.resolve_unwrap(ast.as_any_node_ref());
+
+                let mut bases = Vec::with_capacity(node.bases().len());
+
+                for base in node.bases() {
+                    bases.push(infer_expr_type(db, file_id, base)?);
+                }
+
+                let ty =
+                    Type::Class(type_store.add_class(file_id, &node.name.id, *scope_id, bases));
+                type_store.cache_node_type(file_id, *node_key.erased(), ty);
+                ty
+            }
+        }
+        Definition::FunctionDef(node_key) => {
+            if let Some(ty) = type_store.get_cached_node_type(file_id, node_key.erased()) {
+                ty
+            } else {
+                let parsed = db.parse(file_id)?;
+                let ast = parsed.ast();
+                let node = node_key
+                    .resolve(ast.as_any_node_ref())
+                    .expect("node key should resolve");
+
+                let decorator_tys = node
+                    .decorator_list
+                    .iter()
+                    .map(|decorator| infer_expr_type(db, file_id, &decorator.expression))
+                    .collect::<QueryResult<_>>()?;
+
+                let ty = type_store
+                    .add_function(file_id, &node.name.id, decorator_tys)
+                    .into();
+                type_store.cache_node_type(file_id, *node_key.erased(), ty);
+                ty
+            }
+        }
+        Definition::Assignment(node_key) => {
+            let parsed = db.parse(file_id)?;
+            let ast = parsed.ast();
+            let node = node_key.resolve_unwrap(ast.as_any_node_ref());
+            // TODO handle unpacking assignment correctly
+            infer_expr_type(db, file_id, &node.value)?
+        }
+        _ => todo!("other kinds of definitions"),
+    };
+
+    type_store.cache_symbol_type(file_id, symbol_id, ty);
+
+    // TODO record dependencies
+    Ok(ty)
+}
+
+fn infer_expr_type<Db>(db: &Db, file_id: FileId, expr: &ast::Expr) -> QueryResult<Type>
+where
+    Db: SemanticDb + HasJar<SemanticJar>,
+{
+    // TODO cache the resolution of the type on the node
+    let symbols = db.symbol_table(file_id)?;
+    match expr {
+        ast::Expr::Name(name) => {
+            if let Some(symbol_id) = symbols.root_symbol_id_by_name(&name.id) {
+                db.infer_symbol_type(file_id, symbol_id)
+            } else {
+                Ok(Type::Unknown)
+            }
+        }
+        _ => todo!("full expression type resolution"),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::db::tests::TestDb;
+    use crate::db::{HasJar, SemanticDb, SemanticJar};
+    use crate::module::{ModuleName, ModuleSearchPath, ModuleSearchPathKind};
+    use crate::types::Type;
+    use crate::Name;
+
+    // TODO with virtual filesystem we shouldn't have to write files to disk for these
+    // tests
+
+    struct TestCase {
+        temp_dir: tempfile::TempDir,
+        db: TestDb,
+
+        src: ModuleSearchPath,
+    }
+
+    fn create_test() -> std::io::Result<TestCase> {
+        let temp_dir = tempfile::tempdir()?;
+
+        let src = temp_dir.path().join("src");
+        std::fs::create_dir(&src)?;
+        let src = ModuleSearchPath::new(src.canonicalize()?, ModuleSearchPathKind::FirstParty);
+
+        let roots = vec![src.clone()];
+
+        let mut db = TestDb::default();
+        db.set_module_search_paths(roots);
+
+        Ok(TestCase { temp_dir, db, src })
+    }
+
+    #[test]
+    fn follow_import_to_class() -> anyhow::Result<()> {
+        let case = create_test()?;
+        let db = &case.db;
+
+        let a_path = case.src.path().join("a.py");
+        let b_path = case.src.path().join("b.py");
+        std::fs::write(a_path, "from b import C as D; E = D")?;
+        std::fs::write(b_path, "class C: pass")?;
+        let a_file = db
+            .resolve_module(ModuleName::new("a"))?
+            .expect("module should be found")
+            .path(db)?
+            .file();
+        let a_syms = db.symbol_table(a_file)?;
+        let e_sym = a_syms
+            .root_symbol_id_by_name("E")
+            .expect("E symbol should be found");
+
+        let ty = db.infer_symbol_type(a_file, e_sym)?;
+
+        let jar = HasJar::<SemanticJar>::jar(db)?;
+        assert!(matches!(ty, Type::Class(_)));
+        assert_eq!(format!("{}", ty.display(&jar.type_store)), "Literal[C]");
+
+        Ok(())
+    }
+
+    #[test]
+    fn resolve_base_class_by_name() -> anyhow::Result<()> {
+        let case = create_test()?;
+        let db = &case.db;
+
+        let path = case.src.path().join("mod.py");
+        std::fs::write(path, "class Base: pass\nclass Sub(Base): pass")?;
+        let file = db
+            .resolve_module(ModuleName::new("mod"))?
+            .expect("module should be found")
+            .path(db)?
+            .file();
+        let syms = db.symbol_table(file)?;
+        let sym = syms
+            .root_symbol_id_by_name("Sub")
+            .expect("Sub symbol should be found");
+
+        let ty = db.infer_symbol_type(file, sym)?;
+
+        let Type::Class(class_id) = ty else {
+            panic!("Sub is not a Class")
+        };
+        let jar = HasJar::<SemanticJar>::jar(db)?;
+        let base_names: Vec<_> = jar
+            .type_store
+            .get_class(class_id)
+            .bases()
+            .iter()
+            .map(|base_ty| format!("{}", base_ty.display(&jar.type_store)))
+            .collect();
+
+        assert_eq!(base_names, vec!["Literal[Base]"]);
+
+        Ok(())
+    }
+
+    #[test]
+    fn resolve_method() -> anyhow::Result<()> {
+        let case = create_test()?;
+        let db = &case.db;
+
+        let path = case.src.path().join("mod.py");
+        std::fs::write(path, "class C:\n  def f(self): pass")?;
+        let file = db
+            .resolve_module(ModuleName::new("mod"))?
+            .expect("module should be found")
+            .path(db)?
+            .file();
+        let syms = db.symbol_table(file)?;
+        let sym = syms
+            .root_symbol_id_by_name("C")
+            .expect("C symbol should be found");
+
+        let ty = db.infer_symbol_type(file, sym)?;
+
+        let Type::Class(class_id) = ty else {
+            panic!("C is not a Class");
+        };
+
+        let member_ty = class_id
+            .get_own_class_member(db, &Name::new("f"))
+            .expect("C.f to resolve");
+
+        let Some(Type::Function(func_id)) = member_ty else {
+            panic!("C.f is not a Function");
+        };
+
+        let jar = HasJar::<SemanticJar>::jar(db)?;
+        let function = jar.type_store.get_function(func_id);
+        assert_eq!(function.name(), "f");
+
+        Ok(())
+    }
+}

crates/red_knot/src/watch.rs

@@ -0,0 +1,77 @@
+use std::path::Path;
+
+use anyhow::Context;
+use notify::event::{CreateKind, RemoveKind};
+use notify::{recommended_watcher, Event, EventKind, RecommendedWatcher, RecursiveMode, Watcher};
+
+use crate::program::{FileChangeKind, FileWatcherChange};
+
+pub struct FileWatcher {
+    watcher: RecommendedWatcher,
+}
+
+pub trait EventHandler: Send + 'static {
+    fn handle(&self, changes: Vec<FileWatcherChange>);
+}
+
+impl<F> EventHandler for F
+where
+    F: Fn(Vec<FileWatcherChange>) + Send + 'static,
+{
+    fn handle(&self, changes: Vec<FileWatcherChange>) {
+        let f = self;
+        f(changes);
+    }
+}
+
+impl FileWatcher {
+    pub fn new<E>(handler: E) -> anyhow::Result<Self>
+    where
+        E: EventHandler,
+    {
+        Self::from_handler(Box::new(handler))
+    }
+
+    fn from_handler(handler: Box<dyn EventHandler>) -> anyhow::Result<Self> {
+        let watcher = recommended_watcher(move |changes: notify::Result<Event>| {
+            match changes {
+                Ok(event) => {
+                    // TODO verify that this handles all events correctly
+                    let change_kind = match event.kind {
+                        EventKind::Create(CreateKind::File) => FileChangeKind::Created,
+                        EventKind::Modify(_) => FileChangeKind::Modified,
+                        EventKind::Remove(RemoveKind::File) => FileChangeKind::Deleted,
+                        _ => {
+                            return;
+                        }
+                    };
+
+                    let mut changes = Vec::new();
+
+                    for path in event.paths {
+                        if path.is_file() {
+                            changes.push(FileWatcherChange::new(path, change_kind));
+                        }
+                    }
+
+                    if !changes.is_empty() {
+                        handler.handle(changes);
+                    }
+                }
+                // TODO proper error handling
+                Err(err) => {
+                    panic!("Error: {err}");
+                }
+            }
+        })
+        .context("Failed to create file watcher.")?;
+
+        Ok(Self { watcher })
+    }
+
+    pub fn watch_folder(&mut self, path: &Path) -> anyhow::Result<()> {
+        self.watcher.watch(path, RecursiveMode::Recursive)?;
+
+        Ok(())
+    }
+}

crates/ruff/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ruff"
-version = "0.4.0"
+version = "0.4.3"
 publish = false
 authors = { workspace = true }
 edition = { workspace = true }
@@ -41,7 +41,6 @@ is-macro = { workspace = true }
 itertools = { workspace = true }
 log = { workspace = true }
 notify = { workspace = true }
-num_cpus = { workspace = true }
 path-absolutize = { workspace = true, features = ["once_cell_cache"] }
 rayon = { workspace = true }
 regex = { workspace = true }

crates/ruff/src/args.rs

@@ -338,7 +338,7 @@ pub struct CheckCommand {
     /// The name of the file when passing it through stdin.
     #[arg(long, help_heading = "Miscellaneous")]
     pub stdin_filename: Option<PathBuf>,
-    /// List of mappings from file extension to language (one of ["python", "ipynb", "pyi"]). For
+    /// List of mappings from file extension to language (one of `python`, `ipynb`, `pyi`). For
     /// example, to treat `.ipy` files as IPython notebooks, use `--extension ipy:ipynb`.
     #[arg(long, value_delimiter = ',')]
     pub extension: Option<Vec<ExtensionPair>>,
@@ -466,7 +466,7 @@ pub struct FormatCommand {
     /// The name of the file when passing it through stdin.
     #[arg(long, help_heading = "Miscellaneous")]
     pub stdin_filename: Option<PathBuf>,
-    /// List of mappings from file extension to language (one of ["python", "ipynb", "pyi"]). For
+    /// List of mappings from file extension to language (one of `python`, `ipynb`, `pyi`). For
     /// example, to treat `.ipy` files as IPython notebooks, use `--extension ipy:ipynb`.
     #[arg(long, value_delimiter = ',')]
     pub extension: Option<Vec<ExtensionPair>>,

crates/ruff/src/cache.rs

@@ -23,7 +23,6 @@ use ruff_linter::message::Message;
 use ruff_linter::{warn_user, VERSION};
 use ruff_macros::CacheKey;
 use ruff_notebook::NotebookIndex;
-use ruff_python_ast::imports::ImportMap;
 use ruff_source_file::SourceFileBuilder;
 use ruff_text_size::{TextRange, TextSize};
 use ruff_workspace::resolver::Resolver;
@@ -348,7 +347,7 @@ impl FileCache {
             } else {
                 FxHashMap::default()
             };
-            Diagnostics::new(messages, lint.imports.clone(), notebook_indexes)
+            Diagnostics::new(messages, notebook_indexes)
         })
     }
 }
@@ -394,7 +393,7 @@ pub(crate) fn init(path: &Path) -> Result<()> {
 #[derive(Deserialize, Debug, Serialize, PartialEq)]
 pub(crate) struct LintCacheData {
     /// Imports made.
-    pub(super) imports: ImportMap,
+    // pub(super) imports: ImportMap,
     /// Diagnostic messages.
     pub(super) messages: Vec<CacheMessage>,
     /// Source code of the file.
@@ -410,7 +409,6 @@ pub(crate) struct LintCacheData {
 impl LintCacheData {
     pub(crate) fn from_messages(
         messages: &[Message],
-        imports: ImportMap,
         notebook_index: Option<NotebookIndex>,
     ) -> Self {
         let source = if let Some(msg) = messages.first() {
@@ -438,7 +436,6 @@ impl LintCacheData {
             .collect();
 
