Bump 0.9.10 (#16556)

## Summary

This should give us better coverage for the unsupported syntax error
features and
increases our confidence that the formatter doesn't accidentially
introduce new unsupported
syntax errors. 

A feature like this would have been very useful when working on f-string
formatting
where it took a lot of iteration to find all Python 3.11 or older
incompatibilities.

## Test Plan

I applied my changes on top of
https://github.com/astral-sh/ruff/pull/16523 and
removed the target version check in the with-statement formatting code.
As expected,
the integration tests now failed

.github/ISSUE_TEMPLATE/1_bug_report.yaml

@@ -0,0 +1,31 @@
+name: Bug report
+description: Report an error or unexpected behavior
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thank you for taking the time to report an issue! We're glad to have you involved with Ruff.
+
+        **Before reporting, please make sure to search through [existing issues](https://github.com/astral-sh/ruff/issues?q=is:issue+is:open+label:bug) (including [closed](https://github.com/astral-sh/ruff/issues?q=is:issue%20state:closed%20label:bug)).**
+
+  - type: textarea
+    attributes:
+      label: Summary
+      description: |
+        A clear and concise description of the bug, including a minimal reproducible example.
+
+        Be sure to include the command you invoked (e.g., `ruff check /path/to/file.py --fix`), ideally including the `--isolated` flag and
+        the current Ruff settings (e.g., relevant sections from your `pyproject.toml`).
+
+        If possible, try to include the [playground](https://play.ruff.rs) link that reproduces this issue.
+
+    validations:
+      required: true
+
+  - type: input
+    attributes:
+      label: Version
+      description: What version of ruff are you using? (see `ruff version`)
+      placeholder: e.g., ruff 0.9.3 (90589372d 2025-01-23)
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/2_rule_request.yaml

@@ -0,0 +1,10 @@
+name: Rule request
+description: Anything related to lint rules (proposing new rules, changes to existing rules, auto-fixes, etc.)
+body:
+  - type: textarea
+    attributes:
+      label: Summary
+      description: |
+        A clear and concise description of the relevant request. If applicable, please describe the current behavior as well.
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/3_question.yaml

@@ -0,0 +1,18 @@
+name: Question
+description: Ask a question about Ruff
+labels: ["question"]
+body:
+  - type: textarea
+    attributes:
+      label: Question
+      description: Describe your question in detail.
+    validations:
+      required: true
+
+  - type: input
+    attributes:
+      label: Version
+      description: What version of ruff are you using? (see `ruff version`)
+      placeholder: e.g., ruff 0.9.3 (90589372d 2025-01-23)
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/config.yml

@@ -1,2 +1,8 @@
-# This file cannot use the extension `.yaml`.
-blank_issues_enabled: false
+blank_issues_enabled: true
+contact_links:
+  - name: Documentation
+    url: https://docs.astral.sh/ruff
+    about: Please consult the documentation before creating an issue.
+  - name: Community
+    url: https://discord.com/invite/astral-sh
+    about: Join our Discord community to ask questions and collaborate.

.github/ISSUE_TEMPLATE/issue.yaml

@@ -1,22 +0,0 @@
-name: New issue
-description: A generic issue
-
-body:
-  - type: markdown
-    attributes:
-      value: |
-        Thank you for taking the time to report an issue! We're glad to have you involved with Ruff.
-
-        If you're filing a bug report, please consider including the following information:
-
-        * List of keywords you searched for before creating this issue. Write them down here so that others can find this issue more easily and help provide feedback.
-        e.g. "RUF001", "unused variable", "Jupyter notebook"
-        * A minimal code snippet that reproduces the bug.
-        * The command you invoked (e.g., `ruff /path/to/file.py --fix`), ideally including the `--isolated` flag.
-        * The current Ruff settings (any relevant sections from your `pyproject.toml`).
-        * The current Ruff version (`ruff --version`).
-
-  - type: textarea
-    attributes:
-      label: Description
-      description: A description of the issue

.github/renovate.json5

@@ -58,12 +58,6 @@
       description: "Disable PRs updating GitHub runners (e.g. 'runs-on: macos-14')",
       enabled: false,
     },
-    {
-      // TODO: Remove this once the codebase is upgrade to v4 (https://github.com/astral-sh/ruff/pull/16069)
-      matchPackageNames: ["tailwindcss"],
-      matchManagers: ["npm"],
-      enabled: false,
-    },
     {
       // Disable updates of `zip-rs`; intentionally pinned for now due to ownership change
       // See: https://github.com/astral-sh/uv/issues/3642
@@ -101,14 +95,7 @@
       matchManagers: ["cargo"],
       matchPackageNames: ["strum"],
       description: "Weekly update of strum dependencies",
-    },
-    {
-      groupName: "ESLint",
-      matchManagers: ["npm"],
-      matchPackageNames: ["eslint"],
-      allowedVersions: "<9",
-      description: "Constraint ESLint to version 8 until TypeScript-eslint supports ESLint 9", // https://github.com/typescript-eslint/typescript-eslint/issues/8211
-    },
+    }
   ],
   vulnerabilityAlerts: {
     commitMessageSuffix: "",

.github/workflows/daily_property_tests.yaml

@@ -47,6 +47,7 @@ jobs:
         run: |
           export QUICKCHECK_TESTS=100000
           for _ in {1..5}; do
+            cargo test --locked --release --package red_knot_python_semantic -- --ignored list::property_tests
             cargo test --locked --release --package red_knot_python_semantic -- --ignored types::property_tests::stable
           done
 

.pre-commit-config.yaml

@@ -60,7 +60,7 @@ repos:
           - black==25.1.0
 
   - repo: https://github.com/crate-ci/typos
-    rev: v1.29.7
+    rev: v1.30.0
     hooks:
       - id: typos
 
@@ -74,7 +74,7 @@ repos:
         pass_filenames: false # This makes it a lot faster
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.9.6
+    rev: v0.9.9
     hooks:
       - id: ruff-format
       - id: ruff
@@ -84,7 +84,7 @@ repos:
 
   # Prettier
   - repo: https://github.com/rbubley/mirrors-prettier
-    rev: v3.5.1
+    rev: v3.5.2
     hooks:
       - id: prettier
         types: [yaml]
@@ -92,12 +92,12 @@ repos:
   # zizmor detects security vulnerabilities in GitHub Actions workflows.
   # Additional configuration for the tool is found in `.github/zizmor.yml`
   - repo: https://github.com/woodruffw/zizmor-pre-commit
-    rev: v1.3.1
+    rev: v1.4.1
     hooks:
       - id: zizmor
 
   - repo: https://github.com/python-jsonschema/check-jsonschema
-    rev: 0.31.1
+    rev: 0.31.2
     hooks:
       - id: check-github-workflows
 

CHANGELOG.md

@@ -1,5 +1,83 @@
 # Changelog
 
+## 0.9.10
+
+### Preview features
+
+- \[`ruff`\] Add new rule `RUF059`: Unused unpacked assignment ([#16449](https://github.com/astral-sh/ruff/pull/16449))
+- \[`syntax-errors`\] Detect assignment expressions before Python 3.8 ([#16383](https://github.com/astral-sh/ruff/pull/16383))
+- \[`syntax-errors`\] Named expressions in decorators before Python 3.9 ([#16386](https://github.com/astral-sh/ruff/pull/16386))
+- \[`syntax-errors`\] Parenthesized keyword argument names after Python 3.8 ([#16482](https://github.com/astral-sh/ruff/pull/16482))
+- \[`syntax-errors`\] Positional-only parameters before Python 3.8 ([#16481](https://github.com/astral-sh/ruff/pull/16481))
+- \[`syntax-errors`\] Tuple unpacking in `return` and `yield` before Python 3.8 ([#16485](https://github.com/astral-sh/ruff/pull/16485))
+- \[`syntax-errors`\] Type parameter defaults before Python 3.13 ([#16447](https://github.com/astral-sh/ruff/pull/16447))
+- \[`syntax-errors`\] Type parameter lists before Python 3.12 ([#16479](https://github.com/astral-sh/ruff/pull/16479))
+- \[`syntax-errors`\] `except*` before Python 3.11 ([#16446](https://github.com/astral-sh/ruff/pull/16446))
+- \[`syntax-errors`\] `type` statements before Python 3.12 ([#16478](https://github.com/astral-sh/ruff/pull/16478))
+
+### Bug fixes
+
+- Escape template filenames in glob patterns in configuration ([#16407](https://github.com/astral-sh/ruff/pull/16407))
+- \[`flake8-simplify`\] Exempt unittest context methods for `SIM115` rule ([#16439](https://github.com/astral-sh/ruff/pull/16439))
+- Formatter: Fix syntax error location in notebooks ([#16499](https://github.com/astral-sh/ruff/pull/16499))
+- \[`pyupgrade`\] Do not offer fix when at least one target is `global`/`nonlocal` (`UP028`) ([#16451](https://github.com/astral-sh/ruff/pull/16451))
+- \[`flake8-builtins`\] Ignore variables matching module attribute names (`A001`) ([#16454](https://github.com/astral-sh/ruff/pull/16454))
+- \[`pylint`\] Convert `code` keyword argument to a positional argument in fix for (`PLR1722`) ([#16424](https://github.com/astral-sh/ruff/pull/16424))
+
+### CLI
+
+- Move rule code from `description` to `check_name` in GitLab output serializer ([#16437](https://github.com/astral-sh/ruff/pull/16437))
+
+### Documentation
+
+- \[`pydocstyle`\] Clarify that `D417` only checks docstrings with an arguments section ([#16494](https://github.com/astral-sh/ruff/pull/16494))
+
+## 0.9.9
+
+### Preview features
+
+- Fix caching of unsupported-syntax errors ([#16425](https://github.com/astral-sh/ruff/pull/16425))
+
+### Bug fixes
+
+- Only show unsupported-syntax errors in editors when preview mode is enabled ([#16429](https://github.com/astral-sh/ruff/pull/16429))
+
+## 0.9.8
+
+### Preview features
+
+- Start detecting version-related syntax errors in the parser ([#16090](https://github.com/astral-sh/ruff/pull/16090))
+
+### Rule changes
+
+- \[`pylint`\] Mark fix unsafe (`PLW1507`) ([#16343](https://github.com/astral-sh/ruff/pull/16343))
+- \[`pylint`\] Catch `case np.nan`/`case math.nan` in `match` statements (`PLW0177`) ([#16378](https://github.com/astral-sh/ruff/pull/16378))
+- \[`ruff`\] Add more Pydantic models variants to the list of default copy semantics (`RUF012`) ([#16291](https://github.com/astral-sh/ruff/pull/16291))
+
+### Server
+
+- Avoid indexing the project if `configurationPreference` is `editorOnly` ([#16381](https://github.com/astral-sh/ruff/pull/16381))
+- Avoid unnecessary info at non-trace server log level ([#16389](https://github.com/astral-sh/ruff/pull/16389))
+- Expand `ruff.configuration` to allow inline config ([#16296](https://github.com/astral-sh/ruff/pull/16296))
+- Notify users for invalid client settings ([#16361](https://github.com/astral-sh/ruff/pull/16361))
+
+### Configuration
+
+- Add `per-file-target-version` option ([#16257](https://github.com/astral-sh/ruff/pull/16257))
+
+### Bug fixes
+
+- \[`refurb`\] Do not consider docstring(s) (`FURB156`) ([#16391](https://github.com/astral-sh/ruff/pull/16391))
+- \[`flake8-self`\] Ignore attribute accesses on instance-like variables (`SLF001`) ([#16149](https://github.com/astral-sh/ruff/pull/16149))
+- \[`pylint`\] Fix false positives, add missing methods, and support positional-only parameters (`PLE0302`) ([#16263](https://github.com/astral-sh/ruff/pull/16263))
+- \[`flake8-pyi`\] Mark `PYI030` fix unsafe when comments are deleted ([#16322](https://github.com/astral-sh/ruff/pull/16322))
+
+### Documentation
+
+- Fix example for `S611` ([#16316](https://github.com/astral-sh/ruff/pull/16316))
+- Normalize inconsistent markdown headings in docstrings ([#16364](https://github.com/astral-sh/ruff/pull/16364))
+- Document MSRV policy ([#16384](https://github.com/astral-sh/ruff/pull/16384))
+
 ## 0.9.7
 
 ### Preview features
@@ -13,16 +91,7 @@
 
 ### Rule changes
 
-- \[`flake8-comprehensions`\]: Handle trailing comma in `C403` fix ([#16110](https://github.com/astral-sh/ruff/pull/16110))
 - \[`flake8-debugger`\] Also flag `sys.breakpointhook` and `sys.__breakpointhook__` (`T100`) ([#16191](https://github.com/astral-sh/ruff/pull/16191))
-- \[`pydocstyle`\] Handle arguments with the same names as sections (`D417`) ([#16011](https://github.com/astral-sh/ruff/pull/16011))
-- \[`pylint`\] Correct ordering of arguments in fix for `if-stmt-min-max` (`PLR1730`) ([#16080](https://github.com/astral-sh/ruff/pull/16080))
-- \[`pylint`\] Do not offer fix for raw strings (`PLE251`) ([#16132](https://github.com/astral-sh/ruff/pull/16132))
-- \[`pyupgrade`\] Do not upgrade functional `TypedDicts` with private field names to the class-based syntax (`UP013`) ([#16219](https://github.com/astral-sh/ruff/pull/16219))
-- \[`pyupgrade`\] Handle micro version numbers correctly (`UP036`) ([#16091](https://github.com/astral-sh/ruff/pull/16091))
-- \[`pyupgrade`\] Unwrap unary expressions correctly (`UP018`) ([#15919](https://github.com/astral-sh/ruff/pull/15919))
-- \[`ruff`\] Skip `RUF001` diagnostics when visiting string type definitions ([#16122](https://github.com/astral-sh/ruff/pull/16122))
-- \[`flake8-pyi`\] Avoid flagging `custom-typevar-for-self` on metaclass methods (`PYI019`) ([#16141](https://github.com/astral-sh/ruff/pull/16141))
 - \[`pycodestyle`\] Exempt `site.addsitedir(...)` calls (`E402`) ([#16251](https://github.com/astral-sh/ruff/pull/16251))
 
 ### Formatter
@@ -43,7 +112,16 @@
 
 ### Bug fixes
 
+- \[`flake8-comprehensions`\] Handle trailing comma in `C403` fix ([#16110](https://github.com/astral-sh/ruff/pull/16110))
+- \[`flake8-pyi`\] Avoid flagging `custom-typevar-for-self` on metaclass methods (`PYI019`) ([#16141](https://github.com/astral-sh/ruff/pull/16141))
+- \[`pydocstyle`\] Handle arguments with the same names as sections (`D417`) ([#16011](https://github.com/astral-sh/ruff/pull/16011))
+- \[`pylint`\] Correct ordering of arguments in fix for `if-stmt-min-max` (`PLR1730`) ([#16080](https://github.com/astral-sh/ruff/pull/16080))
+- \[`pylint`\] Do not offer fix for raw strings (`PLE251`) ([#16132](https://github.com/astral-sh/ruff/pull/16132))
+- \[`pyupgrade`\] Do not upgrade functional `TypedDicts` with private field names to the class-based syntax (`UP013`) ([#16219](https://github.com/astral-sh/ruff/pull/16219))
+- \[`pyupgrade`\] Handle micro version numbers correctly (`UP036`) ([#16091](https://github.com/astral-sh/ruff/pull/16091))
+- \[`pyupgrade`\] Unwrap unary expressions correctly (`UP018`) ([#15919](https://github.com/astral-sh/ruff/pull/15919))
 - \[`refurb`\] Correctly handle lengths of literal strings in `slice-to-remove-prefix-or-suffix` (`FURB188`) ([#16237](https://github.com/astral-sh/ruff/pull/16237))
+- \[`ruff`\] Skip `RUF001` diagnostics when visiting string type definitions ([#16122](https://github.com/astral-sh/ruff/pull/16122))
 
 ### Documentation
 

Cargo.lock

@@ -8,18 +8,6 @@ version = "2.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "512761e0bb2578dd7380c6baaa0f4ce03e84f95e960231d1dec8bf4d7d6e2627"
 
-[[package]]
-name = "ahash"
-version = "0.8.11"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "e89da841a80418a9b391ebaea17f5c112ffaaa96f621d2c285b5174da76b9011"
-dependencies = [
- "cfg-if",
- "once_cell",
- "version_check",
- "zerocopy 0.7.35",
-]
-
 [[package]]
 name = "aho-corasick"
 version = "1.1.3"
@@ -136,21 +124,9 @@ dependencies = [
 
 [[package]]
 name = "anyhow"
-version = "1.0.95"
+version = "1.0.96"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "34ac096ce696dc2fcabef30516bb13c0a68a11d30131d3df6f04711467681b04"
-
-[[package]]
-name = "append-only-vec"
-version = "0.1.7"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7992085ec035cfe96992dd31bfd495a2ebd31969bb95f624471cb6c0b349e571"
-
-[[package]]
-name = "arc-swap"
-version = "1.7.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "69f7f8c3906b62b754cd5326047894316021dcfe5a194c8ea52bdd94934a3457"
+checksum = "6b964d184e89d9b6b67dd2715bc8e74cf3107fb2b529990c90cf517326150bf4"
 
 [[package]]
 name = "argfile"
@@ -212,9 +188,9 @@ checksum = "bef38d45163c2f1dde094a7dfd33ccf595c92905c8f8f4fdc18d06fb1037718a"
 
 [[package]]
 name = "bitflags"
-version = "2.8.0"
+version = "2.9.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8f68f53c83ab957f72c32642f3868eec03eb974d1fb82e453128456482613d36"
+checksum = "5c8214115b7bf84099f1309324e63141d4c5d7cc26862f97a0a857dbefe165bd"
 
 [[package]]
 name = "block-buffer"
@@ -227,9 +203,12 @@ dependencies = [
 
 [[package]]
 name = "boxcar"
-version = "0.2.8"
+version = "0.2.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2721c3c5a6f0e7f7e607125d963fedeb765f545f67adc9d71ed934693881eb42"
+checksum = "225450ee9328e1e828319b48a89726cffc1b0ad26fd9211ad435de9fa376acae"
+dependencies = [
+ "loom",
+]
 
 [[package]]
 name = "bstr"
@@ -321,14 +300,14 @@ dependencies = [
 
 [[package]]
 name = "chrono"
-version = "0.4.39"
+version = "0.4.40"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7e36cc9d416881d2e24f9a963be5fb1cd90966419ac844274161d10488b3e825"
+checksum = "1a7964611d71df112cb1730f2ee67324fcf4d0fc6606acbbe9bfe06df124637c"
 dependencies = [
  "android-tzdata",
  "iana-time-zone",
  "num-traits",
- "windows-targets 0.52.6",
+ "windows-link",
 ]
 
 [[package]]
@@ -360,9 +339,9 @@ dependencies = [
 
 [[package]]
 name = "clap"
-version = "4.5.29"
+version = "4.5.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "8acebd8ad879283633b343856142139f2da2317c96b05b4dd6181c61e2480184"
+checksum = "027bb0d98429ae334a8698531da7077bdf906419543a35a55c2cb1b66437d767"
 dependencies = [
  "clap_builder",
  "clap_derive",
@@ -370,9 +349,9 @@ dependencies = [
 
 [[package]]
 name = "clap_builder"
-version = "4.5.29"
+version = "4.5.31"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f6ba32cbda51c7e1dfd49acc1457ba1a7dec5b64fe360e828acb13ca8dc9c2f9"
+checksum = "5589e0cba072e0f3d23791efac0fd8627b49c829c196a492e88168e6a669d863"
 dependencies = [
  "anstream",
  "anstyle",
@@ -444,9 +423,9 @@ dependencies = [
 
 [[package]]
 name = "codspeed"
-version = "2.8.0"
+version = "2.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "25d2f5a6570db487f5258e0bded6352fa2034c2aeb46bb5cc3ff060a0fcfba2f"
+checksum = "de4b67ff8985f3993f06167d71cf4aec178b0a1580f91a987170c59d60021103"
 dependencies = [
  "colored 2.2.0",
  "libc",
@@ -457,9 +436,9 @@ dependencies = [
 
 [[package]]
 name = "codspeed-criterion-compat"
-version = "2.8.0"
+version = "2.8.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f53a55558dedec742b14aae3c5fec389361b8b5ca28c1aadf09dd91faf710074"
+checksum = "68403d768ed1def18a87e2306676781314448393ecf0d3057c4527cabf524a3d"
 dependencies = [
  "codspeed",
  "colored 2.2.0",
@@ -479,7 +458,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "117725a109d387c937a1533ce01b450cbde6b88abceea8473c4d7a85853cda3c"
 dependencies = [
  "lazy_static",
- "windows-sys 0.48.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -488,7 +467,7 @@ version = "3.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fde0e0ec90c9dfb3b4b1a0891a7dcd0e2bffde2f7efed5fe7c9bb00e5bfb915e"
 dependencies = [
- "windows-sys 0.48.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -905,7 +884,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "33d852cb9b869c2a9b3df2f71a3074817f01e1844f839a144f5fcef059a4eb5d"
 dependencies = [
  "libc",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -1013,6 +992,19 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "generator"
+version = "0.8.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cc6bd114ceda131d3b1d665eba35788690ad37f5916457286b32ab6fd3c438dd"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "log",
+ "rustversion",
+ "windows",
+]
+
 [[package]]
 name = "generic-array"
 version = "0.14.7"
@@ -1065,9 +1057,9 @@ checksum = "a8d1add55171497b4705a648c6b583acafb01d58050a51727785f0b2c8e0a2b2"
 
 [[package]]
 name = "globset"
-version = "0.4.15"
+version = "0.4.16"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "15f1ce686646e7f1e19bf7d5533fe443a45dbfb990e00629110797578b42fb19"
+checksum = "54a1028dfc5f5df5da8a56a73e6c153c9a9708ec57232470703592a3f18e49f5"
 dependencies = [
  "aho-corasick",
  "bstr",
@@ -1082,7 +1074,7 @@ version = "0.9.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0bf760ebf69878d9fd8f110c89703d90ce35095324d1f1edcb595c63945ee757"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "ignore",
  "walkdir",
 ]
@@ -1102,10 +1094,6 @@ name = "hashbrown"
 version = "0.14.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
-dependencies = [
- "ahash",
- "allocator-api2",
-]
 
 [[package]]
 name = "hashbrown"
@@ -1113,17 +1101,18 @@ version = "0.15.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "bf151400ff0baff5465007dd2f3e717f3fe502074ca563069ce3a6629d07b289"
 dependencies = [
+ "allocator-api2",
  "equivalent",
  "foldhash",
 ]
 
 [[package]]
 name = "hashlink"
-version = "0.9.1"
+version = "0.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6ba4ff7128dee98c7dc9794b6a411377e1404dba1c97deb8d1a55297bd25d8af"
+checksum = "7382cf6263419f2d8df38c55d7da83da5c18aef87fc7a7fc1fb1e344edfe14c1"
 dependencies = [
- "hashbrown 0.14.5",
+ "hashbrown 0.15.2",
 ]
 
 [[package]]
@@ -1179,7 +1168,7 @@ dependencies = [
  "iana-time-zone-haiku",
  "js-sys",
  "wasm-bindgen",
- "windows-core",
+ "windows-core 0.52.0",
 ]
 
 [[package]]
@@ -1408,7 +1397,7 @@ version = "0.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f37dccff2791ab604f9babef0ba14fbe0be30bd368dc541e2b08d07c8aa908f3"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "inotify-sys",
  "libc",
 ]
@@ -1424,9 +1413,9 @@ dependencies = [
 
 [[package]]
 name = "insta"
-version = "1.42.1"
+version = "1.42.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "71c1b125e30d93896b365e156c33dadfffab45ee8400afcbba4752f59de08a86"
+checksum = "50259abbaa67d11d2bcafc7ba1d094ed7a0c70e3ce893f0d0997f73558cb3084"
 dependencies = [
  "console",
  "globset",
@@ -1482,7 +1471,7 @@ checksum = "e19b23d53f35ce9f56aebc7d1bb4e6ac1e9c0db7ac85c8d1760c04379edced37"
 dependencies = [
  "hermit-abi 0.4.0",
  "libc",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -1587,9 +1576,9 @@ checksum = "bbd2bcb4c963f2ddae06a2efc7e9f3591312473c50c6685e1f298068316e66fe"
 
 [[package]]
 name = "libc"
-version = "0.2.169"
+version = "0.2.170"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b5aba8db14291edd000dfcc4d620c7ebfb122c613afb886ca8803fa4e128a20a"
+checksum = "875b3680cb2f8f71bdcf9a30f38d48282f5d3c95cbf9b3fa57269bb5d5c06828"
 
 [[package]]
 name = "libcst"
@@ -1632,7 +1621,7 @@ version = "0.1.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c0ff37bd590ca25063e35af745c343cb7a0271906fb7b37e4813e8f79f00268d"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "libc",
  "redox_syscall",
 ]
@@ -1679,9 +1668,22 @@ dependencies = [
 
 [[package]]
 name = "log"
-version = "0.4.25"
+version = "0.4.26"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "04cbf5b083de1c7e0222a7a51dbfdba1cbe1c6ab0b15e29fff3f6c077fd9cd9f"
+checksum = "30bde2b3dc3671ae49d8e2e9f044c7c005836e7a023ee57cffa25ab82764bb9e"
+
+[[package]]
+name = "loom"
+version = "0.7.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "419e0dc8046cb947daa77eb95ae174acfbddb7673b4151f56d1eed8e93fbfaca"
+dependencies = [
+ "cfg-if",
+ "generator",
+ "scoped-tls",
+ "tracing",
+ "tracing-subscriber",
+]
 
 [[package]]
 name = "lsp-server"
@@ -1802,7 +1804,7 @@ version = "0.29.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "71e2746dc3a24dd78b3cfcb7be93368c6de9963d30f43a6a73998a9cf4b17b46"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "cfg-if",
  "cfg_aliases",
  "libc",
@@ -1830,7 +1832,7 @@ version = "8.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2fee8403b3d66ac7b26aee6e40a897d85dc5ce26f44da36b8b73e987cc52e943"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "filetime",
  "fsevent-sys",
  "inotify",
@@ -2401,6 +2403,7 @@ name = "red_knot"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "argfile",
  "chrono",
  "clap",
  "colored 3.0.0",
@@ -2425,6 +2428,7 @@ dependencies = [
  "tracing-flame",
  "tracing-subscriber",
  "tracing-tree",
+ "wild",
 ]
 
 [[package]]
@@ -2459,7 +2463,7 @@ name = "red_knot_python_semantic"
 version = "0.0.0"
 dependencies = [
  "anyhow",
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "camino",
  "compact_str",
  "countme",
@@ -2491,6 +2495,8 @@ dependencies = [
  "serde",
  "smallvec",
  "static_assertions",
+ "strum",
+ "strum_macros",
  "tempfile",
  "test-case",
  "thiserror 2.0.11",
@@ -2535,6 +2541,7 @@ dependencies = [
  "regex",
  "ruff_db",
  "ruff_index",
+ "ruff_notebook",
  "ruff_python_ast",
  "ruff_python_trivia",
  "ruff_source_file",
@@ -2543,6 +2550,8 @@ dependencies = [
  "salsa",
  "serde",
  "smallvec",
+ "tempfile",
+ "thiserror 2.0.11",
  "toml",
 ]
 
@@ -2579,7 +2588,7 @@ version = "0.5.8"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "03a862b389f93e68874fbf580b9de08dd02facb9a788ebadaf4a3fd33cf58834"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
 ]
 
 [[package]]
@@ -2650,13 +2659,13 @@ dependencies = [
 
 [[package]]
 name = "ruff"
-version = "0.9.7"
+version = "0.9.10"
 dependencies = [
  "anyhow",
  "argfile",
  "assert_fs",
  "bincode",
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "cachedir",
  "chrono",
  "clap",
@@ -2686,6 +2695,7 @@ dependencies = [
  "ruff_notebook",
  "ruff_python_ast",
  "ruff_python_formatter",
+ "ruff_python_parser",
  "ruff_server",
  "ruff_source_file",
  "ruff_text_size",
@@ -2884,11 +2894,11 @@ dependencies = [
 
 [[package]]
 name = "ruff_linter"
-version = "0.9.7"
+version = "0.9.10"
 dependencies = [
  "aho-corasick",
  "anyhow",
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "chrono",
  "clap",
  "colored 3.0.0",
@@ -2978,7 +2988,7 @@ name = "ruff_python_ast"
 version = "0.0.0"
 dependencies = [
  "aho-corasick",
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "compact_str",
  "is-macro",
  "itertools 0.14.0",
@@ -3062,7 +3072,7 @@ dependencies = [
 name = "ruff_python_literal"
 version = "0.0.0"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "itertools 0.14.0",
  "ruff_python_ast",
  "unic-ucd-category",
@@ -3073,7 +3083,7 @@ name = "ruff_python_parser"
 version = "0.0.0"
 dependencies = [
  "anyhow",
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "bstr",
  "compact_str",
  "insta",
@@ -3084,6 +3094,8 @@ dependencies = [
  "ruff_source_file",
  "ruff_text_size",
  "rustc-hash 2.1.1",
+ "serde",
+ "serde_json",
  "static_assertions",
  "unicode-ident",
  "unicode-normalization",
@@ -3105,7 +3117,7 @@ dependencies = [
 name = "ruff_python_semantic"
 version = "0.0.0"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "is-macro",
  "ruff_cache",
  "ruff_index",
@@ -3124,7 +3136,7 @@ dependencies = [
 name = "ruff_python_stdlib"
 version = "0.0.0"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "unicode-ident",
 ]
 
@@ -3178,6 +3190,7 @@ dependencies = [
  "serde_json",
  "shellexpand",
  "thiserror 2.0.11",
+ "toml",
  "tracing",
  "tracing-subscriber",
 ]
@@ -3203,7 +3216,7 @@ dependencies = [
 
 [[package]]
 name = "ruff_wasm"
-version = "0.9.7"
+version = "0.9.10"
 dependencies = [
  "console_error_panic_hook",
  "console_log",
@@ -3293,11 +3306,11 @@ version = "0.38.44"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fdb5bc1ae2baa591800df16c9ca78619bf65c0488b41b96ccec5d11220d8c154"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
  "errno",
  "libc",
  "linux-raw-sys",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -3315,14 +3328,13 @@ checksum = "6ea1a2d0a644769cc99faa24c3ad26b379b786fe7c36fd3c546254801650e6dd"
 [[package]]
 name = "salsa"
 version = "0.18.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=351d9cf0037be949d17800d0c7b4838e533c2ed6#351d9cf0037be949d17800d0c7b4838e533c2ed6"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=99be5d9917c3dd88e19735a82ef6bf39ba84bd7e#99be5d9917c3dd88e19735a82ef6bf39ba84bd7e"
 dependencies = [
- "append-only-vec",
- "arc-swap",
+ "boxcar",
  "compact_str",
- "crossbeam",
+ "crossbeam-queue",
  "dashmap 6.1.0",
- "hashbrown 0.14.5",
+ "hashbrown 0.15.2",
  "hashlink",
  "indexmap",
  "parking_lot",
@@ -3337,12 +3349,12 @@ dependencies = [
 [[package]]
 name = "salsa-macro-rules"
 version = "0.1.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=351d9cf0037be949d17800d0c7b4838e533c2ed6#351d9cf0037be949d17800d0c7b4838e533c2ed6"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=99be5d9917c3dd88e19735a82ef6bf39ba84bd7e#99be5d9917c3dd88e19735a82ef6bf39ba84bd7e"
 
 [[package]]
 name = "salsa-macros"
 version = "0.18.0"
-source = "git+https://github.com/salsa-rs/salsa.git?rev=351d9cf0037be949d17800d0c7b4838e533c2ed6#351d9cf0037be949d17800d0c7b4838e533c2ed6"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=99be5d9917c3dd88e19735a82ef6bf39ba84bd7e#99be5d9917c3dd88e19735a82ef6bf39ba84bd7e"
 dependencies = [
  "heck",
  "proc-macro2",
@@ -3362,9 +3374,9 @@ dependencies = [
 
 [[package]]
 name = "schemars"
-version = "0.8.21"
+version = "0.8.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "09c024468a378b7e36765cd36702b7a90cc3cba11654f6685c8f233408e89e92"
+checksum = "3fbf2ae1b8bc8e02df939598064d22402220cd5bbcca1c76f7d6a310974d5615"
 dependencies = [
  "dyn-clone",
  "schemars_derive",
@@ -3374,9 +3386,9 @@ dependencies = [
 
 [[package]]
 name = "schemars_derive"
-version = "0.8.21"
+version = "0.8.22"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b1eee588578aff73f856ab961cd2f79e36bc45d7ded33a7562adba4667aecc0e"
+checksum = "32e265784ad618884abaea0600a9adf15393368d840e0222d101a072f3f7534d"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -3384,6 +3396,12 @@ dependencies = [
  "syn 2.0.98",
 ]
 
+[[package]]
+name = "scoped-tls"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e1cf6437eb19a8f4a6cc0f7dca544973b0b78843adbfeb3683d1a94a0024a294"
+
 [[package]]
 name = "scopeguard"
 version = "1.2.0"
@@ -3398,9 +3416,9 @@ checksum = "1c107b6f4780854c8b126e228ea8869f4d7b71260f962fefb57b996b8959ba6b"
 
 [[package]]
 name = "serde"
-version = "1.0.217"
+version = "1.0.218"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "02fc4265df13d6fa1d00ecff087228cc0a2b5f3c0e87e258d8b94a156e984c70"
+checksum = "e8dfc9d19bdbf6d17e22319da49161d5d0108e4188e8b680aef6299eed22df60"
 dependencies = [
  "serde_derive",
 ]
@@ -3418,9 +3436,9 @@ dependencies = [
 
 [[package]]
 name = "serde_derive"
-version = "1.0.217"
+version = "1.0.218"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5a9bf7cf98d04a2b28aead066b7496853d4779c9cc183c440dbac457641e19a0"
+checksum = "f09503e191f4e797cb8aac08e9a4a4695c5edf6a2e70e376d961ddd5c969f82b"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -3440,9 +3458,9 @@ dependencies = [
 
 [[package]]
 name = "serde_json"
-version = "1.0.138"
+version = "1.0.139"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d434192e7da787e94a6ea7e9670b26a036d0ca41e0b7efb2676dd32bae872949"
+checksum = "44f86c3acccc9c65b153fe1b85a3be07fe5515274ec9f0653b4a0875731c72a6"
 dependencies = [
  "itoa",
  "memchr",
@@ -3668,16 +3686,16 @@ dependencies = [
 
