Include scope kind in scope_count metric, add bar graph

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

.cargo/config.toml

@@ -8,3 +8,7 @@ benchmark = "bench -p ruff_benchmark --bench linter --bench formatter --"
 # See: https://github.com/astral-sh/ruff/issues/11503
 [target.'cfg(all(target_env="msvc", target_os = "windows"))']
 rustflags = ["-C", "target-feature=+crt-static"]
+
+[target.'wasm32-unknown-unknown']
+# See https://docs.rs/getrandom/latest/getrandom/#webassembly-support
+rustflags = ["--cfg", 'getrandom_backend="wasm_js"']

.github/workflows/ci.yaml

@@ -280,7 +280,7 @@ jobs:
 
   cargo-build-msrv:
     name: "cargo build (msrv)"
-    runs-on: ubuntu-latest
+    runs-on: depot-ubuntu-latest-8
     needs: determine_changes
     if: ${{ !contains(github.event.pull_request.labels.*.name, 'no-test') && (needs.determine_changes.outputs.code == 'true' || github.ref == 'refs/heads/main') }}
     timeout-minutes: 20
@@ -430,7 +430,7 @@ jobs:
           name: ruff
           path: target/debug
 
-      - uses: dawidd6/action-download-artifact@v7
+      - uses: dawidd6/action-download-artifact@v8
         name: Download baseline Ruff binary
         with:
           name: ruff

.github/workflows/pr-comment.yaml

@@ -16,7 +16,7 @@ jobs:
     permissions:
       pull-requests: write
     steps:
-      - uses: dawidd6/action-download-artifact@v7
+      - uses: dawidd6/action-download-artifact@v8
         name: Download pull request number
         with:
           name: pr-number
@@ -32,7 +32,7 @@ jobs:
             echo "pr-number=$(<pr-number)" >> "$GITHUB_OUTPUT"
           fi
 
-      - uses: dawidd6/action-download-artifact@v7
+      - uses: dawidd6/action-download-artifact@v8
         name: "Download ecosystem results"
         id: download-ecosystem-result
         if: steps.pr-number.outputs.pr-number

.gitignore

@@ -30,7 +30,7 @@ tracing-flamechart.svg
 tracing-flamegraph.svg
 
 # insta
-.rs.pending-snap
+*.rs.pending-snap
 
 
 ###

.pre-commit-config.yaml

@@ -5,6 +5,7 @@ exclude: |
     .github/workflows/release.yml|
     crates/red_knot_vendored/vendor/.*|
     crates/red_knot_project/resources/.*|
+    crates/ruff_benchmark/resources/.*|
     crates/ruff_linter/resources/.*|
     crates/ruff_linter/src/rules/.*/snapshots/.*|
     crates/ruff_notebook/resources/.*|
@@ -23,7 +24,7 @@ repos:
       - id: validate-pyproject
 
   - repo: https://github.com/executablebooks/mdformat
-    rev: 0.7.21
+    rev: 0.7.22
     hooks:
       - id: mdformat
         additional_dependencies:
@@ -36,7 +37,7 @@ repos:
           )$
 
   - repo: https://github.com/igorshubovych/markdownlint-cli
-    rev: v0.43.0
+    rev: v0.44.0
     hooks:
       - id: markdownlint-fix
         exclude: |
@@ -56,10 +57,10 @@ repos:
             .*?invalid(_.+)*_syntax\.md
           )$
         additional_dependencies:
-          - black==24.10.0
+          - black==25.1.0
 
   - repo: https://github.com/crate-ci/typos
-    rev: v1.29.4
+    rev: v1.29.5
     hooks:
       - id: typos
 
@@ -73,7 +74,7 @@ repos:
         pass_filenames: false # This makes it a lot faster
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.9.2
+    rev: v0.9.4
     hooks:
       - id: ruff-format
       - id: ruff
@@ -91,12 +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: v1.2.2
+    rev: v1.3.0
     hooks:
       - id: zizmor
 
   - repo: https://github.com/python-jsonschema/check-jsonschema
-    rev: 0.31.0
+    rev: 0.31.1
     hooks:
       - id: check-github-workflows
 

CHANGELOG.md

@@ -1,5 +1,43 @@
 # Changelog
 
+## 0.9.4
+
+### Preview features
+
+- \[`airflow`\] Extend airflow context parameter check for `BaseOperator.execute` (`AIR302`) ([#15713](https://github.com/astral-sh/ruff/pull/15713))
+- \[`airflow`\] Update `AIR302` to check for deprecated context keys ([#15144](https://github.com/astral-sh/ruff/pull/15144))
+- \[`flake8-bandit`\] Permit suspicious imports within stub files (`S4`) ([#15822](https://github.com/astral-sh/ruff/pull/15822))
+- \[`pylint`\] Do not trigger `PLR6201` on empty collections ([#15732](https://github.com/astral-sh/ruff/pull/15732))
+- \[`refurb`\] Do not emit diagnostic when loop variables are used outside loop body (`FURB122`) ([#15757](https://github.com/astral-sh/ruff/pull/15757))
+- \[`ruff`\] Add support for more `re` patterns (`RUF055`) ([#15764](https://github.com/astral-sh/ruff/pull/15764))
+- \[`ruff`\] Check for shadowed `map` before suggesting fix (`RUF058`) ([#15790](https://github.com/astral-sh/ruff/pull/15790))
+- \[`ruff`\] Do not emit diagnostic when all arguments to `zip()` are variadic (`RUF058`) ([#15744](https://github.com/astral-sh/ruff/pull/15744))
+- \[`ruff`\] Parenthesize fix when argument spans multiple lines for `unnecessary-round` (`RUF057`) ([#15703](https://github.com/astral-sh/ruff/pull/15703))
+
+### Rule changes
+
+- Preserve quote style in generated code ([#15726](https://github.com/astral-sh/ruff/pull/15726), [#15778](https://github.com/astral-sh/ruff/pull/15778), [#15794](https://github.com/astral-sh/ruff/pull/15794))
+- \[`flake8-bugbear`\] Exempt `NewType` calls where the original type is immutable (`B008`) ([#15765](https://github.com/astral-sh/ruff/pull/15765))
+- \[`pylint`\] Honor banned top-level imports by `TID253` in `PLC0415`. ([#15628](https://github.com/astral-sh/ruff/pull/15628))
+- \[`pyupgrade`\] Ignore `is_typeddict` and `TypedDict` for `deprecated-import` (`UP035`) ([#15800](https://github.com/astral-sh/ruff/pull/15800))
+
+### CLI
+
+- Fix formatter warning message for `flake8-quotes` option ([#15788](https://github.com/astral-sh/ruff/pull/15788))
+- Implement tab autocomplete for `ruff config` ([#15603](https://github.com/astral-sh/ruff/pull/15603))
+
+### Bug fixes
+
+- \[`flake8-comprehensions`\] Do not emit `unnecessary-map` diagnostic when lambda has different arity (`C417`) ([#15802](https://github.com/astral-sh/ruff/pull/15802))
+- \[`flake8-comprehensions`\] Parenthesize `sorted` when needed for `unnecessary-call-around-sorted` (`C413`) ([#15825](https://github.com/astral-sh/ruff/pull/15825))
+- \[`pyupgrade`\] Handle end-of-line comments for `quoted-annotation` (`UP037`) ([#15824](https://github.com/astral-sh/ruff/pull/15824))
+
+### Documentation
+
+- Add missing config docstrings ([#15803](https://github.com/astral-sh/ruff/pull/15803))
+- Add references to `trio.run_process` and `anyio.run_process` ([#15761](https://github.com/astral-sh/ruff/pull/15761))
+- Use `uv init --lib` in tutorial ([#15718](https://github.com/astral-sh/ruff/pull/15718))
+
 ## 0.9.3
 
 ### Preview features

Cargo.toml

@@ -22,6 +22,7 @@ ruff_graph = { path = "crates/ruff_graph" }
 ruff_index = { path = "crates/ruff_index" }
 ruff_linter = { path = "crates/ruff_linter" }
 ruff_macros = { path = "crates/ruff_macros" }
+ruff_metrics = { path = "crates/ruff_metrics" }
 ruff_notebook = { path = "crates/ruff_notebook" }
 ruff_python_ast = { path = "crates/ruff_python_ast" }
 ruff_python_codegen = { path = "crates/ruff_python_codegen" }
@@ -74,11 +75,13 @@ env_logger = { version = "0.11.0" }
 etcetera = { version = "0.8.0" }
 fern = { version = "0.7.0" }
 filetime = { version = "0.2.23" }
+getrandom = { version = "0.3.1" }
 glob = { version = "0.3.1" }
 globset = { version = "0.4.14" }
 globwalk = { version = "0.9.1" }
 hashbrown = { version = "0.15.0", default-features = false, features = [
     "raw-entry",
+    "equivalent",
     "inline-more",
 ] }
 ignore = { version = "0.4.22" }
@@ -103,6 +106,8 @@ lsp-types = { git = "https://github.com/astral-sh/lsp-types.git", rev = "3512a9f
 ] }
 matchit = { version = "0.8.1" }
 memchr = { version = "2.7.1" }
+metrics = { version = "0.24.1" }
+metrics-util = { version = "0.19.0" }
 mimalloc = { version = "0.1.39" }
 natord = { version = "1.0.9" }
 notify = { version = "8.0.0" }
@@ -116,7 +121,7 @@ proc-macro2 = { version = "1.0.79" }
 pyproject-toml = { version = "0.13.4" }
 quick-junit = { version = "0.5.0" }
 quote = { version = "1.0.23" }
-rand = { version = "0.8.5" }
+rand = { version = "0.9.0" }
 rayon = { version = "1.10.0" }
 regex = { version = "1.10.2" }
 rustc-hash = { version = "2.0.0" }
@@ -134,7 +139,12 @@ serde_with = { version = "3.6.0", default-features = false, features = [
 shellexpand = { version = "3.0.0" }
 similar = { version = "2.4.0", features = ["inline"] }
 smallvec = { version = "1.13.2" }
-snapbox = { version = "0.6.0", features = ["diff", "term-svg", "cmd", "examples"] }
+snapbox = { version = "0.6.0", features = [
+    "diff",
+    "term-svg",
+    "cmd",
+    "examples",
+] }
 static_assertions = "1.1.0"
 strum = { version = "0.26.0", features = ["strum_macros"] }
 strum_macros = { version = "0.26.0" }
@@ -159,7 +169,6 @@ unicode-ident = { version = "1.0.12" }
 unicode-width = { version = "0.2.0" }
 unicode_names2 = { version = "1.2.2" }
 unicode-normalization = { version = "0.1.23" }
-ureq = { version = "2.9.6" }
 url = { version = "2.5.0" }
 uuid = { version = "1.6.1", features = [
     "v4",
@@ -173,6 +182,10 @@ wasm-bindgen-test = { version = "0.3.42" }
 wild = { version = "2" }
 zip = { version = "0.6.6", default-features = false }
 
+[workspace.metadata.cargo-shear]
+ignored = ["getrandom"]
+
+
 [workspace.lints.rust]
 unsafe_code = "warn"
 unreachable_pub = "warn"
@@ -305,7 +318,11 @@ local-artifacts-jobs = ["./build-binaries", "./build-docker"]
 # Publish jobs to run in CI
 publish-jobs = ["./publish-pypi", "./publish-wasm"]
 # Post-announce jobs to run in CI
-post-announce-jobs = ["./notify-dependents", "./publish-docs", "./publish-playground"]
+post-announce-jobs = [
+    "./notify-dependents",
+    "./publish-docs",
+    "./publish-playground",
+]
 # Custom permissions for GitHub Jobs
 github-custom-job-permissions = { "build-docker" = { packages = "write", contents = "read" }, "publish-wasm" = { contents = "read", id-token = "write", packages = "write" } }
 # Whether to install an updater program