         Self {
-            imports,
             messages,
             source,
             notebook_index,

crates/ruff/src/commands/check.rs

@@ -17,7 +17,6 @@ use ruff_linter::registry::Rule;
 use ruff_linter::settings::types::UnsafeFixes;
 use ruff_linter::settings::{flags, LinterSettings};
 use ruff_linter::{fs, warn_user_once, IOError};
-use ruff_python_ast::imports::ImportMap;
 use ruff_source_file::SourceFileBuilder;
 use ruff_text_size::{TextRange, TextSize};
 use ruff_workspace::resolver::{
@@ -134,7 +133,6 @@ pub(crate) fn check(
                             dummy,
                             TextSize::default(),
                         )],
-                        ImportMap::default(),
                         FxHashMap::default(),
                     )
                 } else {

crates/ruff/src/diagnostics.rs

@@ -23,7 +23,6 @@ use ruff_linter::settings::{flags, LinterSettings};
 use ruff_linter::source_kind::{SourceError, SourceKind};
 use ruff_linter::{fs, IOError, SyntaxError};
 use ruff_notebook::{Notebook, NotebookError, NotebookIndex};
-use ruff_python_ast::imports::ImportMap;
 use ruff_python_ast::{PySourceType, SourceType, TomlSourceType};
 use ruff_source_file::SourceFileBuilder;
 use ruff_text_size::{TextRange, TextSize};
@@ -35,20 +34,17 @@ use crate::cache::{Cache, FileCacheKey, LintCacheData};
 pub(crate) struct Diagnostics {
     pub(crate) messages: Vec<Message>,
     pub(crate) fixed: FixMap,
-    pub(crate) imports: ImportMap,
     pub(crate) notebook_indexes: FxHashMap<String, NotebookIndex>,
 }
 
 impl Diagnostics {
     pub(crate) fn new(
         messages: Vec<Message>,
-        imports: ImportMap,
         notebook_indexes: FxHashMap<String, NotebookIndex>,
     ) -> Self {
         Self {
             messages,
             fixed: FixMap::default(),
-            imports,
             notebook_indexes,
         }
     }
@@ -92,7 +88,6 @@ impl Diagnostics {
                     dummy,
                     TextSize::default(),
                 )],
-                ImportMap::default(),
                 FxHashMap::default(),
             )
         } else {
@@ -127,7 +122,6 @@ impl Add for Diagnostics {
 impl AddAssign for Diagnostics {
     fn add_assign(&mut self, other: Self) {
         self.messages.extend(other.messages);
-        self.imports.extend(other.imports);
         self.fixed += other.fixed;
         self.notebook_indexes.extend(other.notebook_indexes);
     }
@@ -267,7 +261,7 @@ pub(crate) fn lint_path(
     // Lint the file.
     let (
         LinterResult {
-            data: (messages, imports),
+            data: messages,
             error: parse_error,
         },
         transformed,
@@ -335,8 +329,6 @@ pub(crate) fn lint_path(
         (result, transformed, fixed)
     };
 
-    let imports = imports.unwrap_or_default();
-
     if let Some((cache, relative_path, key)) = caching {
         // We don't cache parsing errors.
         if parse_error.is_none() {
@@ -354,7 +346,6 @@ pub(crate) fn lint_path(
                     &key,
                     LintCacheData::from_messages(
                         &messages,
-                        imports.clone(),
                         transformed.as_ipy_notebook().map(Notebook::index).cloned(),
                     ),
                 );
@@ -378,7 +369,6 @@ pub(crate) fn lint_path(
     Ok(Diagnostics {
         messages,
         fixed: FixMap::from_iter([(fs::relativize_path(path), fixed)]),
-        imports,
         notebook_indexes,
     })
 }
@@ -416,7 +406,7 @@ pub(crate) fn lint_stdin(
     // Lint the inputs.
     let (
         LinterResult {
-            data: (messages, imports),
+            data: messages,
             error: parse_error,
         },
         transformed,
@@ -494,8 +484,6 @@ pub(crate) fn lint_stdin(
         (result, transformed, fixed)
     };
 
-    let imports = imports.unwrap_or_default();
-
     if let Some(error) = parse_error {
         error!(
             "{}",
@@ -518,7 +506,6 @@ pub(crate) fn lint_stdin(
             fs::relativize_path(path.unwrap_or_else(|| Path::new("-"))),
             fixed,
         )]),
-        imports,
         notebook_indexes,
     })
 }

crates/ruff/src/lib.rs

@@ -214,13 +214,14 @@ fn format(args: FormatCommand, global_options: GlobalConfigArgs) -> Result<ExitS
 
 fn server(args: ServerCommand, log_level: LogLevel) -> Result<ExitStatus> {
     let ServerCommand { preview } = args;
+
+    let four = NonZeroUsize::new(4).unwrap();
+
     // by default, we set the number of worker threads to `num_cpus`, with a maximum of 4.
-    let worker_threads = num_cpus::get().max(4);
-    commands::server::run_server(
-        preview,
-        NonZeroUsize::try_from(worker_threads).expect("a non-zero worker thread count"),
-        log_level,
-    )
+    let worker_threads = std::thread::available_parallelism()
+        .unwrap_or(four)
+        .max(four);
+    commands::server::run_server(preview, worker_threads, log_level)
 }
 
 pub fn check(args: CheckCommand, global_options: GlobalConfigArgs) -> Result<ExitStatus> {

crates/ruff/tests/lint.rs

@@ -1284,3 +1284,49 @@ fn negated_per_file_ignores_overlap() -> Result<()> {
     "###);
     Ok(())
 }
+
+#[test]
+fn unused_interaction() -> Result<()> {
+    let tempdir = TempDir::new()?;
+    let ruff_toml = tempdir.path().join("ruff.toml");
+    fs::write(
+        &ruff_toml,
+        r#"
+[lint]
+select = ["F"]
+"#,
+    )?;
+
+    insta::with_settings!({
+        filters => vec![(tempdir_filter(&tempdir).as_str(), "[TMP]/")]
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .args(STDIN_BASE_OPTIONS)
+            .arg("--config")
+            .arg(&ruff_toml)
+            .args(["--stdin-filename", "test.py"])
+            .arg("--fix")
+            .arg("-")
+            .pass_stdin(r#"
+import os  # F401
+
+def function():
+    import os  # F811
+    print(os.name)
+"#), @r###"
+        success: true
+        exit_code: 0
+        ----- stdout -----
+
+        import os  # F401
+
+        def function():
+            print(os.name)
+
+        ----- stderr -----
+        Found 1 error (1 fixed, 0 remaining).
+        "###);
+    });
+
+    Ok(())
+}

crates/ruff_cache/Cargo.toml

@@ -11,9 +11,9 @@ repository = { workspace = true }
 license = { workspace = true }
 
 [dependencies]
-itertools = { workspace = true }
 glob = { workspace = true }
 globset = { workspace = true }
+itertools = { workspace = true }
 regex = { workspace = true }
 filetime = { workspace = true }
 seahash = { workspace = true }

crates/ruff_formatter/src/buffer.rs

@@ -344,7 +344,7 @@ pub struct RemoveSoftLinesBuffer<'a, Context> {
 
     /// Caches the interned elements after the soft line breaks have been removed.
     ///
-    /// The `key` is the [Interned] element as it has been passed to [Self::write_element] or the child of another
+    /// The `key` is the [Interned] element as it has been passed to [`Self::write_element`] or the child of another
     /// [Interned] element. The `value` is the matching document of the key where all soft line breaks have been removed.
     ///
     /// It's fine to not snapshot the cache. The worst that can happen is that it holds on interned elements

crates/ruff_formatter/src/diagnostics.rs

@@ -18,10 +18,10 @@ pub enum FormatError {
     InvalidDocument(InvalidDocumentError),
 
     /// Formatting failed because some content encountered a situation where a layout
-    /// choice by an enclosing [crate::Format] resulted in a poor layout for a child [crate::Format].
+    /// choice by an enclosing [`crate::Format`] resulted in a poor layout for a child [`crate::Format`].
     ///
-    /// It's up to an enclosing [crate::Format] to handle the error and pick another layout.
-    /// This error should not be raised if there's no outer [crate::Format] handling the poor layout error,
+    /// It's up to an enclosing [`crate::Format`] to handle the error and pick another layout.
+    /// This error should not be raised if there's no outer [`crate::Format`] handling the poor layout error,
     /// avoiding that formatting of the whole document fails.
     PoorLayout,
 }

crates/ruff_formatter/src/format_element.rs

@@ -19,10 +19,10 @@ use ruff_text_size::TextSize;
 /// Use the helper functions like [`crate::builders::space`], [`crate::builders::soft_line_break`] etc. defined in this file to create elements.
 #[derive(Clone, Eq, PartialEq)]
 pub enum FormatElement {
-    /// A space token, see [crate::builders::space] for documentation.
+    /// A space token, see [`crate::builders::space`] for documentation.
     Space,
 
-    /// A new line, see [crate::builders::soft_line_break], [crate::builders::hard_line_break], and [crate::builders::soft_line_break_or_space] for documentation.
+    /// A new line, see [`crate::builders::soft_line_break`], [`crate::builders::hard_line_break`], and [`crate::builders::soft_line_break_or_space`] for documentation.
     Line(LineMode),
 
     /// Forces the parent group to print in expanded mode.
@@ -108,13 +108,13 @@ impl std::fmt::Debug for FormatElement {
 
 #[derive(Debug, Clone, Copy, Eq, PartialEq)]
 pub enum LineMode {
-    /// See [crate::builders::soft_line_break_or_space] for documentation.
+    /// See [`crate::builders::soft_line_break_or_space`] for documentation.
     SoftOrSpace,
-    /// See [crate::builders::soft_line_break] for documentation.
+    /// See [`crate::builders::soft_line_break`] for documentation.
     Soft,
-    /// See [crate::builders::hard_line_break] for documentation.
+    /// See [`crate::builders::hard_line_break`] for documentation.
     Hard,
-    /// See [crate::builders::empty_line] for documentation.
+    /// See [`crate::builders::empty_line`] for documentation.
     Empty,
 }
 

crates/ruff_formatter/src/format_element/tag.rs

@@ -9,14 +9,14 @@ use std::num::NonZeroU8;
 /// will be applied to all elements in between the start/end tags.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum Tag {
-    /// Indents the content one level deeper, see [crate::builders::indent] for documentation and examples.
+    /// Indents the content one level deeper, see [`crate::builders::indent`] for documentation and examples.
     StartIndent,
     EndIndent,
 
-    /// Variant of [TagKind::Indent] that indents content by a number of spaces. For example, `Align(2)`
+    /// Variant of [`TagKind::Indent`] that indents content by a number of spaces. For example, `Align(2)`
     /// indents any content following a line break by an additional two spaces.
     ///
-    /// Nesting (Aligns)[TagKind::Align] has the effect that all except the most inner align are handled as (Indent)[TagKind::Indent].
+    /// Nesting (Aligns)[`TagKind::Align`] has the effect that all except the most inner align are handled as (Indent)[`TagKind::Indent`].
     StartAlign(Align),
     EndAlign,
 
@@ -29,7 +29,7 @@ pub enum Tag {
     /// - on a single line: Omitting `LineMode::Soft` line breaks and printing spaces for `LineMode::SoftOrSpace`
     /// - on multiple lines: Printing all line breaks
     ///
-    /// See [crate::builders::group] for documentation and examples.
+    /// See [`crate::builders::group`] for documentation and examples.
     StartGroup(Group),
     EndGroup,
 
@@ -44,22 +44,22 @@ pub enum Tag {
     EndConditionalGroup,
 
     /// Allows to specify content that gets printed depending on whatever the enclosing group
-    /// is printed on a single line or multiple lines. See [crate::builders::if_group_breaks] for examples.
+    /// is printed on a single line or multiple lines. See [`crate::builders::if_group_breaks`] for examples.
     StartConditionalContent(Condition),
     EndConditionalContent,
 
-    /// Optimized version of [Tag::StartConditionalContent] for the case where some content
+    /// Optimized version of [`Tag::StartConditionalContent`] for the case where some content
     /// should be indented if the specified group breaks.
     StartIndentIfGroupBreaks(GroupId),
     EndIndentIfGroupBreaks,
 