 [[package]]
 name = "tempfile"
-version = "3.17.0"
+version = "3.17.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a40f762a77d2afa88c2d919489e390a12bdd261ed568e60cfa7e48d4e20f0d33"
+checksum = "22e5a0acb1f3f55f65cc4a866c361b2fb2a0ff6366785ae6fbb5f85df07ba230"
 dependencies = [
  "cfg-if",
  "fastrand",
  "getrandom 0.3.1",
  "once_cell",
  "rustix",
- "windows-sys 0.59.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -3971,6 +3989,7 @@ version = "0.3.19"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e8189decb5ac0fa7bc8b96b7cb9b2701d60d48805aca84a238004d665fcc4008"
 dependencies = [
+ "chrono",
  "matchers",
  "nu-ansi-term 0.46.0",
  "once_cell",
@@ -4068,9 +4087,9 @@ dependencies = [
 
 [[package]]
 name = "unicode-ident"
-version = "1.0.16"
+version = "1.0.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a210d160f08b701c8721ba1c726c11662f877ea6b7094007e1ca9a1041945034"
+checksum = "00e2473a93778eb0bad35909dff6a10d28e63f792f16ed15e404fca9d5eeedbe"
 
 [[package]]
 name = "unicode-normalization"
@@ -4442,7 +4461,7 @@ version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cf221c93e13a30d793f7645a0e7762c55d169dbb0a49671918a2319d289b10bb"
 dependencies = [
- "windows-sys 0.48.0",
+ "windows-sys 0.52.0",
 ]
 
 [[package]]
@@ -4451,6 +4470,16 @@ version = "0.4.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
 
+[[package]]
+name = "windows"
+version = "0.58.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dd04d41d93c4992d421894c18c8b43496aa748dd4c081bac0dc93eb0489272b6"
+dependencies = [
+ "windows-core 0.58.0",
+ "windows-targets 0.52.6",
+]
+
 [[package]]
 name = "windows-core"
 version = "0.52.0"
@@ -4460,6 +4489,66 @@ dependencies = [
  "windows-targets 0.52.6",
 ]
 
+[[package]]
+name = "windows-core"
+version = "0.58.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6ba6d44ec8c2591c134257ce647b7ea6b20335bf6379a27dac5f1641fcf59f99"
+dependencies = [
+ "windows-implement",
+ "windows-interface",
+ "windows-result",
+ "windows-strings",
+ "windows-targets 0.52.6",
+]
+
+[[package]]
+name = "windows-implement"
+version = "0.58.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2bbd5b46c938e506ecbce286b6628a02171d56153ba733b6c741fc627ec9579b"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.98",
+]
+
+[[package]]
+name = "windows-interface"
+version = "0.58.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "053c4c462dc91d3b1504c6fe5a726dd15e216ba718e84a0e46a88fbe5ded3515"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.98",
+]
+
+[[package]]
+name = "windows-link"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6dccfd733ce2b1753b03b6d3c65edf020262ea35e20ccdf3e288043e6dd620e3"
+
+[[package]]
+name = "windows-result"
+version = "0.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1d1043d8214f791817bab27572aaa8af63732e11bf84aa21a45a78d6c317ae0e"
+dependencies = [
+ "windows-targets 0.52.6",
+]
+
+[[package]]
+name = "windows-strings"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4cd9b125c486025df0eabcb585e62173c6c9eddcec5d117d3b6e8c30e2ee4d10"
+dependencies = [
+ "windows-result",
+ "windows-targets 0.52.6",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.48.0"
@@ -4629,7 +4718,7 @@ version = "0.33.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3268f3d866458b787f390cf61f4bbb563b922d091359f9608842999eaee3943c"
 dependencies = [
- "bitflags 2.8.0",
+ "bitflags 2.9.0",
 ]
 
 [[package]]

Cargo.toml

@@ -4,7 +4,7 @@ resolver = "2"
 
 [workspace.package]
 edition = "2021"
-rust-version = "1.80"
+rust-version = "1.83"
 homepage = "https://docs.astral.sh/ruff"
 documentation = "https://docs.astral.sh/ruff"
 repository = "https://github.com/astral-sh/ruff"
@@ -123,7 +123,7 @@ rayon = { version = "1.10.0" }
 regex = { version = "1.10.2" }
 rustc-hash = { version = "2.0.0" }
 # When updating salsa, make sure to also update the revision in `fuzz/Cargo.toml`
-salsa = { git = "https://github.com/salsa-rs/salsa.git", rev = "351d9cf0037be949d17800d0c7b4838e533c2ed6" }
+salsa = { git = "https://github.com/salsa-rs/salsa.git", rev = "99be5d9917c3dd88e19735a82ef6bf39ba84bd7e" }
 schemars = { version = "0.8.16" }
 seahash = { version = "4.1.0" }
 serde = { version = "1.0.197", features = ["derive"] }

README.md

@@ -149,8 +149,8 @@ curl -LsSf https://astral.sh/ruff/install.sh | sh
 powershell -c "irm https://astral.sh/ruff/install.ps1 | iex"
 
 # For a specific version.
-curl -LsSf https://astral.sh/ruff/0.9.7/install.sh | sh
-powershell -c "irm https://astral.sh/ruff/0.9.7/install.ps1 | iex"
+curl -LsSf https://astral.sh/ruff/0.9.10/install.sh | sh
+powershell -c "irm https://astral.sh/ruff/0.9.10/install.ps1 | iex"
 ```
 
 You can also install Ruff via [Homebrew](https://formulae.brew.sh/formula/ruff), [Conda](https://anaconda.org/conda-forge/ruff),
@@ -183,7 +183,7 @@ Ruff can also be used as a [pre-commit](https://pre-commit.com/) hook via [`ruff
 ```yaml
 - repo: https://github.com/astral-sh/ruff-pre-commit
   # Ruff version.
-  rev: v0.9.7
+  rev: v0.9.10
   hooks:
     # Run the linter.
     - id: ruff

_typos.toml

@@ -23,6 +23,10 @@ extend-ignore-re = [
     # Line ignore with trailing "spellchecker:disable-line"
     "(?Rm)^.*#\\s*spellchecker:disable-line$",
     "LICENSEs",
+    # Various third party dependencies uses `typ` as struct field names (e.g., lsp_types::LogMessageParams)
+    "typ",
+    # TODO: Remove this once the `TYP` redirects are removed from `rule_redirects.rs`
+    "TYP",
 ]
 
 [default.extend-identifiers]

crates/red_knot/Cargo.toml

@@ -19,6 +19,7 @@ ruff_db = { workspace = true, features = ["os", "cache"] }
 ruff_python_ast = { workspace = true }
 
 anyhow = { workspace = true }
+argfile = { workspace = true }
 chrono = { workspace = true }
 clap = { workspace = true, features = ["wrap_help"] }
 colored = { workspace = true }
@@ -31,6 +32,7 @@ tracing = { workspace = true, features = ["release_max_level_debug"] }
 tracing-subscriber = { workspace = true, features = ["env-filter", "fmt"] }
 tracing-flame = { workspace = true }
 tracing-tree = { workspace = true }
+wild = { workspace = true }
 
 [dev-dependencies]
 ruff_db = { workspace = true, features = ["testing"] }

crates/red_knot/src/args.rs

@@ -32,6 +32,13 @@ pub(crate) enum Command {
 
 #[derive(Debug, Parser)]
 pub(crate) struct CheckCommand {
+    /// List of files or directories to check.
+    #[clap(
+        help = "List of files or directories to check [default: the project root]",
+        value_name = "PATH"
+    )]
+    pub paths: Vec<SystemPathBuf>,
+
     /// Run the command within the given project directory.
     ///
     /// All `pyproject.toml` files will be discovered by walking up the directory tree from the given project directory,
@@ -41,12 +48,14 @@ pub(crate) struct CheckCommand {
     #[arg(long, value_name = "PROJECT")]
     pub(crate) project: Option<SystemPathBuf>,
 
-    /// Path to the virtual environment the project uses.
+    /// Path to the Python installation from which Red Knot resolves type information and third-party dependencies.
     ///
-    /// If provided, red-knot will use the `site-packages` directory of this virtual environment
-    /// to resolve type information for the project's third-party dependencies.
+    /// Red Knot will search in the path's `site-packages` directories for type information and
+    /// third-party imports.
+    ///
+    /// This option is commonly used to specify the path to a virtual environment.
     #[arg(long, value_name = "PATH")]
-    pub(crate) venv_path: Option<SystemPathBuf>,
+    pub(crate) python: Option<SystemPathBuf>,
 
     /// Custom directory to use for stdlib typeshed stubs.
     #[arg(long, value_name = "PATH", alias = "custom-typeshed-dir")]
@@ -74,7 +83,7 @@ pub(crate) struct CheckCommand {
     #[arg(long)]
     pub(crate) exit_zero: bool,
 
-    /// Run in watch mode by re-running whenever files change.
+    /// Watch files for changes and recheck files related to the changed files.
     #[arg(long, short = 'W')]
     pub(crate) watch: bool,
 }
@@ -97,7 +106,7 @@ impl CheckCommand {
                 python_version: self
                     .python_version
                     .map(|version| RangedValue::cli(version.into())),
-                venv_path: self.venv_path.map(RelativePathBuf::cli),
+                python: self.python.map(RelativePathBuf::cli),
                 typeshed: self.typeshed.map(RelativePathBuf::cli),
                 extra_paths: self.extra_search_path.map(|extra_search_paths| {
                     extra_search_paths

crates/red_knot/src/main.rs

@@ -1,4 +1,4 @@
-use std::io::{self, BufWriter, Write};
+use std::io::{self, stdout, BufWriter, Write};
 use std::process::{ExitCode, Termination};
 
 use anyhow::Result;
@@ -15,8 +15,8 @@ use red_knot_project::watch::ProjectWatcher;
 use red_knot_project::{watch, Db};
 use red_knot_project::{ProjectDatabase, ProjectMetadata};
 use red_knot_server::run_server;
-use ruff_db::diagnostic::{Diagnostic, DisplayDiagnosticConfig, Severity};
-use ruff_db::system::{OsSystem, System, SystemPath, SystemPathBuf};
+use ruff_db::diagnostic::{DisplayDiagnosticConfig, OldDiagnosticTrait, Severity};
+use ruff_db::system::{OsSystem, SystemPath, SystemPathBuf};
 use salsa::plumbing::ZalsaDatabase;
 
 mod args;
@@ -39,6 +39,15 @@ pub fn main() -> ExitStatus {
         // the configuration it is help to chain errors ("resolving configuration failed" ->
         // "failed to read file: subdir/pyproject.toml")
         for cause in error.chain() {
+            // Exit "gracefully" on broken pipe errors.
+            //
+            // See: https://github.com/BurntSushi/ripgrep/blob/bf63fe8f258afc09bae6caa48f0ae35eaf115005/crates/core/main.rs#L47C1-L61C14
+            if let Some(ioerr) = cause.downcast_ref::<io::Error>() {
+                if ioerr.kind() == io::ErrorKind::BrokenPipe {
+                    return ExitStatus::Success;
+                }
+            }
+
             writeln!(stderr, "  {} {cause}", "Cause:".bold()).ok();
         }
 
@@ -47,7 +56,10 @@ pub fn main() -> ExitStatus {
 }
 
 fn run() -> anyhow::Result<ExitStatus> {
-    let args = Args::parse_from(std::env::args());
+    let args = wild::args_os();
+    let args = argfile::expand_args_from(args, argfile::parse_fromfile, argfile::PREFIX)
+        .context("Failed to read CLI arguments from file")?;
+    let args = Args::parse_from(args);
 
     match args.command {
         Command::Server => run_server().map(|()| ExitStatus::Success),
@@ -69,7 +81,7 @@ fn run_check(args: CheckCommand) -> anyhow::Result<ExitStatus> {
     let _guard = setup_tracing(verbosity)?;
 
     // The base path to which all CLI arguments are relative to.
-    let cli_base_path = {
+    let cwd = {
         let cwd = std::env::current_dir().context("Failed to get the current working directory")?;
         SystemPathBuf::from_path_buf(cwd)
             .map_err(|path| {
@@ -80,30 +92,42 @@ fn run_check(args: CheckCommand) -> anyhow::Result<ExitStatus> {
             })?
     };
 
-    let cwd = args
+    let project_path = args
         .project
         .as_ref()
-        .map(|cwd| {
-            if cwd.as_std_path().is_dir() {
-                Ok(SystemPath::absolute(cwd, &cli_base_path))
+        .map(|project| {
+            if project.as_std_path().is_dir() {
+                Ok(SystemPath::absolute(project, &cwd))
             } else {
-                Err(anyhow!("Provided project path `{cwd}` is not a directory"))
+                Err(anyhow!(
+                    "Provided project path `{project}` is not a directory"
+                ))
             }
         })
         .transpose()?
-        .unwrap_or_else(|| cli_base_path.clone());
+        .unwrap_or_else(|| cwd.clone());
+
+    let check_paths: Vec<_> = args
+        .paths
+        .iter()
+        .map(|path| SystemPath::absolute(path, &cwd))
+        .collect();
 
     let system = OsSystem::new(cwd);
     let watch = args.watch;
     let exit_zero = args.exit_zero;
 
     let cli_options = args.into_options();
-    let mut project_metadata = ProjectMetadata::discover(system.current_directory(), &system)?;
+    let mut project_metadata = ProjectMetadata::discover(&project_path, &system)?;
     project_metadata.apply_cli_options(cli_options.clone());
     project_metadata.apply_configuration_files(&system)?;
 
     let mut db = ProjectDatabase::new(project_metadata, system)?;
 
+    if !check_paths.is_empty() {
+        db.project().set_included_paths(&mut db, check_paths);
+    }
+
     let (main_loop, main_loop_cancellation_token) = MainLoop::new(cli_options);
 
     // Listen to Ctrl+C and abort the watch mode.
@@ -119,7 +143,7 @@ fn run_check(args: CheckCommand) -> anyhow::Result<ExitStatus> {
     let exit_status = if watch {
         main_loop.watch(&mut db)?
     } else {
-        main_loop.run(&mut db)
+        main_loop.run(&mut db)?
     };
 
     tracing::trace!("Counts for entire CLI run:\n{}", countme::get_all());
@@ -179,7 +203,7 @@ impl MainLoop {
         )
     }
 
-    fn watch(mut self, db: &mut ProjectDatabase) -> anyhow::Result<ExitStatus> {
+    fn watch(mut self, db: &mut ProjectDatabase) -> Result<ExitStatus> {
         tracing::debug!("Starting watch mode");
         let sender = self.sender.clone();
         let watcher = watch::directory_watcher(move |event| {
@@ -188,12 +212,12 @@ impl MainLoop {
 
         self.watcher = Some(ProjectWatcher::new(watcher, db));
 
-        self.run(db);
+        self.run(db)?;
 
         Ok(ExitStatus::Success)
     }
 
-    fn run(mut self, db: &mut ProjectDatabase) -> ExitStatus {
+    fn run(mut self, db: &mut ProjectDatabase) -> Result<ExitStatus> {
         self.sender.send(MainLoopMessage::CheckWorkspace).unwrap();
 
         let result = self.main_loop(db);
@@ -203,7 +227,7 @@ impl MainLoop {
         result
     }
 
-    fn main_loop(&mut self, db: &mut ProjectDatabase) -> ExitStatus {
+    fn main_loop(&mut self, db: &mut ProjectDatabase) -> Result<ExitStatus> {
         // Schedule the first check.
         tracing::debug!("Starting main loop");
 
@@ -241,14 +265,43 @@ impl MainLoop {
                             Severity::Error
                         };
 
-                    let failed = result
-                        .iter()
-                        .any(|diagnostic| diagnostic.severity() >= min_error_severity);
-
                     if check_revision == revision {
-                        #[allow(clippy::print_stdout)]
-                        for diagnostic in result {
-                            println!("{}", diagnostic.display(db, &display_config));
+                        if db.project().files(db).is_empty() {
+                            tracing::warn!("No python files found under the given path(s)");
+                        }
+
+                        let mut stdout = stdout().lock();
+
+                        if result.is_empty() {
+                            writeln!(stdout, "All checks passed!")?;
+
+                            if self.watcher.is_none() {
+                                return Ok(ExitStatus::Success);
+                            }
+                        } else {
+                            let mut failed = false;
+                            let diagnostics_count = result.len();
+
+                            for diagnostic in result {
+                                writeln!(stdout, "{}", diagnostic.display(db, &display_config))?;
+
+                                failed |= diagnostic.severity() >= min_error_severity;
+                            }
+
+                            writeln!(
+                                stdout,
+                                "Found {} diagnostic{}",
+                                diagnostics_count,
+                                if diagnostics_count > 1 { "s" } else { "" }
+                            )?;
+
+                            if self.watcher.is_none() {
+                                return Ok(if failed {
+                                    ExitStatus::Failure
+                                } else {
+                                    ExitStatus::Success
+                                });
+                            }
                         }
                     } else {
                         tracing::debug!(
@@ -256,14 +309,6 @@ impl MainLoop {
                         );
                     }
 
-                    if self.watcher.is_none() {
-                        return if failed {
-                            ExitStatus::Failure
-                        } else {
-                            ExitStatus::Success
-                        };
-                    }
-
                     tracing::trace!("Counts after last check:\n{}", countme::get_all());
                 }
 
@@ -281,14 +326,14 @@ impl MainLoop {
                     // TODO: Don't use Salsa internal APIs
                     //  [Zulip-Thread](https://salsa.zulipchat.com/#narrow/stream/333573-salsa-3.2E0/topic/Expose.20an.20API.20to.20cancel.20other.20queries)
                     let _ = db.zalsa_mut();
-                    return ExitStatus::Success;
+                    return Ok(ExitStatus::Success);
                 }
             }
 
             tracing::debug!("Waiting for next main loop message.");
         }
 
-        ExitStatus::Success
+        Ok(ExitStatus::Success)
     }
 }
 
@@ -308,7 +353,8 @@ impl MainLoopCancellationToken {
 enum MainLoopMessage {
     CheckWorkspace,
     CheckCompleted {
-        result: Vec<Box<dyn Diagnostic>>,
+        /// The diagnostics that were found during the check.
+        result: Vec<Box<dyn OldDiagnosticTrait>>,
         revision: u64,
     },
     ApplyChanges(Vec<watch::ChangeEvent>),

crates/red_knot/tests/cli.rs

@@ -28,7 +28,7 @@ fn config_override() -> anyhow::Result<()> {
         ),
     ])?;
 
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -40,16 +40,18 @@ fn config_override() -> anyhow::Result<()> {
       |       ^^^^^^^^^^^^ Type `<module 'sys'>` has no attribute `last_exc`
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     assert_cmd_snapshot!(case.command().arg("--python-version").arg("3.12"), @r"
-        success: true
-        exit_code: 0
-        ----- stdout -----
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
 
-        ----- stderr -----
+    ----- stderr -----
     ");
 
     Ok(())
@@ -98,7 +100,7 @@ fn cli_arguments_are_relative_to_the_current_directory() -> anyhow::Result<()> {
     ])?;
 
     // Make sure that the CLI fails when the `libs` directory is not in the search path.
-    assert_cmd_snapshot!(case.command().current_dir(case.root().join("child")), @r###"
+    assert_cmd_snapshot!(case.command().current_dir(case.root().join("child")), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -111,16 +113,18 @@ fn cli_arguments_are_relative_to_the_current_directory() -> anyhow::Result<()> {
     4 | stat = add(10, 15)
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     assert_cmd_snapshot!(case.command().current_dir(case.root().join("child")).arg("--extra-search-path").arg("../libs"), @r"
-        success: true
-        exit_code: 0
-        ----- stdout -----
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
 
-        ----- stderr -----
+    ----- stderr -----
     ");
 
     Ok(())
@@ -168,11 +172,12 @@ fn paths_in_configuration_files_are_relative_to_the_project_root() -> anyhow::Re
     ])?;
 
     assert_cmd_snapshot!(case.command().current_dir(case.root().join("child")), @r"
-        success: true
-        exit_code: 0
-        ----- stdout -----
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
 
-        ----- stderr -----
+    ----- stderr -----
     ");
 
     Ok(())
@@ -195,7 +200,7 @@ fn configuration_rule_severity() -> anyhow::Result<()> {
 
     // Assert that there's a possibly unresolved reference diagnostic
     // and that division-by-zero has a severity of error by default.
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -217,9 +222,10 @@ fn configuration_rule_severity() -> anyhow::Result<()> {
       |       - Name `x` used when possibly not defined
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###);
+    ");
 
     case.write_file(
         "pyproject.toml",
@@ -230,7 +236,7 @@ fn configuration_rule_severity() -> anyhow::Result<()> {
     "#,
     )?;
 
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -243,9 +249,10 @@ fn configuration_rule_severity() -> anyhow::Result<()> {
     4 | for a in range(0, y):
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -269,7 +276,7 @@ fn cli_rule_severity() -> anyhow::Result<()> {
 
     // Assert that there's a possibly unresolved reference diagnostic
     // and that division-by-zero has a severity of error by default.
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -302,9 +309,10 @@ fn cli_rule_severity() -> anyhow::Result<()> {
       |       - Name `x` used when possibly not defined
       |
 
+    Found 3 diagnostics
 
     ----- stderr -----
-    "###);
+    ");
 
     assert_cmd_snapshot!(
         case
@@ -315,7 +323,7 @@ fn cli_rule_severity() -> anyhow::Result<()> {
             .arg("division-by-zero")
             .arg("--warn")
             .arg("unresolved-import"),
-        @r###"
+        @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -339,9 +347,10 @@ fn cli_rule_severity() -> anyhow::Result<()> {
     6 | for a in range(0, y):
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###
+    "
     );
 
     Ok(())
@@ -365,7 +374,7 @@ fn cli_rule_severity_precedence() -> anyhow::Result<()> {
 
     // Assert that there's a possibly unresolved reference diagnostic
     // and that division-by-zero has a severity of error by default.
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -387,9 +396,10 @@ fn cli_rule_severity_precedence() -> anyhow::Result<()> {
       |       - Name `x` used when possibly not defined
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###);
+    ");
 
     assert_cmd_snapshot!(
         case
@@ -401,7 +411,7 @@ fn cli_rule_severity_precedence() -> anyhow::Result<()> {
             // Override the error severity with warning
             .arg("--ignore")
             .arg("possibly-unresolved-reference"),
-        @r###"
+        @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -414,9 +424,10 @@ fn cli_rule_severity_precedence() -> anyhow::Result<()> {
     4 | for a in range(0, y):
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###
+    "
     );
 
     Ok(())
@@ -436,7 +447,7 @@ fn configuration_unknown_rules() -> anyhow::Result<()> {
         ("test.py", "print(10)"),
     ])?;
 
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r#"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -448,9 +459,10 @@ fn configuration_unknown_rules() -> anyhow::Result<()> {
       | --------------- Unknown lint rule `division-by-zer`
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    "#);
 
     Ok(())
 }
@@ -460,15 +472,16 @@ fn configuration_unknown_rules() -> anyhow::Result<()> {
 fn cli_unknown_rules() -> anyhow::Result<()> {
     let case = TestCase::with_file("test.py", "print(10)")?;
 
-    assert_cmd_snapshot!(case.command().arg("--ignore").arg("division-by-zer"), @r###"
+    assert_cmd_snapshot!(case.command().arg("--ignore").arg("division-by-zer"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
     warning: unknown-rule: Unknown lint rule `division-by-zer`
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -477,7 +490,7 @@ fn cli_unknown_rules() -> anyhow::Result<()> {
 fn exit_code_only_warnings() -> anyhow::Result<()> {
     let case = TestCase::with_file("test.py", r"print(x)  # [unresolved-reference]")?;
 
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -488,9 +501,10 @@ fn exit_code_only_warnings() -> anyhow::Result<()> {
       |       - Name `x` used when not defined
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -505,7 +519,7 @@ fn exit_code_only_info() -> anyhow::Result<()> {
         "#,
     )?;
 
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -517,9 +531,10 @@ fn exit_code_only_info() -> anyhow::Result<()> {
       | -------------- info: Revealed type is `Literal[1]`
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -534,7 +549,7 @@ fn exit_code_only_info_and_error_on_warning_is_true() -> anyhow::Result<()> {
         "#,
     )?;
 
-    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r###"
+    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -546,9 +561,10 @@ fn exit_code_only_info_and_error_on_warning_is_true() -> anyhow::Result<()> {
       | -------------- info: Revealed type is `Literal[1]`
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -557,7 +573,7 @@ fn exit_code_only_info_and_error_on_warning_is_true() -> anyhow::Result<()> {
 fn exit_code_no_errors_but_error_on_warning_is_true() -> anyhow::Result<()> {
     let case = TestCase::with_file("test.py", r"print(x)  # [unresolved-reference]")?;
 
-    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r###"
+    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -568,9 +584,10 @@ fn exit_code_no_errors_but_error_on_warning_is_true() -> anyhow::Result<()> {
       |       - Name `x` used when not defined
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -588,7 +605,7 @@ fn exit_code_no_errors_but_error_on_warning_is_enabled_in_configuration() -> any
         ),
     ])?;
 
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -599,9 +616,10 @@ fn exit_code_no_errors_but_error_on_warning_is_enabled_in_configuration() -> any
       |       - Name `x` used when not defined
       |
 
+    Found 1 diagnostic
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -616,7 +634,7 @@ fn exit_code_both_warnings_and_errors() -> anyhow::Result<()> {
         "#,
     )?;
 
-    assert_cmd_snapshot!(case.command(), @r###"
+    assert_cmd_snapshot!(case.command(), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -636,9 +654,10 @@ fn exit_code_both_warnings_and_errors() -> anyhow::Result<()> {
       |       ^ Cannot subscript object of type `Literal[4]` with no `__getitem__` method
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -653,7 +672,7 @@ fn exit_code_both_warnings_and_errors_and_error_on_warning_is_true() -> anyhow::
         "###,
     )?;
 
-    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r###"
+    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -673,9 +692,10 @@ fn exit_code_both_warnings_and_errors_and_error_on_warning_is_true() -> anyhow::
       |       ^ Cannot subscript object of type `Literal[4]` with no `__getitem__` method
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -690,7 +710,7 @@ fn exit_code_exit_zero_is_true() -> anyhow::Result<()> {
         "#,
     )?;
 
-    assert_cmd_snapshot!(case.command().arg("--exit-zero"), @r###"
+    assert_cmd_snapshot!(case.command().arg("--exit-zero"), @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -710,9 +730,10 @@ fn exit_code_exit_zero_is_true() -> anyhow::Result<()> {
       |       ^ Cannot subscript object of type `Literal[4]` with no `__getitem__` method
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###);
+    ");
 
     Ok(())
 }
@@ -749,7 +770,7 @@ fn user_configuration() -> anyhow::Result<()> {
 
     assert_cmd_snapshot!(
         case.command().current_dir(case.root().join("project")).env(config_env_var, config_directory.as_os_str()),
-        @r###"
+        @r"
     success: true
     exit_code: 0
     ----- stdout -----
@@ -771,9 +792,10 @@ fn user_configuration() -> anyhow::Result<()> {
       |       - Name `x` used when possibly not defined
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###
+    "
     );
 
     // The user-level configuration promotes `possibly-unresolved-reference` to an error.
@@ -790,7 +812,7 @@ fn user_configuration() -> anyhow::Result<()> {
 
     assert_cmd_snapshot!(
         case.command().current_dir(case.root().join("project")).env(config_env_var, config_directory.as_os_str()),
-        @r###"
+        @r"
     success: false
     exit_code: 1
     ----- stdout -----
@@ -812,9 +834,134 @@ fn user_configuration() -> anyhow::Result<()> {
       |       ^ Name `x` used when possibly not defined
       |
 
+    Found 2 diagnostics
 
     ----- stderr -----
-    "###
+    "
+    );
+
+    Ok(())
+}
+
+#[test]
+fn check_specific_paths() -> anyhow::Result<()> {
+    let case = TestCase::with_files([
+        (
+            "project/main.py",
+            r#"
+            y = 4 / 0  # error: division-by-zero
+            "#,
+        ),
+        (
+            "project/tests/test_main.py",
+            r#"
+            import does_not_exist  # error: unresolved-import
+            "#,
+        ),
+        (
+            "project/other.py",
+            r#"
+            from main2 import z  # error: unresolved-import
+
+            print(z)
+            "#,
+        ),
+    ])?;
+
+    assert_cmd_snapshot!(
+        case.command(),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: lint:unresolved-import
+     --> <temp_dir>/project/tests/test_main.py:2:8
+      |
+    2 | import does_not_exist  # error: unresolved-import
+      |        ^^^^^^^^^^^^^^ Cannot resolve import `does_not_exist`
+      |
+
+    error: lint:division-by-zero
+     --> <temp_dir>/project/main.py:2:5
+      |
+    2 | y = 4 / 0  # error: division-by-zero
+      |     ^^^^^ Cannot divide object of type `Literal[4]` by zero
+      |
+
+    error: lint:unresolved-import
+     --> <temp_dir>/project/other.py:2:6
+      |
+    2 | from main2 import z  # error: unresolved-import
+      |      ^^^^^ Cannot resolve import `main2`
+    3 |
+    4 | print(z)
+      |
+
+    Found 3 diagnostics
+
+    ----- stderr -----
+    "
+    );
+
+    // Now check only the `tests` and `other.py` files.
+    // We should no longer see any diagnostics related to `main.py`.
+    assert_cmd_snapshot!(
+        case.command().arg("project/tests").arg("project/other.py"),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: lint:unresolved-import
+     --> <temp_dir>/project/tests/test_main.py:2:8
+      |
+    2 | import does_not_exist  # error: unresolved-import
+      |        ^^^^^^^^^^^^^^ Cannot resolve import `does_not_exist`
+      |
+
+    error: lint:unresolved-import
+     --> <temp_dir>/project/other.py:2:6
+      |
+    2 | from main2 import z  # error: unresolved-import
+      |      ^^^^^ Cannot resolve import `main2`
+    3 |
+    4 | print(z)
+      |
+
+    Found 2 diagnostics
+
+    ----- stderr -----
+    "
+    );
+
+    Ok(())
+}
+
+#[test]
+fn check_non_existing_path() -> anyhow::Result<()> {
+    let case = TestCase::with_files([])?;
+
+    let mut settings = insta::Settings::clone_current();
+    settings.add_filter(
+        &regex::escape("The system cannot find the path specified. (os error 3)"),
+        "No such file or directory (os error 2)",
+    );
+    let _s = settings.bind_to_scope();
+
+    assert_cmd_snapshot!(
+        case.command().arg("project/main.py").arg("project/tests"),
+        @r"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: io: `<temp_dir>/project/main.py`: No such file or directory (os error 2)
+
+    error: io: `<temp_dir>/project/tests`: No such file or directory (os error 2)
+
+    Found 2 diagnostics
+
+    ----- stderr -----
+    WARN No python files found under the given path(s)
+    "
     );
 
     Ok(())

crates/red_knot/tests/file_watching.rs

@@ -1,5 +1,6 @@
 #![allow(clippy::disallowed_names)]
 
+use std::collections::HashSet;
 use std::io::Write;
 use std::time::{Duration, Instant};
 
@@ -193,11 +194,29 @@ impl TestCase {
         Ok(())
     }
 
-    fn collect_project_files(&self) -> Vec<File> {
-        let files = self.db().project().files(self.db());
-        let mut collected: Vec<_> = files.into_iter().collect();
-        collected.sort_unstable_by_key(|file| file.path(self.db()).as_system_path().unwrap());
-        collected
+    #[track_caller]
+    fn assert_indexed_project_files(&self, expected: impl IntoIterator<Item = File>) {
+        let mut expected: HashSet<_> = expected.into_iter().collect();
+
+        let actual = self.db().project().files(self.db());
+        for file in &actual {
+            assert!(
+                expected.remove(&file),
+                "Indexed project files contains '{}' which was not expected.",
+                file.path(self.db())
+            );
+        }
+
+        if !expected.is_empty() {
+            let paths: Vec<_> = expected
+                .iter()
+                .map(|file| file.path(self.db()).as_str())
+                .collect();
+            panic!(
+                "Indexed project files are missing the following files: {:?}",
+                paths.join(", ")
+            );
+        }
     }
 
     fn system_file(&self, path: impl AsRef<SystemPath>) -> Result<File, FileError> {
@@ -222,13 +241,15 @@ where
     }
 }
 
-trait SetupFiles {
-    fn setup(self, context: &SetupContext) -> anyhow::Result<()>;
+trait Setup {
+    fn setup(self, context: &mut SetupContext) -> anyhow::Result<()>;
 }
 
 struct SetupContext<'a> {
     system: &'a OsSystem,
     root_path: &'a SystemPath,
+    options: Option<Options>,
+    included_paths: Option<Vec<SystemPathBuf>>,
 }
 
 impl<'a> SetupContext<'a> {
@@ -251,55 +272,77 @@ impl<'a> SetupContext<'a> {
     fn join_root_path(&self, relative: impl AsRef<SystemPath>) -> SystemPathBuf {
         self.root_path().join(relative)
     }
+
+    fn write_project_file(
+        &self,
+        relative_path: impl AsRef<SystemPath>,
+        content: &str,
+    ) -> anyhow::Result<()> {
+        let relative_path = relative_path.as_ref();
+        let absolute_path = self.join_project_path(relative_path);
+        Self::write_file_impl(absolute_path, content)
+    }
+
+    fn write_file(
+        &self,
+        relative_path: impl AsRef<SystemPath>,
+        content: &str,
+    ) -> anyhow::Result<()> {
+        let relative_path = relative_path.as_ref();
+        let absolute_path = self.join_root_path(relative_path);
+        Self::write_file_impl(absolute_path, content)
+    }
+
+    fn write_file_impl(path: impl AsRef<SystemPath>, content: &str) -> anyhow::Result<()> {
+        let path = path.as_ref();
+        if let Some(parent) = path.parent() {
+            std::fs::create_dir_all(parent)
+                .with_context(|| format!("Failed to create parent directory for file `{path}`"))?;
+        }
+
+        let mut file = std::fs::File::create(path.as_std_path())
+            .with_context(|| format!("Failed to open file `{path}`"))?;
+        file.write_all(content.as_bytes())
+            .with_context(|| format!("Failed to write to file `{path}`"))?;
+        file.sync_data()?;
+
+        Ok(())
+    }
+
+    fn set_options(&mut self, options: Options) {
+        self.options = Some(options);
+    }
+
+    fn set_included_paths(&mut self, paths: Vec<SystemPathBuf>) {
+        self.included_paths = Some(paths);
+    }
 }
 