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/0.9.3/install.sh | sh
-powershell -c "irm https://astral.sh/ruff/0.9.3/install.ps1 | iex"
+curl -LsSf https://astral.sh/ruff/0.9.4/install.sh | sh
+powershell -c "irm https://astral.sh/ruff/0.9.4/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: v0.9.3
+  rev: v0.9.4
   hooks:
     # Run the linter.
     - id: ruff

crates/red_knot/Cargo.toml

@@ -16,6 +16,7 @@ red_knot_python_semantic = { workspace = true }
 red_knot_project = { workspace = true, features = ["zstd"] }
 red_knot_server = { workspace = true }
 ruff_db = { workspace = true, features = ["os", "cache"] }
+ruff_metrics = { workspace = true }
 
 anyhow = { workspace = true }
 chrono = { workspace = true }
@@ -24,6 +25,7 @@ colored = { workspace = true }
 countme = { workspace = true, features = ["enable"] }
 crossbeam = { workspace = true }
 ctrlc = { version = "3.4.4" }
+metrics = { workspace = true }
 rayon = { workspace = true }
 salsa = { workspace = true }
 tracing = { workspace = true, features = ["release_max_level_debug"] }

crates/red_knot/build.rs

@@ -0,0 +1,104 @@
+use std::{
+    fs,
+    path::{Path, PathBuf},
+    process::Command,
+};
+
+fn main() {
+    // The workspace root directory is not available without walking up the tree
+    // https://github.com/rust-lang/cargo/issues/3946
+    let workspace_root = Path::new(&std::env::var("CARGO_MANIFEST_DIR").unwrap())
+        .join("..")
+        .join("..");
+
+    commit_info(&workspace_root);
+
+    #[allow(clippy::disallowed_methods)]
+    let target = std::env::var("TARGET").unwrap();
+    println!("cargo::rustc-env=RUST_HOST_TARGET={target}");
+}
+
+fn commit_info(workspace_root: &Path) {
+    // If not in a git repository, do not attempt to retrieve commit information
+    let git_dir = workspace_root.join(".git");
+    if !git_dir.exists() {
+        return;
+    }
+
+    if let Some(git_head_path) = git_head(&git_dir) {
+        println!("cargo:rerun-if-changed={}", git_head_path.display());
+
+        let git_head_contents = fs::read_to_string(git_head_path);
+        if let Ok(git_head_contents) = git_head_contents {
+            // The contents are either a commit or a reference in the following formats
+            // - "<commit>" when the head is detached
+            // - "ref <ref>" when working on a branch
+            // If a commit, checking if the HEAD file has changed is sufficient
+            // If a ref, we need to add the head file for that ref to rebuild on commit
+            let mut git_ref_parts = git_head_contents.split_whitespace();
+            git_ref_parts.next();
+            if let Some(git_ref) = git_ref_parts.next() {
+                let git_ref_path = git_dir.join(git_ref);
+                println!("cargo:rerun-if-changed={}", git_ref_path.display());
+            }
+        }
+    }
+
+    let output = match Command::new("git")
+        .arg("log")
+        .arg("-1")
+        .arg("--date=short")
+        .arg("--abbrev=9")
+        .arg("--format=%H %h %cd %(describe)")
+        .output()
+    {
+        Ok(output) if output.status.success() => output,
+        _ => return,
+    };
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    let mut parts = stdout.split_whitespace();
+    let mut next = || parts.next().unwrap();
+    let _commit_hash = next();
+    println!("cargo::rustc-env=RED_KNOT_COMMIT_SHORT_HASH={}", next());
+    println!("cargo::rustc-env=RED_KNOT_COMMIT_DATE={}", next());
+
+    // Describe can fail for some commits
+    // https://git-scm.com/docs/pretty-formats#Documentation/pretty-formats.txt-emdescribeoptionsem
+    if let Some(describe) = parts.next() {
+        let mut describe_parts = describe.split('-');
+        let _last_tag = describe_parts.next().unwrap();
+
+        // If this is the tagged commit, this component will be missing
+        println!(
+            "cargo::rustc-env=RED_KNOT_LAST_TAG_DISTANCE={}",
+            describe_parts.next().unwrap_or("0")
+        );
+    }
+}
+
+fn git_head(git_dir: &Path) -> Option<PathBuf> {
+    // The typical case is a standard git repository.
+    let git_head_path = git_dir.join("HEAD");
+    if git_head_path.exists() {
+        return Some(git_head_path);
+    }
+    if !git_dir.is_file() {
+        return None;
+    }
+    // If `.git/HEAD` doesn't exist and `.git` is actually a file,
+    // then let's try to attempt to read it as a worktree. If it's
+    // a worktree, then its contents will look like this, e.g.:
+    //
+    //     gitdir: /home/andrew/astral/uv/main/.git/worktrees/pr2
+    //
+    // And the HEAD file we want to watch will be at:
+    //
+    //     /home/andrew/astral/uv/main/.git/worktrees/pr2/HEAD
+    let contents = fs::read_to_string(git_dir).ok()?;
+    let (label, worktree_path) = contents.split_once(':')?;
+    if label != "gitdir" {
+        return None;
+    }
+    let worktree_path = worktree_path.trim();
+    Some(PathBuf::from(worktree_path))
+}

crates/red_knot/src/args.rs

@@ -0,0 +1,208 @@
+use crate::logging::Verbosity;
+use crate::python_version::PythonVersion;
+use clap::{ArgAction, ArgMatches, Error, Parser};
+use red_knot_project::metadata::options::{EnvironmentOptions, Options};
+use red_knot_project::metadata::value::{RangedValue, RelativePathBuf};
+use red_knot_python_semantic::lint;
+use ruff_db::system::SystemPathBuf;
+
+#[derive(Debug, Parser)]
+#[command(
+    author,
+    name = "red-knot",
+    about = "An extremely fast Python type checker."
+)]
+#[command(version)]
+pub(crate) struct Args {
+    #[command(subcommand)]
+    pub(crate) command: Command,
+}
+
+#[derive(Debug, clap::Subcommand)]
+pub(crate) enum Command {
+    /// Check a project for type errors.
+    Check(CheckCommand),
+
+    /// Start the language server
+    Server,
+
+    /// Display Red Knot's version
+    Version,
+}
+
+#[derive(Debug, Parser)]
+pub(crate) struct CheckCommand {
+    /// 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,
+    /// as will the project's virtual environment (`.venv`) unless the `venv-path` option is set.
+    ///
+    /// Other command-line arguments (such as relative paths) will be resolved relative to the current working directory.
+    #[arg(long, value_name = "PROJECT")]
+    pub(crate) project: Option<SystemPathBuf>,
+
+    /// Path to the virtual environment the project uses.
+    ///
+    /// 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.
+    #[arg(long, value_name = "PATH")]
+    pub(crate) venv_path: Option<SystemPathBuf>,
+
+    /// Custom directory to use for stdlib typeshed stubs.
+    #[arg(long, value_name = "PATH", alias = "custom-typeshed-dir")]
+    pub(crate) typeshed: Option<SystemPathBuf>,
+
+    /// Additional path to use as a module-resolution source (can be passed multiple times).
+    #[arg(long, value_name = "PATH")]
+    pub(crate) extra_search_path: Option<Vec<SystemPathBuf>>,
+
+    /// Python version to assume when resolving types.
+    #[arg(long, value_name = "VERSION", alias = "target-version")]
+    pub(crate) python_version: Option<PythonVersion>,
+
+    #[clap(flatten)]
+    pub(crate) verbosity: Verbosity,
+
+    /// Whether to output metrics about type-checking performance. If you provide a path, metrics
+    /// will be written to that file. If you provide this option but don't provide a path, metrics
+    /// will be written to a file called `metrics.json` in the current directory. We will _append_
+    /// metrics to the file if it already exists.
+    #[arg(long, value_name = "PATH", default_missing_value="metrics.json", num_args=0..=1)]
+    pub(crate) metrics: Option<SystemPathBuf>,
+
+    #[clap(flatten)]
+    pub(crate) rules: RulesArg,
+
+    /// Use exit code 1 if there are any warning-level diagnostics.
+    #[arg(long, conflicts_with = "exit_zero")]
+    pub(crate) error_on_warning: bool,
+
+    /// Always use exit code 0, even when there are error-level diagnostics.
+    #[arg(long)]
+    pub(crate) exit_zero: bool,
+
+    /// Run in watch mode by re-running whenever files change.
+    #[arg(long, short = 'W')]
+    pub(crate) watch: bool,
+}
+
+impl CheckCommand {
+    pub(crate) fn into_options(self) -> Options {
+        let rules = if self.rules.is_empty() {
+            None
+        } else {
+            Some(
+                self.rules
+                    .into_iter()
+                    .map(|(rule, level)| (RangedValue::cli(rule), RangedValue::cli(level)))
+                    .collect(),
+            )
+        };
+
+        Options {
+            environment: Some(EnvironmentOptions {
+                python_version: self
+                    .python_version
+                    .map(|version| RangedValue::cli(version.into())),
+                venv_path: self.venv_path.map(RelativePathBuf::cli),
+                typeshed: self.typeshed.map(RelativePathBuf::cli),
+                extra_paths: self.extra_search_path.map(|extra_search_paths| {
+                    extra_search_paths
+                        .into_iter()
+                        .map(RelativePathBuf::cli)
+                        .collect()
+                }),
+                ..EnvironmentOptions::default()
+            }),
+            rules,
+            ..Default::default()
+        }
+    }
+}
+
+/// A list of rules to enable or disable with a given severity.
+///
+/// This type is used to parse the `--error`, `--warn`, and `--ignore` arguments
+/// while preserving the order in which they were specified (arguments last override previous severities).
+#[derive(Debug)]
+pub(crate) struct RulesArg(Vec<(String, lint::Level)>);
+
+impl RulesArg {
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    fn into_iter(self) -> impl Iterator<Item = (String, lint::Level)> {
+        self.0.into_iter()
+    }
+}
+
+impl clap::FromArgMatches for RulesArg {
+    fn from_arg_matches(matches: &ArgMatches) -> Result<Self, Error> {
+        let mut rules = Vec::new();
+
+        for (level, arg_id) in [
+            (lint::Level::Ignore, "ignore"),
+            (lint::Level::Warn, "warn"),
+            (lint::Level::Error, "error"),
+        ] {
+            let indices = matches.indices_of(arg_id).into_iter().flatten();
+            let levels = matches.get_many::<String>(arg_id).into_iter().flatten();
+            rules.extend(
+                indices
+                    .zip(levels)
+                    .map(|(index, rule)| (index, rule, level)),
+            );
+        }
+
+        // Sort by their index so that values specified later override earlier ones.
+        rules.sort_by_key(|(index, _, _)| *index);
+
+        Ok(Self(
+            rules
+                .into_iter()
+                .map(|(_, rule, level)| (rule.to_owned(), level))
+                .collect(),
+        ))
+    }
+
+    fn update_from_arg_matches(&mut self, matches: &ArgMatches) -> Result<(), Error> {
+        self.0 = Self::from_arg_matches(matches)?.0;
+        Ok(())
+    }
+}
+
+impl clap::Args for RulesArg {
+    fn augment_args(cmd: clap::Command) -> clap::Command {
+        const HELP_HEADING: &str = "Enabling / disabling rules";
+
+        cmd.arg(
+            clap::Arg::new("error")
+                .long("error")
+                .action(ArgAction::Append)
+                .help("Treat the given rule as having severity 'error'. Can be specified multiple times.")
+                .value_name("RULE")
+                .help_heading(HELP_HEADING),
+        )
+        .arg(
+            clap::Arg::new("warn")
+                .long("warn")
+                .action(ArgAction::Append)
+                .help("Treat the given rule as having severity 'warn'. Can be specified multiple times.")
+                .value_name("RULE")
+                .help_heading(HELP_HEADING),
+        )
+        .arg(
+            clap::Arg::new("ignore")
+                .long("ignore")
+                .action(ArgAction::Append)
+                .help("Disables the rule. Can be specified multiple times.")
+                .value_name("RULE")
+                .help_heading(HELP_HEADING),
+        )
+    }
+
+    fn augment_args_for_update(cmd: clap::Command) -> clap::Command {
+        Self::augment_args(cmd)
+    }
+}

crates/red_knot/src/logging.rs

@@ -2,8 +2,10 @@
 
 use anyhow::Context;
 use colored::Colorize;
+use ruff_db::system::SystemPathBuf;
+use ruff_metrics::JsonRecorder;
 use std::fmt;
-use std::fs::File;
+use std::fs::{File, OpenOptions};
 use std::io::BufWriter;
 use tracing::{Event, Subscriber};
 use tracing_subscriber::filter::LevelFilter;
@@ -252,3 +254,18 @@ where
         writeln!(writer)
     }
 }
+
+pub(crate) fn setup_metrics(dest: Option<&SystemPathBuf>) {
+    // If --metrics is not provided at all, don't collect any metrics.
+    let Some(dest) = dest else {
+        return;
+    };
+
+    let dest = OpenOptions::new()
+        .append(true)
+        .create(true)
+        .open(dest.as_std_path())
+        .expect("cannot open metrics file");
+    let recorder = JsonRecorder::new(dest);
+    metrics::set_global_recorder(recorder).expect("metrics recorder already registered");
+}

crates/red_knot/src/main.rs

@@ -1,101 +1,29 @@
+use std::io::{self, BufWriter, Write};
 use std::process::{ExitCode, Termination};
+
+use anyhow::Result;
 use std::sync::Mutex;
 