     /// Concatenates multiple elements together with a given separator printed in either
     /// flat or expanded mode to fill the print width. Expect that the content is a list of alternating
-    /// [element, separator] See [crate::Formatter::fill].
+    /// [element, separator] See [`crate::Formatter::fill`].
     StartFill,
     EndFill,
 
-    /// Entry inside of a [Tag::StartFill]
+    /// Entry inside of a [`Tag::StartFill`]
     StartEntry,
     EndEntry,
 
@@ -77,7 +77,7 @@ pub enum Tag {
     /// Special semantic element marking the content with a label.
     /// This does not directly influence how the content will be printed.
     ///
-    /// See [crate::builders::labelled] for documentation.
+    /// See [`crate::builders::labelled`] for documentation.
     StartLabelled(LabelId),
     EndLabelled,
 

crates/ruff_formatter/src/printer/queue.rs

@@ -189,27 +189,6 @@ impl<'a, 'print> Queue<'a> for FitsQueue<'a, 'print> {
     }
 }
 
-/// Iterator that calls [`Queue::pop`] until it reaches the end of the document.
-///
-/// The iterator traverses into the content of any [`FormatElement::Interned`].
-pub(super) struct QueueIterator<'a, 'q, Q: Queue<'a>> {
-    queue: &'q mut Q,
-    lifetime: PhantomData<&'a ()>,
-}
-
-impl<'a, Q> Iterator for QueueIterator<'a, '_, Q>
-where
-    Q: Queue<'a>,
-{
-    type Item = &'a FormatElement;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.queue.pop()
-    }
-}
-
-impl<'a, Q> FusedIterator for QueueIterator<'a, '_, Q> where Q: Queue<'a> {}
-
 pub(super) struct QueueContentIterator<'a, 'q, Q: Queue<'a>> {
     queue: &'q mut Q,
     kind: TagKind,

crates/ruff_formatter/src/printer/stack.rs

@@ -8,9 +8,6 @@ pub(super) trait Stack<T> {
 
     /// Returns the last element if any
     fn top(&self) -> Option<&T>;
-
-    /// Returns `true` if the stack is empty
-    fn is_empty(&self) -> bool;
 }
 
 impl<T> Stack<T> for Vec<T> {
@@ -25,10 +22,6 @@ impl<T> Stack<T> for Vec<T> {
     fn top(&self) -> Option<&T> {
         self.last()
     }
-
-    fn is_empty(&self) -> bool {
-        self.is_empty()
-    }
 }
 
 /// A Stack that is stacked on top of another stack. Guarantees that the underlying stack remains unchanged.
@@ -80,10 +73,6 @@ where
             .last()
             .or_else(|| self.original.as_slice().last())
     }
-
-    fn is_empty(&self) -> bool {
-        self.stack.is_empty() && self.original.len() == 0
-    }
 }
 
 #[cfg(test)]

crates/ruff_linter/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "ruff_linter"
-version = "0.4.0"
+version = "0.4.3"
 publish = false
 authors = { workspace = true }
 edition = { workspace = true }

crates/ruff_linter/resources/test/fixtures/flake8_blind_except/BLE.py

@@ -124,3 +124,8 @@ try:
     pass
 except Exception:
     error("...", exc_info=True)
+
+try:
+    ...
+except Exception as e:
+    raise ValueError from e

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B024.py

@@ -115,25 +115,25 @@ class non_keyword_abcmeta_2(abc.ABCMeta):  # safe
 
 
 # very invalid code, but that's up to mypy et al to check
-class keyword_abc_1(metaclass=ABC):  # safe
+class keyword_abc_1(metaclass=ABC):  # incorrect but outside scope of this check
     def method(self):
         foo()
 
 
-class keyword_abc_2(metaclass=abc.ABC):  # safe
+class keyword_abc_2(metaclass=abc.ABC):  # incorrect but outside scope of this check
     def method(self):
         foo()
 
 
-class abc_set_class_variable_1(ABC):  # safe
+class abc_set_class_variable_1(ABC):  # safe (abstract attribute)
     foo: int
 
 
-class abc_set_class_variable_2(ABC):  # safe
+class abc_set_class_variable_2(ABC):  # error (not an abstract attribute)
     foo = 2
 
 
-class abc_set_class_variable_3(ABC):  # safe
+class abc_set_class_variable_3(ABC):  # error (not an abstract attribute)
     foo: int = 2
 
 

crates/ruff_linter/resources/test/fixtures/flake8_logging_format/G004.py

@@ -7,6 +7,9 @@ logging.log(logging.INFO, f"Hello {name}")
 _LOGGER = logging.getLogger()
 _LOGGER.info(f"{__name__}")
 
+logging.getLogger().info(f"{name}")
+
 from logging import info
+
 info(f"{name}")
 info(f"{__name__}")

crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI026.py

@@ -14,6 +14,15 @@ IntOrStr: TypeAlias = int | str
 IntOrFloat: Foo = int | float
 AliasNone: typing.TypeAlias = None
 
-# these are ok
+class NotAnEnum:
+    NOT_A_STUB_SO_THIS_IS_FINE = None
+
+from enum import Enum
+
+class FooEnum(Enum): ...
+
+class BarEnum(FooEnum):
+    BAR = None
+
 VarAlias = str
 AliasFoo = Foo

crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI026.pyi

@@ -13,6 +13,16 @@ IntOrStr: TypeAlias = int | str
 IntOrFloat: Foo = int | float
 AliasNone: typing.TypeAlias = None
 
+class NotAnEnum:
+    FLAG_THIS = None
+
 # these are ok
+from enum import Enum
+
+class FooEnum(Enum): ...
+
+class BarEnum(FooEnum):
+    BAR = None
+
 VarAlias = str
 AliasFoo = Foo

crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI036.py

@@ -3,7 +3,7 @@ import types
 import typing
 from collections.abc import Awaitable
 from types import TracebackType
-from typing import Any, Type
+from typing import Any, Type, overload
 
 import _typeshed
 import typing_extensions
@@ -73,3 +73,97 @@ class BadFive:
 class BadSix:
     def __exit__(self, typ, exc, tb, weird_extra_arg, extra_arg2 = None) -> None: ... # PYI036: Extra arg must have default
     async def __aexit__(self, typ, exc, tb, *, weird_extra_arg) -> None: ... # PYI036: kwargs must have default
+
+class AllPositionalOnlyArgs:
+    def __exit__(self, typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None, /) -> None: ...
+    async def __aexit__(self, typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None, /) -> None: ...
+
+class BadAllPositionalOnlyArgs:
+    def __exit__(self, typ: type[Exception] | None, exc: BaseException | None, tb: TracebackType | None, /) -> None: ...
+    async def __aexit__(self, typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType, /) -> None: ...
+
+# Definitions not in a class scope can do whatever, we don't care
+def __exit__(self, *args: bool) -> None: ...
+async def __aexit__(self, *, go_crazy: bytes) -> list[str]: ...
+
+# Here come the overloads...
+
+class AcceptableOverload1:
+    @overload
+    def __exit__(self, exc_typ: None, exc: None, exc_tb: None) -> None: ...
+    @overload
+    def __exit__(self, exc_typ: type[BaseException], exc: BaseException, exc_tb: TracebackType) -> None: ...
+    def __exit__(self, exc_typ: type[BaseException] | None, exc: BaseException | None, exc_tb: TracebackType | None) -> None: ...
+
+# Using `object` or `Unused` in an overload definition is kinda strange,
+# but let's allow it to be on the safe side
+class AcceptableOverload2:
+    @overload
+    def __exit__(self, exc_typ: None, exc: None, exc_tb: object) -> None: ...
+    @overload
+    def __exit__(self, exc_typ: Unused, exc: BaseException, exc_tb: object) -> None: ...
+    def __exit__(self, exc_typ: type[BaseException] | None, exc: BaseException | None, exc_tb: TracebackType | None) -> None: ...
+
+class AcceptableOverload3:
+    # Just ignore any overloads that don't have exactly 3 annotated non-self parameters.
+    # We don't have the ability (yet) to do arbitrary checking
+    # of whether one function definition is a subtype of another...
+    @overload
+    def __exit__(self, exc_typ: bool, exc: bool, exc_tb: bool, weird_extra_arg: bool) -> None: ...
+    @overload
+    def __exit__(self, *args: object) -> None: ...
+    def __exit__(self, *args: object) -> None: ...
+    @overload
+    async def __aexit__(self, exc_typ: bool, /, exc: bool, exc_tb: bool, *, keyword_only: str) -> None: ...
+    @overload
+    async def __aexit__(self, *args: object) -> None: ...
+    async def __aexit__(self, *args: object) -> None: ...
+
+class AcceptableOverload4:
+    # Same as above
+    @overload
+    def __exit__(self, exc_typ: type[Exception], exc: type[Exception], exc_tb: types.TracebackType) -> None: ...
+    @overload
+    def __exit__(self, *args: object) -> None: ...
+    def __exit__(self, *args: object) -> None: ...
+    @overload
+    async def __aexit__(self, exc_typ: type[Exception], exc: type[Exception], exc_tb: types.TracebackType, *, extra: str = "foo") -> None: ...
+    @overload
+    async def __aexit__(self, exc_typ: None, exc: None, tb: None) -> None: ...
+    async def __aexit__(self, *args: object) -> None: ...
+
+class StrangeNumberOfOverloads:
+    # Only one overload? Type checkers will emit an error, but we should just ignore it
+    @overload
+    def __exit__(self, exc_typ: bool, exc: bool, tb: bool) -> None: ...
+    def __exit__(self, exc_typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None) -> None: ...
+    # More than two overloads? Anything could be going on; again, just ignore all the overloads
+    @overload
+    async def __aexit__(self, arg: bool) -> None: ...
+    @overload
+    async def __aexit__(self, arg: None, arg2: None, arg3: None) -> None: ...
+    @overload
+    async def __aexit__(self, arg: bool, arg2: bool, arg3: bool) -> None: ...
+    async def __aexit__(self, exc_typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None) -> None: ...
+
+# TODO: maybe we should emit an error on this one as well?
+class BizarreAsyncSyncOverloadMismatch:
+    @overload
+    def __exit__(self, exc_typ: bool, exc: bool, tb: bool) -> None: ...
+    @overload
+    async def __exit__(self, exc_typ: bool, exc: bool, tb: bool) -> None: ...
+    def __exit__(self, *args: object) -> None: ...
+
+class UnacceptableOverload1:
+    @overload
+    def __exit__(self, exc_typ: None, exc: None, tb: None) -> None: ...  # Okay
+    @overload
+    def __exit__(self, exc_typ: Exception, exc: Exception, tb: TracebackType) -> None: ...  # PYI036
+    def __exit__(self, exc_typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None) -> None: ...
+
+class UnacceptableOverload2:
+    @overload
+    def __exit__(self, exc_typ: type[BaseException] | None, exc: None, tb: None) -> None: ...  # PYI036
+    @overload
+    def __exit__(self, exc_typ: object, exc: Exception, tb: builtins.TracebackType) -> None: ...  # PYI036
+    def __exit__(self, exc_typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None) -> None: ...

crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI036.pyi

@@ -3,7 +3,7 @@ import types
 import typing
 from collections.abc import Awaitable
 from types import TracebackType
-from typing import Any, Type
+from typing import Any, Type, overload
 
 import _typeshed
 import typing_extensions
@@ -80,3 +80,86 @@ def isolated_scope():
 
     class ShouldNotError:
         def __exit__(self, typ: Type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None) -> None: ...
+
+class AllPositionalOnlyArgs:
+    def __exit__(self, typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None, /) -> None: ...
+    async def __aexit__(self, typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType | None, /) -> None: ...
+
+class BadAllPositionalOnlyArgs:
+    def __exit__(self, typ: type[Exception] | None, exc: BaseException | None, tb: TracebackType | None, /) -> None: ...
+    async def __aexit__(self, typ: type[BaseException] | None, exc: BaseException | None, tb: TracebackType, /) -> None: ...
+
+# Definitions not in a class scope can do whatever, we don't care
+def __exit__(self, *args: bool) -> None: ...
+async def __aexit__(self, *, go_crazy: bytes) -> list[str]: ...
+
+# Here come the overloads...
+
+class AcceptableOverload1:
+    @overload
+    def __exit__(self, exc_typ: None, exc: None, exc_tb: None) -> None: ...
+    @overload
+    def __exit__(self, exc_typ: type[BaseException], exc: BaseException, exc_tb: TracebackType) -> None: ...
+
+# Using `object` or `Unused` in an overload definition is kinda strange,
+# but let's allow it to be on the safe side
+class AcceptableOverload2:
+    @overload
+    def __exit__(self, exc_typ: None, exc: None, exc_tb: object) -> None: ...
+    @overload
+    def __exit__(self, exc_typ: Unused, exc: BaseException, exc_tb: object) -> None: ...
+
+class AcceptableOverload3:
+    # Just ignore any overloads that don't have exactly 3 annotated non-self parameters.
+    # We don't have the ability (yet) to do arbitrary checking
+    # of whether one function definition is a subtype of another...
+    @overload
+    def __exit__(self, exc_typ: bool, exc: bool, exc_tb: bool, weird_extra_arg: bool) -> None: ...
+    @overload
+    def __exit__(self, *args: object) -> None: ...
+    @overload
+    async def __aexit__(self, exc_typ: bool, /, exc: bool, exc_tb: bool, *, keyword_only: str) -> None: ...
+    @overload
+    async def __aexit__(self, *args: object) -> None: ...
+
+class AcceptableOverload4:
+    # Same as above
+    @overload
+    def __exit__(self, exc_typ: type[Exception], exc: type[Exception], exc_tb: types.TracebackType) -> None: ...
+    @overload
+    def __exit__(self, *args: object) -> None: ...
+    @overload
+    async def __aexit__(self, exc_typ: type[Exception], exc: type[Exception], exc_tb: types.TracebackType, *, extra: str = "foo") -> None: ...
+    @overload
+    async def __aexit__(self, exc_typ: None, exc: None, tb: None) -> None: ...
+
+class StrangeNumberOfOverloads:
+    # Only one overload? Type checkers will emit an error, but we should just ignore it
+    @overload
+    def __exit__(self, exc_typ: bool, exc: bool, tb: bool) -> None: ...
+    # More than two overloads? Anything could be going on; again, just ignore all the overloads
+    @overload
+    async def __aexit__(self, arg: bool) -> None: ...
+    @overload
+    async def __aexit__(self, arg: None, arg2: None, arg3: None) -> None: ...
+    @overload
+    async def __aexit__(self, arg: bool, arg2: bool, arg3: bool) -> None: ...
+
+# TODO: maybe we should emit an error on this one as well?
+class BizarreAsyncSyncOverloadMismatch:
+    @overload
+    def __exit__(self, exc_typ: bool, exc: bool, tb: bool) -> None: ...
+    @overload
+    async def __exit__(self, exc_typ: bool, exc: bool, tb: bool) -> None: ...
+
+class UnacceptableOverload1:
+    @overload
+    def __exit__(self, exc_typ: None, exc: None, tb: None) -> None: ...  # Okay
+    @overload
+    def __exit__(self, exc_typ: Exception, exc: Exception, tb: TracebackType) -> None: ...  # PYI036
+
+class UnacceptableOverload2:
+    @overload
+    def __exit__(self, exc_typ: type[BaseException] | None, exc: None, tb: None) -> None: ...  # PYI036
+    @overload
+    def __exit__(self, exc_typ: object, exc: Exception, tb: builtins.TracebackType) -> None: ...  # PYI036

crates/ruff_linter/resources/test/fixtures/flake8_simplify/SIM911.py

@@ -21,3 +21,5 @@ for k, v in zip(d2.keys(), d2.values()):  # SIM911
     ...
 
 items = zip(x.keys(), x.values())  # OK
+
+items.bar = zip(x.keys(), x.values())  # OK

crates/ruff_linter/resources/test/fixtures/pep8_naming/N805.py

@@ -8,7 +8,6 @@ class Class:
         pass
 
     if False:
-
         def extra_bad_method(this):
             pass
 
@@ -94,6 +93,7 @@ class ModelClass:
     def badstatic(foo):
         pass
 
+
 class SelfInArgsClass:
     def self_as_argument(this, self):
         pass
@@ -110,6 +110,7 @@ class SelfInArgsClass:
     def self_as_kwargs(this, **self):
         pass
 
+
 class RenamingInMethodBodyClass:
     def bad_method(this):
         this = this
@@ -117,3 +118,8 @@ class RenamingInMethodBodyClass:
 
     def bad_method(this):
         self = this
+
+
+class RenamingWithNFKC:
+    def formula(household):
+        hºusehold(1)

crates/ruff_linter/resources/test/fixtures/perflint/PERF401.py

@@ -72,3 +72,18 @@ def f():
     result = Foo()
     for i in items:
         result.append(i)  # Ok
+
+
+def f():
+    items = [1, 2, 3, 4]
+    result = []
+    async for i in items:
+        if i % 2:
+            result.append(i)  # PERF401
+
+
+def f():
+    items = [1, 2, 3, 4]
+    result = []
+    async for i in items:
+        result.append(i)  # PERF401

crates/ruff_linter/resources/test/fixtures/perflint/PERF402.py

@@ -43,3 +43,10 @@ def f():
 
     for path in ("foo", "bar"):
         sys.path.append(path)  # OK
+
+
+def f():
+    items = [1, 2, 3, 4]
+    result = []
+    async for i in items:
+        result.append(i)  # PERF402

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_24/__init__.py

@@ -0,0 +1,36 @@
+"""__init__.py without __all__
+
+Unused stdlib and third party imports are unsafe removals
+
+Unused first party imports get changed to redundant aliases
+"""
+
+
+# stdlib
+
+import os  # Ok: is used
+
+_ = os
+
+
+import argparse as argparse  # Ok: is redundant alias
+
+
+import sys  # F401: remove unused
+
+
+# first-party
+
+
+from . import used  # Ok: is used
+
+_ = used
+
+
+from . import aliased as aliased  # Ok: is redundant alias
+
+
+from . import unused  # F401: change to redundant alias
+
+
+from . import renamed as bees  # F401: no fix

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_24/aliased.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_24/renamed.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_24/unused.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_24/used.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_25__all/__init__.py

@@ -0,0 +1,42 @@
+"""__init__.py with __all__
+
+Unused stdlib and third party imports are unsafe removals
+
+Unused first party imports get added to __all__
+"""
+
+
+# stdlib
+
+import os  # Ok: is used
+
+_ = os
+
+
+import argparse  # Ok: is exported in __all__
+
+
+import sys  # F401: remove unused
+
+
+# first-party
+
+
+from . import used  # Ok: is used
+
+_ = used
+
+
+from . import aliased as aliased  # Ok: is redundant alias
+
+
+from . import exported  # Ok: is exported in __all__
+
+
+# from . import unused # F401: add to __all__
+
+
+# from . import renamed as bees # F401: add to __all__
+
+
+__all__ = ["argparse", "exported"]

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_25__all/aliased.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_25__all/exported.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_25__all/renamed.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_25__all/unused.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_25__all/used.py

@@ -0,0 +1 @@
+# empty module imported by __init__.py for test fixture

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_17.py

@@ -53,7 +53,7 @@ class WithinBody[T](list[T]):   # OK
 
     def foo(self, x: T) -> T:  # OK
         return x
-    
+
     def foo(self):
         T  # OK
 
@@ -76,6 +76,16 @@ type Foo[T: (DoesNotExist1, DoesNotExist2)] = T  # F821: Undefined name `DoesNot
 def foo[T: (DoesNotExist1, DoesNotExist2)](t: T) -> T: return t  # F821: Undefined name `DoesNotExist1`, Undefined name `DoesNotExist2`
 class Foo[T: (DoesNotExist1, DoesNotExist2)](list[T]): ...  # F821: Undefined name `DoesNotExist1`, Undefined name `DoesNotExist2`
 
+# Same in defaults
+
+type Foo[T = DoesNotExist] = T  # F821: Undefined name `DoesNotExist`
+def foo[T = DoesNotExist](t: T) -> T: return t  # F821: Undefined name `DoesNotExist`
+class Foo[T = DoesNotExist](list[T]): ...  # F821: Undefined name `DoesNotExist`
+
+type Foo[T = (DoesNotExist1, DoesNotExist2)] = T  # F821: Undefined name `DoesNotExist1`, Undefined name `DoesNotExist2`
+def foo[T = (DoesNotExist1, DoesNotExist2)](t: T) -> T: return t  # F821: Undefined name `DoesNotExist1`, Undefined name `DoesNotExist2`
+class Foo[T = (DoesNotExist1, DoesNotExist2)](list[T]): ...  # F821: Undefined name `DoesNotExist1`, Undefined name `DoesNotExist2`
+
 # Type parameters in nested classes
 
 class Parent[T]:
@@ -83,21 +93,21 @@ class Parent[T]:
 
     def can_use_class_variable(self, x: t) -> t:  # OK
         return x
-    
+
     class Child:
         def can_access_parent_type_parameter(self, x: T) -> T:  # OK
             T  # OK
             return x
-        
+
         def cannot_access_parent_variable(self, x: t) -> t:  # F821: Undefined name `T`
                 t # F821: Undefined name `t`
                 return x
-            
+
 # Type parameters in nested functions
 
 def can_access_inside_nested[T](t: T) -> T:  # OK
     def bar(x: T) -> T:  # OK
         T # OK
         return x
-    
+
     bar(t)

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_26.py

@@ -1,6 +1,6 @@
 """Tests for constructs allowed in `.pyi` stub files but not at runtime"""
 
-from typing import Optional, TypeAlias, Union
+from typing import Optional, TypeAlias, Union, TYPE_CHECKING
 
 __version__: str
 __author__: str
@@ -42,3 +42,7 @@ class MyClass:
 baz: MyClass
 eggs = baz  # valid in a `.pyi` stub file, not in a `.py` runtime file
 eggs = "baz"  # always okay
+
+if TYPE_CHECKING:
+    class Tree3(list[Tree3 | Leaf]): ...  # Always okay if it's in a `TYPE_CHECKING` block
+    Recursive: TypeAlias = Recursive | None  # Always okay if it's in a `TYPE_CHECKING` block

crates/ruff_linter/resources/test/fixtures/pyflakes/F821_27.py

@@ -1,7 +1,7 @@
-"""Tests for constructs allowed when `__future__` annotations are enabled but not otherwise"""
+"""Tests for forward references (some allowed, some still not) in the context of `from __future__ import annotations`"""
 from __future__ import annotations
 
-from typing import Optional, TypeAlias, Union
+from typing import Optional, TypeAlias, Union, TYPE_CHECKING
 
 __version__: str
 __author__: str
@@ -45,4 +45,8 @@ class D: ...
 # More circular references
 class Leaf: ...
 class Tree(list[Tree | Leaf]): ...  # Still invalid even when `__future__.annotations` are enabled
-class Tree2(list["Tree | Leaf"]): ...  # always okay
+class Tree2(list["Tree2 | Leaf"]): ...  # always okay
+
+if TYPE_CHECKING:
+    class Tree3(list[Tree3 | Leaf]): ...  # Always okay if it's in a `TYPE_CHECKING` block
+    Recursive: TypeAlias = Recursive | None  # Always okay if it's in a `TYPE_CHECKING` block

crates/ruff_linter/resources/test/fixtures/pygrep_hooks/PGH004_1.py

@@ -0,0 +1 @@
+#noqa

crates/ruff_linter/resources/test/fixtures/pylint/invalid_return_type_bytes.py

@@ -21,12 +21,6 @@ class BytesNoReturn:
         print("ruff")  # [invalid-bytes-return]
 