-impl<const N: usize, P> SetupFiles for [(P, &'static str); N]
+impl<const N: usize, P> Setup for [(P, &'static str); N]
 where
     P: AsRef<SystemPath>,
 {
-    fn setup(self, context: &SetupContext) -> anyhow::Result<()> {
+    fn setup(self, context: &mut SetupContext) -> anyhow::Result<()> {
         for (relative_path, content) in self {
-            let relative_path = relative_path.as_ref();
-            let absolute_path = context.join_project_path(relative_path);
-            if let Some(parent) = absolute_path.parent() {
-                std::fs::create_dir_all(parent).with_context(|| {
-                    format!("Failed to create parent directory for file `{relative_path}`")
-                })?;
-            }
-
-            let mut file = std::fs::File::create(absolute_path.as_std_path())
-                .with_context(|| format!("Failed to open file `{relative_path}`"))?;
-            file.write_all(content.as_bytes())
-                .with_context(|| format!("Failed to write to file `{relative_path}`"))?;
-            file.sync_data()?;
+            context.write_project_file(relative_path, content)?;
         }
 
         Ok(())
     }
 }
 
-impl<F> SetupFiles for F
+impl<F> Setup for F
 where
-    F: FnOnce(&SetupContext) -> anyhow::Result<()>,
+    F: FnOnce(&mut SetupContext) -> anyhow::Result<()>,
 {
-    fn setup(self, context: &SetupContext) -> anyhow::Result<()> {
+    fn setup(self, context: &mut SetupContext) -> anyhow::Result<()> {
         self(context)
     }
 }
 
 fn setup<F>(setup_files: F) -> anyhow::Result<TestCase>
 where
-    F: SetupFiles,
-{
-    setup_with_options(setup_files, |_context| None)
-}
-
-fn setup_with_options<F>(
-    setup_files: F,
-    create_options: impl FnOnce(&SetupContext) -> Option<Options>,
-) -> anyhow::Result<TestCase>
-where
-    F: SetupFiles,
+    F: Setup,
 {
     let temp_dir = tempfile::tempdir()?;
 
@@ -325,16 +368,18 @@ where
         .with_context(|| format!("Failed to create project directory `{project_path}`"))?;
 
     let system = OsSystem::new(&project_path);
-    let setup_context = SetupContext {
+    let mut setup_context = SetupContext {
         system: &system,
         root_path: &root_path,
+        options: None,
+        included_paths: None,
     };
 
     setup_files
-        .setup(&setup_context)
+        .setup(&mut setup_context)
         .context("Failed to setup test files")?;
 
-    if let Some(options) = create_options(&setup_context) {
+    if let Some(options) = setup_context.options {
         std::fs::write(
             project_path.join("pyproject.toml").as_std_path(),
             toml::to_string(&PyProject {
@@ -348,6 +393,8 @@ where
         .context("Failed to write configuration")?;
     }
 
+    let included_paths = setup_context.included_paths;
+
     let mut project = ProjectMetadata::discover(&project_path, &system)?;
     project.apply_configuration_files(&system)?;
 
@@ -363,7 +410,11 @@ where
             .with_context(|| format!("Failed to create search path `{path}`"))?;
     }
 
-    let db = ProjectDatabase::new(project, system)?;
+    let mut db = ProjectDatabase::new(project, system)?;
+
+    if let Some(included_paths) = included_paths {
+        db.project().set_included_paths(&mut db, included_paths);
+    }
 
     let (sender, receiver) = crossbeam::channel::unbounded();
     let watcher = directory_watcher(move |events| sender.send(events).unwrap())
@@ -425,7 +476,7 @@ fn new_file() -> anyhow::Result<()> {
     let foo_path = case.project_path("foo.py");
 
     assert_eq!(case.system_file(&foo_path), Err(FileError::NotFound));
-    assert_eq!(&case.collect_project_files(), &[bar_file]);
+    case.assert_indexed_project_files([bar_file]);
 
     std::fs::write(foo_path.as_std_path(), "print('Hello')")?;
 
@@ -435,7 +486,7 @@ fn new_file() -> anyhow::Result<()> {
 
     let foo = case.system_file(&foo_path).expect("foo.py to exist.");
 
-    assert_eq!(&case.collect_project_files(), &[bar_file, foo]);
+    case.assert_indexed_project_files([bar_file, foo]);
 
     Ok(())
 }
@@ -448,7 +499,7 @@ fn new_ignored_file() -> anyhow::Result<()> {
     let foo_path = case.project_path("foo.py");
 
     assert_eq!(case.system_file(&foo_path), Err(FileError::NotFound));
-    assert_eq!(&case.collect_project_files(), &[bar_file]);
+    case.assert_indexed_project_files([bar_file]);
 
     std::fs::write(foo_path.as_std_path(), "print('Hello')")?;
 
@@ -457,7 +508,132 @@ fn new_ignored_file() -> anyhow::Result<()> {
     case.apply_changes(changes);
 
     assert!(case.system_file(&foo_path).is_ok());
-    assert_eq!(&case.collect_project_files(), &[bar_file]);
+    case.assert_indexed_project_files([bar_file]);
+
+    Ok(())
+}
+
+#[test]
+fn new_non_project_file() -> anyhow::Result<()> {
+    let mut case = setup(|context: &mut SetupContext| {
+        context.write_project_file("bar.py", "")?;
+        context.set_options(Options {
+            environment: Some(EnvironmentOptions {
+                extra_paths: Some(vec![RelativePathBuf::cli(
+                    context.join_root_path("site_packages"),
+                )]),
+                ..EnvironmentOptions::default()
+            }),
+            ..Options::default()
+        });
+
+        Ok(())
+    })?;
+
+    let bar_path = case.project_path("bar.py");
+    let bar_file = case.system_file(&bar_path).unwrap();
+
+    case.assert_indexed_project_files([bar_file]);
+
+    // Add a file to site packages
+    let black_path = case.root_path().join("site_packages/black.py");
+    std::fs::write(black_path.as_std_path(), "print('Hello')")?;
+
+    let changes = case.stop_watch(event_for_file("black.py"));
+
+    case.apply_changes(changes);
+
+    assert!(case.system_file(&black_path).is_ok());
+
+    // The file should not have been added to the project files
+    case.assert_indexed_project_files([bar_file]);
+
+    Ok(())
+}
+
+#[test]
+fn new_files_with_explicit_included_paths() -> anyhow::Result<()> {
+    let mut case = setup(|context: &mut SetupContext| {
+        context.write_project_file("src/main.py", "")?;
+        context.write_project_file("src/sub/__init__.py", "")?;
+        context.write_project_file("src/test.py", "")?;
+        context.set_included_paths(vec![
+            context.join_project_path("src/main.py"),
+            context.join_project_path("src/sub"),
+        ]);
+        Ok(())
+    })?;
+
+    let main_path = case.project_path("src/main.py");
+    let main_file = case.system_file(&main_path).unwrap();
+
+    let sub_init_path = case.project_path("src/sub/__init__.py");
+    let sub_init = case.system_file(&sub_init_path).unwrap();
+
+    case.assert_indexed_project_files([main_file, sub_init]);
+
+    // Write a new file to `sub` which is an included path
+    let sub_a_path = case.project_path("src/sub/a.py");
+    std::fs::write(sub_a_path.as_std_path(), "print('Hello')")?;
+
+    // and write a second file in the root directory -- this should not be included
+    let test2_path = case.project_path("src/test2.py");
+    std::fs::write(test2_path.as_std_path(), "print('Hello')")?;
+
+    let changes = case.stop_watch(event_for_file("test2.py"));
+
+    case.apply_changes(changes);
+
+    let sub_a_file = case.system_file(&sub_a_path).expect("sub/a.py to exist");
+
+    case.assert_indexed_project_files([main_file, sub_init, sub_a_file]);
+
+    Ok(())
+}
+
+#[test]
+fn new_file_in_included_out_of_project_directory() -> anyhow::Result<()> {
+    let mut case = setup(|context: &mut SetupContext| {
+        context.write_project_file("src/main.py", "")?;
+        context.write_project_file("script.py", "")?;
+        context.write_file("outside_project/a.py", "")?;
+
+        context.set_included_paths(vec![
+            context.join_root_path("outside_project"),
+            context.join_project_path("src"),
+        ]);
+        Ok(())
+    })?;
+
+    let main_path = case.project_path("src/main.py");
+    let main_file = case.system_file(&main_path).unwrap();
+
+    let outside_a_path = case.root_path().join("outside_project/a.py");
+    let outside_a = case.system_file(&outside_a_path).unwrap();
+
+    case.assert_indexed_project_files([outside_a, main_file]);
+
+    // Write a new file to `src` which should be watched
+    let src_a = case.project_path("src/a.py");
+    std::fs::write(src_a.as_std_path(), "print('Hello')")?;
+
+    // and write a second file to `outside_project` which should be watched too
+    let outside_b_path = case.root_path().join("outside_project/b.py");
+    std::fs::write(outside_b_path.as_std_path(), "print('Hello')")?;
+
+    // and a third file in the project's root that should not be included
+    let script2_path = case.project_path("script2.py");
+    std::fs::write(script2_path.as_std_path(), "print('Hello')")?;
+
+    let changes = case.stop_watch(event_for_file("script2.py"));
+
+    case.apply_changes(changes);
+
+    let src_a_file = case.system_file(&src_a).unwrap();
+    let outside_b_file = case.system_file(&outside_b_path).unwrap();
+
+    // The file should not have been added to the project files
+    case.assert_indexed_project_files([main_file, outside_a, outside_b_file, src_a_file]);
 
     Ok(())
 }
@@ -470,7 +646,7 @@ fn changed_file() -> anyhow::Result<()> {
 
     let foo = case.system_file(&foo_path)?;
     assert_eq!(source_text(case.db(), foo).as_str(), foo_source);
-    assert_eq!(&case.collect_project_files(), &[foo]);
+    case.assert_indexed_project_files([foo]);
 
     update_file(&foo_path, "print('Version 2')")?;
 
@@ -481,7 +657,7 @@ fn changed_file() -> anyhow::Result<()> {
     case.apply_changes(changes);
 
     assert_eq!(source_text(case.db(), foo).as_str(), "print('Version 2')");
-    assert_eq!(&case.collect_project_files(), &[foo]);
+    case.assert_indexed_project_files([foo]);
 
     Ok(())
 }
@@ -495,7 +671,7 @@ fn deleted_file() -> anyhow::Result<()> {
     let foo = case.system_file(&foo_path)?;
 
     assert!(foo.exists(case.db()));
-    assert_eq!(&case.collect_project_files(), &[foo]);
+    case.assert_indexed_project_files([foo]);
 
     std::fs::remove_file(foo_path.as_std_path())?;
 
@@ -504,7 +680,7 @@ fn deleted_file() -> anyhow::Result<()> {
     case.apply_changes(changes);
 
     assert!(!foo.exists(case.db()));
-    assert_eq!(&case.collect_project_files(), &[] as &[File]);
+    case.assert_indexed_project_files([]);
 
     Ok(())
 }
@@ -524,7 +700,7 @@ fn move_file_to_trash() -> anyhow::Result<()> {
     let foo = case.system_file(&foo_path)?;
 
     assert!(foo.exists(case.db()));
-    assert_eq!(&case.collect_project_files(), &[foo]);
+    case.assert_indexed_project_files([foo]);
 
     std::fs::rename(
         foo_path.as_std_path(),
@@ -536,7 +712,7 @@ fn move_file_to_trash() -> anyhow::Result<()> {
     case.apply_changes(changes);
 
     assert!(!foo.exists(case.db()));
-    assert_eq!(&case.collect_project_files(), &[] as &[File]);
+    case.assert_indexed_project_files([]);
 
     Ok(())
 }
@@ -554,7 +730,7 @@ fn move_file_to_project() -> anyhow::Result<()> {
     let foo_in_project = case.project_path("foo.py");
 
     assert!(case.system_file(&foo_path).is_ok());
-    assert_eq!(&case.collect_project_files(), &[bar]);
+    case.assert_indexed_project_files([bar]);
 
     std::fs::rename(foo_path.as_std_path(), foo_in_project.as_std_path())?;
 
@@ -565,7 +741,7 @@ fn move_file_to_project() -> anyhow::Result<()> {
     let foo_in_project = case.system_file(&foo_in_project)?;
 
     assert!(foo_in_project.exists(case.db()));
-    assert_eq!(&case.collect_project_files(), &[bar, foo_in_project]);
+    case.assert_indexed_project_files([bar, foo_in_project]);
 
     Ok(())
 }
@@ -579,7 +755,7 @@ fn rename_file() -> anyhow::Result<()> {
 
     let foo = case.system_file(&foo_path)?;
 
-    assert_eq!(case.collect_project_files(), [foo]);
+    case.assert_indexed_project_files([foo]);
 
     std::fs::rename(foo_path.as_std_path(), bar_path.as_std_path())?;
 
@@ -592,7 +768,7 @@ fn rename_file() -> anyhow::Result<()> {
     let bar = case.system_file(&bar_path)?;
 
     assert!(bar.exists(case.db()));
-    assert_eq!(case.collect_project_files(), [bar]);
+    case.assert_indexed_project_files([bar]);
 
     Ok(())
 }
@@ -618,7 +794,7 @@ fn directory_moved_to_project() -> anyhow::Result<()> {
     );
 
     assert_eq!(sub_a_module, None);
-    assert_eq!(case.collect_project_files(), &[bar]);
+    case.assert_indexed_project_files([bar]);
 
     let sub_new_path = case.project_path("sub");
     std::fs::rename(sub_original_path.as_std_path(), sub_new_path.as_std_path())
@@ -642,7 +818,7 @@ fn directory_moved_to_project() -> anyhow::Result<()> {
     )
     .is_some());
 
-    assert_eq!(case.collect_project_files(), &[bar, init_file, a_file]);
+    case.assert_indexed_project_files([bar, init_file, a_file]);
 
     Ok(())
 }
@@ -670,7 +846,7 @@ fn directory_moved_to_trash() -> anyhow::Result<()> {
         .system_file(sub_path.join("a.py"))
         .expect("a.py to exist");
 
-    assert_eq!(case.collect_project_files(), &[bar, init_file, a_file]);
+    case.assert_indexed_project_files([bar, init_file, a_file]);
 
     std::fs::create_dir(case.root_path().join(".trash").as_std_path())?;
     let trashed_sub = case.root_path().join(".trash/sub");
@@ -691,7 +867,7 @@ fn directory_moved_to_trash() -> anyhow::Result<()> {
     assert!(!init_file.exists(case.db()));
     assert!(!a_file.exists(case.db()));
 
-    assert_eq!(case.collect_project_files(), &[bar]);
+    case.assert_indexed_project_files([bar]);
 
     Ok(())
 }
@@ -725,7 +901,7 @@ fn directory_renamed() -> anyhow::Result<()> {
         .system_file(sub_path.join("a.py"))
         .expect("a.py to exist");
 
-    assert_eq!(case.collect_project_files(), &[bar, sub_init, sub_a]);
+    case.assert_indexed_project_files([bar, sub_init, sub_a]);
 
     let foo_baz = case.project_path("foo/baz");
 
@@ -767,10 +943,7 @@ fn directory_renamed() -> anyhow::Result<()> {
     assert!(foo_baz_init.exists(case.db()));
     assert!(foo_baz_a.exists(case.db()));
 
-    assert_eq!(
-        case.collect_project_files(),
-        &[bar, foo_baz_init, foo_baz_a]
-    );
+    case.assert_indexed_project_files([bar, foo_baz_init, foo_baz_a]);
 
     Ok(())
 }
@@ -799,7 +972,7 @@ fn directory_deleted() -> anyhow::Result<()> {
     let a_file = case
         .system_file(sub_path.join("a.py"))
         .expect("a.py to exist");
-    assert_eq!(case.collect_project_files(), &[bar, init_file, a_file]);
+    case.assert_indexed_project_files([bar, init_file, a_file]);
 
     std::fs::remove_dir_all(sub_path.as_std_path())
         .with_context(|| "Failed to remove the sub directory")?;
@@ -817,15 +990,17 @@ fn directory_deleted() -> anyhow::Result<()> {
 
     assert!(!init_file.exists(case.db()));
     assert!(!a_file.exists(case.db()));
-    assert_eq!(case.collect_project_files(), &[bar]);
+    case.assert_indexed_project_files([bar]);
 
     Ok(())
 }
 
 #[test]
 fn search_path() -> anyhow::Result<()> {
-    let mut case = setup_with_options([("bar.py", "import sub.a")], |context| {
-        Some(Options {
+    let mut case = setup(|context: &mut SetupContext| {
+        context.write_project_file("bar.py", "import sub.a")?;
+
+        context.set_options(Options {
             environment: Some(EnvironmentOptions {
                 extra_paths: Some(vec![RelativePathBuf::cli(
                     context.join_root_path("site_packages"),
@@ -833,7 +1008,8 @@ fn search_path() -> anyhow::Result<()> {
                 ..EnvironmentOptions::default()
             }),
             ..Options::default()
-        })
+        });
+        Ok(())
     })?;
 
     let site_packages = case.root_path().join("site_packages");
@@ -850,10 +1026,7 @@ fn search_path() -> anyhow::Result<()> {
     case.apply_changes(changes);
 
     assert!(resolve_module(case.db().upcast(), &ModuleName::new_static("a").unwrap()).is_some());
-    assert_eq!(
-        case.collect_project_files(),
-        &[case.system_file(case.project_path("bar.py")).unwrap()]
-    );
+    case.assert_indexed_project_files([case.system_file(case.project_path("bar.py")).unwrap()]);
 
     Ok(())
 }
@@ -890,8 +1063,9 @@ fn add_search_path() -> anyhow::Result<()> {
 
 #[test]
 fn remove_search_path() -> anyhow::Result<()> {
-    let mut case = setup_with_options([("bar.py", "import sub.a")], |context| {
-        Some(Options {
+    let mut case = setup(|context: &mut SetupContext| {
+        context.write_project_file("bar.py", "import sub.a")?;
+        context.set_options(Options {
             environment: Some(EnvironmentOptions {
                 extra_paths: Some(vec![RelativePathBuf::cli(
                     context.join_root_path("site_packages"),
@@ -899,7 +1073,9 @@ fn remove_search_path() -> anyhow::Result<()> {
                 ..EnvironmentOptions::default()
             }),
             ..Options::default()
-        })
+        });
+
+        Ok(())
     })?;
 
     // Remove site packages from the search path settings.
@@ -922,30 +1098,30 @@ fn remove_search_path() -> anyhow::Result<()> {
 
 #[test]
 fn change_python_version_and_platform() -> anyhow::Result<()> {
-    let mut case = setup_with_options(
+    let mut case = setup(|context: &mut SetupContext| {
         // `sys.last_exc` is a Python 3.12 only feature
         // `os.getegid()` is Unix only
-        [(
+        context.write_project_file(
             "bar.py",
             r#"
 import sys
 import os
 print(sys.last_exc, os.getegid())
 "#,
-        )],
-        |_context| {
-            Some(Options {
-                environment: Some(EnvironmentOptions {
-                    python_version: Some(RangedValue::cli(PythonVersion::PY311)),
-                    python_platform: Some(RangedValue::cli(PythonPlatform::Identifier(
-                        "win32".to_string(),
-                    ))),
-                    ..EnvironmentOptions::default()
-                }),
-                ..Options::default()
-            })
-        },
-    )?;
+        )?;
+        context.set_options(Options {
+            environment: Some(EnvironmentOptions {
+                python_version: Some(RangedValue::cli(PythonVersion::PY311)),
+                python_platform: Some(RangedValue::cli(PythonPlatform::Identifier(
+                    "win32".to_string(),
+                ))),
+                ..EnvironmentOptions::default()
+            }),
+            ..Options::default()
+        });
+
+        Ok(())
+    })?;
 
     let diagnostics = case.db.check().context("Failed to check project.")?;
 
@@ -980,38 +1156,35 @@ print(sys.last_exc, os.getegid())
 
 #[test]
 fn changed_versions_file() -> anyhow::Result<()> {
-    let mut case = setup_with_options(
-        |context: &SetupContext| {
-            std::fs::write(
-                context.join_project_path("bar.py").as_std_path(),
-                "import sub.a",
-            )?;
-            std::fs::create_dir_all(context.join_root_path("typeshed/stdlib").as_std_path())?;
-            std::fs::write(
-                context
-                    .join_root_path("typeshed/stdlib/VERSIONS")
-                    .as_std_path(),
-                "",
-            )?;
-            std::fs::write(
-                context
-                    .join_root_path("typeshed/stdlib/os.pyi")
-                    .as_std_path(),
-                "# not important",
-            )?;
+    let mut case = setup(|context: &mut SetupContext| {
+        std::fs::write(
+            context.join_project_path("bar.py").as_std_path(),
+            "import sub.a",
+        )?;
+        std::fs::create_dir_all(context.join_root_path("typeshed/stdlib").as_std_path())?;
+        std::fs::write(
+            context
+                .join_root_path("typeshed/stdlib/VERSIONS")
+                .as_std_path(),
+            "",
+        )?;
+        std::fs::write(
+            context
+                .join_root_path("typeshed/stdlib/os.pyi")
+                .as_std_path(),
+            "# not important",
+        )?;
 
-            Ok(())
-        },
-        |context| {
-            Some(Options {
-                environment: Some(EnvironmentOptions {
-                    typeshed: Some(RelativePathBuf::cli(context.join_root_path("typeshed"))),
-                    ..EnvironmentOptions::default()
-                }),
-                ..Options::default()
-            })
-        },
-    )?;
+        context.set_options(Options {
+            environment: Some(EnvironmentOptions {
+                typeshed: Some(RelativePathBuf::cli(context.join_root_path("typeshed"))),
+                ..EnvironmentOptions::default()
+            }),
+            ..Options::default()
+        });
+
+        Ok(())
+    })?;
 
     // Unset the custom typeshed directory.
     assert_eq!(
@@ -1056,7 +1229,7 @@ fn changed_versions_file() -> anyhow::Result<()> {
 /// we're seeing is that Windows only emits a single event, similar to Linux.
 #[test]
 fn hard_links_in_project() -> anyhow::Result<()> {
-    let mut case = setup(|context: &SetupContext| {
+    let mut case = setup(|context: &mut SetupContext| {
         let foo_path = context.join_project_path("foo.py");
         std::fs::write(foo_path.as_std_path(), "print('Version 1')")?;
 
@@ -1075,6 +1248,7 @@ fn hard_links_in_project() -> anyhow::Result<()> {
 
     assert_eq!(source_text(case.db(), foo).as_str(), "print('Version 1')");
     assert_eq!(source_text(case.db(), bar).as_str(), "print('Version 1')");
+    case.assert_indexed_project_files([bar, foo]);
 
     // Write to the hard link target.
     update_file(foo_path, "print('Version 2')").context("Failed to update foo.py")?;
@@ -1127,7 +1301,7 @@ fn hard_links_in_project() -> anyhow::Result<()> {
     ignore = "windows doesn't support observing changes to hard linked files."
 )]
 fn hard_links_to_target_outside_project() -> anyhow::Result<()> {
-    let mut case = setup(|context: &SetupContext| {
+    let mut case = setup(|context: &mut SetupContext| {
         let foo_path = context.join_root_path("foo.py");
         std::fs::write(foo_path.as_std_path(), "print('Version 1')")?;
 
@@ -1235,7 +1409,7 @@ mod unix {
         ignore = "FSEvents doesn't emit change events for symlinked directories outside of the watched paths."
     )]
     fn symlink_target_outside_watched_paths() -> anyhow::Result<()> {
-        let mut case = setup(|context: &SetupContext| {
+        let mut case = setup(|context: &mut SetupContext| {
             // Set up the symlink target.
             let link_target = context.join_root_path("bar");
             std::fs::create_dir_all(link_target.as_std_path())
@@ -1316,7 +1490,7 @@ mod unix {
     /// ```
     #[test]
     fn symlink_inside_project() -> anyhow::Result<()> {
-        let mut case = setup(|context: &SetupContext| {
+        let mut case = setup(|context: &mut SetupContext| {
             // Set up the symlink target.
             let link_target = context.join_project_path("patched/bar");
             std::fs::create_dir_all(link_target.as_std_path())
@@ -1354,6 +1528,8 @@ mod unix {
         );
         assert_eq!(baz.file().path(case.db()).as_system_path(), Some(&*bar_baz));
 
+        case.assert_indexed_project_files([patched_bar_baz_file]);
+
         // Write to the symlink target.
         update_file(&patched_bar_baz, "def baz(): print('Version 2')")
             .context("Failed to update bar/baz.py")?;
@@ -1389,6 +1565,7 @@ mod unix {
             bar_baz_text = bar_baz_text.as_str()
         );
 
+        case.assert_indexed_project_files([patched_bar_baz_file]);
         Ok(())
     }
 
@@ -1406,43 +1583,39 @@ mod unix {
     /// ```
     #[test]
     fn symlinked_module_search_path() -> anyhow::Result<()> {
-        let mut case = setup_with_options(
-            |context: &SetupContext| {
-                // Set up the symlink target.
-                let site_packages = context.join_root_path("site-packages");
-                let bar = site_packages.join("bar");
-                std::fs::create_dir_all(bar.as_std_path())
-                    .context("Failed to create bar directory")?;
-                let baz_original = bar.join("baz.py");
-                std::fs::write(baz_original.as_std_path(), "def baz(): ...")
-                    .context("Failed to write baz.py")?;
+        let mut case = setup(|context: &mut SetupContext| {
+            // Set up the symlink target.
+            let site_packages = context.join_root_path("site-packages");
+            let bar = site_packages.join("bar");
+            std::fs::create_dir_all(bar.as_std_path()).context("Failed to create bar directory")?;
+            let baz_original = bar.join("baz.py");
+            std::fs::write(baz_original.as_std_path(), "def baz(): ...")
+                .context("Failed to write baz.py")?;
 
-                // Symlink the site packages in the venv to the global site packages
-                let venv_site_packages =
-                    context.join_project_path(".venv/lib/python3.12/site-packages");
-                std::fs::create_dir_all(venv_site_packages.parent().unwrap())
-                    .context("Failed to create .venv directory")?;
-                std::os::unix::fs::symlink(
-                    site_packages.as_std_path(),
-                    venv_site_packages.as_std_path(),
-                )
-                .context("Failed to create symlink to site-packages")?;
+            // Symlink the site packages in the venv to the global site packages
+            let venv_site_packages =
+                context.join_project_path(".venv/lib/python3.12/site-packages");
+            std::fs::create_dir_all(venv_site_packages.parent().unwrap())
+                .context("Failed to create .venv directory")?;
+            std::os::unix::fs::symlink(
+                site_packages.as_std_path(),
+                venv_site_packages.as_std_path(),
+            )
+            .context("Failed to create symlink to site-packages")?;
 
-                Ok(())
-            },
-            |_context| {
-                Some(Options {
-                    environment: Some(EnvironmentOptions {
-                        extra_paths: Some(vec![RelativePathBuf::cli(
-                            ".venv/lib/python3.12/site-packages",
-                        )]),
-                        python_version: Some(RangedValue::cli(PythonVersion::PY312)),
-                        ..EnvironmentOptions::default()
-                    }),
-                    ..Options::default()
-                })
-            },
-        )?;
+            context.set_options(Options {
+                environment: Some(EnvironmentOptions {
+                    extra_paths: Some(vec![RelativePathBuf::cli(
+                        ".venv/lib/python3.12/site-packages",
+                    )]),
+                    python_version: Some(RangedValue::cli(PythonVersion::PY312)),
+                    ..EnvironmentOptions::default()
+                }),
+                ..Options::default()
+            });
+
+            Ok(())
+        })?;
 
         let baz = resolve_module(
             case.db().upcast(),
@@ -1469,6 +1642,8 @@ mod unix {
             Some(&*baz_original)
         );
 
+        case.assert_indexed_project_files([]);
+
         // Write to the symlink target.
         update_file(&baz_original, "def baz(): print('Version 2')")
             .context("Failed to update bar/baz.py")?;
@@ -1494,13 +1669,15 @@ mod unix {
             "def baz(): print('Version 2')"
         );
 
+        case.assert_indexed_project_files([]);
+
         Ok(())
     }
 }
 
 #[test]
 fn nested_projects_delete_root() -> anyhow::Result<()> {
-    let mut case = setup(|context: &SetupContext| {
+    let mut case = setup(|context: &mut SetupContext| {
         std::fs::write(
             context.join_project_path("pyproject.toml").as_std_path(),
             r#"
@@ -1542,7 +1719,7 @@ fn nested_projects_delete_root() -> anyhow::Result<()> {
 fn changes_to_user_configuration() -> anyhow::Result<()> {
     let mut _config_dir_override: Option<UserConfigDirectoryOverrideGuard> = None;
 
-    let mut case = setup(|context: &SetupContext| {
+    let mut case = setup(|context: &mut SetupContext| {
         std::fs::write(
             context.join_project_path("pyproject.toml").as_std_path(),
             r#"

crates/red_knot_project/src/combine.rs

@@ -1,6 +1,6 @@
 use std::{collections::HashMap, hash::BuildHasher};
 
-use red_knot_python_semantic::{PythonPlatform, SitePackages};
+use red_knot_python_semantic::{PythonPath, PythonPlatform};
 use ruff_db::system::SystemPathBuf;
 use ruff_python_ast::PythonVersion;
 
@@ -128,7 +128,7 @@ macro_rules! impl_noop_combine {
 
 impl_noop_combine!(SystemPathBuf);
 impl_noop_combine!(PythonPlatform);
-impl_noop_combine!(SitePackages);
+impl_noop_combine!(PythonPath);
 impl_noop_combine!(PythonVersion);
 
 // std types

crates/red_knot_project/src/db.rs

@@ -5,7 +5,7 @@ use crate::DEFAULT_LINT_REGISTRY;
 use crate::{Project, ProjectMetadata};
 use red_knot_python_semantic::lint::{LintRegistry, RuleSelection};
 use red_knot_python_semantic::{Db as SemanticDb, Program};
-use ruff_db::diagnostic::Diagnostic;
+use ruff_db::diagnostic::OldDiagnosticTrait;
 use ruff_db::files::{File, Files};
 use ruff_db::system::System;
 use ruff_db::vendored::VendoredFileSystem;
@@ -55,11 +55,11 @@ impl ProjectDatabase {
     }
 
     /// Checks all open files in the project and its dependencies.
-    pub fn check(&self) -> Result<Vec<Box<dyn Diagnostic>>, Cancelled> {
+    pub fn check(&self) -> Result<Vec<Box<dyn OldDiagnosticTrait>>, Cancelled> {
         self.with_db(|db| db.project().check(db))
     }
 
-    pub fn check_file(&self, file: File) -> Result<Vec<Box<dyn Diagnostic>>, Cancelled> {
+    pub fn check_file(&self, file: File) -> Result<Vec<Box<dyn OldDiagnosticTrait>>, Cancelled> {
         let _span = tracing::debug_span!("check_file", file=%file.path(self)).entered();
 
         self.with_db(|db| self.project().check_file(db, file))

crates/red_knot_project/src/db/changes.rs

@@ -2,20 +2,20 @@ use crate::db::{Db, ProjectDatabase};
 use crate::metadata::options::Options;
 use crate::watch::{ChangeEvent, CreatedKind, DeletedKind};
 use crate::{Project, ProjectMetadata};
+use std::collections::BTreeSet;
 
+use crate::walk::ProjectFilesWalker;
 use red_knot_python_semantic::Program;
-use ruff_db::files::{system_path_to_file, File, Files};
-use ruff_db::system::walk_directory::WalkState;
+use ruff_db::files::{File, Files};
 use ruff_db::system::SystemPath;
 use ruff_db::Db as _;
-use ruff_python_ast::PySourceType;
 use rustc_hash::FxHashSet;
 
 impl ProjectDatabase {
     #[tracing::instrument(level = "debug", skip(self, changes, cli_options))]
     pub fn apply_changes(&mut self, changes: Vec<ChangeEvent>, cli_options: Option<&Options>) {
         let mut project = self.project();
-        let project_path = project.root(self).to_path_buf();
+        let project_root = project.root(self).to_path_buf();
         let program = Program::get(self);
         let custom_stdlib_versions_path = program
             .custom_stdlib_search_path(self)
@@ -30,7 +30,7 @@ impl ProjectDatabase {
 
         // Deduplicate the `sync` calls. Many file watchers emit multiple events for the same path.
         let mut synced_files = FxHashSet::default();
-        let mut synced_recursively = FxHashSet::default();
+        let mut sync_recursively = BTreeSet::default();
 
         let mut sync_path = |db: &mut ProjectDatabase, path: &SystemPath| {
             if synced_files.insert(path.to_path_buf()) {
@@ -38,13 +38,9 @@ impl ProjectDatabase {
             }
         };
 
-        let mut sync_recursively = |db: &mut ProjectDatabase, path: &SystemPath| {
-            if synced_recursively.insert(path.to_path_buf()) {
-                Files::sync_recursively(db, path);
-            }
-        };
-
         for change in changes {
+            tracing::trace!("Handle change: {:?}", change);
+
             if let Some(path) = change.system_path() {
                 if matches!(
                     path.file_name(),
@@ -70,16 +66,27 @@ impl ProjectDatabase {
                     match kind {
                         CreatedKind::File => sync_path(self, &path),
                         CreatedKind::Directory | CreatedKind::Any => {
-                            sync_recursively(self, &path);
+                            sync_recursively.insert(path.clone());
                         }
                     }
 
-                    if self.system().is_file(&path) {
-                        // Add the parent directory because `walkdir` always visits explicitly passed files
-                        // even if they match an exclude filter.
-                        added_paths.insert(path.parent().unwrap().to_path_buf());
-                    } else {
-                        added_paths.insert(path);
+                    // Unlike other files, it's not only important to update the status of existing
+                    // and known `File`s (`sync_recursively`), it's also important to discover new files
+                    // that were added in the project's root (or any of the paths included for checking).
+                    //
+                    // This is important because `Project::check` iterates over all included files.
+                    // The code below walks the `added_paths` and adds all files that
+                    // should be included in the project. We can skip this check for
+                    // paths that aren't part of the project or shouldn't be included
+                    // when checking the project.
+                    if project.is_path_included(self, &path) {
+                        if self.system().is_file(&path) {
+                            // Add the parent directory because `walkdir` always visits explicitly passed files
+                            // even if they match an exclude filter.
+                            added_paths.insert(path.parent().unwrap().to_path_buf());
+                        } else {
+                            added_paths.insert(path);
+                        }
                     }
                 }
 