+use crate::args::{Args, CheckCommand, Command};
+use crate::logging::{setup_metrics, setup_tracing};
 use anyhow::{anyhow, Context};
 use clap::Parser;
 use colored::Colorize;
 use crossbeam::channel as crossbeam_channel;
-use python_version::PythonVersion;
-use red_knot_project::metadata::options::{EnvironmentOptions, Options};
-use red_knot_project::metadata::value::{RangedValue, RelativePathBuf};
+use red_knot_project::metadata::options::Options;
 use red_knot_project::watch;
 use red_knot_project::watch::ProjectWatcher;
 use red_knot_project::{ProjectDatabase, ProjectMetadata};
 use red_knot_server::run_server;
-use ruff_db::diagnostic::Diagnostic;
+use ruff_db::diagnostic::{Diagnostic, Severity};
 use ruff_db::system::{OsSystem, System, SystemPath, SystemPathBuf};
 use salsa::plumbing::ZalsaDatabase;
 
-use crate::logging::{setup_tracing, Verbosity};
-
+mod args;
 mod logging;
 mod python_version;
 mod verbosity;
-
-#[derive(Debug, Parser)]
-#[command(
-    author,
-    name = "red-knot",
-    about = "An extremely fast Python type checker."
-)]
-#[command(version)]
-struct Args {
-    #[command(subcommand)]
-    pub(crate) command: Option<Command>,
-
-    /// 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,
-    /// as will the project's virtual environment (`.venv`) unless the `venv-path` option is set.
-    ///
-    /// Other command-line arguments (such as relative paths) will be resolved relative to the current working directory.
-    #[arg(long, value_name = "PROJECT")]
-    project: Option<SystemPathBuf>,
-
-    /// Path to the virtual environment the project uses.
-    ///
-    /// 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.
-    #[arg(long, value_name = "PATH")]
-    venv_path: Option<SystemPathBuf>,
-
-    /// Custom directory to use for stdlib typeshed stubs.
-    #[arg(long, value_name = "PATH", alias = "custom-typeshed-dir")]
-    typeshed: Option<SystemPathBuf>,
-
-    /// Additional path to use as a module-resolution source (can be passed multiple times).
-    #[arg(long, value_name = "PATH")]
-    extra_search_path: Option<Vec<SystemPathBuf>>,
-
-    /// Python version to assume when resolving types.
-    #[arg(long, value_name = "VERSION", alias = "target-version")]
-    python_version: Option<PythonVersion>,
-
-    #[clap(flatten)]
-    verbosity: Verbosity,
-
-    /// Run in watch mode by re-running whenever files change.
-    #[arg(long, short = 'W')]
-    watch: bool,
-}
-
-impl Args {
-    fn to_options(&self) -> Options {
-        Options {
-            environment: Some(EnvironmentOptions {
-                python_version: self
-                    .python_version
-                    .map(|version| RangedValue::cli(version.into())),
-                venv_path: self.venv_path.as_ref().map(RelativePathBuf::cli),
-                typeshed: self.typeshed.as_ref().map(RelativePathBuf::cli),
-                extra_paths: self.extra_search_path.as_ref().map(|extra_search_paths| {
-                    extra_search_paths
-                        .iter()
-                        .map(RelativePathBuf::cli)
-                        .collect()
-                }),
-                ..EnvironmentOptions::default()
-            }),
-            ..Default::default()
-        }
-    }
-}
-
-#[derive(Debug, clap::Subcommand)]
-pub enum Command {
-    /// Start the language server
-    Server,
-}
+mod version;
 
 #[allow(clippy::print_stdout, clippy::unnecessary_wraps, clippy::print_stderr)]
 pub fn main() -> ExitStatus {
@@ -122,13 +50,25 @@ pub fn main() -> ExitStatus {
 fn run() -> anyhow::Result<ExitStatus> {
     let args = Args::parse_from(std::env::args());
 
-    if matches!(args.command, Some(Command::Server)) {
-        return run_server().map(|()| ExitStatus::Success);
+    match args.command {
+        Command::Server => run_server().map(|()| ExitStatus::Success),
+        Command::Check(check_args) => run_check(check_args),
+        Command::Version => version().map(|()| ExitStatus::Success),
     }
+}
 
+pub(crate) fn version() -> Result<()> {
+    let mut stdout = BufWriter::new(io::stdout().lock());
+    let version_info = crate::version::version();
+    writeln!(stdout, "red knot {}", &version_info)?;
+    Ok(())
+}
+
+fn run_check(args: CheckCommand) -> anyhow::Result<ExitStatus> {
     let verbosity = args.verbosity.level();
     countme::enable(verbosity.is_trace());
     let _guard = setup_tracing(verbosity)?;
+    setup_metrics(args.metrics.as_ref());
 
     // The base path to which all CLI arguments are relative to.
     let cli_base_path = {
@@ -156,13 +96,21 @@ fn run() -> anyhow::Result<ExitStatus> {
         .unwrap_or_else(|| cli_base_path.clone());
 
     let system = OsSystem::new(cwd);
-    let cli_options = args.to_options();
+    let watch = args.watch;
+    let exit_zero = args.exit_zero;
+    let min_error_severity = if args.error_on_warning {
+        Severity::Warning
+    } else {
+        Severity::Error
+    };
+
+    let cli_options = args.into_options();
     let mut workspace_metadata = ProjectMetadata::discover(system.current_directory(), &system)?;
     workspace_metadata.apply_cli_options(cli_options.clone());
 
     let mut db = ProjectDatabase::new(workspace_metadata, system)?;
 
-    let (main_loop, main_loop_cancellation_token) = MainLoop::new(cli_options);
+    let (main_loop, main_loop_cancellation_token) = MainLoop::new(cli_options, min_error_severity);
 
     // Listen to Ctrl+C and abort the watch mode.
     let main_loop_cancellation_token = Mutex::new(Some(main_loop_cancellation_token));
@@ -174,7 +122,7 @@ fn run() -> anyhow::Result<ExitStatus> {
         }
     })?;
 
-    let exit_status = if args.watch {
+    let exit_status = if watch {
         main_loop.watch(&mut db)?
     } else {
         main_loop.run(&mut db)
@@ -184,7 +132,11 @@ fn run() -> anyhow::Result<ExitStatus> {
 
     std::mem::forget(db);
 
-    Ok(exit_status)
+    if exit_zero {
+        Ok(ExitStatus::Success)
+    } else {
+        Ok(exit_status)
+    }
 }
 
 #[derive(Copy, Clone)]
@@ -216,10 +168,18 @@ struct MainLoop {
     watcher: Option<ProjectWatcher>,
 
     cli_options: Options,
+
+    /// The minimum severity to consider an error when deciding the exit status.
+    ///
+    /// TODO(micha): Get from the terminal settings.
+    min_error_severity: Severity,
 }
 
 impl MainLoop {
-    fn new(cli_options: Options) -> (Self, MainLoopCancellationToken) {
+    fn new(
+        cli_options: Options,
+        min_error_severity: Severity,
+    ) -> (Self, MainLoopCancellationToken) {
         let (sender, receiver) = crossbeam_channel::bounded(10);
 
         (
@@ -228,6 +188,7 @@ impl MainLoop {
                 receiver,
                 watcher: None,
                 cli_options,
+                min_error_severity,
             },
             MainLoopCancellationToken { sender },
         )
@@ -285,7 +246,10 @@ impl MainLoop {
                     result,
                     revision: check_revision,
                 } => {
-                    let has_diagnostics = !result.is_empty();
+                    let failed = result
+                        .iter()
+                        .any(|diagnostic| diagnostic.severity() >= self.min_error_severity);
+
                     if check_revision == revision {
                         #[allow(clippy::print_stdout)]
                         for diagnostic in result {
@@ -298,7 +262,7 @@ impl MainLoop {
                     }
 
                     if self.watcher.is_none() {
-                        return if has_diagnostics {
+                        return if failed {
                             ExitStatus::Failure
                         } else {
                             ExitStatus::Success

crates/red_knot/src/version.rs

@@ -0,0 +1,105 @@
+//! Code for representing Red Knot's release version number.
+use std::fmt;
+
+/// Information about the git repository where Red Knot was built from.
+pub(crate) struct CommitInfo {
+    short_commit_hash: String,
+    commit_date: String,
+    commits_since_last_tag: u32,
+}
+
+/// Red Knot's version.
+pub(crate) struct VersionInfo {
+    /// Red Knot's version, such as "0.5.1"
+    version: String,
+    /// Information about the git commit we may have been built from.
+    ///
+    /// `None` if not built from a git repo or if retrieval failed.
+    commit_info: Option<CommitInfo>,
+}
+
+impl fmt::Display for VersionInfo {
+    /// Formatted version information: `<version>[+<commits>] (<commit> <date>)`
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.version)?;
+
+        if let Some(ref ci) = self.commit_info {
+            if ci.commits_since_last_tag > 0 {
+                write!(f, "+{}", ci.commits_since_last_tag)?;
+            }
+            write!(f, " ({} {})", ci.short_commit_hash, ci.commit_date)?;
+        }
+
+        Ok(())
+    }
+}
+
+/// Returns information about Red Knot's version.
+pub(crate) fn version() -> VersionInfo {
+    // Environment variables are only read at compile-time
+    macro_rules! option_env_str {
+        ($name:expr) => {
+            option_env!($name).map(|s| s.to_string())
+        };
+    }
+
+    // This version is pulled from Cargo.toml and set by Cargo
+    let version = option_env_str!("CARGO_PKG_VERSION").unwrap();
+
+    // Commit info is pulled from git and set by `build.rs`
+    let commit_info =
+        option_env_str!("RED_KNOT_COMMIT_SHORT_HASH").map(|short_commit_hash| CommitInfo {
+            short_commit_hash,
+            commit_date: option_env_str!("RED_KNOT_COMMIT_DATE").unwrap(),
+            commits_since_last_tag: option_env_str!("RED_KNOT_LAST_TAG_DISTANCE")
+                .as_deref()
+                .map_or(0, |value| value.parse::<u32>().unwrap_or(0)),
+        });
+
+    VersionInfo {
+        version,
+        commit_info,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use insta::assert_snapshot;
+
+    use super::{CommitInfo, VersionInfo};
+
+    #[test]
+    fn version_formatting() {
+        let version = VersionInfo {
+            version: "0.0.0".to_string(),
+            commit_info: None,
+        };
+        assert_snapshot!(version, @"0.0.0");
+    }
+
+    #[test]
+    fn version_formatting_with_commit_info() {
+        let version = VersionInfo {
+            version: "0.0.0".to_string(),
+            commit_info: Some(CommitInfo {
+                short_commit_hash: "53b0f5d92".to_string(),
+                commit_date: "2023-10-19".to_string(),
+                commits_since_last_tag: 0,
+            }),
+        };
+        assert_snapshot!(version, @"0.0.0 (53b0f5d92 2023-10-19)");
+    }
+
+    #[test]
+    fn version_formatting_with_commits_since_last_tag() {
+        let version = VersionInfo {
+            version: "0.0.0".to_string(),
+            commit_info: Some(CommitInfo {
+                short_commit_hash: "53b0f5d92".to_string(),
+                commit_date: "2023-10-19".to_string(),
+                commits_since_last_tag: 24,
+            }),
+        };
+        assert_snapshot!(version, @"0.0.0+24 (53b0f5d92 2023-10-19)");
+    }
+}

crates/red_knot/tests/cli.rs

@@ -1,5 +1,5 @@
 use anyhow::Context;
-use insta::Settings;
+use insta::internals::SettingsBindDropGuard;
 use insta_cmd::{assert_cmd_snapshot, get_cargo_bin};
 use std::path::{Path, PathBuf};
 use std::process::Command;
@@ -28,24 +28,29 @@ fn config_override() -> anyhow::Result<()> {
         ),
     ])?;
 
-    case.insta_settings().bind(|| {
-        assert_cmd_snapshot!(case.command(), @r"
-            success: false
-            exit_code: 1
-            ----- stdout -----
-            error[lint:unresolved-attribute] <temp_dir>/test.py:5:7 Type `<module 'sys'>` has no attribute `last_exc`
+    assert_cmd_snapshot!(case.command(), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: lint:unresolved-attribute
+     --> <temp_dir>/test.py:5:7
+      |
+    4 | # Access `sys.last_exc` that was only added in Python 3.12
+    5 | print(sys.last_exc)
+      |       ^^^^^^^^^^^^ Type `<module 'sys'>` has no attribute `last_exc`
+      |
 