 
-class BytesWrongRaise:
-    def __bytes__(self):
-        print("raise some error")
-        raise NotImplementedError  # [invalid-bytes-return]
-
-
 # TODO: Once Ruff has better type checking
 def return_bytes():
     return "some string"
@@ -63,3 +57,9 @@ class Bytes4:
 class Bytes5:
     def __bytes__(self):
         raise NotImplementedError
+
+
+class Bytes6:
+    def __bytes__(self):
+        print("raise some error")
+        raise NotImplementedError

crates/ruff_linter/resources/test/fixtures/pylint/invalid_return_type_hash.py

@@ -0,0 +1,65 @@
+# These testcases should raise errors
+
+
+class Bool:
+    def __hash__(self):
+        return True  # [invalid-hash-return]
+
+
+class Float:
+    def __hash__(self):
+        return 3.05  # [invalid-hash-return]
+
+
+class Str:
+    def __hash__(self):
+        return "ruff"  # [invalid-hash-return]
+
+
+class HashNoReturn:
+    def __hash__(self):
+        print("ruff")  # [invalid-hash-return]
+
+
+# TODO: Once Ruff has better type checking
+def return_int():
+    return "3"
+
+
+class ComplexReturn:
+    def __hash__(self):
+        return return_int()  # [invalid-hash-return]
+
+
+# These testcases should NOT raise errors
+
+
+class Hash:
+    def __hash__(self):
+        return 7741
+
+
+class Hash2:
+    def __hash__(self):
+        x = 7741
+        return x
+
+
+class Hash3:
+    def __hash__(self): ...
+
+
+class Has4:
+    def __hash__(self):
+        pass
+
+
+class Hash5:
+    def __hash__(self):
+        raise NotImplementedError
+
+
+class HashWrong6:
+    def __hash__(self):
+        print("raise some error")
+        raise NotImplementedError

crates/ruff_linter/resources/test/fixtures/pylint/invalid_return_type_index.py

@@ -0,0 +1,73 @@
+# These testcases should raise errors
+
+
+class Bool:
+    """pylint would not raise, but ruff does - see explanation in the docs"""
+
+    def __index__(self):
+        return True  # [invalid-index-return]
+
+
+class Float:
+    def __index__(self):
+        return 3.05  # [invalid-index-return]
+
+
+class Dict:
+    def __index__(self):
+        return {"1": "1"}  # [invalid-index-return]
+
+
+class Str:
+    def __index__(self):
+        return "ruff"  # [invalid-index-return]
+
+
+class IndexNoReturn:
+    def __index__(self):
+        print("ruff")  # [invalid-index-return]
+
+
+# TODO: Once Ruff has better type checking
+def return_index():
+    return "3"
+
+
+class ComplexReturn:
+    def __index__(self):
+        return return_index()  # [invalid-index-return]
+
+
+# These testcases should NOT raise errors
+
+
+class Index:
+    def __index__(self):
+        return 0
+
+
+class Index2:
+    def __index__(self):
+        x = 1
+        return x
+
+
+class Index3:
+    def __index__(self):
+        ...
+
+
+class Index4:
+    def __index__(self):
+        pass
+
+
+class Index5:
+    def __index__(self):
+        raise NotImplementedError
+
+
+class Index6:
+    def __index__(self):
+        print("raise some error")
+        raise NotImplementedError

crates/ruff_linter/resources/test/fixtures/pylint/invalid_return_type_length.py

@@ -26,12 +26,6 @@ class LengthNegative:
         return -42  # [invalid-length-return]
 
 
-class LengthWrongRaise:
-    def __len__(self):
-        print("raise some error")
-        raise NotImplementedError  # [invalid-length-return]
-
-
 # TODO: Once Ruff has better type checking
 def return_int():
     return "3"
@@ -68,3 +62,9 @@ class Length4:
 class Length5:
     def __len__(self):
         raise NotImplementedError
+
+
+class Length6:
+    def __len__(self):
+        print("raise some error")
+        raise NotImplementedError

crates/ruff_linter/resources/test/fixtures/pylint/invalid_return_type_str.py

@@ -47,3 +47,14 @@ class Str2:
 
 class Str3:
     def __str__(self): ...
+
+
+class Str4:
+    def __str__(self):
+        raise RuntimeError("__str__ not allowed")
+
+
+class Str5:
+    def __str__(self):  # PLE0307 (returns None if x <= 0)
+        if x > 0:
+            raise RuntimeError("__str__ not allowed")

crates/ruff_linter/resources/test/fixtures/pylint/property_with_parameters.py

@@ -28,3 +28,13 @@ class MyClassBase(metaclass=ABCMeta):
     @abstractmethod
     def example(self, value):
         """Setter."""
+
+
+class VariadicParameters:
+    @property
+    def attribute_var_args(self, *args):  # [property-with-parameters]
+        return sum(args)
+
+    @property
+    def attribute_var_kwargs(self, **kwargs):  #[property-with-parameters]
+        return {key: value * 2 for key, value in kwargs.items()}

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_0.py

@@ -117,3 +117,10 @@ path = "%s-%s-%s.pem" % (
     cert.not_valid_after.date().isoformat().replace("-", ""), # expiration date
     hexlify(cert.fingerprint(hashes.SHA256())).decode("ascii")[0:8], # fingerprint prefix
 )
+
+# UP031 (no longer false negatives; now offer potentially unsafe fixes)
+'Hello %s' % bar
+
+'Hello %s' % bar.baz
+
+'Hello %s' % bar['bop']

crates/ruff_linter/resources/test/fixtures/pyupgrade/UP031_1.py

@@ -32,10 +32,3 @@ pytest.param('"%8s" % (None,)', id="unsafe width-string conversion"),
 "%(1)s" % {1: 2, "1": 2}
 
 "%(and)s" % {"and": 2}
-
-# OK (arguably false negatives)
-'Hello %s' % bar
-
-'Hello %s' % bar.baz
-
-'Hello %s' % bar['bop']

crates/ruff_linter/resources/test/fixtures/refurb/FURB116.py

@@ -0,0 +1,19 @@
+num = 1337
+
+def return_num() -> int:
+    return num
+
+print(oct(num)[2:])  # FURB116
+print(hex(num)[2:])  # FURB116
+print(bin(num)[2:])  # FURB116
+
+print(oct(1337)[2:])  # FURB116
+print(hex(1337)[2:])  # FURB116
+print(bin(1337)[2:])  # FURB116
+
+print(bin(return_num())[2:])  # FURB116 (no autofix)
+print(bin(int(f"{num}"))[2:])  # FURB116 (no autofix)
+
+## invalid
+print(oct(0o1337)[1:])
+print(hex(0x1337)[3:])

crates/ruff_linter/resources/test/fixtures/refurb/FURB118.py

@@ -10,7 +10,7 @@ op_mult = lambda x, y: x * y
 op_matmutl = lambda x, y: x @ y
 op_truediv = lambda x, y: x / y
 op_mod = lambda x, y: x % y
-op_pow = lambda x, y: x**y
+op_pow = lambda x, y: x ** y
 op_lshift = lambda x, y: x << y
 op_rshift = lambda x, y: x >> y
 op_bitor = lambda x, y: x | y
@@ -59,6 +59,7 @@ op_itemgetter = lambda x, y: (x[0], y[0])
 op_itemgetter = lambda x, y: (x[0], y[0])
 op_itemgetter = lambda x: ()
 op_itemgetter = lambda x: (*x[0], x[1])
+op_itemgetter = lambda x: (x[0],)
 
 
 def op_neg3(x, y):
@@ -72,3 +73,10 @@ def op_add4(x, y=1):
 def op_add5(x, y):
     print("op_add5")
     return x + y
+
+
+# OK
+class Class:
+    @staticmethod
+    def add(x, y):
+        return x + y

crates/ruff_linter/resources/test/fixtures/refurb/FURB192.py

@@ -0,0 +1,48 @@
+# Errors
+
+sorted(l)[0]
+
+sorted(l)[-1]
+
+sorted(l, reverse=False)[-1]
+
+sorted(l, key=lambda x: x)[0]
+
+sorted(l, key=key_fn)[0]
+
+sorted([1, 2, 3])[0]
+
+# Unsafe
+
+sorted(l, key=key_fn, reverse=True)[-1]
+
+sorted(l, reverse=True)[0]
+
+sorted(l, reverse=True)[-1]
+
+# Non-errors
+
+sorted(l, reverse=foo())[0]
+
+sorted(l)[1]
+
+sorted(get_list())[1]
+
+sorted()[0]
+
+sorted(l)[1]
+
+sorted(l)[-2]
+
+b = True
+
+sorted(l, reverse=b)[0]
+
+sorted(l, invalid_kwarg=True)[0]
+
+
+def sorted():
+    pass
+
+
+sorted(l)[0]

crates/ruff_linter/resources/test/fixtures/ruff/RUF029.py

@@ -22,7 +22,7 @@ async def pass_3():  # OK: uses an async loop
 
 
 class Foo:
-    async def pass_4():  # OK: method of a class
+    async def pass_4(self):  # OK: method of a class
         pass
 
 
@@ -31,6 +31,10 @@ def foo():
         await bla
 
 
+async def pass_6():  # OK: just a stub
+    ...
+
+
 async def fail_1a():  # RUF029
     time.sleep(1)
 
@@ -58,10 +62,19 @@ async def fail_4a():  # RUF029: the /outer/ function does not await
 
 async def fail_4b():  # RUF029: the /outer/ function does not await
     class Foo:
-        async def foo():
+        async def foo(self):
             await bla
 
 
 def foo():
     async def fail_4c():  # RUF029: the /inner/ function does not await
         pass
+
+
+async def test():
+    return [check async for check in async_func()]
+
+
+async def test() -> str:
+    vals = [str(val) for val in await async_func(1)]
+    return ",".join(vals)

crates/ruff_linter/resources/test/fixtures/ruff/RUF100_0.py

@@ -26,10 +26,10 @@ def f() -> None:
 
     # fmt: off
     # Invalid - no space before #
-    d = 1# noqa: E501
+    d = 1  # noqa: E501
 
     # Invalid - many spaces before #
-    d = 1                       # noqa: E501
+    d = 1  # noqa: E501
     # fmt: on
 
 
@@ -104,5 +104,28 @@ def f():
 
 def f():
     # Invalid - nonexistant error code with multibyte character
-    d = 1  #…noqa: F841, E50
-    e = 1  #…noqa: E50
+    d = 1  # …noqa: F841, E50
+    e = 1  # …noqa: E50
+
+
+def f():
+    # Disabled - check redirects are reported correctly
+    eval(command)  # noqa: PGH001
+
+
+# Check duplicate code detection
+def f():
+    x = 2  # noqa: F841, F841, X200
+
+    y = 2 == bar  # noqa: SIM300, F841, SIM300, SIM300
+
+    z = 2  # noqa: F841 F841  F841, F841, F841
+
+    return
+
+
+# Allow code redirects
+x = eval(command)  # noqa: PGH001, S307
+x = eval(command)  # noqa: S307, PGH001, S307, S307, S307
+x = eval(command)  # noqa: PGH001, S307, PGH001
+x = eval(command)  # noqa: PGH001, S307, PGH001, S307

crates/ruff_linter/resources/test/fixtures/ruff/RUF101.py

@@ -0,0 +1,6 @@
+x = 2 # noqa: RUF940
+x = 2 # noqa: RUF950
+x = 2 # noqa: RUF940, RUF950
+x = 2 # noqa: RUF950, RUF940, RUF950, RUF950, RUF950
+x = 2 # noqa: RUF940, RUF950, RUF940
+x = 2 # noqa: RUF940, RUF950, RUF940, RUF950

crates/ruff_linter/src/checkers/ast/analyze/definitions.rs

@@ -109,7 +109,7 @@ pub(crate) fn definitions(checker: &mut Checker) {
     };
 
     let definitions = std::mem::take(&mut checker.semantic.definitions);
-    let mut overloaded_name: Option<String> = None;
+    let mut overloaded_name: Option<&str> = None;
     for ContextualizedDefinition {
         definition,
         visibility,
@@ -127,7 +127,7 @@ pub(crate) fn definitions(checker: &mut Checker) {
             if !overloaded_name.is_some_and(|overloaded_name| {
                 flake8_annotations::helpers::is_overload_impl(
                     definition,
-                    &overloaded_name,
+                    overloaded_name,
                     &checker.semantic,
                 )
             }) {

crates/ruff_linter/src/checkers/ast/analyze/expression.rs

@@ -32,7 +32,8 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 if let Some(operator) = typing::to_pep604_operator(value, slice, &checker.semantic)
                 {
                     if checker.enabled(Rule::FutureRewritableTypeAnnotation) {
-                        if !checker.semantic.future_annotations_or_stub()
+                        if !checker.semantic.in_typing_only_context()
+                            && !checker.semantic.future_annotations()
                             && checker.settings.target_version < PythonVersion::Py310
                             && checker.settings.target_version >= PythonVersion::Py37
                             && checker.semantic.in_annotation()
@@ -44,10 +45,10 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                         }
                     }
                     if checker.enabled(Rule::NonPEP604Annotation) {
-                        if checker.source_type.is_stub()
+                        if checker.semantic.in_typing_only_context()
                             || checker.settings.target_version >= PythonVersion::Py310
                             || (checker.settings.target_version >= PythonVersion::Py37
-                                && checker.semantic.future_annotations_or_stub()
+                                && checker.semantic.future_annotations()
                                 && checker.semantic.in_annotation()
                                 && !checker.settings.pyupgrade.keep_runtime_typing)
                         {
@@ -59,7 +60,8 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
 