@@ -103,7 +110,7 @@ impl ProjectDatabase {
                             project.remove_file(self, file);
                         }
                     } else {
-                        sync_recursively(self, &path);
+                        sync_recursively.insert(path.clone());
 
                         if custom_stdlib_versions_path
                             .as_ref()
@@ -112,11 +119,19 @@ impl ProjectDatabase {
                             custom_stdlib_change = true;
                         }
 
-                        // Perform a full-reload in case the deleted directory contained the pyproject.toml.
-                        // We may want to make this more clever in the future, to e.g. iterate over the
-                        // indexed files and remove the once that start with the same path, unless
-                        // the deleted path is the project configuration.
-                        project_changed = true;
+                        if project.is_path_included(self, &path) || path == project_root {
+                            // TODO: Shouldn't it be enough to simply traverse the project files and remove all
+                            // that start with the given path?
+                            tracing::debug!(
+                                "Reload project because of a path that could have been a directory."
+                            );
+
+                            // Perform a full-reload in case the deleted directory contained the pyproject.toml.
+                            // We may want to make this more clever in the future, to e.g. iterate over the
+                            // indexed files and remove the once that start with the same path, unless
+                            // the deleted path is the project configuration.
+                            project_changed = true;
+                        }
                     }
                 }
 
@@ -133,13 +148,29 @@ impl ProjectDatabase {
                 ChangeEvent::Rescan => {
                     project_changed = true;
                     Files::sync_all(self);
+                    sync_recursively.clear();
                     break;
                 }
             }
         }
 
+        let sync_recursively = sync_recursively.into_iter();
+        let mut last = None;
+
+        for path in sync_recursively {
+            // Avoid re-syncing paths that are sub-paths of each other.
+            if let Some(last) = &last {
+                if path.starts_with(last) {
+                    continue;
+                }
+            }
+
+            Files::sync_recursively(self, &path);
+            last = Some(path);
+        }
+
         if project_changed {
-            match ProjectMetadata::discover(&project_path, self.system()) {
+            match ProjectMetadata::discover(&project_root, self.system()) {
                 Ok(mut metadata) => {
                     if let Some(cli_options) = cli_options {
                         metadata.apply_cli_options(cli_options.clone());
@@ -186,50 +217,24 @@ impl ProjectDatabase {
             }
         }
 
-        let mut added_paths = added_paths.into_iter();
+        let diagnostics = if let Some(walker) = ProjectFilesWalker::incremental(self, added_paths) {
+            // Use directory walking to discover newly added files.
+            let (files, diagnostics) = walker.collect_vec(self);
 
-        // Use directory walking to discover newly added files.
-        if let Some(path) = added_paths.next() {
-            let mut walker = self.system().walk_directory(&path);
-
-            for extra_path in added_paths {
-                walker = walker.add(&extra_path);
+            for file in files {
+                project.add_file(self, file);
             }
 
-            let added_paths = std::sync::Mutex::new(Vec::default());
+            diagnostics
+        } else {
+            Vec::new()
+        };
 
-            walker.run(|| {
-                Box::new(|entry| {
-                    let Ok(entry) = entry else {
-                        return WalkState::Continue;
-                    };
-
-                    if !entry.file_type().is_file() {
-                        return WalkState::Continue;
-                    }
-
-                    if entry
-                        .path()
-                        .extension()
-                        .and_then(PySourceType::try_from_extension)
-                        .is_some()
-                    {
-                        let mut paths = added_paths.lock().unwrap();
-
-                        paths.push(entry.into_path());
-                    }
-
-                    WalkState::Continue
-                })
-            });
-
-            for path in added_paths.into_inner().unwrap() {
-                let file = system_path_to_file(self, &path);
-
-                if let Ok(file) = file {
-                    project.add_file(self, file);
-                }
-            }
-        }
+        // Note: We simply replace all IO related diagnostics here. This isn't ideal, because
+        // it removes IO errors that may still be relevant. However, tracking IO errors correctly
+        // across revisions doesn't feel essential, considering that they're rare. However, we could
+        // implement a `BTreeMap` or similar and only prune the diagnostics from paths that we've
+        // re-scanned (or that were removed etc).
+        project.replace_index_diagnostics(self, diagnostics);
     }
 }

crates/red_knot_project/src/files.rs

@@ -8,10 +8,7 @@ use salsa::Setter;
 use ruff_db::files::File;
 
 use crate::db::Db;
-use crate::Project;
-
-/// Cheap cloneable hash set of files.
-type FileSet = Arc<FxHashSet<File>>;
+use crate::{IOErrorDiagnostic, Project};
 
 /// The indexed files of a project.
 ///
@@ -35,9 +32,9 @@ impl IndexedFiles {
         }
     }
 
-    fn indexed(files: FileSet) -> Self {
+    fn indexed(inner: Arc<IndexedInner>) -> Self {
         Self {
-            state: std::sync::Mutex::new(State::Indexed(files)),
+            state: std::sync::Mutex::new(State::Indexed(inner)),
         }
     }
 
@@ -46,8 +43,8 @@ impl IndexedFiles {
 
         match &*state {
             State::Lazy => Index::Lazy(LazyFiles { files: state }),
-            State::Indexed(files) => Index::Indexed(Indexed {
-                files: Arc::clone(files),
+            State::Indexed(inner) => Index::Indexed(Indexed {
+                inner: Arc::clone(inner),
                 _lifetime: PhantomData,
             }),
         }
@@ -94,7 +91,7 @@ impl IndexedFiles {
         Some(IndexedMut {
             db: Some(db),
             project,
-            files: indexed,
+            indexed,
             did_change: false,
         })
     }
@@ -112,7 +109,7 @@ enum State {
     Lazy,
 
     /// The files are indexed. Stores the known files of a package.
-    Indexed(FileSet),
+    Indexed(Arc<IndexedInner>),
 }
 
 pub(super) enum Index<'db> {
@@ -129,32 +126,48 @@ pub(super) struct LazyFiles<'db> {
 
 impl<'db> LazyFiles<'db> {
     /// Sets the indexed files of a package to `files`.
-    pub(super) fn set(mut self, files: FxHashSet<File>) -> Indexed<'db> {
+    pub(super) fn set(
+        mut self,
+        files: FxHashSet<File>,
+        diagnostics: Vec<IOErrorDiagnostic>,
+    ) -> Indexed<'db> {
         let files = Indexed {
-            files: Arc::new(files),
+            inner: Arc::new(IndexedInner { files, diagnostics }),
             _lifetime: PhantomData,
         };
-        *self.files = State::Indexed(Arc::clone(&files.files));
+        *self.files = State::Indexed(Arc::clone(&files.inner));
         files
     }
 }
 
-/// The indexed files of a package.
+/// The indexed files of the project.
 ///
 /// Note: This type is intentionally non-cloneable. Making it cloneable requires
 /// revisiting the locking behavior in [`IndexedFiles::indexed_mut`].
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Debug)]
 pub struct Indexed<'db> {
-    files: FileSet,
+    inner: Arc<IndexedInner>,
     // Preserve the lifetime of `PackageFiles`.
     _lifetime: PhantomData<&'db ()>,
 }
 
+#[derive(Debug)]
+struct IndexedInner {
+    files: FxHashSet<File>,
+    diagnostics: Vec<IOErrorDiagnostic>,
+}
+
+impl Indexed<'_> {
+    pub(super) fn diagnostics(&self) -> &[IOErrorDiagnostic] {
+        &self.inner.diagnostics
+    }
+}
+
 impl Deref for Indexed<'_> {
     type Target = FxHashSet<File>;
 
     fn deref(&self) -> &Self::Target {
-        &self.files
+        &self.inner.files
     }
 }
 
@@ -165,7 +178,7 @@ impl<'a> IntoIterator for &'a Indexed<'_> {
     type IntoIter = IndexedIter<'a>;
 
     fn into_iter(self) -> Self::IntoIter {
-        self.files.iter().copied()
+        self.inner.files.iter().copied()
     }
 }
 
@@ -176,13 +189,13 @@ impl<'a> IntoIterator for &'a Indexed<'_> {
 pub(super) struct IndexedMut<'db> {
     db: Option<&'db mut dyn Db>,
     project: Project,
-    files: FileSet,
+    indexed: Arc<IndexedInner>,
     did_change: bool,
 }
 
 impl IndexedMut<'_> {
     pub(super) fn insert(&mut self, file: File) -> bool {
-        if self.files_mut().insert(file) {
+        if self.inner_mut().files.insert(file) {
             self.did_change = true;
             true
         } else {
@@ -191,7 +204,7 @@ impl IndexedMut<'_> {
     }
 
     pub(super) fn remove(&mut self, file: File) -> bool {
-        if self.files_mut().remove(&file) {
+        if self.inner_mut().files.remove(&file) {
             self.did_change = true;
             true
         } else {
@@ -199,8 +212,13 @@ impl IndexedMut<'_> {
         }
     }
 
-    fn files_mut(&mut self) -> &mut FxHashSet<File> {
-        Arc::get_mut(&mut self.files).expect("All references to `FilesSet` to have been dropped")
+    pub(super) fn set_diagnostics(&mut self, diagnostics: Vec<IOErrorDiagnostic>) {
+        self.inner_mut().diagnostics = diagnostics;
+    }
+
+    fn inner_mut(&mut self) -> &mut IndexedInner {
+        Arc::get_mut(&mut self.indexed)
+            .expect("All references to `FilesSet` should have been dropped")
     }
 
     fn set_impl(&mut self) {
@@ -208,16 +226,16 @@ impl IndexedMut<'_> {
             return;
         };
 
-        let files = Arc::clone(&self.files);
+        let indexed = Arc::clone(&self.indexed);
 
         if self.did_change {
             // If there are changes, set the new file_set to trigger a salsa revision change.
             self.project
                 .set_file_set(db)
-                .to(IndexedFiles::indexed(files));
+                .to(IndexedFiles::indexed(indexed));
         } else {
             // The `indexed_mut` replaced the `state` with Lazy. Restore it back to the indexed state.
-            *self.project.file_set(db).state.lock().unwrap() = State::Indexed(files);
+            *self.project.file_set(db).state.lock().unwrap() = State::Indexed(indexed);
         }
     }
 }
@@ -237,7 +255,7 @@ mod tests {
     use crate::files::Index;
     use crate::ProjectMetadata;
     use ruff_db::files::system_path_to_file;
-    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
+    use ruff_db::system::{DbWithWritableSystem as _, SystemPathBuf};
     use ruff_python_ast::name::Name;
 
     #[test]
@@ -252,7 +270,7 @@ mod tests {
         let file = system_path_to_file(&db, "test.py").unwrap();
 
         let files = match project.file_set(&db).get() {
-            Index::Lazy(lazy) => lazy.set(FxHashSet::from_iter([file])),
+            Index::Lazy(lazy) => lazy.set(FxHashSet::from_iter([file]), Vec::new()),
             Index::Indexed(files) => files,
         };
 

crates/red_knot_project/src/lib.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::ref_option)]
 
 use crate::metadata::options::OptionDiagnostic;
+use crate::walk::{ProjectFilesFilter, ProjectFilesWalker};
 pub use db::{Db, ProjectDatabase};
 use files::{Index, Indexed, IndexedFiles};
 use metadata::settings::Settings;
@@ -8,24 +9,24 @@ pub use metadata::{ProjectDiscoveryError, ProjectMetadata};
 use red_knot_python_semantic::lint::{LintRegistry, LintRegistryBuilder, RuleSelection};
 use red_knot_python_semantic::register_lints;
 use red_knot_python_semantic::types::check_types;
-use ruff_db::diagnostic::{Diagnostic, DiagnosticId, ParseDiagnostic, Severity, Span};
-use ruff_db::files::{system_path_to_file, File};
+use ruff_db::diagnostic::{DiagnosticId, OldDiagnosticTrait, OldParseDiagnostic, Severity, Span};
+use ruff_db::files::File;
 use ruff_db::parsed::parsed_module;
 use ruff_db::source::{source_text, SourceTextError};
-use ruff_db::system::walk_directory::WalkState;
-use ruff_db::system::{FileType, SystemPath};
-use ruff_python_ast::PySourceType;
-use rustc_hash::{FxBuildHasher, FxHashSet};
+use ruff_db::system::{SystemPath, SystemPathBuf};
+use rustc_hash::FxHashSet;
 use salsa::Durability;
 use salsa::Setter;
 use std::borrow::Cow;
 use std::sync::Arc;
+use thiserror::Error;
 
 pub mod combine;
 
 mod db;
 mod files;
 pub mod metadata;
+mod walk;
 pub mod watch;
 
 pub static DEFAULT_LINT_REGISTRY: std::sync::LazyLock<LintRegistry> =
@@ -71,6 +72,30 @@ pub struct Project {
     #[return_ref]
     pub settings: Settings,
 
+    /// The paths that should be included when checking this project.
+    ///
+    /// The default (when this list is empty) is to include all files in the project root
+    /// (that satisfy the configured include and exclude patterns).
+    /// However, it's sometimes desired to only check a subset of the project, e.g. to see
+    /// the diagnostics for a single file or a folder.
+    ///
+    /// This list gets initialized by the paths passed to `knot check <paths>`
+    ///
+    /// ## How is this different from `open_files`?
+    ///
+    /// The `included_paths` is closely related to `open_files`. The only difference is that
+    /// `open_files` is already a resolved set of files whereas `included_paths` is only a list of paths
+    /// that are resolved to files by indexing them. The other difference is that
+    /// new files added to any directory in `included_paths` will be indexed and added to the project
+    /// whereas `open_files` needs to be updated manually (e.g. by the IDE).
+    ///
+    /// In short, `open_files` is cheaper in contexts where the set of files is known, like
+    /// in an IDE when the user only wants to check the open tabs. This could be modeled
+    /// with `included_paths` too but it would require an explicit walk dir step that's simply unnecessary.
+    #[default]
+    #[return_ref]
+    included_paths_list: Vec<SystemPathBuf>,
+
     /// Diagnostics that were generated when resolving the project settings.
     #[return_ref]
     settings_diagnostics: Vec<OptionDiagnostic>,
@@ -106,6 +131,16 @@ impl Project {
         self.settings(db).to_rules()
     }
 
+    /// Returns `true` if `path` is both part of the project and included (see `included_paths_list`).
+    ///
+    /// Unlike [Self::files], this method does not respect `.gitignore` files. It only checks
+    /// the project's include and exclude settings as well as the paths that were passed to `knot check <paths>`.
+    /// This means, that this method is an over-approximation of `Self::files` and may return `true` for paths
+    /// that won't be included when checking the project because they're ignored in a `.gitignore` file.
+    pub fn is_path_included(self, db: &dyn Db, path: &SystemPath) -> bool {
+        ProjectFilesFilter::from_project(db, self).is_included(path)
+    }
+
     pub fn reload(self, db: &mut dyn Db, metadata: ProjectMetadata) {
         tracing::debug!("Reloading project");
         assert_eq!(self.root(db), metadata.root());
@@ -128,15 +163,22 @@ impl Project {
     }
 
     /// Checks all open files in the project and its dependencies.
-    pub(crate) fn check(self, db: &ProjectDatabase) -> Vec<Box<dyn Diagnostic>> {
+    pub(crate) fn check(self, db: &ProjectDatabase) -> Vec<Box<dyn OldDiagnosticTrait>> {
         let project_span = tracing::debug_span!("Project::check");
         let _span = project_span.enter();
 
         tracing::debug!("Checking project '{name}'", name = self.name(db));
 
-        let mut diagnostics: Vec<Box<dyn Diagnostic>> = Vec::new();
+        let mut diagnostics: Vec<Box<dyn OldDiagnosticTrait>> = Vec::new();
         diagnostics.extend(self.settings_diagnostics(db).iter().map(|diagnostic| {
-            let diagnostic: Box<dyn Diagnostic> = Box::new(diagnostic.clone());
+            let diagnostic: Box<dyn OldDiagnosticTrait> = Box::new(diagnostic.clone());
+            diagnostic
+        }));
+
+        let files = ProjectFiles::new(db, self);
+
+        diagnostics.extend(files.diagnostics().iter().cloned().map(|diagnostic| {
+            let diagnostic: Box<dyn OldDiagnosticTrait> = Box::new(diagnostic);
             diagnostic
         }));
 
@@ -147,7 +189,6 @@ impl Project {
         let project_span = project_span.clone();
 
         rayon::scope(move |scope| {
-            let files = ProjectFiles::new(&db, self);
             for file in &files {
                 let result = inner_result.clone();
                 let db = db.clone();
@@ -166,12 +207,12 @@ impl Project {
         Arc::into_inner(result).unwrap().into_inner().unwrap()
     }
 
-    pub(crate) fn check_file(self, db: &dyn Db, file: File) -> Vec<Box<dyn Diagnostic>> {
+    pub(crate) fn check_file(self, db: &dyn Db, file: File) -> Vec<Box<dyn OldDiagnosticTrait>> {
         let mut file_diagnostics: Vec<_> = self
             .settings_diagnostics(db)
             .iter()
             .map(|diagnostic| {
-                let diagnostic: Box<dyn Diagnostic> = Box::new(diagnostic.clone());
+                let diagnostic: Box<dyn OldDiagnosticTrait> = Box::new(diagnostic.clone());
                 diagnostic
             })
             .collect();
@@ -207,6 +248,30 @@ impl Project {
         removed
     }
 
+    pub fn set_included_paths(self, db: &mut dyn Db, paths: Vec<SystemPathBuf>) {
+        tracing::debug!("Setting included paths: {paths}", paths = paths.len());
+
+        self.set_included_paths_list(db).to(paths);
+        self.reload_files(db);
+    }
+
+    /// Returns the paths that should be checked.
+    ///
+    /// The default is to check the entire project in which case this method returns
+    /// the project root. However, users can specify to only check specific sub-folders or
+    /// even files of a project by using `knot check <paths>`. In that case, this method
+    /// returns the provided absolute paths.
+    ///
+    /// Note: The CLI doesn't prohibit users from specifying paths outside the project root.
+    /// This can be useful to check arbitrary files, but it isn't something we recommend.
+    /// We should try to support this use case but it's okay if there are some limitations around it.
+    fn included_paths_or_root(self, db: &dyn Db) -> &[SystemPathBuf] {
+        match &**self.included_paths_list(db) {
+            [] => std::slice::from_ref(&self.metadata(db).root),
+            paths => paths,
+        }
+    }
+
     /// Returns the open files in the project or `None` if the entire project should be checked.
     pub fn open_files(self, db: &dyn Db) -> Option<&FxHashSet<File>> {
         self.open_fileset(db).as_deref()
@@ -289,6 +354,17 @@ impl Project {
         index.insert(file);
     }
 
+    /// Replaces the diagnostics from indexing the project files with `diagnostics`.
+    ///
+    /// This is a no-op if the project files haven't been indexed yet.
+    pub fn replace_index_diagnostics(self, db: &mut dyn Db, diagnostics: Vec<IOErrorDiagnostic>) {
+        let Some(mut index) = IndexedFiles::indexed_mut(db, self) else {
+            return;
+        };
+
+        index.set_diagnostics(diagnostics);
+    }
+
     /// Returns the files belonging to this project.
     pub fn files(self, db: &dyn Db) -> Indexed<'_> {
         let files = self.file_set(db);
@@ -296,12 +372,14 @@ impl Project {
         let indexed = match files.get() {
             Index::Lazy(vacant) => {
                 let _entered =
-                    tracing::debug_span!("Project::index_files", package = %self.name(db))
+                    tracing::debug_span!("Project::index_files", project = %self.name(db))
                         .entered();
 
-                let files = discover_project_files(db, self);
-                tracing::info!("Found {} files in project `{}`", files.len(), self.name(db));
-                vacant.set(files)
+                let walker = ProjectFilesWalker::new(db);
+                let (files, diagnostics) = walker.collect_set(db);
+
+                tracing::info!("Indexed {} file(s)", files.len());
+                vacant.set(files, diagnostics)
             }
             Index::Indexed(indexed) => indexed,
         };
@@ -319,28 +397,29 @@ impl Project {
     }
 }
 
-fn check_file_impl(db: &dyn Db, file: File) -> Vec<Box<dyn Diagnostic>> {
-    let mut diagnostics: Vec<Box<dyn Diagnostic>> = Vec::new();
+fn check_file_impl(db: &dyn Db, file: File) -> Vec<Box<dyn OldDiagnosticTrait>> {
+    let mut diagnostics: Vec<Box<dyn OldDiagnosticTrait>> = Vec::new();
 
     // Abort checking if there are IO errors.
     let source = source_text(db.upcast(), file);
 
     if let Some(read_error) = source.read_error() {
         diagnostics.push(Box::new(IOErrorDiagnostic {
-            file,
-            error: read_error.clone(),
+            file: Some(file),
+            error: read_error.clone().into(),
         }));
         return diagnostics;
     }
 
     let parsed = parsed_module(db.upcast(), file);
     diagnostics.extend(parsed.errors().iter().map(|error| {
-        let diagnostic: Box<dyn Diagnostic> = Box::new(ParseDiagnostic::new(file, error.clone()));
+        let diagnostic: Box<dyn OldDiagnosticTrait> =
+            Box::new(OldParseDiagnostic::new(file, error.clone()));
         diagnostic
     }));
 
     diagnostics.extend(check_types(db.upcast(), file).iter().map(|diagnostic| {
-        let boxed: Box<dyn Diagnostic> = Box::new(diagnostic.clone());
+        let boxed: Box<dyn OldDiagnosticTrait> = Box::new(diagnostic.clone());
         boxed
     }));
 
@@ -355,53 +434,6 @@ fn check_file_impl(db: &dyn Db, file: File) -> Vec<Box<dyn Diagnostic>> {
     diagnostics
 }
 
-fn discover_project_files(db: &dyn Db, project: Project) -> FxHashSet<File> {
-    let paths = std::sync::Mutex::new(Vec::new());
-
-    db.system().walk_directory(project.root(db)).run(|| {
-        Box::new(|entry| {
-            match entry {
-                Ok(entry) => {
-                    // Skip over any non python files to avoid creating too many entries in `Files`.
-                    match entry.file_type() {
-                        FileType::File => {
-                            if entry
-                                .path()
-                                .extension()
-                                .and_then(PySourceType::try_from_extension)
-                                .is_some()
-                            {
-                                let mut paths = paths.lock().unwrap();
-                                paths.push(entry.into_path());
-                            }
-                        }
-                        FileType::Directory | FileType::Symlink => {}
-                    }
-                }
-                Err(error) => {
-                    // TODO Handle error
-                    tracing::error!("Failed to walk path: {error}");
-                }
-            }
-
-            WalkState::Continue
-        })
-    });
-
-    let paths = paths.into_inner().unwrap();
-    let mut files = FxHashSet::with_capacity_and_hasher(paths.len(), FxBuildHasher);
-
-    for path in paths {
-        // If this returns `None`, then the file was deleted between the `walk_directory` call and now.
-        // We can ignore this.
-        if let Ok(file) = system_path_to_file(db.upcast(), &path) {
-            files.insert(file);
-        }
-    }
-
-    files
-}
-
 #[derive(Debug)]
 enum ProjectFiles<'a> {
     OpenFiles(&'a FxHashSet<File>),
@@ -416,6 +448,13 @@ impl<'a> ProjectFiles<'a> {
             ProjectFiles::Indexed(project.files(db))
         }
     }
+
+    fn diagnostics(&self) -> &[IOErrorDiagnostic] {
+        match self {
+            ProjectFiles::OpenFiles(_) => &[],
+            ProjectFiles::Indexed(indexed) => indexed.diagnostics(),
+        }
+    }
 }
 
 impl<'a> IntoIterator for &'a ProjectFiles<'a> {
@@ -448,13 +487,13 @@ impl Iterator for ProjectFilesIter<'_> {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct IOErrorDiagnostic {
-    file: File,
-    error: SourceTextError,
+    file: Option<File>,
+    error: IOErrorKind,
 }
 
-impl Diagnostic for IOErrorDiagnostic {
+impl OldDiagnosticTrait for IOErrorDiagnostic {
     fn id(&self) -> DiagnosticId {
         DiagnosticId::Io
     }
@@ -464,7 +503,7 @@ impl Diagnostic for IOErrorDiagnostic {
     }
 
     fn span(&self) -> Option<Span> {
-        Some(Span::from(self.file))
+        self.file.map(Span::from)
     }
 
     fn severity(&self) -> Severity {
@@ -472,15 +511,24 @@ impl Diagnostic for IOErrorDiagnostic {
     }
 }
 
+#[derive(Error, Debug, Clone)]
+enum IOErrorKind {
+    #[error(transparent)]
+    Walk(#[from] walk::WalkError),
+
+    #[error(transparent)]
+    SourceText(#[from] SourceTextError),
+}
+
 #[cfg(test)]
 mod tests {
     use crate::db::tests::TestDb;
     use crate::{check_file_impl, ProjectMetadata};
     use red_knot_python_semantic::types::check_types;
-    use ruff_db::diagnostic::Diagnostic;
+    use ruff_db::diagnostic::OldDiagnosticTrait;
     use ruff_db::files::system_path_to_file;
     use ruff_db::source::source_text;
-    use ruff_db::system::{DbWithTestSystem, SystemPath, SystemPathBuf};
+    use ruff_db::system::{DbWithTestSystem, DbWithWritableSystem as _, SystemPath, SystemPathBuf};
     use ruff_db::testing::assert_function_query_was_not_run;
     use ruff_python_ast::name::Name;
 

crates/red_knot_project/src/metadata.rs

@@ -77,10 +77,10 @@ impl ProjectMetadata {
         // If the `options` don't specify a python version but the `project.requires-python` field is set,
         // use that as a lower bound instead.
         if let Some(project) = project {
-            if !options
+            if options
                 .environment
                 .as_ref()
-                .is_some_and(|env| env.python_version.is_some())
+                .is_none_or(|env| env.python_version.is_none())
             {
                 if let Some(requires_python) = project.resolve_requires_python_lower_bound()? {
                     let mut environment = options.environment.unwrap_or_default();
@@ -321,7 +321,7 @@ mod tests {
 
         system
             .memory_file_system()
-            .write_files([(root.join("foo.py"), ""), (root.join("bar.py"), "")])
+            .write_files_all([(root.join("foo.py"), ""), (root.join("bar.py"), "")])
             .context("Failed to write files")?;
 
         let project =
@@ -349,7 +349,7 @@ mod tests {
 
         system
             .memory_file_system()
-            .write_files([
+            .write_files_all([
                 (
                     root.join("pyproject.toml"),
                     r#"
@@ -393,7 +393,7 @@ mod tests {
 
         system
             .memory_file_system()
-            .write_files([
+            .write_files_all([
                 (
                     root.join("pyproject.toml"),
                     r#"
@@ -432,7 +432,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_files([
+            .write_files_all([
                 (
                     root.join("pyproject.toml"),
                     r#"
@@ -482,7 +482,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_files([
+            .write_files_all([
                 (
                     root.join("pyproject.toml"),
                     r#"
@@ -532,7 +532,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_files([
+            .write_files_all([
                 (
                     root.join("pyproject.toml"),
                     r#"
@@ -572,7 +572,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_files([
+            .write_files_all([
                 (
                     root.join("pyproject.toml"),
                     r#"
@@ -623,7 +623,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_files([
+            .write_files_all([
                 (
                     root.join("pyproject.toml"),
                     r#"
@@ -673,7 +673,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -703,7 +703,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -735,7 +735,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -765,7 +765,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -795,7 +795,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -828,7 +828,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -861,7 +861,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -886,7 +886,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]
@@ -911,7 +911,7 @@ expected `.`, `]`
 
         system
             .memory_file_system()
-            .write_file(
+            .write_file_all(
                 root.join("pyproject.toml"),
                 r#"
                 [project]

crates/red_knot_project/src/metadata/options.rs

@@ -1,8 +1,8 @@
 use crate::metadata::value::{RangedValue, RelativePathBuf, ValueSource, ValueSourceGuard};
 use crate::Db;
 use red_knot_python_semantic::lint::{GetLintError, Level, LintSource, RuleSelection};
-use red_knot_python_semantic::{ProgramSettings, PythonPlatform, SearchPathSettings, SitePackages};
-use ruff_db::diagnostic::{Diagnostic, DiagnosticId, Severity, Span};
+use red_knot_python_semantic::{ProgramSettings, PythonPath, PythonPlatform, SearchPathSettings};
+use ruff_db::diagnostic::{DiagnosticId, OldDiagnosticTrait, Severity, Span};
 use ruff_db::files::system_path_to_file;
 use ruff_db::system::{System, SystemPath};
 use ruff_macros::Combine;
@@ -90,7 +90,7 @@ impl Options {
             .map(|env| {
                 (
                     env.extra_paths.clone(),
-                    env.venv_path.clone(),
+                    env.python.clone(),
                     env.typeshed.clone(),
                 )
             })
@@ -104,11 +104,11 @@ impl Options {
                 .collect(),
             src_roots,
             custom_typeshed: typeshed.map(|path| path.absolute(project_root, system)),
-            site_packages: python
-                .map(|venv_path| SitePackages::Derived {
-                    venv_path: venv_path.absolute(project_root, system),
+            python_path: python
+                .map(|python_path| {
+                    PythonPath::SysPrefix(python_path.absolute(project_root, system))
                 })
-                .unwrap_or(SitePackages::Known(vec![])),
+                .unwrap_or(PythonPath::KnownSitePackages(vec![])),
         }
     }
 
@@ -236,10 +236,14 @@ pub struct EnvironmentOptions {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub typeshed: Option<RelativePathBuf>,
 
-    // TODO: Rename to python, see https://github.com/astral-sh/ruff/issues/15530
-    /// The path to the user's `site-packages` directory, where third-party packages from ``PyPI`` are installed.
+    /// Path to the Python installation from which Red Knot resolves type information and third-party dependencies.
+    ///
+    /// Red Knot will search in the path's `site-packages` directories for type information and
+    /// third-party imports.
+    ///
+    /// This option is commonly used to specify the path to a virtual environment.
     #[serde(skip_serializing_if = "Option::is_none")]
-    pub venv_path: Option<RelativePathBuf>,
+    pub python: Option<RelativePathBuf>,
 }
 
 #[derive(Debug, Default, Clone, Eq, PartialEq, Combine, Serialize, Deserialize)]
@@ -372,7 +376,7 @@ impl OptionDiagnostic {
     }
 }
 