-            ----- stderr -----
-        ");
 
-        assert_cmd_snapshot!(case.command().arg("--python-version").arg("3.12"), @r"
-            success: true
-            exit_code: 0
-            ----- stdout -----
+    ----- stderr -----
+    "###);
 
-            ----- stderr -----
-        ");
-    });
+    assert_cmd_snapshot!(case.command().arg("--python-version").arg("3.12"), @r"
+        success: true
+        exit_code: 0
+        ----- stdout -----
+
+        ----- stderr -----
+    ");
 
     Ok(())
 }
@@ -92,25 +97,31 @@ fn cli_arguments_are_relative_to_the_current_directory() -> anyhow::Result<()> {
         ),
     ])?;
 
-    case.insta_settings().bind(|| {
-        // Make sure that the CLI fails when the `libs` directory is not in the search path.
-        assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")), @r#"
-            success: false
-            exit_code: 1
-            ----- stdout -----
-            error[lint:unresolved-import] <temp_dir>/child/test.py:2:1 Cannot resolve import `utils`
+    // Make sure that the CLI fails when the `libs` directory is not in the search path.
+    assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: lint:unresolved-import
+     --> <temp_dir>/child/test.py:2:6
+      |
+    2 | from utils import add
+      |      ^^^^^ Cannot resolve import `utils`
+    3 |
+    4 | stat = add(10, 15)
+      |
 
-            ----- stderr -----
-        "#);
 
-        assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")).arg("--extra-search-path").arg("../libs"), @r"
-            success: true
-            exit_code: 0
-            ----- stdout -----
+    ----- stderr -----
+    "###);
 
-            ----- stderr -----
-        ");
-    });
+    assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")).arg("--extra-search-path").arg("../libs"), @r"
+        success: true
+        exit_code: 0
+        ----- stdout -----
+
+        ----- stderr -----
+    ");
 
     Ok(())
 }
@@ -156,22 +167,20 @@ fn paths_in_configuration_files_are_relative_to_the_project_root() -> anyhow::Re
         ),
     ])?;
 
-    case.insta_settings().bind(|| {
-        assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")), @r"
-            success: true
-            exit_code: 0
-            ----- stdout -----
+    assert_cmd_snapshot!(case.command().current_dir(case.project_dir().join("child")), @r"
+        success: true
+        exit_code: 0
+        ----- stdout -----
 
-            ----- stderr -----
-        ");
-    });
+        ----- stderr -----
+    ");
 
     Ok(())
 }
 
 /// The rule severity can be changed in the configuration file
 #[test]
-fn rule_severity() -> anyhow::Result<()> {
+fn configuration_rule_severity() -> anyhow::Result<()> {
     let case = TestCase::with_file(
         "test.py",
         r#"
@@ -184,41 +193,238 @@ fn rule_severity() -> anyhow::Result<()> {
             "#,
     )?;
 
-    case.insta_settings().bind(|| {
-        // 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"
-            success: false
-            exit_code: 1
-            ----- stdout -----
-            error[lint:division-by-zero] <temp_dir>/test.py:2:5 Cannot divide object of type `Literal[4]` by zero
-            warning[lint:possibly-unresolved-reference] <temp_dir>/test.py:7:7 Name `x` used when possibly not defined
+    // 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###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: lint:division-by-zero
+     --> <temp_dir>/test.py:2:5
+      |
+    2 | y = 4 / 0
+      |     ^^^^^ Cannot divide object of type `Literal[4]` by zero
+    3 |
+    4 | for a in range(0, y):
+      |
 
-            ----- stderr -----
-        ");
+    warning: lint:possibly-unresolved-reference
+     --> <temp_dir>/test.py:7:7
+      |
+    5 |     x = a
+    6 |
+    7 | print(x)  # possibly-unresolved-reference
+      |       - Name `x` used when possibly not defined
+      |
 
-        case.write_file("pyproject.toml", r#"
-            [tool.knot.rules]
-            division-by-zero = "warn" # demote to warn
-            possibly-unresolved-reference = "ignore"
-        "#)?;
 
-        assert_cmd_snapshot!(case.command(), @r"
-            success: false
-            exit_code: 1
-            ----- stdout -----
-            warning[lint:division-by-zero] <temp_dir>/test.py:2:5 Cannot divide object of type `Literal[4]` by zero
+    ----- stderr -----
+    "###);
 
-            ----- stderr -----
-        ");
+    case.write_file(
+        "pyproject.toml",
+        r#"
+        [tool.knot.rules]
+        division-by-zero = "warn" # demote to warn
+        possibly-unresolved-reference = "ignore"
+    "#,
+    )?;
 
-        Ok(())
-    })
+    assert_cmd_snapshot!(case.command(), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: lint:division-by-zero
+     --> <temp_dir>/test.py:2:5
+      |
+    2 | y = 4 / 0
+      |     ----- Cannot divide object of type `Literal[4]` by zero
+    3 |
+    4 | for a in range(0, y):
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
 }
 
-/// Red Knot warns about unknown rules
+/// The rule severity can be changed using `--ignore`, `--warn`, and `--error`
 #[test]
-fn unknown_rules() -> anyhow::Result<()> {
+fn cli_rule_severity() -> anyhow::Result<()> {
+    let case = TestCase::with_file(
+        "test.py",
+        r#"
+        import does_not_exit
+
+        y = 4 / 0
+
+        for a in range(0, y):
+            x = a
+
+        print(x)  # possibly-unresolved-reference
+        "#,
+    )?;
+
+    // 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###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: lint:unresolved-import
+     --> <temp_dir>/test.py:2:8
+      |
+    2 | import does_not_exit
+      |        ^^^^^^^^^^^^^ Cannot resolve import `does_not_exit`
+    3 |
+    4 | y = 4 / 0
+      |
+
+    error: lint:division-by-zero
+     --> <temp_dir>/test.py:4:5
+      |
+    2 | import does_not_exit
+    3 |
+    4 | y = 4 / 0
+      |     ^^^^^ Cannot divide object of type `Literal[4]` by zero
+    5 |
+    6 | for a in range(0, y):
+      |
+
+    warning: lint:possibly-unresolved-reference
+     --> <temp_dir>/test.py:9:7
+      |
+    7 |     x = a
+    8 |
+    9 | print(x)  # possibly-unresolved-reference
+      |       - Name `x` used when possibly not defined
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    assert_cmd_snapshot!(
+        case
+            .command()
+            .arg("--ignore")
+            .arg("possibly-unresolved-reference")
+            .arg("--warn")
+            .arg("division-by-zero")
+            .arg("--warn")
+            .arg("unresolved-import"),
+        @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: lint:unresolved-import
+     --> <temp_dir>/test.py:2:8
+      |
+    2 | import does_not_exit
+      |        ------------- Cannot resolve import `does_not_exit`
+    3 |
+    4 | y = 4 / 0
+      |
+
+    warning: lint:division-by-zero
+     --> <temp_dir>/test.py:4:5
+      |
+    2 | import does_not_exit
+    3 |
+    4 | y = 4 / 0
+      |     ----- Cannot divide object of type `Literal[4]` by zero
+    5 |
+    6 | for a in range(0, y):
+      |
+
+
+    ----- stderr -----
+    "###
+    );
+
+    Ok(())
+}
+
+/// The rule severity can be changed using `--ignore`, `--warn`, and `--error` and
+/// values specified last override previous severities.
+#[test]
+fn cli_rule_severity_precedence() -> anyhow::Result<()> {
+    let case = TestCase::with_file(
+        "test.py",
+        r#"
+        y = 4 / 0
+
+        for a in range(0, y):
+            x = a
+
+        print(x)  # possibly-unresolved-reference
+        "#,
+    )?;
+
+    // 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###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    error: lint:division-by-zero
+     --> <temp_dir>/test.py:2:5
+      |
+    2 | y = 4 / 0
+      |     ^^^^^ Cannot divide object of type `Literal[4]` by zero
+    3 |
+    4 | for a in range(0, y):
+      |
+
+    warning: lint:possibly-unresolved-reference
+     --> <temp_dir>/test.py:7:7
+      |
+    5 |     x = a
+    6 |
+    7 | print(x)  # possibly-unresolved-reference
+      |       - Name `x` used when possibly not defined
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    assert_cmd_snapshot!(
+        case
+            .command()
+            .arg("--error")
+            .arg("possibly-unresolved-reference")
+            .arg("--warn")
+            .arg("division-by-zero")
+            // Override the error severity with warning
+            .arg("--ignore")
+            .arg("possibly-unresolved-reference"),
+        @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: lint:division-by-zero
+     --> <temp_dir>/test.py:2:5
+      |
+    2 | y = 4 / 0
+      |     ----- Cannot divide object of type `Literal[4]` by zero
+    3 |
+    4 | for a in range(0, y):
+      |
+
+
+    ----- stderr -----
+    "###
+    );
+
+    Ok(())
+}
+
+/// Red Knot warns about unknown rules specified in a configuration file
+#[test]
+fn configuration_unknown_rules() -> anyhow::Result<()> {
     let case = TestCase::with_files([
         (
             "pyproject.toml",
@@ -230,22 +436,259 @@ fn unknown_rules() -> anyhow::Result<()> {
         ("test.py", "print(10)"),
     ])?;
 
-    case.insta_settings().bind(|| {
-        assert_cmd_snapshot!(case.command(), @r"
-            success: false
-            exit_code: 1
-            ----- stdout -----
-            warning[unknown-rule] <temp_dir>/pyproject.toml:3:1 Unknown lint rule `division-by-zer`
+    assert_cmd_snapshot!(case.command(), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: unknown-rule
+     --> <temp_dir>/pyproject.toml:3:1
+      |
+    2 | [tool.knot.rules]
+    3 | division-by-zer = "warn" # incorrect rule name
+      | --------------- Unknown lint rule `division-by-zer`
+      |
 
-            ----- stderr -----
-        ");
-    });
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+/// Red Knot warns about unknown rules specified in a CLI argument
+#[test]
+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###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: unknown-rule: Unknown lint rule `division-by-zer`
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+#[test]
+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###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: lint:unresolved-reference
+     --> <temp_dir>/test.py:1:7
+      |
+    1 | print(x)  # [unresolved-reference]
+      |       - Name `x` used when not defined
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+#[test]
+fn exit_code_only_info() -> anyhow::Result<()> {
+    let case = TestCase::with_file(
+        "test.py",
+        r#"
+        from typing_extensions import reveal_type
+        reveal_type(1)
+        "#,
+    )?;
+
+    assert_cmd_snapshot!(case.command(), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    info: revealed-type
+     --> <temp_dir>/test.py:3:1
+      |
+    2 | from typing_extensions import reveal_type
+    3 | reveal_type(1)
+      | -------------- info: Revealed type is `Literal[1]`
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+#[test]
+fn exit_code_only_info_and_error_on_warning_is_true() -> anyhow::Result<()> {
+    let case = TestCase::with_file(
+        "test.py",
+        r#"
+        from typing_extensions import reveal_type
+        reveal_type(1)
+        "#,
+    )?;
+
+    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    info: revealed-type
+     --> <temp_dir>/test.py:3:1
+      |
+    2 | from typing_extensions import reveal_type
+    3 | reveal_type(1)
+      | -------------- info: Revealed type is `Literal[1]`
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+#[test]
+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###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    warning: lint:unresolved-reference
+     --> <temp_dir>/test.py:1:7
+      |
+    1 | print(x)  # [unresolved-reference]
+      |       - Name `x` used when not defined
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+#[test]
+fn exit_code_both_warnings_and_errors() -> anyhow::Result<()> {
+    let case = TestCase::with_file(
+        "test.py",
+        r#"
+        print(x)     # [unresolved-reference]
+        print(4[1])  # [non-subscriptable]
+        "#,
+    )?;
+
+    assert_cmd_snapshot!(case.command(), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    warning: lint:unresolved-reference
+     --> <temp_dir>/test.py:2:7
+      |
+    2 | print(x)     # [unresolved-reference]
+      |       - Name `x` used when not defined
+    3 | print(4[1])  # [non-subscriptable]
+      |
+
+    error: lint:non-subscriptable
+     --> <temp_dir>/test.py:3:7
+      |
+    2 | print(x)     # [unresolved-reference]
+    3 | print(4[1])  # [non-subscriptable]
+      |       ^ Cannot subscript object of type `Literal[4]` with no `__getitem__` method
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+#[test]
+fn exit_code_both_warnings_and_errors_and_error_on_warning_is_true() -> anyhow::Result<()> {
+    let case = TestCase::with_file(
+        "test.py",
+        r###"
+        print(x)     # [unresolved-reference]
+        print(4[1])  # [non-subscriptable]
+        "###,
+    )?;
+
+    assert_cmd_snapshot!(case.command().arg("--error-on-warning"), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    warning: lint:unresolved-reference
+     --> <temp_dir>/test.py:2:7
+      |
+    2 | print(x)     # [unresolved-reference]
+      |       - Name `x` used when not defined
+    3 | print(4[1])  # [non-subscriptable]
+      |
+
+    error: lint:non-subscriptable
+     --> <temp_dir>/test.py:3:7
+      |
+    2 | print(x)     # [unresolved-reference]
+    3 | print(4[1])  # [non-subscriptable]
+      |       ^ Cannot subscript object of type `Literal[4]` with no `__getitem__` method
+      |
+
+
+    ----- stderr -----
+    "###);
+
+    Ok(())
+}
+
+#[test]
+fn exit_code_exit_zero_is_true() -> anyhow::Result<()> {
+    let case = TestCase::with_file(
+        "test.py",
+        r#"
+        print(x)     # [unresolved-reference]
+        print(4[1])  # [non-subscriptable]
+        "#,
+    )?;
+
+    assert_cmd_snapshot!(case.command().arg("--exit-zero"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    warning: lint:unresolved-reference
+     --> <temp_dir>/test.py:2:7
+      |
+    2 | print(x)     # [unresolved-reference]
+      |       - Name `x` used when not defined
+    3 | print(4[1])  # [non-subscriptable]
+      |
+
+    error: lint:non-subscriptable
+     --> <temp_dir>/test.py:3:7
+      |
+    2 | print(x)     # [unresolved-reference]
+    3 | print(4[1])  # [non-subscriptable]
+      |       ^ Cannot subscript object of type `Literal[4]` with no `__getitem__` method
+      |
+
+
+    ----- stderr -----
+    "###);
 