             // Ex) list[...]
             if checker.enabled(Rule::FutureRequiredTypeAnnotation) {
-                if !checker.semantic.future_annotations_or_stub()
+                if !checker.semantic.in_typing_only_context()
+                    && !checker.semantic.future_annotations()
                     && checker.settings.target_version < PythonVersion::Py39
                     && checker.semantic.in_annotation()
                     && typing::is_pep585_generic(value, &checker.semantic)
@@ -78,7 +80,6 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                 Rule::DuplicateUnionMember,
                 Rule::RedundantLiteralUnion,
                 Rule::UnnecessaryTypeUnion,
-                Rule::NeverUnion,
             ]) {
                 // Avoid duplicate checks if the parent is a union, since these rules already
                 // traverse nested unions.
@@ -125,6 +126,12 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::PotentialIndexError) {
                 pylint::rules::potential_index_error(checker, value, slice);
             }
+            if checker.enabled(Rule::SortedMinMax) {
+                refurb::rules::sorted_min_max(checker, subscript);
+            }
+            if checker.enabled(Rule::FStringNumberFormat) {
+                refurb::rules::fstring_number_format(checker, subscript);
+            }
 
             pandas_vet::rules::subscript(checker, value, expr);
         }
@@ -184,7 +191,8 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                             typing::to_pep585_generic(expr, &checker.semantic)
                         {
                             if checker.enabled(Rule::FutureRewritableTypeAnnotation) {
-                                if !checker.semantic.future_annotations_or_stub()
+                                if !checker.semantic.in_typing_only_context()
+                                    && !checker.semantic.future_annotations()
                                     && checker.settings.target_version < PythonVersion::Py39
                                     && checker.settings.target_version >= PythonVersion::Py37
                                     && checker.semantic.in_annotation()
@@ -194,10 +202,10 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                                 }
                             }
                             if checker.enabled(Rule::NonPEP585Annotation) {
-                                if checker.source_type.is_stub()
+                                if checker.semantic.in_typing_only_context()
                                     || checker.settings.target_version >= PythonVersion::Py39
                                     || (checker.settings.target_version >= PythonVersion::Py37
-                                        && checker.semantic.future_annotations_or_stub()
+                                        && checker.semantic.future_annotations()
                                         && checker.semantic.in_annotation()
                                         && !checker.settings.pyupgrade.keep_runtime_typing)
                                 {
@@ -267,7 +275,8 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             ]) {
                 if let Some(replacement) = typing::to_pep585_generic(expr, &checker.semantic) {
                     if checker.enabled(Rule::FutureRewritableTypeAnnotation) {
-                        if !checker.semantic.future_annotations_or_stub()
+                        if !checker.semantic.in_typing_only_context()
+                            && !checker.semantic.future_annotations()
                             && checker.settings.target_version < PythonVersion::Py39
                             && checker.settings.target_version >= PythonVersion::Py37
                             && checker.semantic.in_annotation()
@@ -279,10 +288,10 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
                         }
                     }
                     if checker.enabled(Rule::NonPEP585Annotation) {
-                        if checker.source_type.is_stub()
+                        if checker.semantic.in_typing_only_context()
                             || checker.settings.target_version >= PythonVersion::Py39
                             || (checker.settings.target_version >= PythonVersion::Py37
-                                && checker.semantic.future_annotations_or_stub()
+                                && checker.semantic.future_annotations()
                                 && checker.semantic.in_annotation()
                                 && !checker.settings.pyupgrade.keep_runtime_typing)
                         {
@@ -1172,7 +1181,8 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
         }) => {
             // Ex) `str | None`
             if checker.enabled(Rule::FutureRequiredTypeAnnotation) {
-                if !checker.semantic.future_annotations_or_stub()
+                if !checker.semantic.in_typing_only_context()
+                    && !checker.semantic.future_annotations()
                     && checker.settings.target_version < PythonVersion::Py310
                     && checker.semantic.in_annotation()
                 {

crates/ruff_linter/src/checkers/ast/analyze/statement.rs

@@ -103,6 +103,12 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
             if checker.enabled(Rule::InvalidBytesReturnType) {
                 pylint::rules::invalid_bytes_return(checker, function_def);
             }
+            if checker.enabled(Rule::InvalidIndexReturnType) {
+                pylint::rules::invalid_index_return(checker, function_def);
+            }
+            if checker.enabled(Rule::InvalidHashReturnType) {
+                pylint::rules::invalid_hash_return(checker, function_def);
+            }
             if checker.enabled(Rule::InvalidStrReturnType) {
                 pylint::rules::invalid_str_return(checker, function_def);
             }
@@ -152,7 +158,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                     decorator_list,
                     returns.as_ref().map(AsRef::as_ref),
                     parameters,
-                    type_params.as_ref(),
+                    type_params.as_deref(),
                 );
             }
             if checker.source_type.is_stub() {
@@ -168,7 +174,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                 }
             }
             if checker.enabled(Rule::BadExitAnnotation) {
-                flake8_pyi::rules::bad_exit_annotation(checker, *is_async, name, parameters);
+                flake8_pyi::rules::bad_exit_annotation(checker, function_def);
             }
             if checker.enabled(Rule::RedundantNumericUnion) {
                 flake8_pyi::rules::redundant_numeric_union(checker, parameters);
@@ -871,7 +877,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                         if !matches!(checker.semantic.current_scope().kind, ScopeKind::Module) {
                             checker.diagnostics.push(Diagnostic::new(
                                 pyflakes::rules::UndefinedLocalWithNestedImportStarUsage {
-                                    name: helpers::format_import_from(level, module),
+                                    name: helpers::format_import_from(level, module).to_string(),
                                 },
                                 stmt.range(),
                             ));
@@ -880,7 +886,7 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                     if checker.enabled(Rule::UndefinedLocalWithImportStar) {
                         checker.diagnostics.push(Diagnostic::new(
                             pyflakes::rules::UndefinedLocalWithImportStar {
-                                name: helpers::format_import_from(level, module),
+                                name: helpers::format_import_from(level, module).to_string(),
                             },
                             stmt.range(),
                         ));
@@ -1317,10 +1323,10 @@ pub(crate) fn statement(stmt: &Stmt, checker: &mut Checker) {
                 pylint::rules::dict_iter_missing_items(checker, target, iter);
             }
             if checker.enabled(Rule::ManualListComprehension) {
-                perflint::rules::manual_list_comprehension(checker, target, body);
+                perflint::rules::manual_list_comprehension(checker, for_stmt);
             }
             if checker.enabled(Rule::ManualListCopy) {
-                perflint::rules::manual_list_copy(checker, target, body);
+                perflint::rules::manual_list_copy(checker, for_stmt);
             }
             if checker.enabled(Rule::ManualDictComprehension) {
                 perflint::rules::manual_dict_comprehension(checker, target, body);

crates/ruff_linter/src/checkers/ast/annotation.rs

@@ -56,10 +56,11 @@ impl AnnotationContext {
             _ => {}
         }
 
-        // If `__future__` annotations are enabled or it's a stub file,
+        // If `__future__` annotations are enabled or it's a stub file
+        // or we're in an `if TYPE_CHECKING` block,
         // then annotations are never evaluated at runtime,
         // so we can treat them as typing-only.
-        if semantic.future_annotations_or_stub() {
+        if semantic.in_typing_only_context() || semantic.future_annotations() {
             return Self::TypingOnly;
         }
 
@@ -88,7 +89,7 @@ impl AnnotationContext {
             semantic,
         ) {
             Self::RuntimeRequired
-        } else if semantic.future_annotations_or_stub() {
+        } else if semantic.in_typing_only_context() || semantic.future_annotations() {
             Self::TypingOnly
         } else {
             Self::RuntimeEvaluated

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -31,8 +31,8 @@ use std::path::Path;
 use itertools::Itertools;
 use log::debug;
 use ruff_python_ast::{
-    self as ast, Comprehension, ElifElseClause, ExceptHandler, Expr, ExprContext, FStringElement,
-    Keyword, MatchCase, Parameter, ParameterWithDefault, Parameters, Pattern, Stmt, Suite, UnaryOp,
+    self as ast, AnyParameterRef, Comprehension, ElifElseClause, ExceptHandler, Expr, ExprContext,
+    FStringElement, Keyword, MatchCase, Parameter, Parameters, Pattern, Stmt, Suite, UnaryOp,
 };
 use ruff_text_size::{Ranged, TextRange, TextSize};
 
@@ -464,7 +464,7 @@ impl<'a> Visitor<'a> for Checker<'a> {
                 let level = *level;
 
                 // Mark the top-level module as "seen" by the semantic model.
-                if level.map_or(true, |level| level == 0) {
+                if level == 0 {
                     if let Some(module) = module.and_then(|module| module.split('.').next()) {
                         self.semantic.add_module(module);
                     }
@@ -604,15 +604,11 @@ impl<'a> Visitor<'a> for Checker<'a> {
                     self.visit_type_params(type_params);
                 }
 
-                for parameter_with_default in parameters
-                    .posonlyargs
-                    .iter()
-                    .chain(&parameters.args)
-                    .chain(&parameters.kwonlyargs)
-                {
-                    if let Some(expr) = &parameter_with_default.parameter.annotation {
-                        if singledispatch {
+                for parameter in &**parameters {
+                    if let Some(expr) = parameter.annotation() {
+                        if singledispatch && !parameter.is_variadic() {
                             self.visit_runtime_required_annotation(expr);
+                            singledispatch = false;
                         } else {
                             match annotation {
                                 AnnotationContext::RuntimeRequired => {
@@ -625,42 +621,11 @@ impl<'a> Visitor<'a> for Checker<'a> {
                                     self.visit_annotation(expr);
                                 }
                             }
-                        };
+                        }
                     }
-                    if let Some(expr) = &parameter_with_default.default {
+                    if let Some(expr) = parameter.default() {
                         self.visit_expr(expr);
                     }
-                    singledispatch = false;
-                }
-                if let Some(arg) = &parameters.vararg {
-                    if let Some(expr) = &arg.annotation {
-                        match annotation {
-                            AnnotationContext::RuntimeRequired => {
-                                self.visit_runtime_required_annotation(expr);
-                            }
-                            AnnotationContext::RuntimeEvaluated => {
-                                self.visit_runtime_evaluated_annotation(expr);
-                            }
-                            AnnotationContext::TypingOnly => {
-                                self.visit_annotation(expr);
-                            }
-                        }
-                    }
-                }
-                if let Some(arg) = &parameters.kwarg {
-                    if let Some(expr) = &arg.annotation {
-                        match annotation {
-                            AnnotationContext::RuntimeRequired => {
-                                self.visit_runtime_required_annotation(expr);
-                            }
-                            AnnotationContext::RuntimeEvaluated => {
-                                self.visit_runtime_evaluated_annotation(expr);
-                            }
-                            AnnotationContext::TypingOnly => {
-                                self.visit_annotation(expr);
-                            }
-                        }
-                    }
                 }
                 for expr in returns {
                     match annotation {
@@ -928,7 +893,7 @@ impl<'a> Visitor<'a> for Checker<'a> {
 
     fn visit_expr(&mut self, expr: &'a Expr) {
         // Step 0: Pre-processing
-        if self.source_type.is_stub()
+        if self.semantic.in_typing_only_context()
             && self.semantic.in_class_base()
             && !self.semantic.in_deferred_class_base()
         {
@@ -938,15 +903,16 @@ impl<'a> Visitor<'a> for Checker<'a> {
             return;
         }
 