-impl Diagnostic for OptionDiagnostic {
+impl OldDiagnosticTrait for OptionDiagnostic {
     fn id(&self) -> DiagnosticId {
         self.id
     }

crates/red_knot_project/src/walk.rs

@@ -0,0 +1,256 @@
+use crate::{Db, IOErrorDiagnostic, IOErrorKind, Project};
+use ruff_db::files::{system_path_to_file, File};
+use ruff_db::system::walk_directory::{ErrorKind, WalkDirectoryBuilder, WalkState};
+use ruff_db::system::{FileType, SystemPath, SystemPathBuf};
+use ruff_python_ast::PySourceType;
+use rustc_hash::{FxBuildHasher, FxHashSet};
+use std::path::PathBuf;
+use thiserror::Error;
+
+/// Filter that decides which files are included in the project.
+///
+/// In the future, this will hold a reference to the `include` and `exclude` pattern.
+///
+/// This struct mainly exists because `dyn Db` isn't `Send` or `Sync`, making it impossible
+/// to access fields from within the walker.
+#[derive(Default, Debug)]
+pub(crate) struct ProjectFilesFilter<'a> {
+    /// The same as [`Project::included_paths_or_root`].
+    included_paths: &'a [SystemPathBuf],
+
+    /// The filter skips checking if the path is in `included_paths` if set to `true`.
+    ///
+    /// Skipping this check is useful when the walker only walks over `included_paths`.
+    skip_included_paths: bool,
+}
+
+impl<'a> ProjectFilesFilter<'a> {
+    pub(crate) fn from_project(db: &'a dyn Db, project: Project) -> Self {
+        Self {
+            included_paths: project.included_paths_or_root(db),
+            skip_included_paths: false,
+        }
+    }
+
+    /// Returns `true` if a file is part of the project and included in the paths to check.
+    ///
+    /// A file is included in the checked files if it is a sub path of the project's root
+    /// (when no CLI path arguments are specified) or if it is a sub path of any path provided on the CLI (`knot check <paths>`) AND:
+    ///
+    /// * It matches a positive `include` pattern and isn't excluded by a later negative `include` pattern.
+    /// * It doesn't match a positive `exclude` pattern or is re-included by a later negative `exclude` pattern.
+    ///
+    /// ## Note
+    ///
+    /// This method may return `true` for files that don't end up being included when walking the
+    /// project tree because it doesn't consider `.gitignore` and other ignore files when deciding
+    /// if a file's included.
+    pub(crate) fn is_included(&self, path: &SystemPath) -> bool {
+        #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+        enum CheckPathMatch {
+            /// The path is a partial match of the checked path (it's a sub path)
+            Partial,
+
+            /// The path matches a check path exactly.
+            Full,
+        }
+
+        let m = if self.skip_included_paths {
+            Some(CheckPathMatch::Partial)
+        } else {
+            self.included_paths
+                .iter()
+                .filter_map(|included_path| {
+                    if let Ok(relative_path) = path.strip_prefix(included_path) {
+                        // Exact matches are always included
+                        if relative_path.as_str().is_empty() {
+                            Some(CheckPathMatch::Full)
+                        } else {
+                            Some(CheckPathMatch::Partial)
+                        }
+                    } else {
+                        None
+                    }
+                })
+                .max()
+        };
+
+        match m {
+            None => false,
+            Some(CheckPathMatch::Partial) => {
+                // TODO: For partial matches, only include the file if it is included by the project's include/exclude settings.
+                true
+            }
+            Some(CheckPathMatch::Full) => true,
+        }
+    }
+}
+
+pub(crate) struct ProjectFilesWalker<'a> {
+    walker: WalkDirectoryBuilder,
+
+    filter: ProjectFilesFilter<'a>,
+}
+
+impl<'a> ProjectFilesWalker<'a> {
+    pub(crate) fn new(db: &'a dyn Db) -> Self {
+        let project = db.project();
+
+        let mut filter = ProjectFilesFilter::from_project(db, project);
+        // It's unnecessary to filter on included paths because it only iterates over those to start with.
+        filter.skip_included_paths = true;
+
+        Self::from_paths(db, project.included_paths_or_root(db), filter)
+            .expect("included_paths_or_root to never return an empty iterator")
+    }
+
+    /// Creates a walker for indexing the project files incrementally.
+    ///
+    /// The main difference to a full project walk is that `paths` may contain paths
+    /// that aren't part of the included files.
+    pub(crate) fn incremental<P>(db: &'a dyn Db, paths: impl IntoIterator<Item = P>) -> Option<Self>
+    where
+        P: AsRef<SystemPath>,
+    {
+        let project = db.project();
+
+        let filter = ProjectFilesFilter::from_project(db, project);
+
+        Self::from_paths(db, paths, filter)
+    }
+
+    fn from_paths<P>(
+        db: &'a dyn Db,
+        paths: impl IntoIterator<Item = P>,
+        filter: ProjectFilesFilter<'a>,
+    ) -> Option<Self>
+    where
+        P: AsRef<SystemPath>,
+    {
+        let mut paths = paths.into_iter();
+
+        let mut walker = db.system().walk_directory(paths.next()?.as_ref());
+
+        for path in paths {
+            walker = walker.add(path);
+        }
+
+        Some(Self { walker, filter })
+    }
+
+    /// Walks the project paths and collects the paths of all files that
+    /// are included in the project.
+    pub(crate) fn walk_paths(self) -> (Vec<SystemPathBuf>, Vec<IOErrorDiagnostic>) {
+        let paths = std::sync::Mutex::new(Vec::new());
+        let diagnostics = std::sync::Mutex::new(Vec::new());
+
+        self.walker.run(|| {
+            Box::new(|entry| {
+                match entry {
+                    Ok(entry) => {
+                        if !self.filter.is_included(entry.path()) {
+                            tracing::debug!("Ignoring not-included path: {}", entry.path());
+                            return WalkState::Skip;
+                        }
+
+                        // Skip over any non python files to avoid creating too many entries in `Files`.
+                        match entry.file_type() {
+                            FileType::File => {
+                                if entry
+                                    .path()
+                                    .extension()
+                                    .and_then(PySourceType::try_from_extension)
+                                    .is_some()
+                                {
+                                    let mut paths = paths.lock().unwrap();
+                                    paths.push(entry.into_path());
+                                }
+                            }
+                            FileType::Directory | FileType::Symlink => {}
+                        }
+                    }
+                    Err(error) => match error.kind() {
+                        ErrorKind::Loop { .. } => {
+                            unreachable!("Loops shouldn't be possible without following symlinks.")
+                        }
+                        ErrorKind::Io { path, err } => {
+                            let mut diagnostics = diagnostics.lock().unwrap();
+                            let error = if let Some(path) = path {
+                                WalkError::IOPathError {
+                                    path: path.clone(),
+                                    error: err.to_string(),
+                                }
+                            } else {
+                                WalkError::IOError {
+                                    error: err.to_string(),
+                                }
+                            };
+
+                            diagnostics.push(IOErrorDiagnostic {
+                                file: None,
+                                error: IOErrorKind::Walk(error),
+                            });
+                        }
+                        ErrorKind::NonUtf8Path { path } => {
+                            diagnostics.lock().unwrap().push(IOErrorDiagnostic {
+                                file: None,
+                                error: IOErrorKind::Walk(WalkError::NonUtf8Path {
+                                    path: path.clone(),
+                                }),
+                            });
+                        }
+                    },
+                }
+
+                WalkState::Continue
+            })
+        });
+
+        (
+            paths.into_inner().unwrap(),
+            diagnostics.into_inner().unwrap(),
+        )
+    }
+
+    pub(crate) fn collect_vec(self, db: &dyn Db) -> (Vec<File>, Vec<IOErrorDiagnostic>) {
+        let (paths, diagnostics) = self.walk_paths();
+
+        (
+            paths
+                .into_iter()
+                .filter_map(move |path| {
+                    // If this returns `None`, then the file was deleted between the `walk_directory` call and now.
+                    // We can ignore this.
+                    system_path_to_file(db.upcast(), &path).ok()
+                })
+                .collect(),
+            diagnostics,
+        )
+    }
+
+    pub(crate) fn collect_set(self, db: &dyn Db) -> (FxHashSet<File>, Vec<IOErrorDiagnostic>) {
+        let (paths, diagnostics) = self.walk_paths();
+
+        let mut files = FxHashSet::with_capacity_and_hasher(paths.len(), FxBuildHasher);
+
+        for path in paths {
+            if let Ok(file) = system_path_to_file(db.upcast(), &path) {
+                files.insert(file);
+            }
+        }
+
+        (files, diagnostics)
+    }
+}
+
+#[derive(Error, Debug, Clone)]
+pub(crate) enum WalkError {
+    #[error("`{path}`: {error}")]
+    IOPathError { path: SystemPathBuf, error: String },
+
+    #[error("Failed to walk project directory: {error}")]
+    IOError { error: String },
+
+    #[error("`{path}` is not a valid UTF-8 path")]
+    NonUtf8Path { path: PathBuf },
+}

crates/red_knot_project/src/watch/project_watcher.rs

@@ -6,7 +6,7 @@ use tracing::info;
 use red_knot_python_semantic::system_module_search_paths;
 use ruff_cache::{CacheKey, CacheKeyHasher};
 use ruff_db::system::{SystemPath, SystemPathBuf};
-use ruff_db::{Db as _, Upcast};
+use ruff_db::Upcast;
 
 use crate::db::{Db, ProjectDatabase};
 use crate::watch::Watcher;
@@ -42,9 +42,9 @@ impl ProjectWatcher {
 
     pub fn update(&mut self, db: &ProjectDatabase) {
         let search_paths: Vec<_> = system_module_search_paths(db.upcast()).collect();
-        let project_path = db.project().root(db).to_path_buf();
+        let project_path = db.project().root(db);
 
-        let new_cache_key = Self::compute_cache_key(&project_path, &search_paths);
+        let new_cache_key = Self::compute_cache_key(project_path, &search_paths);
 
         if self.cache_key == Some(new_cache_key) {
             return;
@@ -68,41 +68,47 @@ impl ProjectWatcher {
 
         self.has_errored_paths = false;
 
-        let project_path = db
-            .system()
-            .canonicalize_path(&project_path)
-            .unwrap_or(project_path);
-
         let config_paths = db
             .project()
             .metadata(db)
             .extra_configuration_paths()
             .iter()
-            .cloned();
+            .map(SystemPathBuf::as_path);
+
+        // Watch both the project root and any paths provided by the user on the CLI (removing any redundant nested paths).
+        // This is necessary to observe changes to files that are outside the project root.
+        // We always need to watch the project root to observe changes to its configuration.
+        let included_paths = ruff_db::system::deduplicate_nested_paths(
+            std::iter::once(project_path).chain(
+                db.project()
+                    .included_paths_list(db)
+                    .iter()
+                    .map(SystemPathBuf::as_path),
+            ),
+        );
 
         // Find the non-overlapping module search paths and filter out paths that are already covered by the project.
         // Module search paths are already canonicalized.
         let unique_module_paths = ruff_db::system::deduplicate_nested_paths(
             search_paths
                 .into_iter()
-                .filter(|path| !path.starts_with(&project_path)),
-        )
-        .map(SystemPath::to_path_buf);
+                .filter(|path| !path.starts_with(project_path)),
+        );
 
         // Now add the new paths, first starting with the project path and then
         // adding the library search paths, and finally the paths for configurations.
-        for path in std::iter::once(project_path)
+        for path in included_paths
             .chain(unique_module_paths)
             .chain(config_paths)
         {
             // Log a warning. It's not worth aborting if registering a single folder fails because
             // Ruff otherwise stills works as expected.
-            if let Err(error) = self.watcher.watch(&path) {
+            if let Err(error) = self.watcher.watch(path) {
                 // TODO: Log a user-facing warning.
                 tracing::warn!("Failed to setup watcher for path `{path}`: {error}. You have to restart Ruff after making changes to files under this path or you might see stale results.");
                 self.has_errored_paths = true;
             } else {
-                self.watched_paths.push(path);
+                self.watched_paths.push(path.to_path_buf());
             }
         }
 

crates/red_knot_project/tests/check.rs

@@ -117,7 +117,7 @@ fn run_corpus_tests(pattern: &str) -> anyhow::Result<()> {
         let code = std::fs::read_to_string(source)?;
 
         let mut check_with_file_name = |path: &SystemPath| {
-            memory_fs.write_file(path, &code).unwrap();
+            memory_fs.write_file_all(path, &code).unwrap();
             File::sync_path(&mut db, path);
 
             // this test is only asserting that we can pull every expression type without a panic
@@ -283,4 +283,9 @@ const KNOWN_FAILURES: &[(&str, bool, bool)] = &[
     // related to circular references in f-string annotations (invalid syntax)
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_15.py", true, true),
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_14.py", false, true),
+    // related to circular references in stub type annotations (salsa cycle panic):
+    ("crates/ruff_linter/resources/test/fixtures/pycodestyle/E501_4.py", false, true),
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F401_0.py", false, true),
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F401_12.py", false, true),
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F401_14.py", false, true),
 ];

crates/red_knot_python_semantic/Cargo.toml

@@ -42,6 +42,8 @@ smallvec = { workspace = true }
 static_assertions = { workspace = true }
 test-case = { workspace = true }
 memchr = { workspace = true }
+strum = { workspace = true}
+strum_macros = { workspace = true}
 
 [dev-dependencies]
 ruff_db = { workspace = true, features = ["testing", "os"] }

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

@@ -0,0 +1,45 @@
+# Tests for invalid types in type expressions
+
+## Invalid types are rejected
+
+Many types are illegal in the context of a type expression:
+
+```py
+import typing
+from knot_extensions import AlwaysTruthy, AlwaysFalsy
+from typing_extensions import Literal, Never
+
+def _(
+    a: type[int],
+    b: AlwaysTruthy,
+    c: AlwaysFalsy,
+    d: Literal[True],
+    e: Literal["bar"],
+    f: Literal[b"foo"],
+    g: tuple[int, str],
+    h: Never,
+):
+    def foo(): ...
+    def invalid(
+        i: a,  # error: [invalid-type-form] "Variable of type `type[int]` is not allowed in a type expression"
+        j: b,  # error: [invalid-type-form]
+        k: c,  # error: [invalid-type-form]
+        l: d,  # error: [invalid-type-form]
+        m: e,  # error: [invalid-type-form]
+        n: f,  # error: [invalid-type-form]
+        o: g,  # error: [invalid-type-form]
+        p: h,  # error: [invalid-type-form]
+        q: typing,  # error: [invalid-type-form]
+        r: foo,  # error: [invalid-type-form]
+    ):
+        reveal_type(i)  # revealed: Unknown
+        reveal_type(j)  # revealed: Unknown
+        reveal_type(k)  # revealed: Unknown
+        reveal_type(l)  # revealed: Unknown
+        reveal_type(m)  # revealed: Unknown
+        reveal_type(n)  # revealed: Unknown
+        reveal_type(o)  # revealed: Unknown
+        reveal_type(p)  # revealed: Unknown
+        reveal_type(q)  # revealed: Unknown
+        reveal_type(r)  # revealed: Unknown
+```

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

@@ -73,12 +73,12 @@ qux = (foo, bar)
 reveal_type(qux)  # revealed: tuple[Literal["foo"], Literal["bar"]]
 
 # TODO: Infer "LiteralString"
-reveal_type(foo.join(qux))  # revealed: @Todo(decorated method)
+reveal_type(foo.join(qux))  # revealed: @Todo(overloaded method)
 
 template: LiteralString = "{}, {}"
 reveal_type(template)  # revealed: Literal["{}, {}"]
 # TODO: Infer `LiteralString`
-reveal_type(template.format(foo, bar))  # revealed: @Todo(decorated method)
+reveal_type(template.format(foo, bar))  # revealed: @Todo(overloaded method)
 ```
 
 ### Assignability

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

@@ -116,8 +116,8 @@ MyType = int
 class Aliases:
     MyType = str
 
-    forward: "MyType"
-    not_forward: MyType
+    forward: "MyType" = "value"
+    not_forward: MyType = "value"
 
 reveal_type(Aliases.forward)  # revealed: str
 reveal_type(Aliases.not_forward)  # revealed: str

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

@@ -18,7 +18,7 @@ def f(*args: Unpack[Ts]) -> tuple[Unpack[Ts]]:
     # TODO: should understand the annotation
     reveal_type(args)  # revealed: tuple
 
-    reveal_type(Alias)  # revealed: @Todo(Unsupported or invalid type in a type expression)
+    reveal_type(Alias)  # revealed: @Todo(Invalid or unsupported `KnownInstanceType` in `Type::to_type_expression`)
 
 def g() -> TypeGuard[int]: ...
 def h() -> TypeIs[int]: ...
@@ -33,7 +33,7 @@ def i(callback: Callable[Concatenate[int, P], R_co], *args: P.args, **kwargs: P.
 
 class Foo:
     def method(self, x: Self):
-        reveal_type(x)  # revealed: @Todo(Unsupported or invalid type in a type expression)
+        reveal_type(x)  # revealed: @Todo(Invalid or unsupported `KnownInstanceType` in `Type::to_type_expression`)
 ```
 
 ## Inheritance

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -54,13 +54,12 @@ c_instance.declared_and_bound = False
 # error: [invalid-assignment] "Object of type `Literal["incompatible"]` is not assignable to attribute `declared_and_bound` of type `bool`"
 c_instance.declared_and_bound = "incompatible"
 
-# TODO: we already show an error here but the message might be improved?
 # mypy shows no error here, but pyright raises "reportAttributeAccessIssue"
-# error: [unresolved-attribute] "Type `Literal[C]` has no attribute `inferred_from_value`"
+# error: [unresolved-attribute] "Attribute `inferred_from_value` can only be accessed on instances, not on the class object `Literal[C]` itself."
 reveal_type(C.inferred_from_value)  # revealed: Unknown
 
-# TODO: this should be an error (pure instance variables cannot be accessed on the class)
 # mypy shows no error here, but pyright raises "reportAttributeAccessIssue"
+# error: [invalid-attribute-access] "Cannot assign to instance attribute `inferred_from_value` from the class object `Literal[C]`"
 C.inferred_from_value = "overwritten on class"
 
 # This assignment is fine:
@@ -90,13 +89,13 @@ c_instance = C()
 
 reveal_type(c_instance.declared_and_bound)  # revealed: str | None
 
-# TODO: we currently plan to emit a diagnostic here. Note that both mypy
-# and pyright show no error in this case! So we may reconsider this in
-# the future, if it turns out to produce too many false positives.
-reveal_type(C.declared_and_bound)  # revealed: str | None
+# Note that both mypy and pyright show no error in this case! So we may reconsider this in
+# the future, if it turns out to produce too many false positives. We currently emit:
+# error: [unresolved-attribute] "Attribute `declared_and_bound` can only be accessed on instances, not on the class object `Literal[C]` itself."
+reveal_type(C.declared_and_bound)  # revealed: Unknown
 
-# TODO: same as above. We plan to emit a diagnostic here, even if both mypy
-# and pyright allow this.
+# Same as above. Mypy and pyright do not show an error here.
+# error: [invalid-attribute-access] "Cannot assign to instance attribute `declared_and_bound` from the class object `Literal[C]`"
 C.declared_and_bound = "overwritten on class"
 
 # error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to attribute `declared_and_bound` of type `str | None`"
@@ -116,11 +115,11 @@ c_instance = C()
 
 reveal_type(c_instance.only_declared)  # revealed: str
 
-# TODO: mypy and pyright do not show an error here, but we plan to emit a diagnostic.
-# The type could be changed to 'Unknown' if we decide to emit an error?
-reveal_type(C.only_declared)  # revealed: str
+# Mypy and pyright do not show an error here. We treat this as a pure instance variable.
+# error: [unresolved-attribute] "Attribute `only_declared` can only be accessed on instances, not on the class object `Literal[C]` itself."
+reveal_type(C.only_declared)  # revealed: Unknown
 
-# TODO: mypy and pyright do not show an error here, but we plan to emit one.
+# error: [invalid-attribute-access] "Cannot assign to instance attribute `only_declared` from the class object `Literal[C]`"
 C.only_declared = "overwritten on class"
 ```
 
@@ -191,11 +190,10 @@ reveal_type(c_instance.declared_only)  # revealed: bytes
 
 reveal_type(c_instance.declared_and_bound)  # revealed: bool
 
-# TODO: We already show an error here, but the message might be improved?
-# error: [unresolved-attribute]
+# error: [unresolved-attribute] "Attribute `inferred_from_value` can only be accessed on instances, not on the class object `Literal[C]` itself."
 reveal_type(C.inferred_from_value)  # revealed: Unknown
 
-# TODO: this should be an error
+# error: [invalid-attribute-access] "Cannot assign to instance attribute `inferred_from_value` from the class object `Literal[C]`"
 C.inferred_from_value = "overwritten on class"
 ```
 
@@ -598,6 +596,9 @@ C.class_method()
 # error: [unresolved-attribute]
 reveal_type(C.pure_class_variable)  # revealed: Unknown
 
+# TODO: should be no error when descriptor protocol is supported
+# and the assignment is properly attributed to the class method.
+# error: [invalid-attribute-access] "Cannot assign to instance attribute `pure_class_variable` from the class object `Literal[C]`"
 C.pure_class_variable = "overwritten on class"
 
 # TODO: should be  `Unknown | Literal["value set in class method"]` or
@@ -782,6 +783,9 @@ def _(flag1: bool, flag2: bool):
 
     # error: [possibly-unbound-attribute] "Attribute `x` on type `Literal[C1, C2, C3]` is possibly unbound"
     reveal_type(C.x)  # revealed: Unknown | Literal[1, 3]
+
+    # error: [possibly-unbound-attribute] "Attribute `x` on type `C1 | C2 | C3` is possibly unbound"
+    reveal_type(C().x)  # revealed: Unknown | Literal[1, 3]
 ```
 
 ### Possibly-unbound within a class
@@ -805,6 +809,28 @@ def _(flag: bool, flag1: bool, flag2: bool):
 
     # error: [possibly-unbound-attribute] "Attribute `x` on type `Literal[C1, C2, C3]` is possibly unbound"
     reveal_type(C.x)  # revealed: Unknown | Literal[1, 2, 3]
+
+    # Note: we might want to consider ignoring possibly-unbound diagnostics for instance attributes eventually,
+    # see the "Possibly unbound/undeclared instance attribute" section below.
+    # error: [possibly-unbound-attribute] "Attribute `x` on type `C1 | C2 | C3` is possibly unbound"
+    reveal_type(C().x)  # revealed: Unknown | Literal[1, 2, 3]
+```
+
+### Possibly-unbound within gradual types
+
+```py
+from typing import Any
+
+def _(flag: bool):
+    class Base:
+        x: Any
+
+    class Derived(Base):
+        if flag:
+            # Redeclaring `x` with a more static type is okay in terms of LSP.
+            x: int
+
+    reveal_type(Derived().x)  # revealed: int | Any
 ```
 
 ### Attribute possibly unbound on a subclass but not on a superclass
@@ -819,6 +845,8 @@ def _(flag: bool):
             x = 2
 
     reveal_type(Bar.x)  # revealed: Unknown | Literal[2, 1]
+
+    reveal_type(Bar().x)  # revealed: Unknown | Literal[2, 1]
 ```
 
 ### Attribute possibly unbound on a subclass and on a superclass
@@ -835,6 +863,41 @@ def _(flag: bool):
 
     # error: [possibly-unbound-attribute]
     reveal_type(Bar.x)  # revealed: Unknown | Literal[2, 1]
+
+    # error: [possibly-unbound-attribute]
+    reveal_type(Bar().x)  # revealed: Unknown | Literal[2, 1]
+```
+
+### Possibly unbound/undeclared instance attribute
+
+#### Possibly unbound and undeclared
+
+```py
+def _(flag: bool):
+    class Foo:
+        if flag:
+            x: int
+
+        def __init(self):
+            if flag:
+                self.x = 1
+
+    # error: [possibly-unbound-attribute]
+    reveal_type(Foo().x)  # revealed: int
+```
+
+#### Possibly unbound
+
+```py
+def _(flag: bool):
+    class Foo:
+        def __init(self):
+            if flag:
+                self.x = 1
+
+    # Emitting a diagnostic in a case like this is not something we support, and it's unclear
+    # if we ever will (or want to)
+    reveal_type(Foo().x)  # revealed: Unknown | Literal[1]
 ```
 
 ### Attribute access on `Any`
@@ -884,13 +947,18 @@ def _(flag: bool):
 
 ## Objects of all types have a `__class__` method
 
+The type of `x.__class__` is the same as `x`'s meta-type. `x.__class__` is always the same value as
+`type(x)`.
+
 ```py
 import typing_extensions
 
 reveal_type(typing_extensions.__class__)  # revealed: Literal[ModuleType]
+reveal_type(type(typing_extensions))  # revealed: Literal[ModuleType]
 
 a = 42
 reveal_type(a.__class__)  # revealed: Literal[int]
+reveal_type(type(a))  # revealed: Literal[int]
 
 b = "42"
 reveal_type(b.__class__)  # revealed: Literal[str]
@@ -906,8 +974,13 @@ reveal_type(e.__class__)  # revealed: Literal[tuple]
 
 def f(a: int, b: typing_extensions.LiteralString, c: int | str, d: type[str]):
     reveal_type(a.__class__)  # revealed: type[int]
+    reveal_type(type(a))  # revealed: type[int]
+
     reveal_type(b.__class__)  # revealed: Literal[str]
+    reveal_type(type(b))  # revealed: Literal[str]
+
     reveal_type(c.__class__)  # revealed: type[int] | type[str]
+    reveal_type(type(c))  # revealed: type[int] | type[str]
 
     # `type[type]`, a.k.a., either the class `type` or some subclass of `type`.
     # It would be incorrect to infer `Literal[type]` here,
@@ -1032,8 +1105,8 @@ Most attribute accesses on bool-literal types are delegated to `builtins.bool`,
 bools are instances of that class:
 
 ```py
-reveal_type(True.__and__)  # revealed: @Todo(decorated method)
-reveal_type(False.__or__)  # revealed: @Todo(decorated method)
+reveal_type(True.__and__)  # revealed: @Todo(overloaded method)
+reveal_type(False.__or__)  # revealed: @Todo(overloaded method)
 ```
 
 Some attributes are special-cased, however:
@@ -1136,6 +1209,20 @@ class C:
 reveal_type(C().x)  # revealed: Unknown
 ```
 
+### Accessing attributes on `Never`
+
+Arbitrary attributes can be accessed on `Never` without emitting any errors:
+
+```py
+from typing_extensions import Never
+
+def f(never: Never):
+    reveal_type(never.arbitrary_attribute)  # revealed: Never
+
+    # Assigning `Never` to an attribute on `Never` is also allowed:
+    never.another_attribute = never
+```
+
 ### Builtin types attributes
 
 This test can probably be removed eventually, but we currently include it because we do not yet

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

@@ -259,11 +259,17 @@ class A:
 class B:
     __add__ = A()
 
-# TODO: this could be `int` if we declare `B.__add__` using a `Callable` type
-# TODO: Should not be an error: `A` instance is not a method descriptor, don't prepend `self` arg.
-#   Revealed type should be `Unknown | int`.
-# error: [unsupported-operator] "Operator `+` is unsupported between objects of type `B` and `B`"
-reveal_type(B() + B())  # revealed: Unknown
+reveal_type(B() + B())  # revealed: Unknown | int
+```
+
+Note that we union with `Unknown` here because `__add__` is not declared. We do infer just `int` if
+the callable is declared:
+
+```py
+class B2:
+    __add__: A = A()
+
+reveal_type(B2() + B2())  # revealed: int
 ```
 
 ## Integration test: numbers from typeshed
@@ -306,7 +312,7 @@ reveal_type(1 + A())  # revealed: A
 reveal_type(A() + "foo")  # revealed: A
 # TODO should be `A` since `str.__add__` doesn't support `A` instances
 # TODO overloads
-reveal_type("foo" + A())  # revealed: @Todo(return type)
+reveal_type("foo" + A())  # revealed: @Todo(return type of decorated function)
 
 reveal_type(A() + b"foo")  # revealed: A
 # TODO should be `A` since `bytes.__add__` doesn't support `A` instances
@@ -314,7 +320,7 @@ reveal_type(b"foo" + A())  # revealed: bytes
 
 reveal_type(A() + ())  # revealed: A
 # TODO this should be `A`, since `tuple.__add__` doesn't support `A` instances
-reveal_type(() + A())  # revealed: @Todo(return type)
+reveal_type(() + A())  # revealed: @Todo(return type of decorated function)
 
 literal_string_instance = "foo" * 1_000_000_000
 # the test is not testing what it's meant to be testing if this isn't a `LiteralString`:
@@ -323,7 +329,7 @@ reveal_type(literal_string_instance)  # revealed: LiteralString
 reveal_type(A() + literal_string_instance)  # revealed: A
 # TODO should be `A` since `str.__add__` doesn't support `A` instances
 # TODO overloads
-reveal_type(literal_string_instance + A())  # revealed: @Todo(return type)
+reveal_type(literal_string_instance + A())  # revealed: @Todo(return type of decorated function)
 ```
 
 ## Operations involving instances of classes inheriting from `Any`
@@ -351,6 +357,20 @@ class Y(Foo): ...
 reveal_type(X() + Y())  # revealed: int
 ```
 
+## Operations involving types with invalid `__bool__` methods
+
+<!-- snapshot-diagnostics -->
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+a = NotBoolable()
+
+# error: [unsupported-bool-conversion]
+10 and a and True
+```
+
 ## Unsupported
 
 ### Dunder as instance attribute

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

@@ -51,9 +51,9 @@ reveal_type(1 ** (largest_u32 + 1))  # revealed: int
 reveal_type(2**largest_u32)  # revealed: int
 
 def variable(x: int):
-    reveal_type(x**2)  # revealed: @Todo(return type)
-    reveal_type(2**x)  # revealed: @Todo(return type)
-    reveal_type(x**x)  # revealed: @Todo(return type)
+    reveal_type(x**2)  # revealed: @Todo(return type of decorated function)
+    reveal_type(2**x)  # revealed: @Todo(return type of decorated function)
+    reveal_type(x**x)  # revealed: @Todo(return type of decorated function)
 ```
 
 ## Division by Zero

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

@@ -0,0 +1,37 @@
+# Calling builtins
+
+## `bool` with incorrect arguments
+
+```py
+class NotBool:
+    __bool__ = None
+
+# TODO: We should emit an `invalid-argument` error here for `2` because `bool` only takes one argument.
+bool(1, 2)
+
+# TODO: We should emit an `unsupported-bool-conversion` error here because the argument doesn't implement `__bool__` correctly.
+bool(NotBool())
+```
+
+## Calls to `type()`
+
+A single-argument call to `type()` returns an object that has the argument's meta-type. (This is
+tested more extensively in `crates/red_knot_python_semantic/resources/mdtest/attributes.md`,
+alongside the tests for the `__class__` attribute.)
+
+```py
+reveal_type(type(1))  # revealed: Literal[int]
+```
+
+But a three-argument call to type creates a dynamic instance of the `type` class:
+
+```py
+reveal_type(type("Foo", (), {}))  # revealed: type
+```
+
+Other numbers of arguments are invalid (TODO -- these should emit a diagnostic)
+
+```py
+type("Foo", ())
+type("Foo", (), {}, weird_other_arg=42)
+```

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

@@ -82,7 +82,7 @@ class C:
 
 c = C()
 
-# error: 15 [invalid-argument-type] "Object of type `Literal["foo"]` cannot be assigned to parameter 2 (`x`) of function `__call__`; expected type `int`"
+# error: 15 [invalid-argument-type] "Object of type `Literal["foo"]` cannot be assigned to parameter 2 (`x`) of bound method `__call__`; expected type `int`"
 reveal_type(c("foo"))  # revealed: int
 ```
 
@@ -96,7 +96,7 @@ class C:
 
 c = C()
 
-# error: 13 [invalid-argument-type] "Object of type `C` cannot be assigned to parameter 1 (`self`) of function `__call__`; expected type `int`"
+# error: 13 [invalid-argument-type] "Object of type `C` cannot be assigned to parameter 1 (`self`) of bound method `__call__`; expected type `int`"
 reveal_type(c())  # revealed: int
 ```
 

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

@@ -0,0 +1,128 @@
+# Dunder calls
+
+## Introduction
+
+This test suite explains and documents how dunder methods are looked up and called. Throughout the
+document, we use `__getitem__` as an example, but the same principles apply to other dunder methods.
+
+Dunder methods are implicitly called when using certain syntax. For example, the index operator
+`obj[key]` calls the `__getitem__` method under the hood. Exactly *how* a dunder method is looked up
+and called works slightly different from regular methods. Dunder methods are not looked up on `obj`
+directly, but rather on `type(obj)`. But in many ways, they still *act* as if they were called on
+`obj` directly. If the `__getitem__` member of `type(obj)` is a descriptor, it is called with `obj`
+as the `instance` argument to `__get__`. A desugared version of `obj[key]` is roughly equivalent to
+`getitem_desugared(obj, key)` as defined below:
+
+```py
+from typing import Any
+
+def find_name_in_mro(typ: type, name: str) -> Any:
+    # See implementation in https://docs.python.org/3/howto/descriptor.html#invocation-from-an-instance
+    pass
+
+def getitem_desugared(obj: object, key: object) -> object:
+    getitem_callable = find_name_in_mro(type(obj), "__getitem__")
+    if hasattr(getitem_callable, "__get__"):
+        getitem_callable = getitem_callable.__get__(obj, type(obj))
+
+    return getitem_callable(key)
+```
+
+In the following tests, we demonstrate that we implement this behavior correctly.
+
+## Operating on class objects
+
+If we invoke a dunder method on a class, it is looked up on the *meta* class, since any class is an
+instance of its metaclass:
+
+```py
+class Meta(type):
+    def __getitem__(cls, key: int) -> str:
+        return str(key)
+
+class DunderOnMetaClass(metaclass=Meta):
+    pass
+
+reveal_type(DunderOnMetaClass[0])  # revealed: str
+```
+
+## Operating on instances
+
+When invoking a dunder method on an instance of a class, it is looked up on the class:
+
+```py
+class ClassWithNormalDunder:
+    def __getitem__(self, key: int) -> str:
+        return str(key)
+
+class_with_normal_dunder = ClassWithNormalDunder()
+
+reveal_type(class_with_normal_dunder[0])  # revealed: str
+```
+
+Which can be demonstrated by trying to attach a dunder method to an instance, which will not work:
+
+```py
+def external_getitem(instance, key: int) -> str:
+    return str(key)
+
+class ThisFails:
+    def __init__(self):
+        self.__getitem__ = external_getitem
+
+this_fails = ThisFails()
+
+# error: [non-subscriptable] "Cannot subscript object of type `ThisFails` with no `__getitem__` method"
+reveal_type(this_fails[0])  # revealed: Unknown
+```
+
+However, the attached dunder method *can* be called if accessed directly:
+
+```py
+# TODO: `this_fails.__getitem__` is incorrectly treated as a bound method. This
+# should be fixed with https://github.com/astral-sh/ruff/issues/16367
+# error: [too-many-positional-arguments]
+# error: [invalid-argument-type]
+reveal_type(this_fails.__getitem__(this_fails, 0))  # revealed: Unknown | str
+```
+
+## When the dunder is not a method
+
+A dunder can also be a non-method callable:
+
+```py
+class SomeCallable:
+    def __call__(self, key: int) -> str:
+        return str(key)
+
+class ClassWithNonMethodDunder:
+    __getitem__: SomeCallable = SomeCallable()
+
+class_with_callable_dunder = ClassWithNonMethodDunder()
+
+reveal_type(class_with_callable_dunder[0])  # revealed: str
+```
+
+## Dunders are looked up using the descriptor protocol
+
+Here, we demonstrate that the descriptor protocol is invoked when looking up a dunder method. Note
+that the `instance` argument is on object of type `ClassWithDescriptorDunder`:
+
+```py
+from __future__ import annotations
+
+class SomeCallable:
+    def __call__(self, key: int) -> str:
+        return str(key)
+
+class Descriptor:
+    def __get__(self, instance: ClassWithDescriptorDunder, owner: type[ClassWithDescriptorDunder]) -> SomeCallable:
+        return SomeCallable()
+
+class ClassWithDescriptorDunder:
+    __getitem__: Descriptor = Descriptor()
+
+class_with_descriptor_dunder = ClassWithDescriptorDunder()
+
+reveal_type(class_with_descriptor_dunder[0])  # revealed: str
+```

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

@@ -44,7 +44,7 @@ def bar() -> str:
     return "bar"
 
 # TODO: should reveal `int`, as the decorator replaces `bar` with `foo`
-reveal_type(bar())  # revealed: @Todo(return type)
+reveal_type(bar())  # revealed: @Todo(return type of decorated function)
 ```
 
 ## Invalid callable

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

@@ -239,11 +239,11 @@ method_wrapper(None, C)
 method_wrapper(None)
 
 # Passing something that is not assignable to `type` as the `owner` argument is an
-# error: [invalid-argument-type] "Object of type `Literal[1]` cannot be assigned to parameter 2 (`owner`); expected type `type`"
+# error: [invalid-argument-type] "Object of type `Literal[1]` cannot be assigned to parameter 2 (`owner`) of method wrapper `__get__` of function `f`; expected type `type`"
 method_wrapper(None, 1)
 
 # Passing `None` as the `owner` argument when `instance` is `None` is an
-# error: [invalid-argument-type] "Object of type `None` cannot be assigned to parameter 2 (`owner`); expected type `type`"
+# error: [invalid-argument-type] "Object of type `None` cannot be assigned to parameter 2 (`owner`) of method wrapper `__get__` of function `f`; expected type `type`"
 method_wrapper(None, None)
 
 # Calling `__get__` without any arguments is an
@@ -251,8 +251,130 @@ method_wrapper(None, None)
 method_wrapper()
 
 # Calling `__get__` with too many positional arguments is an
-# error: [too-many-positional-arguments] "Too many positional arguments: expected 2, got 3"
+# error: [too-many-positional-arguments] "Too many positional arguments to method wrapper `__get__` of function `f`: expected 2, got 3"
 method_wrapper(C(), C, "one too many")
 ```
 