     Ok(())
 }
 
 struct TestCase {
     _temp_dir: TempDir,
+    _settings_scope: SettingsBindDropGuard,
     project_dir: PathBuf,
 }
 
@@ -260,9 +703,16 @@ impl TestCase {
             .canonicalize()
             .context("Failed to canonicalize project path")?;
 
+        let mut settings = insta::Settings::clone_current();
+        settings.add_filter(&tempdir_filter(&project_dir), "<temp_dir>/");
+        settings.add_filter(r#"\\(\w\w|\s|\.|")"#, "/$1");
+
+        let settings_scope = settings.bind_to_scope();
+
         Ok(Self {
             project_dir,
             _temp_dir: temp_dir,
+            _settings_scope: settings_scope,
         })
     }
 
@@ -307,17 +757,9 @@ impl TestCase {
         &self.project_dir
     }
 
-    // Returns the insta filters to escape paths in snapshots
-    fn insta_settings(&self) -> Settings {
-        let mut settings = insta::Settings::clone_current();
-        settings.add_filter(&tempdir_filter(&self.project_dir), "<temp_dir>/");
-        settings.add_filter(r#"\\(\w\w|\s|\.|")"#, "/$1");
-        settings
-    }
-
     fn command(&self) -> Command {
         let mut command = Command::new(get_cargo_bin("red_knot"));
-        command.current_dir(&self.project_dir);
+        command.current_dir(&self.project_dir).arg("check");
         command
     }
 }

crates/red_knot/tests/file_watching.rs

@@ -47,7 +47,7 @@ impl TestCase {
         #[track_caller]
         fn panic_with_formatted_events(events: Vec<ChangeEvent>) -> Vec<ChangeEvent> {
             panic!(
-                "Didn't observe expected change:\n{}",
+                "Didn't observe the expected event. The following events occurred:\n{}",
                 events
                     .into_iter()
                     .map(|event| format!("  - {event:?}"))

crates/red_knot_project/src/metadata/options.rs

@@ -149,6 +149,16 @@ impl Options {
                             format!("Unknown lint rule `{rule_name}`"),
                             Severity::Warning,
                         ),
+                        GetLintError::PrefixedWithCategory { suggestion, .. } => {
+                            OptionDiagnostic::new(
+                                DiagnosticId::UnknownRule,
+                                format!(
+                                    "Unknown lint rule `{rule_name}`. Did you mean `{suggestion}`?"
+                                ),
+                                Severity::Warning,
+                            )
+                        }
+
                         GetLintError::Removed(_) => OptionDiagnostic::new(
                             DiagnosticId::UnknownRule,
                             format!("Unknown lint rule `{rule_name}`"),
@@ -206,6 +216,16 @@ pub struct Rules {
     inner: FxHashMap<RangedValue<String>, RangedValue<Level>>,
 }
 
+impl FromIterator<(RangedValue<String>, RangedValue<Level>)> for Rules {
+    fn from_iter<T: IntoIterator<Item = (RangedValue<String>, RangedValue<Level>)>>(
+        iter: T,
+    ) -> Self {
+        Self {
+            inner: iter.into_iter().collect(),
+        }
+    }
+}
+
 #[derive(Error, Debug)]
 pub enum KnotTomlError {
     #[error(transparent)]

crates/red_knot_project/tests/check.rs

@@ -270,6 +270,8 @@ impl SourceOrderVisitor<'_> for PullTypesVisitor<'_> {
 /// Whether or not the .py/.pyi version of this file is expected to fail
 #[rustfmt::skip]
 const KNOWN_FAILURES: &[(&str, bool, bool)] = &[
+    // related to circular references in nested functions
+    ("crates/ruff_linter/resources/test/fixtures/flake8_return/RET503.py", false, true),
     // related to circular references in class definitions
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_26.py", true, true),
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_27.py", true, true),

crates/red_knot_python_semantic/Cargo.toml

@@ -30,6 +30,7 @@ countme = { workspace = true }
 drop_bomb = { workspace = true }
 indexmap = { workspace = true }
 itertools = { workspace = true }
+metrics = { workspace = true }
 ordermap = { workspace = true }
 salsa = { workspace = true }
 thiserror = { workspace = true }

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

@@ -2,7 +2,9 @@
 
 ## Deferred annotations in stubs always resolve
 
-```pyi path=mod.pyi
+`mod.pyi`:
+
+```pyi
 def get_foo() -> Foo: ...
 class Foo: ...
 ```

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

@@ -106,7 +106,7 @@ def union_example(
         Literal["B"],
         Literal[True],
         None,
-    ]
+    ],
 ):
     reveal_type(x)  # revealed: Unknown | Literal[-1, "A", b"A", b"\x00", b"\x07", 0, 1, "B", "foo", "bar", True] | None
 ```
@@ -116,7 +116,9 @@ def union_example(
 Only Literal that is defined in typing and typing_extension modules is detected as the special
 Literal.
 
-```pyi path=other.pyi
+`other.pyi`:
+
+```pyi
 from typing import _SpecialForm
 
 Literal: _SpecialForm

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

@@ -25,7 +25,9 @@ x = "foo"  # error: [invalid-assignment] "Object of type `Literal["foo"]` is not
 
 ## Tuple annotations are understood
 
-```py path=module.py
+`module.py`:
+
+```py
 from typing_extensions import Unpack
 
 a: tuple[()] = ()
@@ -40,7 +42,9 @@ i: tuple[str | int, str | int] = (42, 42)
 j: tuple[str | int] = (42,)
 ```
 
-```py path=script.py
+`script.py`:
+
+```py
 from module import a, b, c, d, e, f, g, h, i, j
 
 reveal_type(a)  # revealed: tuple[()]
@@ -114,7 +118,7 @@ reveal_type(x)  # revealed: Foo
 
 ## Annotations in stub files are deferred
 
-```pyi path=main.pyi
+```pyi
 x: Foo
 
 class Foo: ...
@@ -125,7 +129,7 @@ reveal_type(x)  # revealed: Foo
 
 ## Annotated assignments in stub files are inferred correctly
 
-```pyi path=main.pyi
+```pyi
 x: int = 1
 reveal_type(x) # revealed: Literal[1]
 ```

crates/red_knot_python_semantic/resources/mdtest/attributes.md

@@ -13,123 +13,90 @@ accessed on the class itself.
 
 ```py
 class C:
-    def __init__(self, value2: int, flag: bool = False) -> None:
-        # bound but not declared
-        self.pure_instance_variable1 = "value set in __init__"
-
-        # bound but not declared - with type inferred from parameter
-        self.pure_instance_variable2 = value2
-
-        # declared but not bound
-        self.pure_instance_variable3: bytes
-
-        # declared and bound
-        self.pure_instance_variable4: bool = True
-
-        # possibly undeclared/unbound
+    def __init__(self, param: int | None, flag: bool = False) -> None:
+        value = 1 if flag else "a"
+        self.inferred_from_value = value
+        self.inferred_from_other_attribute = self.inferred_from_value
+        self.inferred_from_param = param
+        self.declared_only: bytes
+        self.declared_and_bound: bool = True
         if flag:
-            self.pure_instance_variable5: str = "possibly set in __init__"
+            self.possibly_undeclared_unbound: str = "possibly set in __init__"
 
 c_instance = C(1)
 
-# TODO: should be `Literal["value set in __init__"]`, or `Unknown | Literal[…]` to allow
-# assignments to this unannotated attribute from other scopes.
-reveal_type(c_instance.pure_instance_variable1)  # revealed: @Todo(implicit instance attribute)
+reveal_type(c_instance.inferred_from_value)  # revealed: Unknown | Literal[1, "a"]
 
-# TODO: should be `int`
-reveal_type(c_instance.pure_instance_variable2)  # revealed: @Todo(implicit instance attribute)
+# TODO: Same here. This should be `Unknown | Literal[1, "a"]`
+reveal_type(c_instance.inferred_from_other_attribute)  # revealed: Unknown
 
-# TODO: should be `bytes`
-reveal_type(c_instance.pure_instance_variable3)  # revealed: @Todo(implicit instance attribute)
+# TODO: should be `int | None`
+reveal_type(c_instance.inferred_from_param)  # revealed: Unknown | int | None
 
-# TODO: should be `bool`
-reveal_type(c_instance.pure_instance_variable4)  # revealed: @Todo(implicit instance attribute)
+reveal_type(c_instance.declared_only)  # revealed: bytes
+
+reveal_type(c_instance.declared_and_bound)  # revealed: bool
 
-# TODO: should be `str`
 # We probably don't want to emit a diagnostic for this being possibly undeclared/unbound.
 # mypy and pyright do not show an error here.
-reveal_type(c_instance.pure_instance_variable5)  # revealed: @Todo(implicit instance attribute)
+reveal_type(c_instance.possibly_undeclared_unbound)  # revealed: str
 
-# TODO: If we choose to infer a precise `Literal[…]` type for the instance attribute (see
-# above), this should be an error: incompatible types in assignment. If we choose to infer
-# a gradual `Unknown | Literal[…]` type, this assignment is fine.
-c_instance.pure_instance_variable1 = "value set on instance"
+# This assignment is fine, as we infer `Unknown | Literal[1, "a"]` for `inferred_from_value`.
+c_instance.inferred_from_value = "value set on instance"
+
+# This assignment is also fine:
+c_instance.inferred_from_param = None
 
 # TODO: this should be an error (incompatible types in assignment)
-c_instance.pure_instance_variable2 = "incompatible"
+c_instance.inferred_from_param = "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 `pure_instance_variable1`"
-reveal_type(C.pure_instance_variable1)  # revealed: Unknown
+# error: [unresolved-attribute] "Type `Literal[C]` has no attribute `inferred_from_value`"
+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"
-C.pure_instance_variable1 = "overwritten on class"
+C.inferred_from_value = "overwritten on class"
 
-c_instance.pure_instance_variable4 = False
+# This assignment is fine:
+c_instance.declared_and_bound = False
 
 # TODO: After this assignment to the attribute within this scope, we may eventually want to narrow
 # the `bool` type (see above) for this instance variable to `Literal[False]` here. This is unsound
 # in general (we don't know what else happened to `c_instance` between the assignment and the use
 # here), but mypy and pyright support this. In conclusion, this could be `bool` but should probably
 # be `Literal[False]`.
-reveal_type(c_instance.pure_instance_variable4)  # revealed: @Todo(implicit instance attribute)
+reveal_type(c_instance.declared_and_bound)  # revealed: bool
 ```
 
-#### Variable declared in class body and declared/bound in `__init__`
+#### Variable declared in class body and possibly bound in `__init__`
 