-        if !self.semantic.in_typing_literal()
-            // `in_deferred_type_definition()` will only be `true` if we're now visiting the deferred nodes
-            // after having already traversed the source tree once. If we're now visiting the deferred nodes,
-            // we can't defer again, or we'll infinitely recurse!
+        // `in_deferred_type_definition()` will only be `true` if we're now visiting the deferred nodes
+        // after having already traversed the source tree once. If we're now visiting the deferred nodes,
+        // we can't defer again, or we'll infinitely recurse!
+        let defer_probable_type_definition = self.semantic.in_type_definition()
             && !self.semantic.in_deferred_type_definition()
-            && self.semantic.in_type_definition()
-            && self.semantic.future_annotations_or_stub()
-            && (self.semantic.in_annotation() || self.source_type.is_stub())
-        {
+            && !self.semantic.in_typing_literal()
+            && (self.semantic.in_typing_only_context()
+                || (self.semantic.future_annotations() && self.semantic.in_annotation()));
+
+        if defer_probable_type_definition {
             if let Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) = expr {
                 self.visit.string_type_definitions.push((
                     expr.range(),
@@ -1043,19 +1009,11 @@ impl<'a> Visitor<'a> for Checker<'a> {
             ) => {
                 // Visit the default arguments, but avoid the body, which will be deferred.
                 if let Some(parameters) = parameters {
-                    for ParameterWithDefault {
-                        default,
-                        parameter: _,
-                        range: _,
-                    } in parameters
-                        .posonlyargs
-                        .iter()
-                        .chain(&parameters.args)
-                        .chain(&parameters.kwonlyargs)
+                    for default in parameters
+                        .iter_non_variadic_params()
+                        .filter_map(|param| param.default.as_deref())
                     {
-                        if let Some(expr) = &default {
-                            self.visit_expr(expr);
-                        }
+                        self.visit_expr(default);
                     }
                 }
 
@@ -1483,20 +1441,8 @@ impl<'a> Visitor<'a> for Checker<'a> {
         // Step 1: Binding.
         // Bind, but intentionally avoid walking default expressions, as we handle them
         // upstream.
-        for parameter_with_default in &parameters.posonlyargs {
-            self.visit_parameter(&parameter_with_default.parameter);
-        }
-        for parameter_with_default in &parameters.args {
-            self.visit_parameter(&parameter_with_default.parameter);
-        }
-        if let Some(arg) = &parameters.vararg {
-            self.visit_parameter(arg);
-        }
-        for parameter_with_default in &parameters.kwonlyargs {
-            self.visit_parameter(&parameter_with_default.parameter);
-        }
-        if let Some(arg) = &parameters.kwarg {
-            self.visit_parameter(arg);
+        for parameter in parameters.iter().map(AnyParameterRef::as_parameter) {
+            self.visit_parameter(parameter);
         }
 
         // Step 4: Analysis
@@ -1568,8 +1514,8 @@ impl<'a> Visitor<'a> for Checker<'a> {
         // Step 1: Binding
         match type_param {
             ast::TypeParam::TypeVar(ast::TypeParamTypeVar { name, range, .. })
-            | ast::TypeParam::TypeVarTuple(ast::TypeParamTypeVarTuple { name, range })
-            | ast::TypeParam::ParamSpec(ast::TypeParamParamSpec { name, range }) => {
+            | ast::TypeParam::TypeVarTuple(ast::TypeParamTypeVarTuple { name, range, .. })
+            | ast::TypeParam::ParamSpec(ast::TypeParamParamSpec { name, range, .. }) => {
                 self.add_binding(
                     name.as_str(),
                     *range,
@@ -1579,13 +1525,46 @@ impl<'a> Visitor<'a> for Checker<'a> {
             }
         }
         // Step 2: Traversal
-        if let ast::TypeParam::TypeVar(ast::TypeParamTypeVar {
-            bound: Some(bound), ..
-        }) = type_param
-        {
-            self.visit
-                .type_param_definitions
-                .push((bound, self.semantic.snapshot()));
+        match type_param {
+            ast::TypeParam::TypeVar(ast::TypeParamTypeVar {
+                bound,
+                default,
+                name: _,
+                range: _,
+            }) => {
+                if let Some(expr) = bound {
+                    self.visit
+                        .type_param_definitions
+                        .push((expr, self.semantic.snapshot()));
+                }
+                if let Some(expr) = default {
+                    self.visit
+                        .type_param_definitions
+                        .push((expr, self.semantic.snapshot()));
+                }
+            }
+            ast::TypeParam::TypeVarTuple(ast::TypeParamTypeVarTuple {
+                default,
+                name: _,
+                range: _,
+            }) => {
+                if let Some(expr) = default {
+                    self.visit
+                        .type_param_definitions
+                        .push((expr, self.semantic.snapshot()));
+                }
+            }
+            ast::TypeParam::ParamSpec(ast::TypeParamParamSpec {
+                default,
+                name: _,
+                range: _,
+            }) => {
+                if let Some(expr) = default {
+                    self.visit
+                        .type_param_definitions
+                        .push((expr, self.semantic.snapshot()));
+                }
+            }
         }
     }
 
@@ -1975,9 +1954,10 @@ impl<'a> Checker<'a> {
 
     /// After initial traversal of the AST, visit all class bases that were deferred.
     ///
-    /// This method should only be relevant in stub files, where forward references are
-    /// legal in class bases. For other kinds of Python files, using a forward reference
-    /// in a class base is never legal, so `self.visit.class_bases` should always be empty.
+    /// This method should only be relevant in stub files or `TYPE_CHECKING` blocks,
+    /// where forward references are legal in class bases. In other contexts,
+    /// using a forward reference in a class base is never legal,
+    /// so `self.visit.class_bases` should always be empty.
     ///
     /// For example, in a stub file:
     /// ```python
@@ -1987,12 +1967,9 @@ impl<'a> Checker<'a> {
     fn visit_deferred_class_bases(&mut self) {
         let snapshot = self.semantic.snapshot();
         let deferred_bases = std::mem::take(&mut self.visit.class_bases);
-        debug_assert!(
-            self.source_type.is_stub() || deferred_bases.is_empty(),
-            "Class bases should never be deferred outside of stub files"
-        );
         for (expr, snapshot) in deferred_bases {
             self.semantic.restore(snapshot);
+            debug_assert!(self.semantic.in_typing_only_context());
             // Set this flag to avoid infinite recursion, or we'll just defer it again:
             self.semantic.flags |= SemanticModelFlags::DEFERRED_CLASS_BASE;
             self.visit_expr(expr);
@@ -2003,8 +1980,9 @@ impl<'a> Checker<'a> {
     /// After initial traversal of the AST, visit all "future type definitions".
     ///
     /// A "future type definition" is a type definition where [PEP 563] semantics
-    /// apply (i.e., an annotation in a module that has `from __future__ import annotations`
-    /// at the top of the file, or an annotation in a stub file). These type definitions
+    /// apply (i.e., a definition in a module that has `from __future__ import annotations`
+    /// at the top of the file, a definition in a `TYPE_CHECKING` block,
+    /// or an annotation in a stub file). These type definitions
     /// support forward references, so they are deferred on initial traversal
     /// of the source tree.
     ///
@@ -2025,15 +2003,6 @@ impl<'a> Checker<'a> {
             let type_definitions = std::mem::take(&mut self.visit.future_type_definitions);
             for (expr, snapshot) in type_definitions {
                 self.semantic.restore(snapshot);
-
-                // Type definitions should only be considered "`__future__` type definitions"
-                // if they are annotations in a module where `from __future__ import
-                // annotations` is active, or they are type definitions in a stub file.
-                debug_assert!(
-                    self.semantic.future_annotations_or_stub()
-                        && (self.source_type.is_stub() || self.semantic.in_annotation())
-                );
-
                 self.semantic.flags |= SemanticModelFlags::TYPE_DEFINITION
                     | SemanticModelFlags::FUTURE_TYPE_DEFINITION;
                 self.visit_expr(expr);
@@ -2093,7 +2062,10 @@ impl<'a> Checker<'a> {
 
                     self.semantic.restore(snapshot);
 
-                    if self.semantic.in_annotation() && self.semantic.future_annotations_or_stub() {
+                    if self.semantic.in_annotation()
+                        && (self.semantic.in_typing_only_context()
+                            || self.semantic.future_annotations())
+                    {
                         if self.enabled(Rule::QuotedAnnotation) {
                             pyupgrade::rules::quoted_annotation(self, value, range);
                         }

crates/ruff_linter/src/checkers/imports.rs

@@ -1,17 +1,13 @@
 //! Lint rules based on import analysis.
-use std::borrow::Cow;
 use std::path::Path;
 
 use ruff_diagnostics::Diagnostic;
 use ruff_notebook::CellOffsets;
-use ruff_python_ast::helpers::to_module_path;
-use ruff_python_ast::imports::{ImportMap, ModuleImport};
 use ruff_python_ast::statement_visitor::StatementVisitor;
-use ruff_python_ast::{self as ast, PySourceType, Stmt, Suite};
+use ruff_python_ast::{PySourceType, Suite};
 use ruff_python_codegen::Stylist;
 use ruff_python_index::Indexer;
 use ruff_source_file::Locator;
-use ruff_text_size::Ranged;
 
 use crate::directives::IsortDirectives;
 use crate::registry::Rule;
@@ -19,57 +15,6 @@ use crate::rules::isort;
 use crate::rules::isort::block::{Block, BlockBuilder};
 use crate::settings::LinterSettings;
 
-fn extract_import_map(path: &Path, package: Option<&Path>, blocks: &[&Block]) -> Option<ImportMap> {
-    let module_path = to_module_path(package?, path)?;
-
-    let num_imports = blocks.iter().map(|block| block.imports.len()).sum();
-    let mut module_imports = Vec::with_capacity(num_imports);
-    for stmt in blocks.iter().flat_map(|block| &block.imports) {
-        match stmt {
-            Stmt::Import(ast::StmtImport { names, range: _ }) => {
-                module_imports.extend(
-                    names
-                        .iter()
-                        .map(|name| ModuleImport::new(name.name.to_string(), stmt.range())),
-                );
-            }
-            Stmt::ImportFrom(ast::StmtImportFrom {
-                module,
-                names,
-                level,
-                range: _,
-            }) => {
-                let level = level.unwrap_or_default() as usize;
-                let module = if let Some(module) = module {
-                    let module: &String = module.as_ref();
-                    if level == 0 {
-                        Cow::Borrowed(module)
-                    } else {
-                        if module_path.len() <= level {
-                            continue;
-                        }
-                        let prefix = module_path[..module_path.len() - level].join(".");
-                        Cow::Owned(format!("{prefix}.{module}"))
-                    }
-                } else {
-                    if module_path.len() <= level {
-                        continue;
-                    }
-                    Cow::Owned(module_path[..module_path.len() - level].join("."))
-                };
-                module_imports.extend(names.iter().map(|name| {
-                    ModuleImport::new(format!("{}.{}", module, name.name), name.range())
-                }));
-            }
-            _ => panic!("Expected Stmt::Import | Stmt::ImportFrom"),
-        }
-    }
-
-    let mut import_map = ImportMap::default();
-    import_map.insert(module_path.join("."), module_imports);
-    Some(import_map)
-}
-
 #[allow(clippy::too_many_arguments)]
 pub(crate) fn check_imports(
     python_ast: &Suite,
@@ -78,11 +23,10 @@ pub(crate) fn check_imports(
     directives: &IsortDirectives,
     settings: &LinterSettings,
     stylist: &Stylist,
-    path: &Path,
     package: Option<&Path>,
     source_type: PySourceType,
     cell_offsets: Option<&CellOffsets>,
-) -> (Vec<Diagnostic>, Option<ImportMap>) {
+) -> Vec<Diagnostic> {
     // Extract all import blocks from the AST.
     let tracker = {
         let mut tracker =
@@ -122,8 +66,5 @@ pub(crate) fn check_imports(
         ));
     }
 