+## `@classmethod`
+
+### Basic
+
+When a `@classmethod` attribute is accessed, it returns a bound method object, even when accessed on
+the class object itself:
+
+```py
+from __future__ import annotations
+
+class C:
+    @classmethod
+    def f(cls: type[C], x: int) -> str:
+        return "a"
+
+reveal_type(C.f)  # revealed: <bound method `f` of `Literal[C]`>
+reveal_type(C().f)  # revealed: <bound method `f` of `type[C]`>
+```
+
+The `cls` method argument is then implicitly passed as the first argument when calling the method:
+
+```py
+reveal_type(C.f(1))  # revealed: str
+reveal_type(C().f(1))  # revealed: str
+```
+
+When the class method is called incorrectly, we detect it:
+
+```py
+C.f("incorrect")  # error: [invalid-argument-type]
+C.f()  # error: [missing-argument]
+C.f(1, 2)  # error: [too-many-positional-arguments]
+```
+
+If the `cls` parameter is wrongly annotated, we emit an error at the call site:
+
+```py
+class D:
+    @classmethod
+    def f(cls: D):
+        # This function is wrongly annotated, it should be `type[D]` instead of `D`
+        pass
+
+# error: [invalid-argument-type] "Object of type `Literal[D]` cannot be assigned to parameter 1 (`cls`) of bound method `f`; expected type `D`"
+D.f()
+```
+
+When a class method is accessed on a derived class, it is bound to that derived class:
+
+```py
+class Derived(C):
+    pass
+
+reveal_type(Derived.f)  # revealed: <bound method `f` of `Literal[Derived]`>
+reveal_type(Derived().f)  # revealed: <bound method `f` of `type[Derived]`>
+
+reveal_type(Derived.f(1))  # revealed: str
+reveal_type(Derived().f(1))  # revealed: str
+```
+
+### Accessing the classmethod as a static member
+
+Accessing a `@classmethod`-decorated function at runtime returns a `classmethod` object. We
+currently don't model this explicitly:
+
+```py
+from inspect import getattr_static
+
+class C:
+    @classmethod
+    def f(cls): ...
+
+reveal_type(getattr_static(C, "f"))  # revealed: Literal[f]
+reveal_type(getattr_static(C, "f").__get__)  # revealed: <method-wrapper `__get__` of `f`>
+```
+
+But we correctly model how the `classmethod` descriptor works:
+
+```py
+reveal_type(getattr_static(C, "f").__get__(None, C))  # revealed: <bound method `f` of `Literal[C]`>
+reveal_type(getattr_static(C, "f").__get__(C(), C))  # revealed: <bound method `f` of `Literal[C]`>
+reveal_type(getattr_static(C, "f").__get__(C()))  # revealed: <bound method `f` of `type[C]`>
+```
+
+The `owner` argument takes precedence over the `instance` argument:
+
+```py
+reveal_type(getattr_static(C, "f").__get__("dummy", C))  # revealed: <bound method `f` of `Literal[C]`>
+```
+
+### Classmethods mixed with other decorators
+
+When a `@classmethod` is additionally decorated with another decorator, it is still treated as a
+class method:
+
+```py
+from __future__ import annotations
+
+def does_nothing[T](f: T) -> T:
+    return f
+
+class C:
+    @classmethod
+    @does_nothing
+    def f1(cls: type[C], x: int) -> str:
+        return "a"
+
+    @does_nothing
+    @classmethod
+    def f2(cls: type[C], x: int) -> str:
+        return "a"
+
+# TODO: We do not support decorators yet (only limited special cases). Eventually,
+# these should all return `str`:
+
+reveal_type(C.f1(1))  # revealed: @Todo(return type of decorated function)
+reveal_type(C().f1(1))  # revealed: @Todo(decorated method)
+
+reveal_type(C.f2(1))  # revealed: @Todo(return type of decorated function)
+reveal_type(C().f2(1))  # revealed: @Todo(decorated method)
+```
+
 [functions and methods]: https://docs.python.org/3/howto/descriptor.html#functions-and-methods

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

@@ -0,0 +1,12 @@
+# Never is callable
+
+The type `Never` is callable with an arbitrary set of arguments. The result is always `Never`.
+
+```py
+from typing_extensions import Never
+
+def f(never: Never):
+    reveal_type(never())  # revealed: Never
+    reveal_type(never(1))  # revealed: Never
+    reveal_type(never(1, "a", never, x=None))  # revealed: Never
+```

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

@@ -56,6 +56,7 @@ def _(flag: bool, flag2: bool):
     else:
         def f() -> int:
             return 1
+    # TODO we should mention all non-callable elements of the union
     # error: [call-non-callable] "Object of type `Literal[1]` is not callable"
     # revealed: int | Unknown
     reveal_type(f())
@@ -108,3 +109,38 @@ def _(flag: bool):
     x = f(3)
     reveal_type(x)  # revealed: Unknown
 ```
+
+## Union of binding errors
+
+```py
+def f1(): ...
+def f2(): ...
+def _(flag: bool):
+    if flag:
+        f = f1
+    else:
+        f = f2
+
+    # TODO: we should show all errors from the union, not arbitrarily pick one union element
+    # error: [too-many-positional-arguments] "Too many positional arguments to function `f1`: expected 0, got 1"
+    x = f(3)
+    reveal_type(x)  # revealed: Unknown
+```
+
+## One not-callable, one wrong argument
+
+```py
+class C: ...
+
+def f1(): ...
+def _(flag: bool):
+    if flag:
+        f = f1
+    else:
+        f = C()
+
+    # TODO: we should either show all union errors here, or prioritize the not-callable error
+    # error: [too-many-positional-arguments] "Too many positional arguments to function `f1`: expected 0, got 1"
+    x = f(3)
+    reveal_type(x)  # revealed: Unknown
+```

crates/red_knot_python_semantic/resources/mdtest/comparison/instances/membership_test.md

@@ -160,3 +160,45 @@ reveal_type(42 in A())  # revealed: bool
 # error: [unsupported-operator] "Operator `in` is not supported for types `str` and `A`, in comparing `Literal["hello"]` with `A`"
 reveal_type("hello" in A())  # revealed: bool
 ```
+
+## Return type that doesn't implement `__bool__` correctly
+
+`in` and `not in` operations will fail at runtime if the object on the right-hand side of the
+operation has a `__contains__` method that returns a type which is not convertible to `bool`. This
+is because of the way these operations are handled by the Python interpreter at runtime. If we
+assume that `y` is an object that has a `__contains__` method, the Python expression `x in y`
+desugars to a `contains(y, x)` call, where `contains` looks something like this:
+
+```ignore
+def contains(y, x):
+    return bool(type(y).__contains__(y, x))
+```
+
+where the `bool()` conversion itself implicitly calls `__bool__` under the hood.
+
+TODO: Ideally the message would explain to the user what's wrong. E.g,
+
+```ignore
+error: [operator] cannot use `in` operator on object of type `WithContains`
+    note: This is because the `in` operator implicitly calls `WithContains.__contains__`, but `WithContains.__contains__` is invalidly defined
+    note: `WithContains.__contains__` is invalidly defined because it returns an instance of `NotBoolable`, which cannot be evaluated in a boolean context
+    note: `NotBoolable` cannot be evaluated in a boolean context because its `__bool__` attribute is not callable
+```
+
+It may also be more appropriate to use `unsupported-operator` as the error code.
+
+<!-- snapshot-diagnostics -->
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+class WithContains:
+    def __contains__(self, item) -> NotBoolable:
+        return NotBoolable()
+
+# error: [unsupported-bool-conversion]
+10 in WithContains()
+# error: [unsupported-bool-conversion]
+10 not in WithContains()
+```

crates/red_knot_python_semantic/resources/mdtest/comparison/instances/rich_comparison.md

@@ -345,3 +345,47 @@ def f(x: bool, y: int):
     reveal_type(4.2 < x)  # revealed: bool
     reveal_type(x < 4.2)  # revealed: bool
 ```
+
+## Chained comparisons with objects that don't implement `__bool__` correctly
+
+<!-- snapshot-diagnostics -->
+
+Python implicitly calls `bool` on the comparison result of preceding elements (but not for the last
+element) of a chained comparison.
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+class Comparable:
+    def __lt__(self, item) -> NotBoolable:
+        return NotBoolable()
+
+    def __gt__(self, item) -> NotBoolable:
+        return NotBoolable()
+
+# error: [unsupported-bool-conversion]
+10 < Comparable() < 20
+# error: [unsupported-bool-conversion]
+10 < Comparable() < Comparable()
+
+Comparable() < Comparable()  # fine
+```
+
+## Callables as comparison dunders
+
+```py
+from typing import Literal
+
+class AlwaysTrue:
+    def __call__(self, other: object) -> Literal[True]:
+        return True
+
+class A:
+    __eq__: AlwaysTrue = AlwaysTrue()
+    __lt__: AlwaysTrue = AlwaysTrue()
+
+reveal_type(A() == A())  # revealed: Literal[True]
+reveal_type(A() < A())  # revealed: Literal[True]
+reveal_type(A() > A())  # revealed: Literal[True]
+```

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

@@ -334,3 +334,61 @@ reveal_type(a is not c)  # revealed: Literal[True]
 For tuples like `tuple[int, ...]`, `tuple[Any, ...]`
 
 // TODO
+
+## Chained comparisons with elements that incorrectly implement `__bool__`
+
+<!-- snapshot-diagnostics -->
+
+For an operation `A() < A()` to succeed at runtime, the `A.__lt__` method does not necessarily need
+to return an object that is convertible to a `bool`. However, the return type _does_ need to be
+convertible to a `bool` for the operation `A() < A() < A()` (a _chained_ comparison) to succeed.
+This is because `A() < A() < A()` desugars to something like this, which involves several implicit
+conversions to `bool`:
+
+```ignore
+def compute_chained_comparison():
+  a1 = A()
+  a2 = A()
+  first_comparison = a1 < a2
+  return first_comparison and (a2 < A())
+```
+
+```py
+class NotBoolable:
+    __bool__ = 5
+
+class Comparable:
+    def __lt__(self, other) -> NotBoolable:
+        return NotBoolable()
+
+    def __gt__(self, other) -> NotBoolable:
+        return NotBoolable()
+
+a = (1, Comparable())
+b = (1, Comparable())
+
+# error: [unsupported-bool-conversion]
+a < b < b
+
+a < b  # fine
+```
+
+## Equality with elements that incorrectly implement `__bool__`
+
+<!-- snapshot-diagnostics -->
+
+Python does not generally attempt to coerce the result of `==` and `!=` operations between two
+arbitrary objects to a `bool`, but a comparison of tuples will fail if the result of comparing any
+pair of elements at equivalent positions cannot be converted to a `bool`:
+
+```py
+class A:
+    def __eq__(self, other) -> NotBoolable:
+        return NotBoolable()
+
+class NotBoolable:
+    __bool__ = None
+
+# error: [unsupported-bool-conversion]
+(A(),) == (A(),)
+```

crates/red_knot_python_semantic/resources/mdtest/conditional/if_expression.md

@@ -35,3 +35,13 @@ def _(flag: bool):
     x = 1 if flag else None
     reveal_type(x)  # revealed: Literal[1] | None
 ```
+
+## Condition with object that implements `__bool__` incorrectly
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+# error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable"
+3 if NotBoolable() else 4
+```

crates/red_knot_python_semantic/resources/mdtest/conditional/if_statement.md

@@ -147,3 +147,17 @@ def _(flag: bool):
 
     reveal_type(y)  # revealed: Literal[0, 1]
 ```
+
+## Condition with object that implements `__bool__` incorrectly
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+# error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable"
+if NotBoolable():
+    ...
+# error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable"
+elif NotBoolable():
+    ...
+```

crates/red_knot_python_semantic/resources/mdtest/conditional/match.md

@@ -43,3 +43,21 @@ def _(target: int):
 
     reveal_type(y)  # revealed: Literal[2, 3, 4]
 ```
+
+## Guard with object that implements `__bool__` incorrectly
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+def _(target: int, flag: NotBoolable):
+    y = 1
+    match target:
+        # error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable"
+        case 1 if flag:
+            y = 2
+        case 2:
+            y = 3
+
+    reveal_type(y)  # revealed: Literal[1, 2, 3]
+```

crates/red_knot_python_semantic/resources/mdtest/descriptor_protocol.md

@@ -201,14 +201,11 @@ class C:
 
 c1 = C.factory("test")  # okay
 
-# TODO: should be `C`
-reveal_type(c1)  # revealed: @Todo(return type)
+reveal_type(c1)  # revealed: C
 
-# TODO: should be `str`
-reveal_type(C.get_name())  # revealed: @Todo(return type)
+reveal_type(C.get_name())  # revealed: str
 
-# TODO: should be `str`
-reveal_type(C("42").get_name())  # revealed: @Todo(decorated method)
+reveal_type(C("42").get_name())  # revealed: str
 ```
 
 ## Descriptors only work when used as class variables
@@ -285,6 +282,31 @@ C.descriptor = "something else"
 reveal_type(C.descriptor)  # revealed: Unknown | int
 ```
 
+## `__get__` is called with correct arguments
+
+```py
+from __future__ import annotations
+
+class TailoredForClassObjectAccess:
+    def __get__(self, instance: None, owner: type[C]) -> int:
+        return 1
+
+class TailoredForInstanceAccess:
+    def __get__(self, instance: C, owner: type[C] | None = None) -> str:
+        return "a"
+
+class C:
+    class_object_access: TailoredForClassObjectAccess = TailoredForClassObjectAccess()
+    instance_access: TailoredForInstanceAccess = TailoredForInstanceAccess()
+
+reveal_type(C.class_object_access)  # revealed: int
+reveal_type(C().instance_access)  # revealed: str
+
+# TODO: These should emit a diagnostic
+reveal_type(C().class_object_access)  # revealed: TailoredForClassObjectAccess
+reveal_type(C.instance_access)  # revealed: TailoredForInstanceAccess
+```
+
 ## Descriptors with incorrect `__get__` signature
 
 ```py
@@ -403,15 +425,15 @@ wrapper_descriptor(f, None)
 wrapper_descriptor(f, C())
 
 # Calling it with something that is not a `FunctionType` as the first argument is an
-# error: [invalid-argument-type] "Object of type `Literal[1]` cannot be assigned to parameter 1 (`self`); expected type `FunctionType`"
+# error: [invalid-argument-type] "Object of type `Literal[1]` cannot be assigned to parameter 1 (`self`) of wrapper descriptor `FunctionType.__get__`; expected type `FunctionType`"
 wrapper_descriptor(1, None, type(f))
 
 # Calling it with something that is not a `type` as the `owner` argument is an
-# error: [invalid-argument-type] "Object of type `Literal[f]` cannot be assigned to parameter 3 (`owner`); expected type `type`"
+# error: [invalid-argument-type] "Object of type `Literal[f]` cannot be assigned to parameter 3 (`owner`) of wrapper descriptor `FunctionType.__get__`; expected type `type`"
 wrapper_descriptor(f, None, f)
 
 # Calling it with too many positional arguments is an
-# error: [too-many-positional-arguments] "Too many positional arguments: expected 3, got 4"
+# error: [too-many-positional-arguments] "Too many positional arguments to wrapper descriptor `FunctionType.__get__`: expected 3, got 4"
 wrapper_descriptor(f, None, type(f), "one too many")
 ```
 

crates/red_knot_python_semantic/resources/mdtest/diagnostics/invalid_argument_type.md

@@ -182,3 +182,16 @@ class C:
 c = C()
 c("wrong")  # error: [invalid-argument-type]
 ```
+
+## Calls to methods
+
+Tests that we also see a reference to a function if the callable is a bound method.
+
+```py
+class C:
+    def square(self, x: int) -> int:
+        return x * x
+
+c = C()
+c.square("hello")  # error: [invalid-argument-type]
+```

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

@@ -0,0 +1,9 @@
+## Condition with object that implements `__bool__` incorrectly
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+# error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable"
+assert NotBoolable()
+```

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

@@ -101,3 +101,55 @@ reveal_type(bool([]))  # revealed: bool
 reveal_type(bool({}))  # revealed: bool
 reveal_type(bool(set()))  # revealed: bool
 ```
+
+## `__bool__` returning `NoReturn`
+
+```py
+from typing import NoReturn
+
+class NotBoolable:
+    def __bool__(self) -> NoReturn:
+        raise NotImplementedError("This object can't be converted to a boolean")
+
+# TODO: This should emit an error that `NotBoolable` can't be converted to a bool but it currently doesn't
+#   because `Never` is assignable to `bool`. This probably requires dead code analysis to fix.
+if NotBoolable():
+    ...
+```
+
+## Not callable `__bool__`
+
+```py
+class NotBoolable:
+    __bool__ = None
+
+# error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable"
+if NotBoolable():
+    ...
+```
+
+## Not-boolable union
+
+```py
+def test(cond: bool):
+    class NotBoolable:
+        __bool__ = None if cond else 3
+
+    # error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; it incorrectly implements `__bool__`"
+    if NotBoolable():
+        ...
+```
+
+## Union with some variants implementing `__bool__` incorrectly
+
+```py
+def test(cond: bool):
+    class NotBoolable:
+        __bool__ = None
+
+    a = 10 if cond else NotBoolable()
+
+    # error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `Literal[10] | NotBoolable`; its `__bool__` method isn't callable"
+    if a:
+        ...
+```

crates/red_knot_python_semantic/resources/mdtest/generics.md

@@ -1,81 +0,0 @@
-# PEP 695 Generics
-
-## Class Declarations
-
-Basic PEP 695 generics
-
-```py
-class MyBox[T]:
-    data: T
-    box_model_number = 695
-
-    def __init__(self, data: T):
-        self.data = data
-
-box: MyBox[int] = MyBox(5)
-
-# TODO should emit a diagnostic here (str is not assignable to int)
-wrong_innards: MyBox[int] = MyBox("five")
-
-# TODO reveal int, do not leak the typevar
-reveal_type(box.data)  # revealed: T
-
-reveal_type(MyBox.box_model_number)  # revealed: Unknown | Literal[695]
-```
-
-## Subclassing
-
-```py
-class MyBox[T]:
-    data: T
-
-    def __init__(self, data: T):
-        self.data = data
-
-# TODO not error on the subscripting
-# error: [non-subscriptable]
-class MySecureBox[T](MyBox[T]): ...
-
-secure_box: MySecureBox[int] = MySecureBox(5)
-reveal_type(secure_box)  # revealed: MySecureBox
-# TODO reveal int
-# The @Todo(…) is misleading here. We currently treat `MyBox[T]` as a dynamic base class because we
-# don't understand generics and therefore infer `Unknown` for the `MyBox[T]` base of `MySecureBox[T]`.
-reveal_type(secure_box.data)  # revealed: @Todo(instance attribute on class with dynamic base)
-```
-
-## Cyclical class definition
-
-In type stubs, classes can reference themselves in their base class definitions. For example, in
-`typeshed`, we have `class str(Sequence[str]): ...`.
-
-This should hold true even with generics at play.
-
-```pyi
-class Seq[T]: ...
-
-# TODO not error on the subscripting
-class S[T](Seq[S]): ...  # error: [non-subscriptable]
-
-reveal_type(S)  # revealed: Literal[S]
-```
-
-## Type params
-
-A PEP695 type variable defines a value of type `typing.TypeVar`.
-
-```py
-def f[T]():
-    reveal_type(T)  # revealed: T
-    reveal_type(T.__name__)  # revealed: Literal["T"]
-```
-
-## Minimum two constraints
-
-A typevar with less than two constraints emits a diagnostic:
-
-```py
-# error: [invalid-type-variable-constraints] "TypeVar must have at least two constrained types"
-def f[T: (int,)]():
-    pass
-```

crates/red_knot_python_semantic/resources/mdtest/generics/classes.md

@@ -0,0 +1,186 @@
+# Generic classes
+
+## PEP 695 syntax
+
+TODO: Add a `red_knot_extension` function that asserts whether a function or class is generic.
+
+This is a generic class defined using PEP 695 syntax:
+
+```py
+class C[T]: ...
+```
+
+A class that inherits from a generic class, and fills its type parameters with typevars, is generic:
+
+```py
+# TODO: no error
+# error: [non-subscriptable]
+class D[U](C[U]): ...
+```
+
+A class that inherits from a generic class, but fills its type parameters with concrete types, is
+_not_ generic:
+
+```py
+# TODO: no error
+# error: [non-subscriptable]
+class E(C[int]): ...
+```
+
+A class that inherits from a generic class, and doesn't fill its type parameters at all, implicitly
+uses the default value for the typevar. In this case, that default type is `Unknown`, so `F`
+inherits from `C[Unknown]` and is not itself generic.
+
+```py
+class F(C): ...
+```
+
+## Legacy syntax
+
+This is a generic class defined using the legacy syntax:
+
+```py
+from typing import Generic, TypeVar
+
+T = TypeVar("T")
+
+# TODO: no error
+# error: [invalid-base]
+class C(Generic[T]): ...
+```
+
+A class that inherits from a generic class, and fills its type parameters with typevars, is generic.
+
+```py
+class D(C[T]): ...
+```
+
+(Examples `E` and `F` from above do not have analogues in the legacy syntax.)
+
+## Inferring generic class parameters
+
+The type parameter can be specified explicitly:
+
+```py
+class C[T]:
+    x: T
+
+# TODO: no error
+# TODO: revealed: C[int]
+# error: [non-subscriptable]
+reveal_type(C[int]())  # revealed: Unknown
+```
+
+We can infer the type parameter from a type context:
+
+```py
+c: C[int] = C()
+# TODO: revealed: C[int]
+reveal_type(c)  # revealed: C
+```
+
+The typevars of a fully specialized generic class should no longer be visible:
+
+```py
+# TODO: revealed: int
+reveal_type(c.x)  # revealed: T
+```
+
+If the type parameter is not specified explicitly, and there are no constraints that let us infer a
+specific type, we infer the typevar's default type:
+
+```py
+class D[T = int]: ...
+
+# TODO: revealed: D[int]
+reveal_type(D())  # revealed: D
+```
+
+If a typevar does not provide a default, we use `Unknown`:
+
+```py
+# TODO: revealed: C[Unknown]
+reveal_type(C())  # revealed: C
+```
+
+If the type of a constructor parameter is a class typevar, we can use that to infer the type
+parameter:
+
+```py
+class E[T]:
+    def __init__(self, x: T) -> None: ...
+
+# TODO: revealed: E[int] or E[Literal[1]]
+reveal_type(E(1))  # revealed: E
+```
+
+The types inferred from a type context and from a constructor parameter must be consistent with each
+other:
+
+```py
+# TODO: error
+wrong_innards: E[int] = E("five")
+```
+
+## Generic subclass
+
+When a generic subclass fills its superclass's type parameter with one of its own, the actual types
+propagate through:
+
+```py
+class Base[T]:
+    x: T
+
+# TODO: no error
+# error: [non-subscriptable]
+class Sub[U](Base[U]): ...
+
+# TODO: no error
+# TODO: revealed: int
+# error: [non-subscriptable]
+reveal_type(Base[int].x)  # revealed: Unknown
+# TODO: revealed: int
+reveal_type(Sub[int].x)  # revealed: Unknown
+```
+
+## Cyclic class definition
+
+A class can use itself as the type parameter of one of its superclasses. (This is also known as the
+[curiously recurring template pattern][crtp] or [F-bounded quantification][f-bound].)
+
+Here, `Sub` is not a generic class, since it fills its superclass's type parameter (with itself).
+
+`stub.pyi`:
+
+```pyi
+class Base[T]: ...
+# TODO: no error
+# error: [non-subscriptable]
+class Sub(Base[Sub]): ...
+
+reveal_type(Sub)  # revealed: Literal[Sub]
+```
+
+`string_annotation.py`:
+
+```py
+class Base[T]: ...
+
+# TODO: no error
+# error: [non-subscriptable]
+class Sub(Base["Sub"]): ...
+
+reveal_type(Sub)  # revealed: Literal[Sub]
+```
+
+`bare_annotation.py`:
+
+```py
+class Base[T]: ...
+
+# TODO: error: [unresolved-reference]
+class Sub(Base[Sub]): ...
+```
+
+[crtp]: https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
+[f-bound]: https://en.wikipedia.org/wiki/Bounded_quantification#F-bounded_quantification

crates/red_knot_python_semantic/resources/mdtest/generics/functions.md

@@ -0,0 +1,244 @@
+# Generic functions
+
+## Typevar must be used at least twice
+
+If you're only using a typevar for a single parameter, you don't need the typevar — just use
+`object` (or the typevar's upper bound):
+
+```py
+# TODO: error, should be (x: object)
+def typevar_not_needed[T](x: T) -> None:
+    pass
+
+# TODO: error, should be (x: int)
+def bounded_typevar_not_needed[T: int](x: T) -> None:
+    pass
+```
+
+Typevars are only needed if you use them more than once. For instance, to specify that two
+parameters must both have the same type:
+
+```py
+def two_params[T](x: T, y: T) -> T:
+    return x
+```
+
+or to specify that a return value is the same as a parameter:
+
+```py
+def return_value[T](x: T) -> T:
+    return x
+```
+
+Each typevar must also appear _somewhere_ in the parameter list:
+
+```py
+def absurd[T]() -> T:
+    # There's no way to construct a T!
+    ...
+```
+
+## Inferring generic function parameter types
+
+If the type of a generic function parameter is a typevar, then we can infer what type that typevar
+is bound to at each call site.
+
+TODO: Note that some of the TODO revealed types have two options, since we haven't decided yet
+whether we want to infer a more specific `Literal` type where possible, or use heuristics to weaken
+the inferred type to e.g. `int`.
+
+```py
+def f[T](x: T) -> T: ...
+
+# TODO: no error
+# TODO: revealed: int or Literal[1]
+# error: [invalid-argument-type]
+reveal_type(f(1))  # revealed: T
+
+# TODO: no error
+# TODO: revealed: float
+# error: [invalid-argument-type]
+reveal_type(f(1.0))  # revealed: T
+
+# TODO: no error
+# TODO: revealed: bool or Literal[true]
+# error: [invalid-argument-type]
+reveal_type(f(True))  # revealed: T
+
+# TODO: no error
+# TODO: revealed: str or Literal["string"]
+# error: [invalid-argument-type]
+reveal_type(f("string"))  # revealed: T
+```
+
+## Inferring “deep” generic parameter types
+
+The matching up of call arguments and discovery of constraints on typevars can be a recursive
+process for arbitrarily-nested generic types in parameters.
+
+```py
+def f[T](x: list[T]) -> T: ...
+
+# TODO: revealed: float
+reveal_type(f([1.0, 2.0]))  # revealed: T
+```
+
+## Typevar constraints
+
+If a type parameter has an upper bound, that upper bound constrains which types can be used for that
+typevar. This effectively adds the upper bound as an intersection to every appearance of the typevar
+in the function.
+
+```py
+def good_param[T: int](x: T) -> None:
+    # TODO: revealed: T & int
+    reveal_type(x)  # revealed: T
+```
+
+If the function is annotated as returning the typevar, this means that the upper bound is _not_
+assignable to that typevar, since return types are contravariant. In `bad`, we can infer that
+`x + 1` has type `int`. But `T` might be instantiated with a narrower type than `int`, and so the
+return value is not guaranteed to be compatible for all `T: int`.
+
+```py
+def good_return[T: int](x: T) -> T:
+    return x
+
+def bad_return[T: int](x: T) -> T:
+    # TODO: error: int is not assignable to T
+    # error: [unsupported-operator] "Operator `+` is unsupported between objects of type `T` and `Literal[1]`"
+    return x + 1
+```
+
+## All occurrences of the same typevar have the same type
+
+If a typevar appears multiple times in a function signature, all occurrences have the same type.
+
+```py
+def different_types[T, S](cond: bool, t: T, s: S) -> T:
+    if cond:
+        return t
+    else:
+        # TODO: error: S is not assignable to T
+        return s
+
+def same_types[T](cond: bool, t1: T, t2: T) -> T:
+    if cond:
+        return t1
+    else:
+        return t2
+```
+
+## All occurrences of the same constrained typevar have the same type
+
+The above is true even when the typevars are constrained. Here, both `int` and `str` have `__add__`
+methods that are compatible with the return type, so the `return` expression is always well-typed:
+
+```py
+def same_constrained_types[T: (int, str)](t1: T, t2: T) -> T:
+    # TODO: no error
+    # error: [unsupported-operator] "Operator `+` is unsupported between objects of type `T` and `T`"
+    return t1 + t2
+```
+
+This is _not_ the same as a union type, because of this additional constraint that the two
+occurrences have the same type. In `unions_are_different`, `t1` and `t2` might have different types,
+and an `int` and a `str` cannot be added together:
+
+```py
+def unions_are_different(t1: int | str, t2: int | str) -> int | str:
+    # error: [unsupported-operator] "Operator `+` is unsupported between objects of type `int | str` and `int | str`"
+    return t1 + t2
+```
+
+## Typevar inference is a unification problem
+
+When inferring typevar assignments in a generic function call, we cannot simply solve constraints
+eagerly for each parameter in turn. We must solve a unification problem involving all of the
+parameters simultaneously.
+
+```py
+def two_params[T](x: T, y: T) -> T:
+    return x
+
+# TODO: no error
+# TODO: revealed: str
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(two_params("a", "b"))  # revealed: T
+
+# TODO: no error
+# TODO: revealed: str | int
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(two_params("a", 1))  # revealed: T
+```
+
+```py
+def param_with_union[T](x: T | int, y: T) -> T:
+    return y
+
+# TODO: no error
+# TODO: revealed: str
+# error: [invalid-argument-type]
+reveal_type(param_with_union(1, "a"))  # revealed: T
+
+# TODO: no error
+# TODO: revealed: str
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(param_with_union("a", "a"))  # revealed: T
+
+# TODO: no error
+# TODO: revealed: int
+# error: [invalid-argument-type]
+reveal_type(param_with_union(1, 1))  # revealed: T
+
+# TODO: no error
+# TODO: revealed: str | int
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(param_with_union("a", 1))  # revealed: T
+```
+
+```py
+def tuple_param[T, S](x: T | S, y: tuple[T, S]) -> tuple[T, S]:
+    return y
+
+# TODO: no error
+# TODO: revealed: tuple[str, int]
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(tuple_param("a", ("a", 1)))  # revealed: tuple[T, S]
+
+# TODO: no error
+# TODO: revealed: tuple[str, int]
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(tuple_param(1, ("a", 1)))  # revealed: tuple[T, S]
+```
+
+## Inferring nested generic function calls
+
+We can infer type assignments in nested calls to multiple generic functions. If they use the same
+type variable, we do not confuse the two; `T@f` and `T@g` have separate types in each example below.
+
+```py
+def f[T](x: T) -> tuple[T, int]:
+    return (x, 1)
+
+def g[T](x: T) -> T | None:
+    return x
+
+# TODO: no error
+# TODO: revealed: tuple[str | None, int]
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(f(g("a")))  # revealed: tuple[T, int]
+
+# TODO: no error
+# TODO: revealed: tuple[str, int] | None
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(g(f("a")))  # revealed: T | None
+```

crates/red_knot_python_semantic/resources/mdtest/generics/legacy.md

@@ -0,0 +1,72 @@
+# Legacy type variables
+
+The tests in this file focus on how type variables are defined using the legacy notation. Most
+_uses_ of type variables are tested in other files in this directory; we do not duplicate every test
+for both type variable syntaxes.
+
+Unless otherwise specified, all quotations come from the [Generics] section of the typing spec.
+
+## Type variables
+
+### Defining legacy type variables
+
+> Generics can be parameterized by using a factory available in `typing` called `TypeVar`.
+
+This was the only way to create type variables prior to PEP 695/Python 3.12. It is still available
+in newer Python releases.
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+```
+
+### Directly assigned to a variable
+
+> A `TypeVar()` expression must always directly be assigned to a variable (it should not be used as
+> part of a larger expression).
+
+```py
+from typing import TypeVar
+
+# TODO: error
+TestList = list[TypeVar("W")]
+```
+
+### `TypeVar` parameter must match variable name
+
+> The argument to `TypeVar()` must be a string equal to the variable name to which it is assigned.
+
+```py
+from typing import TypeVar
+
+# TODO: error
+T = TypeVar("Q")
+```
+
+### No redefinition
+
+> Type variables must not be redefined.
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+
+# TODO: error
+T = TypeVar("T")
+```
+
+### Cannot have only one constraint
+
+> `TypeVar` supports constraining parametric types to a fixed set of possible types...There should
+> be at least two constraints, if any; specifying a single constraint is disallowed.
+
+```py
+from typing import TypeVar
+
+# TODO: error: [invalid-type-variable-constraints]
+T = TypeVar("T", int)
+```
+
+[generics]: https://typing.readthedocs.io/en/latest/spec/generics.html

crates/red_knot_python_semantic/resources/mdtest/generics/pep695.md

@@ -0,0 +1,51 @@
+# PEP 695 Generics
+
+[PEP 695] and Python 3.12 introduced new, more ergonomic syntax for type variables.
+
+## Type variables
+
+### Defining PEP 695 type variables
+
+PEP 695 introduces a new syntax for defining type variables. The resulting type variables are
+instances of `typing.TypeVar`, just like legacy type variables.
+
+```py
+def f[T]():
+    reveal_type(type(T))  # revealed: Literal[TypeVar]
+    reveal_type(T)  # revealed: T
+    reveal_type(T.__name__)  # revealed: Literal["T"]
+```
+
+### Cannot have only one constraint
+
+> `TypeVar` supports constraining parametric types to a fixed set of possible types...There should
+> be at least two constraints, if any; specifying a single constraint is disallowed.
+
+```py
+# error: [invalid-type-variable-constraints] "TypeVar must have at least two constrained types"
+def f[T: (int,)]():
+    pass
+```
+
+## Invalid uses
+
+Note that many of the invalid uses of legacy typevars do not apply to PEP 695 typevars, since the
+PEP 695 syntax is only allowed places where typevars are allowed.
+
+## Displaying typevars
+
+We use a suffix when displaying the typevars of a generic function or class. This helps distinguish
+different uses of the same typevar.
+
+```py
+def f[T](x: T, y: T) -> None:
+    # TODO: revealed: T@f
+    reveal_type(x)  # revealed: T
+
+class C[T]:
+    def m(self, x: T) -> None:
+        # TODO: revealed: T@c
+        reveal_type(x)  # revealed: T
+```
+
+[pep 695]: https://peps.python.org/pep-0695/