 The same rule applies even if the variable is *declared* (not bound!) in the class body: it is still
 a pure instance variable.
 
 ```py
 class C:
-    pure_instance_variable: str
+    declared_and_bound: str | None
 
     def __init__(self) -> None:
-        self.pure_instance_variable = "value set in __init__"
+        self.declared_and_bound = "value set in __init__"
 
 c_instance = C()
 
-reveal_type(c_instance.pure_instance_variable)  # revealed: str
+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.pure_instance_variable)  # revealed: str
+reveal_type(C.declared_and_bound)  # revealed: str | None
 
 # TODO: same as above. We plan to emit a diagnostic here, even if both mypy
 # and pyright allow this.
-C.pure_instance_variable = "overwritten on class"
+C.declared_and_bound = "overwritten on class"
 
-# error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to attribute `pure_instance_variable` of type `str`"
-c_instance.pure_instance_variable = 1
-```
-
-#### Variable only defined in unrelated method
-
-We also recognize pure instance variables if they are defined in a method that is not `__init__`.
-
-```py
-class C:
-    def set_instance_variable(self) -> None:
-        self.pure_instance_variable = "value set in method"
-
-c_instance = C()
-
-# Not that we would use this in static analysis, but for a more realistic example, let's actually
-# call the method, so that the attribute is bound if this example is actually run.
-c_instance.set_instance_variable()
-
-# TODO: should be `Literal["value set in method"]` or `Unknown | Literal[…]` (see above).
-reveal_type(c_instance.pure_instance_variable)  # revealed: @Todo(implicit instance attribute)
-
-# TODO: We already show an error here, but the message might be improved?
-# error: [unresolved-attribute]
-reveal_type(C.pure_instance_variable)  # revealed: Unknown
-
-# TODO: this should be an error
-C.pure_instance_variable = "overwritten on class"
+# error: [invalid-assignment] "Object of type `Literal[1]` is not assignable to attribute `declared_and_bound` of type `str | None`"
+c_instance.declared_and_bound = 1
 ```
 
 #### Variable declared in class body and not bound anywhere
@@ -139,18 +106,341 @@ instance variable and allow access to it via instances.
 
 ```py
 class C:
-    pure_instance_variable: str
+    only_declared: str
 
 c_instance = C()
 
-reveal_type(c_instance.pure_instance_variable)  # revealed: str
+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.pure_instance_variable)  # revealed: str
+reveal_type(C.only_declared)  # revealed: str
 
 # TODO: mypy and pyright do not show an error here, but we plan to emit one.
-C.pure_instance_variable = "overwritten on class"
+C.only_declared = "overwritten on class"
+```
+
+#### Mixed declarations/bindings in class body and `__init__`
+
+```py
+class C:
+    only_declared_in_body: str | None
+    declared_in_body_and_init: str | None
+
+    declared_in_body_defined_in_init: str | None
+
+    bound_in_body_declared_in_init = "a"
+
+    bound_in_body_and_init = None
+
+    def __init__(self, flag) -> None:
+        self.only_declared_in_init: str | None
+        self.declared_in_body_and_init: str | None = None
+
+        self.declared_in_body_defined_in_init = "a"
+
+        self.bound_in_body_declared_in_init: str | None
+
+        if flag:
+            self.bound_in_body_and_init = "a"
+
+c_instance = C(True)
+
+reveal_type(c_instance.only_declared_in_body)  # revealed: str | None
+reveal_type(c_instance.only_declared_in_init)  # revealed: str | None
+reveal_type(c_instance.declared_in_body_and_init)  # revealed: str | None
+
+reveal_type(c_instance.declared_in_body_defined_in_init)  # revealed: str | None
+
+reveal_type(c_instance.bound_in_body_declared_in_init)  # revealed: str | None
+
+reveal_type(c_instance.bound_in_body_and_init)  # revealed: Unknown | None | Literal["a"]
+```
+
+#### Variable defined in non-`__init__` method
+
+We also recognize pure instance variables if they are defined in a method that is not `__init__`.
+
+```py
+class C:
+    def __init__(self, param: int | None, flag: bool = False) -> None:
+        self.initialize(param, flag)
+
+    def initialize(self, param: int | None, flag: bool) -> None:
+        value = 1 if flag else "a"
+        self.inferred_from_value = value
+        self.inferred_from_other_attribute = self.inferred_from_value
+        self.inferred_from_param = param
+        self.declared_only: bytes
+        self.declared_and_bound: bool = True
+
+c_instance = C(1)
+
+reveal_type(c_instance.inferred_from_value)  # revealed: Unknown | Literal[1, "a"]
+
+# TODO: Should be `Unknown | Literal[1, "a"]`
+reveal_type(c_instance.inferred_from_other_attribute)  # revealed: Unknown
+
+# TODO: Should be `int | None`
+reveal_type(c_instance.inferred_from_param)  # revealed: Unknown | int | None
+
+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]
+reveal_type(C.inferred_from_value)  # revealed: Unknown
+
+# TODO: this should be an error
+C.inferred_from_value = "overwritten on class"
+```
+
+#### Variable defined in multiple methods
+
+If we see multiple un-annotated assignments to a single attribute (`self.x` below), we build the
+union of all inferred types (and `Unknown`). If we see multiple conflicting declarations of the same
+attribute, that should be an error.
+
+```py
+def get_int() -> int:
+    return 0
+
+def get_str() -> str:
+    return "a"
+
+class C:
+    z: int
+
+    def __init__(self) -> None:
+        self.x = get_int()
+        self.y: int = 1
+
+    def other_method(self):
+        self.x = get_str()
+
+        # TODO: this redeclaration should be an error
+        self.y: str = "a"
+
+        # TODO: this redeclaration should be an error
+        self.z: str = "a"
+
+c_instance = C()
+
+reveal_type(c_instance.x)  # revealed: Unknown | int | str
+reveal_type(c_instance.y)  # revealed: int
+reveal_type(c_instance.z)  # revealed: int
+```
+
+#### Attributes defined in tuple unpackings
+
+```py
+def returns_tuple() -> tuple[int, str]:
+    return (1, "a")
+
+class C:
+    a1, b1 = (1, "a")
+    c1, d1 = returns_tuple()
+
+    def __init__(self) -> None:
+        self.a2, self.b2 = (1, "a")
+        self.c2, self.d2 = returns_tuple()
+
+c_instance = C()
+
+reveal_type(c_instance.a1)  # revealed: Unknown | Literal[1]
+reveal_type(c_instance.b1)  # revealed: Unknown | Literal["a"]
+reveal_type(c_instance.c1)  # revealed: Unknown | int
+reveal_type(c_instance.d1)  # revealed: Unknown | str
+
+# TODO: This should be supported (no error; type should be: `Unknown | Literal[1]`)
+# error: [unresolved-attribute]
+reveal_type(c_instance.a2)  # revealed: Unknown
+
+# TODO: This should be supported (no error; type should be: `Unknown | Literal["a"]`)
+# error: [unresolved-attribute]
+reveal_type(c_instance.b2)  # revealed: Unknown
+
+# TODO: Similar for these two (should be `Unknown | int` and `Unknown | str`, respectively)
+# error: [unresolved-attribute]
+reveal_type(c_instance.c2)  # revealed: Unknown
+# error: [unresolved-attribute]
+reveal_type(c_instance.d2)  # revealed: Unknown
+```
+
+#### Attributes defined in for-loop (unpacking)
+
+```py
+class IntIterator:
+    def __next__(self) -> int:
+        return 1
+
+class IntIterable:
+    def __iter__(self) -> IntIterator:
+        return IntIterator()
+
+class TupleIterator:
+    def __next__(self) -> tuple[int, str]:
+        return (1, "a")
+
+class TupleIterable:
+    def __iter__(self) -> TupleIterator:
+        return TupleIterator()
+
+class C:
+    def __init__(self):
+        for self.x in IntIterable():
+            pass
+
+        for _, self.y in TupleIterable():
+            pass
+
+# TODO: Pyright fully supports these, mypy detects the presence of the attributes,
+# but infers type `Any` for both of them. We should infer `int` and `str` here:
+
+# error: [unresolved-attribute]
+reveal_type(C().x)  # revealed: Unknown
+
+# error: [unresolved-attribute]
+reveal_type(C().y)  # revealed: Unknown
+```
+
+#### Conditionally declared / bound attributes
+
+We currently do not raise a diagnostic or change behavior if an attribute is only conditionally
+defined. This is consistent with what mypy and pyright do.
+
+```py
+def flag() -> bool:
+    return True
+
+class C:
+    def f(self) -> None:
+        if flag():
+            self.a1: str | None = "a"
+            self.b1 = 1
+    if flag():
+        def f(self) -> None:
+            self.a2: str | None = "a"
+            self.b2 = 1
+
+c_instance = C()
+
+reveal_type(c_instance.a1)  # revealed: str | None
+reveal_type(c_instance.a2)  # revealed: str | None
+reveal_type(c_instance.b1)  # revealed: Unknown | Literal[1]
+reveal_type(c_instance.b2)  # revealed: Unknown | Literal[1]
+```
+
+#### Methods that does not use `self` as a first parameter
+
+```py
+class C:
+    # This might trigger a stylistic lint like `invalid-first-argument-name-for-method`, but
+    # it should be supported in general:
+    def __init__(this) -> None:
+        this.declared_and_bound: str | None = "a"
+
+reveal_type(C().declared_and_bound)  # revealed: str | None
+```
+
+#### Aliased `self` parameter
+
+```py
+class C:
+    def __init__(self) -> None:
+        this = self
+        this.declared_and_bound: str | None = "a"
+
+# This would ideally be `str | None`, but mypy/pyright don't support this either,
+# so `Unknown` + a diagnostic is also fine.
+# error: [unresolved-attribute]
+reveal_type(C().declared_and_bound)  # revealed: Unknown
+```
+
+#### Static methods do not influence implicitly defined attributes
+
+```py
+class Other:
+    x: int
+
+class C:
+    @staticmethod
+    def f(other: Other) -> None:
+        other.x = 1
+
+# error: [unresolved-attribute]
+reveal_type(C.x)  # revealed: Unknown
+
+# TODO: this should raise `unresolved-attribute` as well, and the type should be `Unknown`
+reveal_type(C().x)  # revealed: Unknown | Literal[1]
+
+# This also works if `staticmethod` is aliased:
+
+my_staticmethod = staticmethod
+
+class D:
+    @my_staticmethod
+    def f(other: Other) -> None:
+        other.x = 1
+
+# error: [unresolved-attribute]
+reveal_type(D.x)  # revealed: Unknown
+
+# TODO: this should raise `unresolved-attribute` as well, and the type should be `Unknown`
+reveal_type(D().x)  # revealed: Unknown | Literal[1]
+```
+
+If `staticmethod` is something else, that should not influence the behavior:
+
+```py
+def staticmethod(f):
+    return f
+
+class C:
+    @staticmethod
+    def f(self) -> None:
+        self.x = 1
+
+reveal_type(C().x)  # revealed: Unknown | Literal[1]
+```
+
+And if `staticmethod` is fully qualified, that should also be recognized:
+
+```py
+import builtins
+
+class Other:
+    x: int
+
+class C:
+    @builtins.staticmethod
+    def f(other: Other) -> None:
+        other.x = 1
+
+# error: [unresolved-attribute]
+reveal_type(C.x)  # revealed: Unknown
+
+# TODO: this should raise `unresolved-attribute` as well, and the type should be `Unknown`
+reveal_type(C().x)  # revealed: Unknown | Literal[1]
+```
+
+#### Attributes defined in statically-known-to-be-false branches
+
+```py
+class C:
+    def __init__(self) -> None:
+        # We use a "significantly complex" condition here (instead of just `False`)
+        # for a proper comparison with mypy and pyright, which distinguish between
+        # conditions that can be resolved from a simple pattern matching and those
+        # that need proper type inference.
+        if (2 + 3) < 4:
+            self.x: str = "a"
+
+# TODO: Ideally, this would result in a `unresolved-attribute` error. But mypy and pyright
+# do not support this either (for conditions that can only be resolved to `False` in type
+# inference), so it does not seem to be particularly important.
+reveal_type(C().x)  # revealed: str
 ```
 
 ### Pure class variables (`ClassVar`)
@@ -221,13 +511,13 @@ reveal_type(C.pure_class_variable)  # revealed: Unknown
 
 C.pure_class_variable = "overwritten on class"
 
-# TODO: should be `Literal["overwritten on class"]`
+# TODO: should be  `Unknown | Literal["value set in class method"]` or
+# Literal["overwritten on class"]`, once/if we support local narrowing.
 # error: [unresolved-attribute]
 reveal_type(C.pure_class_variable)  # revealed: Unknown
 
 c_instance = C()
-# TODO: should be `Literal["overwritten on class"]`
-reveal_type(c_instance.pure_class_variable)  # revealed: @Todo(implicit instance attribute)
+reveal_type(c_instance.pure_class_variable)  # revealed: Unknown | Literal["value set in class method"]
 