-    // Extract import map.
-    let imports = extract_import_map(path, package, &blocks);
-
-    (diagnostics, imports)
+    diagnostics
 }

crates/ruff_linter/src/checkers/noqa.rs

@@ -3,17 +3,19 @@
 use std::path::Path;
 
 use itertools::Itertools;
-use ruff_text_size::Ranged;
+use rustc_hash::FxHashSet;
 
 use ruff_diagnostics::{Diagnostic, Edit, Fix};
 use ruff_python_trivia::CommentRanges;
 use ruff_source_file::Locator;
+use ruff_text_size::Ranged;
 
 use crate::fix::edits::delete_comment;
-use crate::noqa;
-use crate::noqa::{Directive, FileExemption, NoqaDirectives, NoqaMapping};
+use crate::noqa::{Code, Directive, FileExemption, NoqaDirectives, NoqaMapping};
 use crate::registry::{AsRule, Rule, RuleSet};
 use crate::rule_redirects::get_redirect_target;
+use crate::rules::pygrep_hooks;
+use crate::rules::ruff;
 use crate::rules::ruff::rules::{UnusedCodes, UnusedNOQA};
 use crate::settings::LinterSettings;
 
@@ -84,7 +86,7 @@ pub(crate) fn check_noqa(
                         true
                     }
                     Directive::Codes(directive) => {
-                        if noqa::includes(diagnostic.kind.rule(), directive.codes()) {
+                        if directive.includes(diagnostic.kind.rule()) {
                             directive_line
                                 .matches
                                 .push(diagnostic.kind.rule().noqa_code());
@@ -127,33 +129,37 @@ pub(crate) fn check_noqa(
                 }
                 Directive::Codes(directive) => {
                     let mut disabled_codes = vec![];
+                    let mut duplicated_codes = vec![];
                     let mut unknown_codes = vec![];
                     let mut unmatched_codes = vec![];
                     let mut valid_codes = vec![];
+                    let mut seen_codes = FxHashSet::default();
                     let mut self_ignore = false;
-                    for code in directive.codes() {
-                        let code = get_redirect_target(code).unwrap_or(code);
+                    for original_code in directive.iter().map(Code::as_str) {
+                        let code = get_redirect_target(original_code).unwrap_or(original_code);
                         if Rule::UnusedNOQA.noqa_code() == code {
                             self_ignore = true;
                             break;
                         }
 
-                        if line.matches.iter().any(|match_| *match_ == code)
+                        if !seen_codes.insert(original_code) {
+                            duplicated_codes.push(original_code);
+                        } else if line.matches.iter().any(|match_| *match_ == code)
                             || settings
                                 .external
                                 .iter()
                                 .any(|external| code.starts_with(external))
                         {
-                            valid_codes.push(code);
+                            valid_codes.push(original_code);
                         } else {
                             if let Ok(rule) = Rule::from_code(code) {
                                 if settings.rules.enabled(rule) {
-                                    unmatched_codes.push(code);
+                                    unmatched_codes.push(original_code);
                                 } else {
-                                    disabled_codes.push(code);
+                                    disabled_codes.push(original_code);
                                 }
                             } else {
-                                unknown_codes.push(code);
+                                unknown_codes.push(original_code);
                             }
                         }
                     }
@@ -163,6 +169,7 @@ pub(crate) fn check_noqa(
                     }
 
                     if !(disabled_codes.is_empty()
+                        && duplicated_codes.is_empty()
                         && unknown_codes.is_empty()
                         && unmatched_codes.is_empty())
                     {
@@ -173,6 +180,10 @@ pub(crate) fn check_noqa(
                                         .iter()
                                         .map(|code| (*code).to_string())
                                         .collect(),
+                                    duplicated: duplicated_codes
+                                        .iter()
+                                        .map(|code| (*code).to_string())
+                                        .collect(),
                                     unknown: unknown_codes
                                         .iter()
                                         .map(|code| (*code).to_string())
@@ -203,6 +214,14 @@ pub(crate) fn check_noqa(
         }
     }
 
+    if settings.rules.enabled(Rule::BlanketNOQA) {
+        pygrep_hooks::rules::blanket_noqa(diagnostics, &noqa_directives, locator);
+    }
+
+    if settings.rules.enabled(Rule::RedirectedNOQA) {
+        ruff::rules::redirected_noqa(diagnostics, &noqa_directives);
+    }
+
     ignored_diagnostics.sort_unstable();
     ignored_diagnostics
 }

crates/ruff_linter/src/checkers/tokens.rs

@@ -45,10 +45,6 @@ pub(crate) fn check_tokens(
             .check_lines(tokens, &mut diagnostics);
     }
 
-    if settings.rules.enabled(Rule::BlanketNOQA) {
-        pygrep_hooks::rules::blanket_noqa(&mut diagnostics, indexer, locator);
-    }
-
     if settings.rules.enabled(Rule::BlanketTypeIgnore) {
         pygrep_hooks::rules::blanket_type_ignore(&mut diagnostics, indexer, locator);
     }

crates/ruff_linter/src/codes.rs

@@ -243,8 +243,10 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "E0302") => (RuleGroup::Stable, rules::pylint::rules::UnexpectedSpecialMethodSignature),
         (Pylint, "E0303") => (RuleGroup::Preview, rules::pylint::rules::InvalidLengthReturnType),
         (Pylint, "E0304") => (RuleGroup::Preview, rules::pylint::rules::InvalidBoolReturnType),
+        (Pylint, "E0305") => (RuleGroup::Preview, rules::pylint::rules::InvalidIndexReturnType),
         (Pylint, "E0307") => (RuleGroup::Stable, rules::pylint::rules::InvalidStrReturnType),
         (Pylint, "E0308") => (RuleGroup::Preview, rules::pylint::rules::InvalidBytesReturnType),
+        (Pylint, "E0309") => (RuleGroup::Preview, rules::pylint::rules::InvalidHashReturnType),
         (Pylint, "E0604") => (RuleGroup::Stable, rules::pylint::rules::InvalidAllObject),
         (Pylint, "E0605") => (RuleGroup::Stable, rules::pylint::rules::InvalidAllFormat),
         (Pylint, "E0643") => (RuleGroup::Preview, rules::pylint::rules::PotentialIndexError),
@@ -967,33 +969,34 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "028") => (RuleGroup::Preview, rules::ruff::rules::InvalidFormatterSuppressionComment),
         (Ruff, "029") => (RuleGroup::Preview, rules::ruff::rules::UnusedAsync),
         (Ruff, "100") => (RuleGroup::Stable, rules::ruff::rules::UnusedNOQA),
+        (Ruff, "101") => (RuleGroup::Preview, rules::ruff::rules::RedirectedNOQA),
         (Ruff, "200") => (RuleGroup::Stable, rules::ruff::rules::InvalidPyprojectToml),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "900") => (RuleGroup::Stable, rules::ruff::rules::StableTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "901") => (RuleGroup::Stable, rules::ruff::rules::StableTestRuleSafeFix),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "902") => (RuleGroup::Stable, rules::ruff::rules::StableTestRuleUnsafeFix),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "903") => (RuleGroup::Stable, rules::ruff::rules::StableTestRuleDisplayOnlyFix),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "911") => (RuleGroup::Preview, rules::ruff::rules::PreviewTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         #[allow(deprecated)]
         (Ruff, "912") => (RuleGroup::Nursery, rules::ruff::rules::NurseryTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "920") => (RuleGroup::Deprecated, rules::ruff::rules::DeprecatedTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "921") => (RuleGroup::Deprecated, rules::ruff::rules::AnotherDeprecatedTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "930") => (RuleGroup::Removed, rules::ruff::rules::RemovedTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "931") => (RuleGroup::Removed, rules::ruff::rules::AnotherRemovedTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "940") => (RuleGroup::Removed, rules::ruff::rules::RedirectedFromTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "950") => (RuleGroup::Stable, rules::ruff::rules::RedirectedToTestRule),
-        #[cfg(feature = "test-rules")]
+        #[cfg(any(feature = "test-rules", test))]
         (Ruff, "960") => (RuleGroup::Removed, rules::ruff::rules::RedirectedFromPrefixTestRule),
 
 
@@ -1048,6 +1051,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Refurb, "110") => (RuleGroup::Preview, rules::refurb::rules::IfExpInsteadOfOrOperator),
         #[allow(deprecated)]
         (Refurb, "113") => (RuleGroup::Nursery, rules::refurb::rules::RepeatedAppend),
+        (Refurb, "116") => (RuleGroup::Preview, rules::refurb::rules::FStringNumberFormat),
         (Refurb, "118") => (RuleGroup::Preview, rules::refurb::rules::ReimplementedOperator),
         (Refurb, "129") => (RuleGroup::Preview, rules::refurb::rules::ReadlinesInFor),
         #[allow(deprecated)]
@@ -1073,6 +1077,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Refurb, "180") => (RuleGroup::Preview, rules::refurb::rules::MetaClassABCMeta),
         (Refurb, "181") => (RuleGroup::Preview, rules::refurb::rules::HashlibDigestHex),
         (Refurb, "187") => (RuleGroup::Preview, rules::refurb::rules::ListReverseCopy),
+        (Refurb, "192") => (RuleGroup::Preview, rules::refurb::rules::SortedMinMax),
 
         // flake8-logging
         (Flake8Logging, "001") => (RuleGroup::Stable, rules::flake8_logging::rules::DirectLoggerInstantiation),

crates/ruff_linter/src/directives.rs

@@ -273,7 +273,7 @@ pub(crate) struct TodoComment<'a> {
     pub(crate) directive: TodoDirective<'a>,
     /// The comment's actual [`TextRange`].
     pub(crate) range: TextRange,
-    /// The comment range's position in [`Indexer`].comment_ranges()
+    /// The comment range's position in [`Indexer::comment_ranges`]
     pub(crate) range_index: usize,
 }
 

crates/ruff_linter/src/fix/edits.rs

@@ -122,6 +122,28 @@ pub(crate) fn remove_unused_imports<'a>(
     }
 }
 
+/// Edits to make the specified imports explicit, e.g. change `import x` to `import x as x`.
+pub(crate) fn make_redundant_alias<'a>(
+    member_names: impl Iterator<Item = &'a str>,
+    stmt: &Stmt,
+) -> Vec<Edit> {
+    let aliases = match stmt {
+        Stmt::Import(ast::StmtImport { names, .. }) => names,
+        Stmt::ImportFrom(ast::StmtImportFrom { names, .. }) => names,
+        _ => {
+            return Vec::new();
+        }
+    };
+    member_names
+        .filter_map(|name| {
+            aliases
+                .iter()
+                .find(|alias| alias.asname.is_none() && name == alias.name.id)
+                .map(|alias| Edit::range_replacement(format!("{name} as {name}"), alias.range))
+        })
+        .collect()
+}
+
 #[derive(Debug, Copy, Clone)]
 pub(crate) enum Parentheses {
     /// Remove parentheses, if the removed argument is the only argument left.
@@ -457,11 +479,12 @@ fn all_lines_fit(
 mod tests {
     use anyhow::Result;
 
+    use ruff_diagnostics::Edit;
     use ruff_python_parser::parse_suite;
     use ruff_source_file::Locator;
-    use ruff_text_size::{Ranged, TextSize};
+    use ruff_text_size::{Ranged, TextRange, TextSize};
 
-    use crate::fix::edits::{next_stmt_break, trailing_semicolon};
+    use crate::fix::edits::{make_redundant_alias, next_stmt_break, trailing_semicolon};
 
     #[test]
     fn find_semicolon() -> Result<()> {
@@ -532,4 +555,35 @@ x = 1 \
             TextSize::from(12)
         );
     }
+
+    #[test]
+    fn redundant_alias() {
+        let contents = "import x, y as y, z as bees";
+        let program = parse_suite(contents).unwrap();
+        let stmt = program.first().unwrap();
+        assert_eq!(
+            make_redundant_alias(["x"].into_iter(), stmt),
+            vec![Edit::range_replacement(
+                String::from("x as x"),
+                TextRange::new(TextSize::new(7), TextSize::new(8)),
+            )],
+            "make just one item redundant"
+        );
+        assert_eq!(
+            make_redundant_alias(vec!["x", "y"].into_iter(), stmt),
+            vec![Edit::range_replacement(
+                String::from("x as x"),
+                TextRange::new(TextSize::new(7), TextSize::new(8)),
+            )],
+            "the second item is already a redundant alias"
+        );
+        assert_eq!(
+            make_redundant_alias(vec!["x", "z"].into_iter(), stmt),
+            vec![Edit::range_replacement(
+                String::from("x as x"),
+                TextRange::new(TextSize::new(7), TextSize::new(8)),
+            )],
+            "the third item is already aliased to something else"
+        );
+    }
 }