crates/red_knot_python_semantic/resources/mdtest/generics/scoping.md

@@ -0,0 +1,255 @@
+# Scoping rules for type variables
+
+Most of these tests come from the [Scoping rules for type variables][scoping] section of the typing
+spec.
+
+## Typevar used outside of generic function or class
+
+Typevars may only be used in generic function or class definitions.
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+
+# TODO: error
+x: T
+
+class C:
+    # TODO: error
+    x: T
+
+def f() -> None:
+    # TODO: error
+    x: T
+```
+
+## Legacy typevar used multiple times
+
+> A type variable used in a generic function could be inferred to represent different types in the
+> same code block.
+
+This only applies to typevars defined using the legacy syntax, since the PEP 695 syntax creates a
+new distinct typevar for each occurrence.
+
+```py
+from typing import TypeVar
+
+T = TypeVar("T")
+
+def f1(x: T) -> T: ...
+def f2(x: T) -> T: ...
+
+f1(1)
+f2("a")
+```
+
+## Typevar inferred multiple times
+
+> A type variable used in a generic function could be inferred to represent different types in the
+> same code block.
+
+This also applies to a single generic function being used multiple times, instantiating the typevar
+to a different type each time.
+
+```py
+def f[T](x: T) -> T: ...
+
+# TODO: no error
+# TODO: revealed: int or Literal[1]
+# error: [invalid-argument-type]
+reveal_type(f(1))  # revealed: T
+# TODO: no error
+# TODO: revealed: str or Literal["a"]
+# error: [invalid-argument-type]
+reveal_type(f("a"))  # revealed: T
+```
+
+## Methods can mention class typevars
+
+> A type variable used in a method of a generic class that coincides with one of the variables that
+> parameterize this class is always bound to that variable.
+
+```py
+class C[T]:
+    def m1(self, x: T) -> T: ...
+    def m2(self, x: T) -> T: ...
+
+c: C[int] = C()
+# TODO: no error
+# error: [invalid-argument-type]
+c.m1(1)
+# TODO: no error
+# error: [invalid-argument-type]
+c.m2(1)
+# TODO: expected type `int`
+# error: [invalid-argument-type] "Object of type `Literal["string"]` cannot be assigned to parameter 2 (`x`) of bound method `m2`; expected type `T`"
+c.m2("string")
+```
+
+## Methods can mention other typevars
+
+> A type variable used in a method that does not match any of the variables that parameterize the
+> class makes this method a generic function in that variable.
+
+```py
+from typing import TypeVar, Generic
+
+T = TypeVar("T")
+S = TypeVar("S")
+
+# TODO: no error
+# error: [invalid-base]
+class Legacy(Generic[T]):
+    def m(self, x: T, y: S) -> S: ...
+
+legacy: Legacy[int] = Legacy()
+# TODO: revealed: str
+reveal_type(legacy.m(1, "string"))  # revealed: @Todo(Invalid or unsupported `Instance` in `Type::to_type_expression`)
+```
+
+With PEP 695 syntax, it is clearer that the method uses a separate typevar:
+
+```py
+class C[T]:
+    def m[S](self, x: T, y: S) -> S: ...
+
+c: C[int] = C()
+# TODO: no errors
+# TODO: revealed: str
+# error: [invalid-argument-type]
+# error: [invalid-argument-type]
+reveal_type(c.m(1, "string"))  # revealed: S
+```
+
+## Unbound typevars
+
+> Unbound type variables should not appear in the bodies of generic functions, or in the class
+> bodies apart from method definitions.
+
+This is true with the legacy syntax:
+
+```py
+from typing import TypeVar, Generic
+
+T = TypeVar("T")
+S = TypeVar("S")
+
+def f(x: T) -> None:
+    x: list[T] = []
+    # TODO: error
+    y: list[S] = []
+
+# TODO: no error
+# error: [invalid-base]
+class C(Generic[T]):
+    # TODO: error
+    x: list[S] = []
+
+    # This is not an error, as shown in the previous test
+    def m(self, x: S) -> S: ...
+```
+
+This is true with PEP 695 syntax, as well, though we must use the legacy syntax to define the
+unbound typevars:
+
+`pep695.py`:
+
+```py
+from typing import TypeVar
+
+S = TypeVar("S")
+
+def f[T](x: T) -> None:
+    x: list[T] = []
+    # TODO: error
+    y: list[S] = []
+
+class C[T]:
+    # TODO: error
+    x: list[S] = []
+
+    def m1(self, x: S) -> S: ...
+    def m2[S](self, x: S) -> S: ...
+```
+
+## Nested formal typevars must be distinct
+
+Generic functions and classes can be nested in each other, but it is an error for the same typevar
+to be used in nested generic definitions.
+
+Note that the typing spec only mentions two specific versions of this rule:
+
+> A generic class definition that appears inside a generic function should not use type variables
+> that parameterize the generic function.
+
+and
+
+> A generic class nested in another generic class cannot use the same type variables.
+
+We assume that the more general form holds.
+
+### Generic function within generic function
+
+```py
+def f[T](x: T, y: T) -> None:
+    def ok[S](a: S, b: S) -> None: ...
+
+    # TODO: error
+    def bad[T](a: T, b: T) -> None: ...
+```
+
+### Generic method within generic class
+
+```py
+class C[T]:
+    def ok[S](self, a: S, b: S) -> None: ...
+
+    # TODO: error
+    def bad[T](self, a: T, b: T) -> None: ...
+```
+
+### Generic class within generic function
+
+```py
+from typing import Iterable
+
+def f[T](x: T, y: T) -> None:
+    class Ok[S]: ...
+    # TODO: error
+    class Bad1[T]: ...
+    # TODO: error
+    class Bad2(Iterable[T]): ...
+```
+
+### Generic class within generic class
+
+```py
+from typing import Iterable
+
+class C[T]:
+    class Ok1[S]: ...
+    # TODO: error
+    class Bad1[T]: ...
+    # TODO: error
+    class Bad2(Iterable[T]): ...
+```
+
+## Class scopes do not cover inner scopes
+
+Just like regular symbols, the typevars of a generic class are only available in that class's scope,
+and are not available in nested scopes.
+
+```py
+class C[T]:
+    ok1: list[T] = []
+
+    class Bad:
+        # TODO: error
+        bad: list[T] = []
+
+    class Inner[S]: ...
+    ok2: Inner[T]
+```
+
+[scoping]: https://typing.readthedocs.io/en/latest/spec/generics.html#scoping-rules-for-type-variables

crates/red_knot_python_semantic/resources/mdtest/import/case_sensitive.md

@@ -0,0 +1,131 @@
+# Case Sensitive Imports
+
+```toml
+# TODO: This test should use the real file system instead of the memory file system.
+# but we can't change the file system yet because the tests would then start failing for 
+# case-insensitive file systems.
+#system = "os"
+```
+
+Python's import system is case-sensitive even on case-insensitive file system. This means, importing
+a module `a` should fail if the file in the search paths is named `A.py`. See
+[PEP 235](https://peps.python.org/pep-0235/).
+
+## Correct casing
+
+Importing a module where the name matches the file name's casing should succeed.
+
+`a.py`:
+
+```py
+class Foo:
+    x: int = 1
+```
+
+```python
+from a import Foo
+
+reveal_type(Foo().x)  # revealed: int
+```
+
+## Incorrect casing
+
+Importing a module where the name does not match the file name's casing should fail.
+
+`A.py`:
+
+```py
+class Foo:
+    x: int = 1
+```
+
+```python
+# error: [unresolved-import]
+from a import Foo
+```
+
+## Multiple search paths with different cased modules
+
+The resolved module is the first matching the file name's casing but Python falls back to later
+search paths if the file name's casing does not match.
+
+```toml
+[environment]
+extra-paths = ["/search-1", "/search-2"]
+```
+
+`/search-1/A.py`:
+
+```py
+class Foo:
+    x: int = 1
+```
+
+`/search-2/a.py`:
+
+```py
+class Bar:
+    x: str = "test"
+```
+
+```python
+from A import Foo
+from a import Bar
+
+reveal_type(Foo().x)  # revealed: int
+reveal_type(Bar().x)  # revealed: str
+```
+
+## Intermediate segments
+
+`db/__init__.py`:
+
+```py
+```
+
+`db/a.py`:
+
+```py
+class Foo:
+    x: int = 1
+```
+
+`correctly_cased.py`:
+
+```python
+from db.a import Foo
+
+reveal_type(Foo().x)  # revealed: int
+```
+
+Imports where some segments are incorrectly cased should fail.
+
+`incorrectly_cased.py`:
+
+```python
+# error: [unresolved-import]
+from DB.a import Foo
+
+# error: [unresolved-import]
+from DB.A import Foo
+
+# error: [unresolved-import]
+from db.A import Foo
+```
+
+## Incorrect extension casing
+
+The extension of imported python modules must be `.py` or `.pyi` but not `.PY` or `Py` or any
+variant where some characters are uppercase.
+
+`a.PY`:
+
+```py
+class Foo:
+    x: int = 1
+```
+
+```python
+# error: [unresolved-import]
+from a import Foo
+```

crates/red_knot_python_semantic/resources/mdtest/known_constants.md

@@ -26,23 +26,6 @@ from typing import TYPE_CHECKING as TC
 reveal_type(TC)  # revealed: Literal[True]
 ```
 
-### Must originate from `typing`
-
-Make sure we only use our special handling for `typing.TYPE_CHECKING` and not for other constants
-with the same name:
-
-`constants.py`:
-
-```py
-TYPE_CHECKING: bool = False
-```
-
-```py
-from constants import TYPE_CHECKING
-
-reveal_type(TYPE_CHECKING)  # revealed: bool
-```
-
 ### `typing_extensions` re-export
 
 This should behave in the same way as `typing.TYPE_CHECKING`:
@@ -52,3 +35,117 @@ from typing_extensions import TYPE_CHECKING
 
 reveal_type(TYPE_CHECKING)  # revealed: Literal[True]
 ```
+
+## User-defined `TYPE_CHECKING`
+
+If we set `TYPE_CHECKING = False` directly instead of importing it from the `typing` module, it will
+still be treated as `True` during type checking. This behavior is for compatibility with other major
+type checkers, e.g. mypy and pyright.
+
+### With no type annotation
+
+```py
+TYPE_CHECKING = False
+reveal_type(TYPE_CHECKING)  # revealed: Literal[True]
+if TYPE_CHECKING:
+    type_checking = True
+if not TYPE_CHECKING:
+    runtime = True
+
+# type_checking is treated as unconditionally assigned.
+reveal_type(type_checking)  # revealed: Literal[True]
+# error: [unresolved-reference]
+reveal_type(runtime)  # revealed: Unknown
+```
+
+### With a type annotation
+
+We can also define `TYPE_CHECKING` with a type annotation. The type must be one to which `bool` can
+be assigned. Even in this case, the type of `TYPE_CHECKING` is still inferred to be `Literal[True]`.
+
+```py
+TYPE_CHECKING: bool = False
+reveal_type(TYPE_CHECKING)  # revealed: Literal[True]
+if TYPE_CHECKING:
+    type_checking = True
+if not TYPE_CHECKING:
+    runtime = True
+
+reveal_type(type_checking)  # revealed: Literal[True]
+# error: [unresolved-reference]
+reveal_type(runtime)  # revealed: Unknown
+```
+
+### Importing user-defined `TYPE_CHECKING`
+
+`constants.py`:
+
+```py
+TYPE_CHECKING = False
+```
+
+`stub.pyi`:
+
+```pyi
+TYPE_CHECKING: bool
+# or
+TYPE_CHECKING: bool = ...
+```
+
+```py
+from constants import TYPE_CHECKING
+
+reveal_type(TYPE_CHECKING)  # revealed: Literal[True]
+
+from stub import TYPE_CHECKING
+
+reveal_type(TYPE_CHECKING)  # revealed: Literal[True]
+```
+
+### Invalid assignment to `TYPE_CHECKING`
+
+Only `False` can be assigned to `TYPE_CHECKING`; any assignment other than `False` will result in an
+error. A type annotation to which `bool` is not assignable is also an error.
+
+```py
+from typing import Literal
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING = True
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: bool = True
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: int = 1
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: str = "str"
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: str = False
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: Literal[False] = False
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: Literal[True] = False
+```
+
+The same rules apply in a stub file:
+
+```pyi
+from typing import Literal
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: str
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: str = False
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: Literal[False] = ...
+
+# error: [invalid-type-checking-constant]
+TYPE_CHECKING: object = "str"
+```

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

@@ -105,7 +105,11 @@ reveal_type(x)
 
 ## With non-callable iterator
 
+<!-- snapshot-diagnostics -->
+
 ```py
+from typing_extensions import reveal_type
+
 def _(flag: bool):
     class NotIterable:
         if flag:
@@ -113,7 +117,8 @@ def _(flag: bool):
         else:
             __iter__: None = None
 
-    for x in NotIterable():  # error: "Object of type `NotIterable` is not iterable"
+    # error: [not-iterable]
+    for x in NotIterable():
         pass
 
     # revealed: Unknown
@@ -123,21 +128,25 @@ def _(flag: bool):
 
 ## Invalid iterable
 
+<!-- snapshot-diagnostics -->
+
 ```py
 nonsense = 123
-for x in nonsense:  # error: "Object of type `Literal[123]` is not iterable"
+for x in nonsense:  # error: [not-iterable]
     pass
 ```
 
 ## New over old style iteration protocol
 
+<!-- snapshot-diagnostics -->
+
 ```py
 class NotIterable:
     def __getitem__(self, key: int) -> int:
         return 42
     __iter__: None = None
 
-for x in NotIterable():  # error: "Object of type `NotIterable` is not iterable"
+for x in NotIterable():  # error: [not-iterable]
     pass
 ```
 
@@ -221,7 +230,11 @@ def _(flag: bool):
 
 ## Union type as iterable where one union element has no `__iter__` method
 
+<!-- snapshot-diagnostics -->
+
 ```py
+from typing_extensions import reveal_type
+
 class TestIter:
     def __next__(self) -> int:
         return 42
@@ -231,14 +244,18 @@ class Test:
         return TestIter()
 
 def _(flag: bool):
-    # error: [not-iterable] "Object of type `Test | Literal[42]` is not iterable because its `__iter__` method is possibly unbound"
+    # error: [not-iterable]
     for x in Test() if flag else 42:
         reveal_type(x)  # revealed: int
 ```
 
 ## Union type as iterable where one union element has invalid `__iter__` method
 
+<!-- snapshot-diagnostics -->
+
 ```py
+from typing_extensions import reveal_type
+
 class TestIter:
     def __next__(self) -> int:
         return 42
@@ -253,7 +270,7 @@ class Test2:
 
 def _(flag: bool):
     # TODO: Improve error message to state which union variant isn't iterable (https://github.com/astral-sh/ruff/issues/13989)
-    # error: "Object of type `Test | Test2` is not iterable"
+    # error: [not-iterable]
     for x in Test() if flag else Test2():
         reveal_type(x)  # revealed: int
 ```
@@ -269,7 +286,464 @@ class Test:
     def __iter__(self) -> TestIter | int:
         return TestIter()
 
-# error: [not-iterable] "Object of type `Test` is not iterable"
+# error: [not-iterable] "Object of type `Test` may not be iterable because its `__iter__` method returns an object of type `TestIter | int`, which may not have a `__next__` method"
 for x in Test():
     reveal_type(x)  # revealed: int
 ```
+
+## Possibly-not-callable `__iter__` method
+
+```py
+def _(flag: bool):
+    class Iterator:
+        def __next__(self) -> int:
+            return 42
+
+    class CustomCallable:
+        if flag:
+            def __call__(self, *args, **kwargs) -> Iterator:
+                return Iterator()
+        else:
+            __call__: None = None
+
+    class Iterable1:
+        __iter__: CustomCallable = CustomCallable()
+
+    class Iterable2:
+        if flag:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+        else:
+            __iter__: None = None
+
+    # error: [not-iterable] "Object of type `Iterable1` may not be iterable because its `__iter__` attribute (with type `CustomCallable`) may not be callable"
+    for x in Iterable1():
+        # TODO... `int` might be ideal here?
+        reveal_type(x)  # revealed: int | Unknown
+
+    # error: [not-iterable] "Object of type `Iterable2` may not be iterable because its `__iter__` attribute (with type `<bound method `__iter__` of `Iterable2`> | None`) may not be callable"
+    for y in Iterable2():
+        # TODO... `int` might be ideal here?
+        reveal_type(y)  # revealed: int | Unknown
+```
+
+## `__iter__` method with a bad signature
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Iterator:
+    def __next__(self) -> int:
+        return 42
+
+class Iterable:
+    def __iter__(self, extra_arg) -> Iterator:
+        return Iterator()
+
+# error: [not-iterable]
+for x in Iterable():
+    reveal_type(x)  # revealed: int
+```
+
+## `__iter__` does not return an iterator
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Bad:
+    def __iter__(self) -> int:
+        return 42
+
+# error: [not-iterable]
+for x in Bad():
+    reveal_type(x)  # revealed: Unknown
+```
+
+## `__iter__` returns an object with a possibly unbound `__next__` method
+
+```py
+def _(flag: bool):
+    class Iterator:
+        if flag:
+            def __next__(self) -> int:
+                return 42
+
+    class Iterable:
+        def __iter__(self) -> Iterator:
+            return Iterator()
+
+    # error: [not-iterable] "Object of type `Iterable` may not be iterable because its `__iter__` method returns an object of type `Iterator`, which may not have a `__next__` method"
+    for x in Iterable():
+        reveal_type(x)  # revealed: int
+```
+
+## `__iter__` returns an iterator with an invalid `__next__` method
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Iterator1:
+    def __next__(self, extra_arg) -> int:
+        return 42
+
+class Iterator2:
+    __next__: None = None
+
+class Iterable1:
+    def __iter__(self) -> Iterator1:
+        return Iterator1()
+
+class Iterable2:
+    def __iter__(self) -> Iterator2:
+        return Iterator2()
+
+# error: [not-iterable]
+for x in Iterable1():
+    reveal_type(x)  # revealed: int
+
+# error: [not-iterable]
+for y in Iterable2():
+    reveal_type(y)  # revealed: Unknown
+```
+
+## Possibly unbound `__iter__` and bad `__getitem__` method
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+def _(flag: bool):
+    class Iterator:
+        def __next__(self) -> int:
+            return 42
+
+    class Iterable:
+        if flag:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+        # invalid signature because it only accepts a `str`,
+        # but the old-style iteration protocol will pass it an `int`
+        def __getitem__(self, key: str) -> bytes:
+            return 42
+
+    # error: [not-iterable]
+    for x in Iterable():
+        reveal_type(x)  # revealed: int | bytes
+```
+
+## Possibly unbound `__iter__` and not-callable `__getitem__`
+
+This snippet tests that we infer the element type correctly in the following edge case:
+
+- `__iter__` is a method with the correct parameter spec that returns a valid iterator; BUT
+- `__iter__` is possibly unbound; AND
+- `__getitem__` is set to a non-callable type
+
+It's important that we emit a diagnostic here, but it's also important that we still use the return
+type of the iterator's `__next__` method as the inferred type of `x` in the `for` loop:
+
+```py
+def _(flag: bool):
+    class Iterator:
+        def __next__(self) -> int:
+            return 42
+
+    class Iterable:
+        if flag:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+        __getitem__: None = None
+
+    # error: [not-iterable] "Object of type `Iterable` may not be iterable because it may not have an `__iter__` method and its `__getitem__` attribute has type `None`, which is not callable"
+    for x in Iterable():
+        reveal_type(x)  # revealed: int
+```
+
+## Possibly unbound `__iter__` and possibly unbound `__getitem__`
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Iterator:
+    def __next__(self) -> int:
+        return 42
+
+def _(flag1: bool, flag2: bool):
+    class Iterable:
+        if flag1:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+        if flag2:
+            def __getitem__(self, key: int) -> bytes:
+                return 42
+
+    # error: [not-iterable]
+    for x in Iterable():
+        reveal_type(x)  # revealed: int | bytes
+```
+
+## No `__iter__` method and `__getitem__` is not callable
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Bad:
+    __getitem__: None = None
+
+# error: [not-iterable]
+for x in Bad():
+    reveal_type(x)  # revealed: Unknown
+```
+
+## Possibly-not-callable `__getitem__` method
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+def _(flag: bool):
+    class CustomCallable:
+        if flag:
+            def __call__(self, *args, **kwargs) -> int:
+                return 42
+        else:
+            __call__: None = None
+
+    class Iterable1:
+        __getitem__: CustomCallable = CustomCallable()
+
+    class Iterable2:
+        if flag:
+            def __getitem__(self, key: int) -> int:
+                return 42
+        else:
+            __getitem__: None = None
+
+    # error: [not-iterable]
+    for x in Iterable1():
+        # TODO... `int` might be ideal here?
+        reveal_type(x)  # revealed: int | Unknown
+
+    # error: [not-iterable]
+    for y in Iterable2():
+        # TODO... `int` might be ideal here?
+        reveal_type(y)  # revealed: int | Unknown
+```
+
+## Bad `__getitem__` method
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Iterable:
+    # invalid because it will implicitly be passed an `int`
+    # by the interpreter
+    def __getitem__(self, key: str) -> int:
+        return 42
+
+# error: [not-iterable]
+for x in Iterable():
+    reveal_type(x)  # revealed: int
+```
+
+## Possibly unbound `__iter__` but definitely bound `__getitem__`
+
+Here, we should not emit a diagnostic: if `__iter__` is unbound, we should fallback to
+`__getitem__`:
+
+```py
+class Iterator:
+    def __next__(self) -> str:
+        return "foo"
+
+def _(flag: bool):
+    class Iterable:
+        if flag:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+
+        def __getitem__(self, key: int) -> bytes:
+            return b"foo"
+
+    for x in Iterable():
+        reveal_type(x)  # revealed: str | bytes
+```
+
+## Possibly invalid `__iter__` methods
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Iterator:
+    def __next__(self) -> int:
+        return 42
+
+def _(flag: bool):
+    class Iterable1:
+        if flag:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+        else:
+            def __iter__(self, invalid_extra_arg) -> Iterator:
+                return Iterator()
+
+    # error: [not-iterable]
+    for x in Iterable1():
+        reveal_type(x)  # revealed: int
+
+    class Iterable2:
+        if flag:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+        else:
+            __iter__: None = None
+
+    # error: [not-iterable]
+    for x in Iterable2():
+        # TODO: `int` would probably be better here:
+        reveal_type(x)  # revealed: int | Unknown
+```
+
+## Possibly invalid `__next__` method
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+def _(flag: bool):
+    class Iterator1:
+        if flag:
+            def __next__(self) -> int:
+                return 42
+        else:
+            def __next__(self, invalid_extra_arg) -> str:
+                return "foo"
+
+    class Iterator2:
+        if flag:
+            def __next__(self) -> int:
+                return 42
+        else:
+            __next__: None = None
+
+    class Iterable1:
+        def __iter__(self) -> Iterator1:
+            return Iterator1()
+
+    class Iterable2:
+        def __iter__(self) -> Iterator2:
+            return Iterator2()
+
+    # error: [not-iterable]
+    for x in Iterable1():
+        reveal_type(x)  # revealed: int | str
+
+    # error: [not-iterable]
+    for y in Iterable2():
+        # TODO: `int` would probably be better here:
+        reveal_type(y)  # revealed: int | Unknown
+```
+
+## Possibly invalid `__getitem__` methods
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+def _(flag: bool):
+    class Iterable1:
+        if flag:
+            def __getitem__(self, item: int) -> str:
+                return "foo"
+        else:
+            __getitem__: None = None
+
+    class Iterable2:
+        if flag:
+            def __getitem__(self, item: int) -> str:
+                return "foo"
+        else:
+            def __getitem__(self, item: str) -> int:
+                return "foo"
+
+    # error: [not-iterable]
+    for x in Iterable1():
+        # TODO: `str` might be better
+        reveal_type(x)  # revealed: str | Unknown
+
+    # error: [not-iterable]
+    for y in Iterable2():
+        reveal_type(y)  # revealed: str | int
+```
+
+## Possibly unbound `__iter__` and possibly invalid `__getitem__`
+
+<!-- snapshot-diagnostics -->
+
+```py
+from typing_extensions import reveal_type
+
+class Iterator:
+    def __next__(self) -> bytes:
+        return b"foo"
+
+def _(flag: bool, flag2: bool):
+    class Iterable1:
+        if flag:
+            def __getitem__(self, item: int) -> str:
+                return "foo"
+        else:
+            __getitem__: None = None
+
+        if flag2:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+
+    class Iterable2:
+        if flag:
+            def __getitem__(self, item: int) -> str:
+                return "foo"
+        else:
+            def __getitem__(self, item: str) -> int:
+                return "foo"
+        if flag2:
+            def __iter__(self) -> Iterator:
+                return Iterator()
+
+    # error: [not-iterable]
+    for x in Iterable1():
+        # TODO: `bytes | str` might be better
+        reveal_type(x)  # revealed: bytes | str | Unknown
+
+    # error: [not-iterable]
+    for y in Iterable2():
+        reveal_type(y)  # revealed: bytes | str | int
+```
+
+## Never is iterable
+
+```py
+from typing_extensions import Never
+
+def f(never: Never):
+    for x in never:
+        reveal_type(x)  # revealed: Never
+```

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

@@ -116,3 +116,14 @@ def _(flag: bool, flag2: bool):
     # error: [possibly-unresolved-reference]
     y
 ```
+
+## Condition with object that implements `__bool__` incorrectly
+
+```py
+class NotBoolable:
+    __bool__ = 3
+
+# error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable"
+while NotBoolable():
+    ...
+```

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

@@ -97,12 +97,7 @@ else:
 ## No narrowing for instances of `builtins.type`
 
 ```py
-def _(flag: bool):
-    t = type("t", (), {})
-
-    # This isn't testing what we want it to test if we infer anything more precise here:
-    reveal_type(t)  # revealed: type
-
+def _(flag: bool, t: type):
     x = 1 if flag else "foo"
 
     if isinstance(x, t):

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

@@ -112,8 +112,7 @@ def _(flag: bool):
         reveal_type(t)  # revealed: Literal[NoneType]
 
     if issubclass(t, type(None)):
-        # TODO: this should be just `Literal[NoneType]`
-        reveal_type(t)  # revealed: Literal[int, NoneType]
+        reveal_type(t)  # revealed: Literal[NoneType]
 ```
 
 ## `classinfo` contains multiple types

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

@@ -266,7 +266,7 @@ def _(
     if af:
         reveal_type(af)  # revealed: type[AmbiguousClass] & ~AlwaysFalsy
 
-    # TODO: Emit a diagnostic (`d` is not valid in boolean context)
+    # error: [unsupported-bool-conversion] "Boolean conversion is unsupported for type `MetaDeferred`; the return type of its bool method (`MetaAmbiguous`) isn't assignable to `bool"
     if d:
         # TODO: Should be `Unknown`
         reveal_type(d)  # revealed: type[DeferredClass] & ~AlwaysFalsy

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

@@ -341,10 +341,12 @@ annotation are looked up lazily, even if they occur in an eager scope.
 ### Eager annotations in a Python file
 
 ```py
+from typing import ClassVar
+
 x = int
 
 class C:
-    var: x
+    var: ClassVar[x]
 
 reveal_type(C.var)  # revealed: int
 
@@ -356,10 +358,12 @@ x = str
 ```py
 from __future__ import annotations
 
+from typing import ClassVar
+
 x = int
 
 class C:
-    var: x
+    var: ClassVar[x]
 
 reveal_type(C.var)  # revealed: Unknown | str
 
@@ -369,10 +373,12 @@ x = str
 ### Deferred annotations in a stub file
 
 ```pyi
+from typing import ClassVar
+
 x = int
 
 class C:
-    var: x
+    var: ClassVar[x]
 