 # TODO: should raise an error.
 c_instance.pure_class_variable = "value set on instance"
@@ -277,6 +567,53 @@ reveal_type(C.variable_with_class_default1)  # revealed: str
 reveal_type(c_instance.variable_with_class_default1)  # revealed: str
 ```
 
+### Inheritance of class/instance attributes
+
+#### Instance variable defined in a base class
+
+```py
+class Base:
+    declared_in_body: int | None = 1
+
+    base_class_attribute_1: str | None
+    base_class_attribute_2: str | None
+    base_class_attribute_3: str | None
+
+    def __init__(self) -> None:
+        self.defined_in_init: str | None = "value in base"
+
+class Intermediate(Base):
+    # Re-declaring base class attributes with the *same *type is fine:
+    base_class_attribute_1: str | None = None
+
+    # Re-declaring them with a *narrower type* is unsound, because modifications
+    # through a `Base` reference could violate that constraint.
+    #
+    # Mypy does not report an error here, but pyright does: "… overrides symbol
+    # of same name in class "Base". Variable is mutable so its type is invariant"
+    #
+    # We should introduce a diagnostic for this. Whether or not that should be
+    # enabled by default can still be discussed.
+    #
+    # TODO: This should be an error
+    base_class_attribute_2: str
+
+    # Re-declaring attributes with a *wider type* directly violates LSP.
+    #
+    # In this case, both mypy and pyright report an error.
+    #
+    # TODO: This should be an error
+    base_class_attribute_3: str | int | None
+
+class Derived(Intermediate): ...
+
+reveal_type(Derived.declared_in_body)  # revealed: int | None
+
+reveal_type(Derived().declared_in_body)  # revealed: int | None
+
+reveal_type(Derived().defined_in_init)  # revealed: str | None
+```
+
 ## Union of attributes
 
 ```py
@@ -437,7 +774,9 @@ reveal_type(Foo.__class__)  # revealed: Literal[type]
 
 ## Module attributes
 
-```py path=mod.py
+`mod.py`:
+
+```py
 global_symbol: str = "a"
 ```
 
@@ -471,13 +810,19 @@ for mod.global_symbol in IntIterable():
 
 ## Nested attributes
 
-```py path=outer/__init__.py
+`outer/__init__.py`:
+
+```py
 ```
 
-```py path=outer/nested/__init__.py
+`outer/nested/__init__.py`:
+
+```py
 ```
 
-```py path=outer/nested/inner.py
+`outer/nested/inner.py`:
+
+```py
 class Outer:
     class Nested:
         class Inner:
@@ -500,7 +845,7 @@ outer.nested.inner.Outer.Nested.Inner.attr = "a"
 Most attribute accesses on function-literal types are delegated to `types.FunctionType`, since all
 functions are instances of that class:
 
-```py path=a.py
+```py
 def f(): ...
 
 reveal_type(f.__defaults__)  # revealed: @Todo(full tuple[...] support) | None
@@ -509,9 +854,7 @@ reveal_type(f.__kwdefaults__)  # revealed: @Todo(generics) | None
 
 Some attributes are special-cased, however:
 
-```py path=b.py
-def f(): ...
-
+```py
 reveal_type(f.__get__)  # revealed: @Todo(`__get__` method on functions)
 reveal_type(f.__call__)  # revealed: @Todo(`__call__` method on functions)
 ```
@@ -521,14 +864,14 @@ reveal_type(f.__call__)  # revealed: @Todo(`__call__` method on functions)
 Most attribute accesses on int-literal types are delegated to `builtins.int`, since all literal
 integers are instances of that class:
 
-```py path=a.py
+```py
 reveal_type((2).bit_length)  # revealed: @Todo(bound method)
 reveal_type((2).denominator)  # revealed: @Todo(@property)
 ```
 
 Some attributes are special-cased, however:
 
-```py path=b.py
+```py
 reveal_type((2).numerator)  # revealed: Literal[2]
 reveal_type((2).real)  # revealed: Literal[2]
 ```
@@ -538,14 +881,14 @@ reveal_type((2).real)  # revealed: Literal[2]
 Most attribute accesses on bool-literal types are delegated to `builtins.bool`, since all literal
 bols are instances of that class:
 
-```py path=a.py
+```py
 reveal_type(True.__and__)  # revealed: @Todo(bound method)
 reveal_type(False.__or__)  # revealed: @Todo(bound method)
 ```
 
 Some attributes are special-cased, however:
 
-```py path=b.py
+```py
 reveal_type(True.numerator)  # revealed: Literal[1]
 reveal_type(False.real)  # revealed: Literal[0]
 ```
@@ -559,6 +902,90 @@ reveal_type(b"foo".join)  # revealed: @Todo(bound method)
 reveal_type(b"foo".endswith)  # revealed: @Todo(bound method)
 ```
 
+## Instance attribute edge cases
+
+### Assignment to attribute that does not correspond to the instance
+
+```py
+class Other:
+    x: int = 1
+
+class C:
+    def __init__(self, other: Other) -> None:
+        other.x = 1
+
+def f(c: C):
+    # error: [unresolved-attribute]
+    reveal_type(c.x)  # revealed: Unknown
+```
+
+### Nested classes
+
+```py
+class Outer:
+    def __init__(self):
+        self.x: int = 1
+
+    class Middle:
+        # has no 'x' attribute
+
+        class Inner:
+            def __init__(self):
+                self.x: str = "a"
+
+reveal_type(Outer().x)  # revealed: int
+
+# error: [unresolved-attribute]
+Outer.Middle().x
+
+reveal_type(Outer.Middle.Inner().x)  # revealed: str
+```
+
+### Shadowing of `self`
+
+```py
+class Other:
+    x: int = 1
+
+class C:
+    def __init__(self) -> None:
+        # Redeclaration of self. `self` does not refer to the instance anymore.
+        self: Other = Other()
+        self.x: int = 1
+
+# TODO: this should be an error
+C().x
+```
+
+### Assignment to `self` after nested function
+
+```py
+class Other:
+    x: str = "a"
+
+class C:
+    def __init__(self) -> None:
+        def nested_function(self: Other):
+            self.x = "b"
+        self.x: int = 1
+
+reveal_type(C().x)  # revealed: int
+```
+
+### Assignment to `self` from nested function
+
+```py
+class C:
+    def __init__(self) -> None:
+        def set_attribute(value: str):
+            self.x: str = value
+        set_attribute("a")
+
+# TODO: ideally, this would be `str`. Mypy supports this, pyright does not.
+# error: [unresolved-attribute]
+reveal_type(C().x)  # revealed: Unknown
+```
+
 ## References
 
 Some of the tests in the *Class and instance variables* section draw inspiration from

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

@@ -3,6 +3,8 @@
 ## Class instances
 
 ```py
+from typing import Literal
+
 class Yes:
     def __add__(self, other) -> Literal["+"]:
         return "+"
@@ -136,6 +138,8 @@ reveal_type(No() // Yes())  # revealed: Unknown
 ## Subclass reflections override superclass dunders
 
 ```py
+from typing import Literal
+
 class Yes:
     def __add__(self, other) -> Literal["+"]:
         return "+"
@@ -294,6 +298,8 @@ itself. (For these operators to work on the class itself, they would have to be
 class's type, i.e. `type`.)
 
 ```py
+from typing import Literal
+
 class Yes:
     def __add__(self, other) -> Literal["+"]:
         return "+"
@@ -312,6 +318,8 @@ reveal_type(No + No)  # revealed: Unknown
 ## Subclass
 
 ```py
+from typing import Literal
+
 class Yes:
     def __add__(self, other) -> Literal["+"]:
         return "+"

crates/red_knot_python_semantic/resources/mdtest/boundness_declaredness/public.md

@@ -1,6 +1,6 @@
 # Boundness and declaredness: public uses
 
-This document demonstrates how type-inference and diagnostics works for *public* uses of a symbol,
+This document demonstrates how type-inference and diagnostics work for *public* uses of a symbol,
 that is, a use of a symbol from another scope. If a symbol has a declared type in its local scope
 (e.g. `int`), we use that as the symbol's "public type" (the type of the symbol from the perspective
 of other scopes) even if there is a more precise local inferred type for the symbol (`Literal[1]`).
@@ -34,20 +34,28 @@ In particular, we should raise errors in the "possibly-undeclared-and-unbound" a
 ### Declared and bound
 
 If a symbol has a declared type (`int`), we use that even if there is a more precise inferred type
-(`Literal[1]`), or a conflicting inferred type (`Literal[2]`):
+(`Literal[1]`), or a conflicting inferred type (`str` vs. `Literal[2]` below):
 
-```py path=mod.py
-x: int = 1
+`mod.py`:
 
-# error: [invalid-assignment]
-y: str = 2
+```py
+from typing import Any
+
+def any() -> Any: ...
+
+a: int = 1
+b: str = 2  # error: [invalid-assignment]
+c: Any = 3
+d: int = any()
 ```
 
 ```py
-from mod import x, y
+from mod import a, b, c, d
 
-reveal_type(x)  # revealed: int
-reveal_type(y)  # revealed: str
+reveal_type(a)  # revealed: int
+reveal_type(b)  # revealed: str
+reveal_type(c)  # revealed: Any
+reveal_type(d)  # revealed: int
 ```
 
 ### Declared and possibly unbound
@@ -55,22 +63,33 @@ reveal_type(y)  # revealed: str
 If a symbol is declared and *possibly* unbound, we trust that other module and use the declared type
 without raising an error.
 
-```py path=mod.py
+`mod.py`:
+
+```py
+from typing import Any
+
+def any() -> Any: ...
 def flag() -> bool: ...
 
-x: int
-y: str
+a: int
+b: str
+c: Any
+d: int
+
 if flag:
-    x = 1
-    # error: [invalid-assignment]
-    y = 2
+    a = 1
+    b = 2  # error: [invalid-assignment]
+    c = 3
+    d = any()
 ```
 
 ```py
-from mod import x, y
+from mod import a, b, c, d
 
-reveal_type(x)  # revealed: int
-reveal_type(y)  # revealed: str
+reveal_type(a)  # revealed: int
+reveal_type(b)  # revealed: str
+reveal_type(c)  # revealed: Any
+reveal_type(d)  # revealed: int
 ```
 
 ### Declared and unbound
@@ -78,14 +97,20 @@ reveal_type(y)  # revealed: str
 Similarly, if a symbol is declared but unbound, we do not raise an error. We trust that this symbol
 is available somehow and simply use the declared type.
 
-```py path=mod.py
-x: int
+`mod.py`:
+
+```py
+from typing import Any
+
+a: int
+b: Any
 ```
 
 ```py
-from mod import x
+from mod import a, b
 
-reveal_type(x)  # revealed: int
+reveal_type(a)  # revealed: int
+reveal_type(b)  # revealed: Any
 ```
 
 ## Possibly undeclared
@@ -95,61 +120,70 @@ reveal_type(x)  # revealed: int
 If a symbol is possibly undeclared but definitely bound, we use the union of the declared and
 inferred types:
 
-```py path=mod.py
+`mod.py`:
+
+```py
 from typing import Any
 
+def any() -> Any: ...
 def flag() -> bool: ...
 