 reveal_type(C.var)  # revealed: Unknown | str
 

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

@@ -136,3 +136,42 @@ if returns_bool():
 reveal_type(__file__)  # revealed: Literal[42]
 reveal_type(__name__)  # revealed: Literal[1] | str
 ```
+
+## Implicit global attributes in the current module override implicit globals from builtins
+
+Here, we take the type of the implicit global symbol `__name__` from the `types.ModuleType` stub
+(which in this custom typeshed specifies the type as `bytes`). This is because the `main` module has
+an implicit `__name__` global that shadows the builtin `__name__` symbol.
+
+```toml
+[environment]
+typeshed = "/typeshed"
+```
+
+`/typeshed/stdlib/builtins.pyi`:
+
+```pyi
+class int: ...
+class bytes: ...
+
+__name__: int = 42
+```
+
+`/typeshed/stdlib/types.pyi`:
+
+```pyi
+class ModuleType:
+    __name__: bytes
+```
+
+`/typeshed/stdlib/typing_extensions.pyi`:
+
+```pyi
+def reveal_type(obj, /): ...
+```
+
+`main.py`:
+
+```py
+reveal_type(__name__)  # revealed: bytes
+```

crates/red_knot_python_semantic/resources/mdtest/slots.md

@@ -167,7 +167,7 @@ class A:
     __slots__ = ()
     __slots__ += ("a", "b")
 
-reveal_type(A.__slots__)  # revealed: @Todo(return type)
+reveal_type(A.__slots__)  # revealed: @Todo(return type of decorated function)
 
 class B:
     __slots__ = ("c", "d")

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Bad_`__getitem__`_method.snap

@@ -0,0 +1,52 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Bad `__getitem__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class Iterable:
+ 4 |     # invalid because it will implicitly be passed an `int`
+ 5 |     # by the interpreter
+ 6 |     def __getitem__(self, key: str) -> int:
+ 7 |         return 42
+ 8 | 
+ 9 | # error: [not-iterable]
+10 | for x in Iterable():
+11 |     reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:10:10
+   |
+ 9 | # error: [not-iterable]
+10 | for x in Iterable():
+   |          ^^^^^^^^^^ Object of type `Iterable` is not iterable because it has no `__iter__` method and its `__getitem__` method has an incorrect signature for the old-style iteration protocol (expected a signature at least as permissive as `def __getitem__(self, key: int): ...`)
+11 |     reveal_type(x)  # revealed: int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:11:5
+   |
+ 9 | # error: [not-iterable]
+10 | for x in Iterable():
+11 |     reveal_type(x)  # revealed: int
+   |     -------------- info: Revealed type is `int`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Invalid_iterable.snap

@@ -0,0 +1,32 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Invalid iterable
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | nonsense = 123
+2 | for x in nonsense:  # error: [not-iterable]
+3 |     pass
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+ --> /src/mdtest_snippet.py:2:10
+  |
+1 | nonsense = 123
+2 | for x in nonsense:  # error: [not-iterable]
+  |          ^^^^^^^^ Object of type `Literal[123]` is not iterable because it doesn't have an `__iter__` method or a `__getitem__` method
+3 |     pass
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_New_over_old_style_iteration_protocol.snap

@@ -0,0 +1,37 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - New over old style iteration protocol
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | class NotIterable:
+2 |     def __getitem__(self, key: int) -> int:
+3 |         return 42
+4 |     __iter__: None = None
+5 | 
+6 | for x in NotIterable():  # error: [not-iterable]
+7 |     pass
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+ --> /src/mdtest_snippet.py:6:10
+  |
+4 |     __iter__: None = None
+5 |
+6 | for x in NotIterable():  # error: [not-iterable]
+  |          ^^^^^^^^^^^^^ Object of type `NotIterable` is not iterable because its `__iter__` attribute has type `None`, which is not callable
+7 |     pass
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_No_`__iter__`_method_and_`__getitem__`_is_not_callable.snap

@@ -0,0 +1,49 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - No `__iter__` method and `__getitem__` is not callable
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from typing_extensions import reveal_type
+2 | 
+3 | class Bad:
+4 |     __getitem__: None = None
+5 | 
+6 | # error: [not-iterable]
+7 | for x in Bad():
+8 |     reveal_type(x)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+ --> /src/mdtest_snippet.py:7:10
+  |
+6 | # error: [not-iterable]
+7 | for x in Bad():
+  |          ^^^^^ Object of type `Bad` is not iterable because it has no `__iter__` method and its `__getitem__` attribute has type `None`, which is not callable
+8 |     reveal_type(x)  # revealed: Unknown
+  |
+
+```
+
+```
+info: revealed-type
+ --> /src/mdtest_snippet.py:8:5
+  |
+6 | # error: [not-iterable]
+7 | for x in Bad():
+8 |     reveal_type(x)  # revealed: Unknown
+  |     -------------- info: Revealed type is `Unknown`
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly-not-callable_`__getitem__`_method.snap

@@ -0,0 +1,98 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly-not-callable `__getitem__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | def _(flag: bool):
+ 4 |     class CustomCallable:
+ 5 |         if flag:
+ 6 |             def __call__(self, *args, **kwargs) -> int:
+ 7 |                 return 42
+ 8 |         else:
+ 9 |             __call__: None = None
+10 | 
+11 |     class Iterable1:
+12 |         __getitem__: CustomCallable = CustomCallable()
+13 | 
+14 |     class Iterable2:
+15 |         if flag:
+16 |             def __getitem__(self, key: int) -> int:
+17 |                 return 42
+18 |         else:
+19 |             __getitem__: None = None
+20 | 
+21 |     # error: [not-iterable]
+22 |     for x in Iterable1():
+23 |         # TODO... `int` might be ideal here?
+24 |         reveal_type(x)  # revealed: int | Unknown
+25 | 
+26 |     # error: [not-iterable]
+27 |     for y in Iterable2():
+28 |         # TODO... `int` might be ideal here?
+29 |         reveal_type(y)  # revealed: int | Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:22:14
+   |
+21 |     # error: [not-iterable]
+22 |     for x in Iterable1():
+   |              ^^^^^^^^^^^ Object of type `Iterable1` may not be iterable because it has no `__iter__` method and its `__getitem__` attribute (with type `CustomCallable`) may not be callable
+23 |         # TODO... `int` might be ideal here?
+24 |         reveal_type(x)  # revealed: int | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:24:9
+   |
+22 |     for x in Iterable1():
+23 |         # TODO... `int` might be ideal here?
+24 |         reveal_type(x)  # revealed: int | Unknown
+   |         -------------- info: Revealed type is `int | Unknown`
+25 |
+26 |     # error: [not-iterable]
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:27:14
+   |
+26 |     # error: [not-iterable]
+27 |     for y in Iterable2():
+   |              ^^^^^^^^^^^ Object of type `Iterable2` may not be iterable because it has no `__iter__` method and its `__getitem__` attribute (with type `<bound method `__getitem__` of `Iterable2`> | None`) may not be callable
+28 |         # TODO... `int` might be ideal here?
+29 |         reveal_type(y)  # revealed: int | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:29:9
+   |
+27 |     for y in Iterable2():
+28 |         # TODO... `int` might be ideal here?
+29 |         reveal_type(y)  # revealed: int | Unknown
+   |         -------------- info: Revealed type is `int | Unknown`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_invalid_`__getitem__`_methods.snap

@@ -0,0 +1,94 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly invalid `__getitem__` methods
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | def _(flag: bool):
+ 4 |     class Iterable1:
+ 5 |         if flag:
+ 6 |             def __getitem__(self, item: int) -> str:
+ 7 |                 return "foo"
+ 8 |         else:
+ 9 |             __getitem__: None = None
+10 | 
+11 |     class Iterable2:
+12 |         if flag:
+13 |             def __getitem__(self, item: int) -> str:
+14 |                 return "foo"
+15 |         else:
+16 |             def __getitem__(self, item: str) -> int:
+17 |                 return "foo"
+18 | 
+19 |     # error: [not-iterable]
+20 |     for x in Iterable1():
+21 |         # TODO: `str` might be better
+22 |         reveal_type(x)  # revealed: str | Unknown
+23 | 
+24 |     # error: [not-iterable]
+25 |     for y in Iterable2():
+26 |         reveal_type(y)  # revealed: str | int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:20:14
+   |
+19 |     # error: [not-iterable]
+20 |     for x in Iterable1():
+   |              ^^^^^^^^^^^ Object of type `Iterable1` may not be iterable because it has no `__iter__` method and its `__getitem__` attribute (with type `<bound method `__getitem__` of `Iterable1`> | None`) may not be callable
+21 |         # TODO: `str` might be better
+22 |         reveal_type(x)  # revealed: str | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:22:9
+   |
+20 |     for x in Iterable1():
+21 |         # TODO: `str` might be better
+22 |         reveal_type(x)  # revealed: str | Unknown
+   |         -------------- info: Revealed type is `str | Unknown`
+23 |
+24 |     # error: [not-iterable]
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:25:14
+   |
+24 |     # error: [not-iterable]
+25 |     for y in Iterable2():
+   |              ^^^^^^^^^^^ Object of type `Iterable2` may not be iterable because it has no `__iter__` method and its `__getitem__` method (with type `<bound method `__getitem__` of `Iterable2`> | <bound method `__getitem__` of `Iterable2`>`) may have an incorrect signature for the old-style iteration protocol (expected a signature at least as permissive as `def __getitem__(self, key: int): ...`)
+26 |         reveal_type(y)  # revealed: str | int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:26:9
+   |
+24 |     # error: [not-iterable]
+25 |     for y in Iterable2():
+26 |         reveal_type(y)  # revealed: str | int
+   |         -------------- info: Revealed type is `str | int`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_invalid_`__iter__`_methods.snap

@@ -0,0 +1,98 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly invalid `__iter__` methods
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class Iterator:
+ 4 |     def __next__(self) -> int:
+ 5 |         return 42
+ 6 | 
+ 7 | def _(flag: bool):
+ 8 |     class Iterable1:
+ 9 |         if flag:
+10 |             def __iter__(self) -> Iterator:
+11 |                 return Iterator()
+12 |         else:
+13 |             def __iter__(self, invalid_extra_arg) -> Iterator:
+14 |                 return Iterator()
+15 | 
+16 |     # error: [not-iterable]
+17 |     for x in Iterable1():
+18 |         reveal_type(x)  # revealed: int
+19 | 
+20 |     class Iterable2:
+21 |         if flag:
+22 |             def __iter__(self) -> Iterator:
+23 |                 return Iterator()
+24 |         else:
+25 |             __iter__: None = None
+26 | 
+27 |     # error: [not-iterable]
+28 |     for x in Iterable2():
+29 |         # TODO: `int` would probably be better here:
+30 |         reveal_type(x)  # revealed: int | Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:17:14
+   |
+16 |     # error: [not-iterable]
+17 |     for x in Iterable1():
+   |              ^^^^^^^^^^^ Object of type `Iterable1` may not be iterable because its `__iter__` method (with type `<bound method `__iter__` of `Iterable1`> | <bound method `__iter__` of `Iterable1`>`) may have an invalid signature (expected `def __iter__(self): ...`)
+18 |         reveal_type(x)  # revealed: int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:18:9
+   |
+16 |     # error: [not-iterable]
+17 |     for x in Iterable1():
+18 |         reveal_type(x)  # revealed: int
+   |         -------------- info: Revealed type is `int`
+19 |
+20 |     class Iterable2:
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:28:14
+   |
+27 |     # error: [not-iterable]
+28 |     for x in Iterable2():
+   |              ^^^^^^^^^^^ Object of type `Iterable2` may not be iterable because its `__iter__` attribute (with type `<bound method `__iter__` of `Iterable2`> | None`) may not be callable
+29 |         # TODO: `int` would probably be better here:
+30 |         reveal_type(x)  # revealed: int | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:30:9
+   |
+28 |     for x in Iterable2():
+29 |         # TODO: `int` would probably be better here:
+30 |         reveal_type(x)  # revealed: int | Unknown
+   |         -------------- info: Revealed type is `int | Unknown`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_invalid_`__next__`_method.snap

@@ -0,0 +1,102 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly invalid `__next__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | def _(flag: bool):
+ 4 |     class Iterator1:
+ 5 |         if flag:
+ 6 |             def __next__(self) -> int:
+ 7 |                 return 42
+ 8 |         else:
+ 9 |             def __next__(self, invalid_extra_arg) -> str:
+10 |                 return "foo"
+11 | 
+12 |     class Iterator2:
+13 |         if flag:
+14 |             def __next__(self) -> int:
+15 |                 return 42
+16 |         else:
+17 |             __next__: None = None
+18 | 
+19 |     class Iterable1:
+20 |         def __iter__(self) -> Iterator1:
+21 |             return Iterator1()
+22 | 
+23 |     class Iterable2:
+24 |         def __iter__(self) -> Iterator2:
+25 |             return Iterator2()
+26 | 
+27 |     # error: [not-iterable]
+28 |     for x in Iterable1():
+29 |         reveal_type(x)  # revealed: int | str
+30 | 
+31 |     # error: [not-iterable]
+32 |     for y in Iterable2():
+33 |         # TODO: `int` would probably be better here:
+34 |         reveal_type(y)  # revealed: int | Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:28:14
+   |
+27 |     # error: [not-iterable]
+28 |     for x in Iterable1():
+   |              ^^^^^^^^^^^ Object of type `Iterable1` may not be iterable because its `__iter__` method returns an object of type `Iterator1`, which may have an invalid `__next__` method (expected `def __next__(self): ...`)
+29 |         reveal_type(x)  # revealed: int | str
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:29:9
+   |
+27 |     # error: [not-iterable]
+28 |     for x in Iterable1():
+29 |         reveal_type(x)  # revealed: int | str
+   |         -------------- info: Revealed type is `int | str`
+30 |
+31 |     # error: [not-iterable]
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:32:14
+   |
+31 |     # error: [not-iterable]
+32 |     for y in Iterable2():
+   |              ^^^^^^^^^^^ Object of type `Iterable2` may not be iterable because its `__iter__` method returns an object of type `Iterator2`, which has a `__next__` attribute that may not be callable
+33 |         # TODO: `int` would probably be better here:
+34 |         reveal_type(y)  # revealed: int | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:34:9
+   |
+32 |     for y in Iterable2():
+33 |         # TODO: `int` would probably be better here:
+34 |         reveal_type(y)  # revealed: int | Unknown
+   |         -------------- info: Revealed type is `int | Unknown`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_unbound_`__iter__`_and_bad_`__getitem__`_method.snap

@@ -0,0 +1,60 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly unbound `__iter__` and bad `__getitem__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | def _(flag: bool):
+ 4 |     class Iterator:
+ 5 |         def __next__(self) -> int:
+ 6 |             return 42
+ 7 | 
+ 8 |     class Iterable:
+ 9 |         if flag:
+10 |             def __iter__(self) -> Iterator:
+11 |                 return Iterator()
+12 |         # invalid signature because it only accepts a `str`,
+13 |         # but the old-style iteration protocol will pass it an `int`
+14 |         def __getitem__(self, key: str) -> bytes:
+15 |             return 42
+16 | 
+17 |     # error: [not-iterable]
+18 |     for x in Iterable():
+19 |         reveal_type(x)  # revealed: int | bytes
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:18:14
+   |
+17 |     # error: [not-iterable]
+18 |     for x in Iterable():
+   |              ^^^^^^^^^^ Object of type `Iterable` may not be iterable because it may not have an `__iter__` method and its `__getitem__` method has an incorrect signature for the old-style iteration protocol (expected a signature at least as permissive as `def __getitem__(self, key: int): ...`)
+19 |         reveal_type(x)  # revealed: int | bytes
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:19:9
+   |
+17 |     # error: [not-iterable]
+18 |     for x in Iterable():
+19 |         reveal_type(x)  # revealed: int | bytes
+   |         -------------- info: Revealed type is `int | bytes`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_unbound_`__iter__`_and_possibly_invalid_`__getitem__`.snap

@@ -0,0 +1,106 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly unbound `__iter__` and possibly invalid `__getitem__`
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class Iterator:
+ 4 |     def __next__(self) -> bytes:
+ 5 |         return b"foo"
+ 6 | 
+ 7 | def _(flag: bool, flag2: bool):
+ 8 |     class Iterable1:
+ 9 |         if flag:
+10 |             def __getitem__(self, item: int) -> str:
+11 |                 return "foo"
+12 |         else:
+13 |             __getitem__: None = None
+14 | 
+15 |         if flag2:
+16 |             def __iter__(self) -> Iterator:
+17 |                 return Iterator()
+18 | 
+19 |     class Iterable2:
+20 |         if flag:
+21 |             def __getitem__(self, item: int) -> str:
+22 |                 return "foo"
+23 |         else:
+24 |             def __getitem__(self, item: str) -> int:
+25 |                 return "foo"
+26 |         if flag2:
+27 |             def __iter__(self) -> Iterator:
+28 |                 return Iterator()
+29 | 
+30 |     # error: [not-iterable]
+31 |     for x in Iterable1():
+32 |         # TODO: `bytes | str` might be better
+33 |         reveal_type(x)  # revealed: bytes | str | Unknown
+34 | 
+35 |     # error: [not-iterable]
+36 |     for y in Iterable2():
+37 |         reveal_type(y)  # revealed: bytes | str | int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:31:14
+   |
+30 |     # error: [not-iterable]
+31 |     for x in Iterable1():
+   |              ^^^^^^^^^^^ Object of type `Iterable1` may not be iterable because it may not have an `__iter__` method and its `__getitem__` attribute (with type `<bound method `__getitem__` of `Iterable1`> | None`) may not be callable
+32 |         # TODO: `bytes | str` might be better
+33 |         reveal_type(x)  # revealed: bytes | str | Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:33:9
+   |
+31 |     for x in Iterable1():
+32 |         # TODO: `bytes | str` might be better
+33 |         reveal_type(x)  # revealed: bytes | str | Unknown
+   |         -------------- info: Revealed type is `bytes | str | Unknown`
+34 |
+35 |     # error: [not-iterable]
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:36:14
+   |
+35 |     # error: [not-iterable]
+36 |     for y in Iterable2():
+   |              ^^^^^^^^^^^ Object of type `Iterable2` may not be iterable because it may not have an `__iter__` method and its `__getitem__` method (with type `<bound method `__getitem__` of `Iterable2`> | <bound method `__getitem__` of `Iterable2`>`)
+                            may have an incorrect signature for the old-style iteration protocol (expected a signature at least as permissive as `def __getitem__(self, key: int): ...`)
+37 |         reveal_type(y)  # revealed: bytes | str | int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:37:9
+   |
+35 |     # error: [not-iterable]
+36 |     for y in Iterable2():
+37 |         reveal_type(y)  # revealed: bytes | str | int
+   |         -------------- info: Revealed type is `bytes | str | int`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Possibly_unbound_`__iter__`_and_possibly_unbound_`__getitem__`.snap

@@ -0,0 +1,59 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Possibly unbound `__iter__` and possibly unbound `__getitem__`
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class Iterator:
+ 4 |     def __next__(self) -> int:
+ 5 |         return 42
+ 6 | 
+ 7 | def _(flag1: bool, flag2: bool):
+ 8 |     class Iterable:
+ 9 |         if flag1:
+10 |             def __iter__(self) -> Iterator:
+11 |                 return Iterator()
+12 |         if flag2:
+13 |             def __getitem__(self, key: int) -> bytes:
+14 |                 return 42
+15 | 
+16 |     # error: [not-iterable]
+17 |     for x in Iterable():
+18 |         reveal_type(x)  # revealed: int | bytes
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:17:14
+   |
+16 |     # error: [not-iterable]
+17 |     for x in Iterable():
+   |              ^^^^^^^^^^ Object of type `Iterable` may not be iterable because it may not have an `__iter__` method or a `__getitem__` method
+18 |         reveal_type(x)  # revealed: int | bytes
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:18:9
+   |
+16 |     # error: [not-iterable]
+17 |     for x in Iterable():
+18 |         reveal_type(x)  # revealed: int | bytes
+   |         -------------- info: Revealed type is `int | bytes`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Union_type_as_iterable_where_one_union_element_has_invalid_`__iter__`_method.snap

@@ -0,0 +1,61 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Union type as iterable where one union element has invalid `__iter__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class TestIter:
+ 4 |     def __next__(self) -> int:
+ 5 |         return 42
+ 6 | 
+ 7 | class Test:
+ 8 |     def __iter__(self) -> TestIter:
+ 9 |         return TestIter()
+10 | 
+11 | class Test2:
+12 |     def __iter__(self) -> int:
+13 |         return 42
+14 | 
+15 | def _(flag: bool):
+16 |     # TODO: Improve error message to state which union variant isn't iterable (https://github.com/astral-sh/ruff/issues/13989)
+17 |     # error: [not-iterable]
+18 |     for x in Test() if flag else Test2():
+19 |         reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:18:14
+   |
+16 |     # TODO: Improve error message to state which union variant isn't iterable (https://github.com/astral-sh/ruff/issues/13989)
+17 |     # error: [not-iterable]
+18 |     for x in Test() if flag else Test2():
+   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Object of type `Test | Test2` may not be iterable because its `__iter__` method returns an object of type `TestIter | int`, which may not have a `__next__` method
+19 |         reveal_type(x)  # revealed: int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:19:9
+   |
+17 |     # error: [not-iterable]
+18 |     for x in Test() if flag else Test2():
+19 |         reveal_type(x)  # revealed: int
+   |         -------------- info: Revealed type is `int`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_Union_type_as_iterable_where_one_union_element_has_no_`__iter__`_method.snap

@@ -0,0 +1,56 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - Union type as iterable where one union element has no `__iter__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class TestIter:
+ 4 |     def __next__(self) -> int:
+ 5 |         return 42
+ 6 | 
+ 7 | class Test:
+ 8 |     def __iter__(self) -> TestIter:
+ 9 |         return TestIter()
+10 | 
+11 | def _(flag: bool):
+12 |     # error: [not-iterable]
+13 |     for x in Test() if flag else 42:
+14 |         reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:13:14
+   |
+11 | def _(flag: bool):
+12 |     # error: [not-iterable]
+13 |     for x in Test() if flag else 42:
+   |              ^^^^^^^^^^^^^^^^^^^^^^ Object of type `Test | Literal[42]` may not be iterable because it may not have an `__iter__` method and it doesn't have a `__getitem__` method
+14 |         reveal_type(x)  # revealed: int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:14:9
+   |
+12 |     # error: [not-iterable]
+13 |     for x in Test() if flag else 42:
+14 |         reveal_type(x)  # revealed: int
+   |         -------------- info: Revealed type is `int`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_With_non-callable_iterator.snap

@@ -0,0 +1,69 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - With non-callable iterator
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | def _(flag: bool):
+ 4 |     class NotIterable:
+ 5 |         if flag:
+ 6 |             __iter__: int = 1
+ 7 |         else:
+ 8 |             __iter__: None = None
+ 9 | 
+10 |     # error: [not-iterable]
+11 |     for x in NotIterable():
+12 |         pass
+13 | 
+14 |     # revealed: Unknown
+15 |     # error: [possibly-unresolved-reference]
+16 |     reveal_type(x)
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:11:14
+   |
+10 |     # error: [not-iterable]
+11 |     for x in NotIterable():
+   |              ^^^^^^^^^^^^^ Object of type `NotIterable` is not iterable because its `__iter__` attribute has type `int | None`, which is not callable
+12 |         pass
+   |
+
+```
+
+```
+warning: lint:possibly-unresolved-reference
+  --> /src/mdtest_snippet.py:16:17
+   |
+14 |     # revealed: Unknown
+15 |     # error: [possibly-unresolved-reference]
+16 |     reveal_type(x)
+   |                 - Name `x` used when possibly not defined
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:16:5
+   |
+14 |     # revealed: Unknown
+15 |     # error: [possibly-unresolved-reference]
+16 |     reveal_type(x)
+   |     -------------- info: Revealed type is `Unknown`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_`__iter__`_does_not_return_an_iterator.snap

@@ -0,0 +1,50 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - `__iter__` does not return an iterator
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from typing_extensions import reveal_type
+2 | 
+3 | class Bad:
+4 |     def __iter__(self) -> int:
+5 |         return 42
+6 | 
+7 | # error: [not-iterable]
+8 | for x in Bad():
+9 |     reveal_type(x)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+ --> /src/mdtest_snippet.py:8:10
+  |
+7 | # error: [not-iterable]
+8 | for x in Bad():
+  |          ^^^^^ Object of type `Bad` is not iterable because its `__iter__` method returns an object of type `int`, which has no `__next__` method
+9 |     reveal_type(x)  # revealed: Unknown
+  |
+
+```
+
+```
+info: revealed-type
+ --> /src/mdtest_snippet.py:9:5
+  |
+7 | # error: [not-iterable]
+8 | for x in Bad():
+9 |     reveal_type(x)  # revealed: Unknown
+  |     -------------- info: Revealed type is `Unknown`
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_`__iter__`_method_with_a_bad_signature.snap

@@ -0,0 +1,54 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - `__iter__` method with a bad signature
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class Iterator:
+ 4 |     def __next__(self) -> int:
+ 5 |         return 42
+ 6 | 
+ 7 | class Iterable:
+ 8 |     def __iter__(self, extra_arg) -> Iterator:
+ 9 |         return Iterator()
+10 | 
+11 | # error: [not-iterable]
+12 | for x in Iterable():
+13 |     reveal_type(x)  # revealed: int
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:12:10
+   |
+11 | # error: [not-iterable]
+12 | for x in Iterable():
+   |          ^^^^^^^^^^ Object of type `Iterable` is not iterable because its `__iter__` method has an invalid signature (expected `def __iter__(self): ...`)
+13 |     reveal_type(x)  # revealed: int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:13:5
+   |
+11 | # error: [not-iterable]
+12 | for x in Iterable():
+13 |     reveal_type(x)  # revealed: int
+   |     -------------- info: Revealed type is `int`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/for.md_-_For_loops_-_`__iter__`_returns_an_iterator_with_an_invalid_`__next__`_method.snap

@@ -0,0 +1,91 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: for.md - For loops - `__iter__` returns an iterator with an invalid `__next__` method
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/loops/for.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | from typing_extensions import reveal_type
+ 2 | 
+ 3 | class Iterator1:
+ 4 |     def __next__(self, extra_arg) -> int:
+ 5 |         return 42
+ 6 | 
+ 7 | class Iterator2:
+ 8 |     __next__: None = None
+ 9 | 
+10 | class Iterable1:
+11 |     def __iter__(self) -> Iterator1:
+12 |         return Iterator1()
+13 | 
+14 | class Iterable2:
+15 |     def __iter__(self) -> Iterator2:
+16 |         return Iterator2()
+17 | 
+18 | # error: [not-iterable]
+19 | for x in Iterable1():
+20 |     reveal_type(x)  # revealed: int
+21 | 
+22 | # error: [not-iterable]
+23 | for y in Iterable2():
+24 |     reveal_type(y)  # revealed: Unknown
+```
+
+# Diagnostics
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:19:10
+   |
+18 | # error: [not-iterable]
+19 | for x in Iterable1():
+   |          ^^^^^^^^^^^ Object of type `Iterable1` is not iterable because its `__iter__` method returns an object of type `Iterator1`, which has an invalid `__next__` method (expected `def __next__(self): ...`)
+20 |     reveal_type(x)  # revealed: int
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:20:5
+   |
+18 | # error: [not-iterable]
+19 | for x in Iterable1():
+20 |     reveal_type(x)  # revealed: int
+   |     -------------- info: Revealed type is `int`
+21 |
+22 | # error: [not-iterable]
+   |
+
+```
+
+```
+error: lint:not-iterable
+  --> /src/mdtest_snippet.py:23:10
+   |
+22 | # error: [not-iterable]
+23 | for y in Iterable2():
+   |          ^^^^^^^^^^^ Object of type `Iterable2` is not iterable because its `__iter__` method returns an object of type `Iterator2`, which has a `__next__` attribute that is not callable
+24 |     reveal_type(y)  # revealed: Unknown
+   |
+
+```
+
+```
+info: revealed-type
+  --> /src/mdtest_snippet.py:24:5
+   |
+22 | # error: [not-iterable]
+23 | for y in Iterable2():
+24 |     reveal_type(y)  # revealed: Unknown
+   |     -------------- info: Revealed type is `Unknown`
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/instances.md_-_Binary_operations_on_instances_-_Operations_involving_types_with_invalid_`__bool__`_methods.snap

@@ -0,0 +1,35 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: instances.md - Binary operations on instances - Operations involving types with invalid `__bool__` methods
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/binary/instances.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | class NotBoolable:
+2 |     __bool__ = 3
+3 | 
+4 | a = NotBoolable()
+5 | 
+6 | # error: [unsupported-bool-conversion]
+7 | 10 and a and True
+```
+
+# Diagnostics
+
+```
+error: lint:unsupported-bool-conversion
+ --> /src/mdtest_snippet.py:7:8
+  |
+6 | # error: [unsupported-bool-conversion]
+7 | 10 and a and True
+  |        ^ Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Calls_to_methods.snap

@@ -0,0 +1,41 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: invalid_argument_type.md - Invalid argument type diagnostics - Calls to methods
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/invalid_argument_type.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | class C:
+2 |     def square(self, x: int) -> int:
+3 |         return x * x
+4 | 
+5 | c = C()
+6 | c.square("hello")  # error: [invalid-argument-type]
+```
+
+# Diagnostics
+
+```
+error: lint:invalid-argument-type
+ --> /src/mdtest_snippet.py:6:10
+  |
+5 | c = C()
+6 | c.square("hello")  # error: [invalid-argument-type]
+  |          ^^^^^^^ Object of type `Literal["hello"]` cannot be assigned to parameter 2 (`x`) of bound method `square`; expected type `int`
+  |
+ ::: /src/mdtest_snippet.py:2:22
+  |
+1 | class C:
+2 |     def square(self, x: int) -> int:
+  |                      ------ info: parameter declared in function definition here
+3 |         return x * x
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/invalid_argument_type.md_-_Invalid_argument_type_diagnostics_-_Tests_for_a_variety_of_argument_types_-_Synthetic_arguments.snap

@@ -28,7 +28,7 @@ error: lint:invalid-argument-type
   |
 5 | c = C()
 6 | c("wrong")  # error: [invalid-argument-type]
-  |   ^^^^^^^ Object of type `Literal["wrong"]` cannot be assigned to parameter 2 (`x`) of function `__call__`; expected type `int`
+  |   ^^^^^^^ Object of type `Literal["wrong"]` cannot be assigned to parameter 2 (`x`) of bound method `__call__`; expected type `int`
   |
  ::: /src/mdtest_snippet.py:2:24
   |

crates/red_knot_python_semantic/resources/mdtest/snapshots/membership_test.md_-_Comparison___Membership_Test_-_Return_type_that_doesn't_implement_`__bool__`_correctly.snap

@@ -0,0 +1,53 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: membership_test.md - Comparison: Membership Test - Return type that doesn't implement `__bool__` correctly
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/comparison/instances/membership_test.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | class NotBoolable:
+ 2 |     __bool__ = 3
+ 3 | 
+ 4 | class WithContains:
+ 5 |     def __contains__(self, item) -> NotBoolable:
+ 6 |         return NotBoolable()
+ 7 | 
+ 8 | # error: [unsupported-bool-conversion]
+ 9 | 10 in WithContains()
+10 | # error: [unsupported-bool-conversion]
+11 | 10 not in WithContains()
+```
+
+# Diagnostics
+
+```
+error: lint:unsupported-bool-conversion
+  --> /src/mdtest_snippet.py:9:1
+   |
+ 8 | # error: [unsupported-bool-conversion]
+ 9 | 10 in WithContains()
+   | ^^^^^^^^^^^^^^^^^^^^ Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable
+10 | # error: [unsupported-bool-conversion]
+11 | 10 not in WithContains()
+   |
+
+```
+
+```
+error: lint:unsupported-bool-conversion
+  --> /src/mdtest_snippet.py:11:1
+   |
+ 9 | 10 in WithContains()
+10 | # error: [unsupported-bool-conversion]
+11 | 10 not in WithContains()
+   | ^^^^^^^^^^^^^^^^^^^^^^^^ Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/not.md_-_Unary_not_-_Object_that_implements_`__bool__`_incorrectly.snap

@@ -0,0 +1,33 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: not.md - Unary not - Object that implements `__bool__` incorrectly
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/unary/not.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | class NotBoolable:
+2 |     __bool__ = 3
+3 | 
+4 | # error: [unsupported-bool-conversion]
+5 | not NotBoolable()
+```
+
+# Diagnostics
+
+```
+error: lint:unsupported-bool-conversion
+ --> /src/mdtest_snippet.py:5:1
+  |
+4 | # error: [unsupported-bool-conversion]
+5 | not NotBoolable()
+  | ^^^^^^^^^^^^^^^^^ Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/rich_comparison.md_-_Comparison___Rich_Comparison_-_Chained_comparisons_with_objects_that_don't_implement_`__bool__`_correctly.snap

@@ -0,0 +1,60 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: rich_comparison.md - Comparison: Rich Comparison - Chained comparisons with objects that don't implement `__bool__` correctly
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/comparison/instances/rich_comparison.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | class NotBoolable:
+ 2 |     __bool__ = 3
+ 3 | 
+ 4 | class Comparable:
+ 5 |     def __lt__(self, item) -> NotBoolable:
+ 6 |         return NotBoolable()
+ 7 | 
+ 8 |     def __gt__(self, item) -> NotBoolable:
+ 9 |         return NotBoolable()
+10 | 
+11 | # error: [unsupported-bool-conversion]
+12 | 10 < Comparable() < 20
+13 | # error: [unsupported-bool-conversion]
+14 | 10 < Comparable() < Comparable()
+15 | 
+16 | Comparable() < Comparable()  # fine
+```
+
+# Diagnostics
+
+```
+error: lint:unsupported-bool-conversion
+  --> /src/mdtest_snippet.py:12:1
+   |
+11 | # error: [unsupported-bool-conversion]
+12 | 10 < Comparable() < 20
+   | ^^^^^^^^^^^^^^^^^ Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable
+13 | # error: [unsupported-bool-conversion]
+14 | 10 < Comparable() < Comparable()
+   |
+
+```
+
+```
+error: lint:unsupported-bool-conversion
+  --> /src/mdtest_snippet.py:14:1
+   |
+12 | 10 < Comparable() < 20
+13 | # error: [unsupported-bool-conversion]
+14 | 10 < Comparable() < Comparable()
+   | ^^^^^^^^^^^^^^^^^ Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable
+15 |
+16 | Comparable() < Comparable()  # fine
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/tuples.md_-_Comparison___Tuples_-_Chained_comparisons_with_elements_that_incorrectly_implement_`__bool__`.snap

@@ -0,0 +1,47 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: tuples.md - Comparison: Tuples - Chained comparisons with elements that incorrectly implement `__bool__`
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+ 1 | class NotBoolable:
+ 2 |     __bool__ = 5
+ 3 | 
+ 4 | class Comparable:
+ 5 |     def __lt__(self, other) -> NotBoolable:
+ 6 |         return NotBoolable()
+ 7 | 
+ 8 |     def __gt__(self, other) -> NotBoolable:
+ 9 |         return NotBoolable()
+10 | 
+11 | a = (1, Comparable())
+12 | b = (1, Comparable())
+13 | 
+14 | # error: [unsupported-bool-conversion]
+15 | a < b < b
+16 | 
+17 | a < b  # fine
+```
+
+# Diagnostics
+
+```
+error: lint:unsupported-bool-conversion
+  --> /src/mdtest_snippet.py:15:1
+   |
+14 | # error: [unsupported-bool-conversion]
+15 | a < b < b
+   | ^^^^^ Boolean conversion is unsupported for type `NotBoolable | Literal[False]`; its `__bool__` method isn't callable
+16 |
+17 | a < b  # fine
+   |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/tuples.md_-_Comparison___Tuples_-_Equality_with_elements_that_incorrectly_implement_`__bool__`.snap

@@ -0,0 +1,37 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: tuples.md - Comparison: Tuples - Equality with elements that incorrectly implement `__bool__`
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/comparison/tuples.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | class A:
+2 |     def __eq__(self, other) -> NotBoolable:
+3 |         return NotBoolable()
+4 | 
+5 | class NotBoolable:
+6 |     __bool__ = None
+7 | 
+8 | # error: [unsupported-bool-conversion]
+9 | (A(),) == (A(),)
+```
+
+# Diagnostics
+
+```
+error: lint:unsupported-bool-conversion
+ --> /src/mdtest_snippet.py:9:1
+  |
+8 | # error: [unsupported-bool-conversion]
+9 | (A(),) == (A(),)
+  | ^^^^^^^^^^^^^^^^ Boolean conversion is unsupported for type `NotBoolable`; its `__bool__` method isn't callable
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/unpacking.md_-_Unpacking_-_Right_hand_side_not_iterable.snap

@@ -22,7 +22,7 @@ error: lint:not-iterable
  --> /src/mdtest_snippet.py:1:8
   |
 1 | a, b = 1  # error: [not-iterable]
-  |        ^ Object of type `Literal[1]` is not iterable
+  |        ^ Object of type `Literal[1]` is not iterable because it doesn't have an `__iter__` method or a `__getitem__` method
   |
 
 ```

crates/red_knot_python_semantic/resources/mdtest/stubs/class.md

@@ -15,3 +15,30 @@ class Bar(Foo[Bar]): ...
 reveal_type(Bar)  # revealed: Literal[Bar]
 reveal_type(Bar.__mro__)  # revealed: tuple[Literal[Bar], Unknown, Literal[object]]
 ```
+
+## Access to attributes declarated in stubs
+
+Unlike regular Python modules, stub files often omit the right-hand side in declarations, including
+in class scope. However, from the perspective of the type checker, we have to treat them as bindings
+too. That is, `symbol: type` is the same as `symbol: type = ...`.
+
+One implication of this is that we'll always treat symbols in class scope as safe to be accessed
+from the class object itself. We'll never infer a "pure instance attribute" from a stub.
+
+`b.pyi`:
+
+```pyi
+from typing import ClassVar
+
+class C:
+    class_or_instance_var: int
+```
+
+```py
+from typing import ClassVar, Literal
+
+from b import C
+
+# No error here, since we treat `class_or_instance_var` as bound on the class.
+reveal_type(C.class_or_instance_var)  # revealed: int
+```

crates/red_knot_python_semantic/resources/mdtest/stubs/locals.md

@@ -0,0 +1,17 @@
+# Declarations in stubs
+
+Unlike regular Python modules, stub files often declare module-global variables without initializing
+them. If these symbols are then used in the same stub, applying regular logic would lead to an
+undefined variable access error.
+
+However, from the perspective of the type checker, we should treat something like `symbol: type` the
+same as `symbol: type = ...`. In other words, assume these are bindings too.
+
+```pyi
+from typing import Literal
+
+CONSTANT: Literal[42]
+
+# No error here, even though the variable is not initialized.
+uses_constant: int = CONSTANT
+```

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

@@ -25,7 +25,7 @@ reveal_type(y)  # revealed: Unknown
 def _(n: int):
     a = b"abcde"[n]
     # TODO: Support overloads... Should be `bytes`
-    reveal_type(a)  # revealed: @Todo(return type)
+    reveal_type(a)  # revealed: @Todo(return type of decorated function)
 ```
 
 ## Slices
@@ -44,10 +44,10 @@ b[::0]  # error: [zero-stepsize-in-slice]
 def _(m: int, n: int):
     byte_slice1 = b[m:n]
     # TODO: Support overloads... Should be `bytes`
-    reveal_type(byte_slice1)  # revealed: @Todo(return type)
+    reveal_type(byte_slice1)  # revealed: @Todo(return type of decorated function)
 
 def _(s: bytes) -> bytes:
     byte_slice2 = s[0:5]
     # TODO: Support overloads... Should be `bytes`
-    reveal_type(byte_slice2)  # revealed: @Todo(return type)
+    reveal_type(byte_slice2)  # revealed: @Todo(return type of decorated function)
 ```

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

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

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

@@ -22,7 +22,7 @@ reveal_type(b)  # revealed: Unknown
 def _(n: int):
     a = "abcde"[n]
     # TODO: Support overloads... Should be `str`
-    reveal_type(a)  # revealed: @Todo(return type)
+    reveal_type(a)  # revealed: @Todo(return type of decorated function)
 ```
 
 ## Slices
@@ -76,11 +76,11 @@ def _(m: int, n: int, s2: str):
 
     substring1 = s[m:n]
     # TODO: Support overloads... Should be `LiteralString`
-    reveal_type(substring1)  # revealed: @Todo(return type)
+    reveal_type(substring1)  # revealed: @Todo(return type of decorated function)
 
     substring2 = s2[0:5]
     # TODO: Support overloads... Should be `str`
-    reveal_type(substring2)  # revealed: @Todo(return type)
+    reveal_type(substring2)  # revealed: @Todo(return type of decorated function)
 ```
 
 ## Unsupported slice types