-x = 1
-y = 2
-z = 3
+a = 1
+b = 2
+c = 3
+d = any()
 if flag():
-    x: int
-    y: Any
-    # error: [invalid-declaration]
-    z: str
+    a: int
+    b: Any
+    c: str  # error: [invalid-declaration]
+    d: int
 ```
 
 ```py
-from mod import x, y, z
+from mod import a, b, c, d
 
-reveal_type(x)  # revealed: int
-reveal_type(y)  # revealed: Literal[2] | Any
-reveal_type(z)  # revealed: Literal[3] | Unknown
+reveal_type(a)  # revealed: int
+reveal_type(b)  # revealed: Literal[2] | Any
+reveal_type(c)  # revealed: Literal[3] | Unknown
+reveal_type(d)  # revealed: Any | int
 
-# External modifications of `x` that violate the declared type are not allowed:
+# External modifications of `a` that violate the declared type are not allowed:
 # error: [invalid-assignment]
-x = None
+a = None
 ```
 
 ### Possibly undeclared and possibly unbound
 
 If a symbol is possibly undeclared and possibly unbound, we also use the union of the declared and
 inferred types. This case is interesting because the "possibly declared" definition might not be the
-same as the "possibly bound" definition (symbol `y`). Note that we raise a `possibly-unbound-import`
-error for both `x` and `y`:
+same as the "possibly bound" definition (symbol `b`). Note that we raise a `possibly-unbound-import`
+error for both `a` and `b`:
+
+`mod.py`:
+
+```py
+from typing import Any
 
-```py path=mod.py
 def flag() -> bool: ...
 
 if flag():
-    x: Any = 1
-    y = 2
+    a: Any = 1
+    b = 2
 else:
-    y: str
+    b: str
 ```
 
 ```py
 # error: [possibly-unbound-import]
 # error: [possibly-unbound-import]
-from mod import x, y
+from mod import a, b
 
-reveal_type(x)  # revealed: Literal[1] | Any
-reveal_type(y)  # revealed: Literal[2] | str
+reveal_type(a)  # revealed: Literal[1] | Any
+reveal_type(b)  # revealed: Literal[2] | str
 
-# External modifications of `y` that violate the declared type are not allowed:
+# External modifications of `b` that violate the declared type are not allowed:
 # error: [invalid-assignment]
-y = None
+b = None
 ```
 
 ### Possibly undeclared and unbound
@@ -157,40 +191,53 @@ y = None
 If a symbol is possibly undeclared and definitely unbound, we currently do not raise an error. This
 seems inconsistent when compared to the case just above.
 
-```py path=mod.py
+`mod.py`:
+
+```py
 def flag() -> bool: ...
 
 if flag():
-    x: int
+    a: int
 ```
 
 ```py
 # TODO: this should raise an error. Once we fix this, update the section description and the table
 # on top of this document.
-from mod import x
+from mod import a
 
-reveal_type(x)  # revealed: int
+reveal_type(a)  # revealed: int
 
-# External modifications to `x` that violate the declared type are not allowed:
+# External modifications to `a` that violate the declared type are not allowed:
 # error: [invalid-assignment]
-x = None
+a = None
 ```
 
 ## Undeclared
 
 ### Undeclared but bound
 
-```py path=mod.py
-x = 1
+If a symbol is *undeclared*, we use the union of `Unknown` with the inferred type. Note that we
+treat this case differently from the case where a symbol is implicitly declared with `Unknown`,
+possibly due to the usage of an unknown name in the annotation:
+
+`mod.py`:
+
+```py
+# Undeclared:
+a = 1
+
+# Implicitly declared with `Unknown`, due to the usage of an unknown name in the annotation:
+b: SomeUnknownName = 1  # error: [unresolved-reference]
 ```
 
 ```py
-from mod import x
+from mod import a, b
 
-reveal_type(x)  # revealed: Unknown | Literal[1]
+reveal_type(a)  # revealed: Unknown | Literal[1]
+reveal_type(b)  # revealed: Unknown
 
-# All external modifications of `x` are allowed:
-x = None
+# All external modifications of `a` are allowed:
+a = None
 ```
 
 ### Undeclared and possibly unbound
@@ -198,39 +245,45 @@ x = None
 If a symbol is undeclared and *possibly* unbound, we currently do not raise an error. This seems
 inconsistent when compared to the "possibly-undeclared-and-possibly-unbound" case.
 
-```py path=mod.py
+`mod.py`:
+
+```py
 def flag() -> bool: ...
 
 if flag:
-    x = 1
+    a = 1
+    b: SomeUnknownName = 1  # error: [unresolved-reference]
 ```
 
 ```py
 # TODO: this should raise an error. Once we fix this, update the section description and the table
 # on top of this document.
-from mod import x
+from mod import a, b
 
-reveal_type(x)  # revealed: Unknown | Literal[1]
+reveal_type(a)  # revealed: Unknown | Literal[1]
+reveal_type(b)  # revealed: Unknown
 
-# All external modifications of `x` are allowed:
-x = None
+# All external modifications of `a` are allowed:
+a = None
 ```
 
 ### Undeclared and unbound
 
 If a symbol is undeclared *and* unbound, we infer `Unknown` and raise an error.
 
-```py path=mod.py
+`mod.py`:
+
+```py
 if False:
-    x: int = 1
+    a: int = 1
 ```
 
 ```py
 # error: [unresolved-import]
-from mod import x
+from mod import a
 
-reveal_type(x)  # revealed: Unknown
+reveal_type(a)  # revealed: Unknown
 
 # Modifications allowed in this case:
-x = None
+a = None
 ```

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

@@ -6,6 +6,8 @@ If we have an intersection type `A & B` and we get a definitive true/false answe
 types, we can infer that the result for the intersection type is also true/false:
 
 ```py
+from typing import Literal
+
 class Base: ...
 
 class Child1(Base):

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

@@ -33,7 +33,7 @@ reveal_type(a >= b)  # revealed: Literal[False]
 
 Even when tuples have different lengths, comparisons should be handled appropriately.
 
-```py path=different_length.py
+```py
 a = (1, 2, 3)
 b = (1, 2, 3, 4)
 
@@ -102,7 +102,7 @@ reveal_type(a >= b)  # revealed: bool
 However, if the lexicographic comparison completes without reaching a point where str and int are
 compared, Python will still produce a result based on the prior elements.
 
-```py path=short_circuit.py
+```py
 a = (1, 2)
 b = (999999, "hello")
 

crates/red_knot_python_semantic/resources/mdtest/descriptor_protocol.md

@@ -0,0 +1,199 @@
+# Descriptor protocol
+
+[Descriptors] let objects customize attribute lookup, storage, and deletion.
+
+A descriptor is an attribute value that has one of the methods in the descriptor protocol. Those
+methods are `__get__()`, `__set__()`, and `__delete__()`. If any of those methods are defined for an
+attribute, it is said to be a descriptor.
+
+## Basic example
+
+An introductory example, modeled after a [simple example] in the primer on descriptors, involving a
+descriptor that returns a constant value:
+
+```py
+from typing import Literal
+
+class Ten:
+    def __get__(self, instance: object, owner: type | None = None) -> Literal[10]:
+        return 10
+
+    def __set__(self, instance: object, value: Literal[10]) -> None:
+        pass
+
+class C:
+    ten = Ten()
+
+c = C()
+
+# TODO: this should be `Literal[10]`
+reveal_type(c.ten)  # revealed: Unknown | Ten
+
+# TODO: This should `Literal[10]`
+reveal_type(C.ten)  # revealed: Unknown | Ten
+
+# These are fine:
+c.ten = 10
+C.ten = 10
+
+# TODO: Both of these should be errors
+c.ten = 11
+C.ten = 11
+```
+
+## Different types for `__get__` and `__set__`
+
+The return type of `__get__` and the value type of `__set__` can be different:
+
+```py
+class FlexibleInt:
+    def __init__(self):
+        self._value: int | None = None
+
+    def __get__(self, instance: object, owner: type | None = None) -> int | None:
+        return self._value
+
+    def __set__(self, instance: object, value: int | str) -> None:
+        self._value = int(value)
+
+class C:
+    flexible_int = FlexibleInt()
+
+c = C()
+
+# TODO: should be `int | None`
+reveal_type(c.flexible_int)  # revealed: Unknown | FlexibleInt
+
+c.flexible_int = 42  # okay
+c.flexible_int = "42"  # also okay!
+
+# TODO: should be `int | None`
+reveal_type(c.flexible_int)  # revealed: Unknown | FlexibleInt
+
+# TODO: should be an error
+c.flexible_int = None  # not okay
+
+# TODO: should be `int | None`
+reveal_type(c.flexible_int)  # revealed: Unknown | FlexibleInt
+```
+
+## Built-in `property` descriptor
+
+The built-in `property` decorator creates a descriptor. The names for attribute reads/writes are
+determined by the return type of the `name` method and the parameter type of the setter,
+respectively.
+
+```py
+class C:
+    _name: str | None = None
+
+    @property
+    def name(self) -> str:
+        return self._name or "Unset"
+    # TODO: No diagnostic should be emitted here
+    # error: [unresolved-attribute] "Type `Literal[name]` has no attribute `setter`"
+    @name.setter
+    def name(self, value: str | None) -> None:
+        self._value = value
+
+c = C()
+
+reveal_type(c._name)  # revealed: str | None
+
+# Should be `str`
+reveal_type(c.name)  # revealed: @Todo(bound method)
+
+# Should be `builtins.property`
+reveal_type(C.name)  # revealed: Literal[name]
+
+# This is fine:
+c.name = "new"
+
+c.name = None
+
+# TODO: this should be an error
+c.name = 42
+```
+
+## Built-in `classmethod` descriptor
+
+Similarly to `property`, `classmethod` decorator creates an implicit descriptor that binds the first
+argument to the class instead of the instance.
+
+```py
+class C:
+    def __init__(self, value: str) -> None:
+        self._name: str = value
+
+    @classmethod
+    def factory(cls, value: str) -> "C":
+        return cls(value)
+
+    @classmethod
+    def get_name(cls) -> str:
+        return cls.__name__
+
+c1 = C.factory("test")  # okay
+
+# TODO: should be `C`
+reveal_type(c1)  # revealed: @Todo(return type)
+
+# TODO: should be `str`
+reveal_type(C.get_name())  # revealed: @Todo(return type)
+
+# TODO: should be `str`
+reveal_type(C("42").get_name())  # revealed: @Todo(bound method)
+```
+
+## Descriptors only work when used as class variables
+
+From the descriptor guide:
+
+> Descriptors only work when used as class variables. When put in instances, they have no effect.
+
+```py
+from typing import Literal
+
+class Ten:
+    def __get__(self, instance: object, owner: type | None = None) -> Literal[10]:
+        return 10
+
+class C:
+    def __init__(self):
+        self.ten = Ten()
+
+reveal_type(C().ten)  # revealed: Unknown | Ten
+```
+
+## Descriptors distinguishing between class and instance access
+
+Overloads can be used to distinguish between when a descriptor is accessed on a class object and
+when it is accessed on an instance. A real-world example of this is the `__get__` method on
+`types.FunctionType`.
+
+```py
+from typing_extensions import Literal, LiteralString, overload
+
+class Descriptor:
+    @overload
+    def __get__(self, instance: None, owner: type, /) -> Literal["called on class object"]: ...
+    @overload
+    def __get__(self, instance: object, owner: type | None = None, /) -> Literal["called on instance"]: ...
+    def __get__(self, instance, owner=None, /) -> LiteralString:
+        if instance:
+            return "called on instance"
+        else:
+            return "called on class object"
+
+class C:
+    d = Descriptor()
+
+# TODO: should be `Literal["called on class object"]
+reveal_type(C.d)  # revealed: Unknown | Descriptor
+
+# TODO: should be `Literal["called on instance"]
+reveal_type(C().d)  # revealed: Unknown | Descriptor
+```
+
+[descriptors]: https://docs.python.org/3/howto/descriptor.html
+[simple example]: https://docs.python.org/3/howto/descriptor.html#simple-example-a-descriptor-that-returns-a-constant

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

@@ -0,0 +1,21 @@
+# Unpacking
+
+<!-- snapshot-diagnostics -->
+
+## Right hand side not iterable
+
+```py
+a, b = 1  # error: [not-iterable]
+```
+
+## Too many values to unpack
+
+```py
+a, b = (1, 2, 3)  # error: [invalid-assignment]
+```
+
+## Too few values to unpack
+
+```py
+a, b = (1,)  # error: [invalid-assignment]
+```

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

@@ -0,0 +1,87 @@
+# Unresolved import diagnostics
+
+<!-- snapshot-diagnostics -->
+
+## Using `from` with an unresolvable module
+
+This example demonstrates the diagnostic when a `from` style import is used with a module that could
+not be found:
+
+```py
+from does_not_exist import add  # error: [unresolved-import]
+
+stat = add(10, 15)
+```
+
+## Using `from` with too many leading dots
+
+This example demonstrates the diagnostic when a `from` style import is used with a presumptively
+valid path, but where there are too many leading dots.
+
+`package/__init__.py`:
+
+```py
+```
+
+`package/foo.py`:
+
+```py
+def add(x, y):
+    return x + y
+```
+
+`package/subpackage/subsubpackage/__init__.py`:
+
+```py
+from ....foo import add  # error: [unresolved-import]
+
+stat = add(10, 15)
+```
+
+## Using `from` with an unknown current module
+
+This is another case handled separately in Red Knot, where a `.` provokes relative module name
+resolution, but where the module name is not resolvable.
+
+```py
+from .does_not_exist import add  # error: [unresolved-import]
+
+stat = add(10, 15)
+```
+
+## Using `from` with an unknown nested module
+
+Like the previous test, but with sub-modules to ensure the span is correct.
+
+```py
+from .does_not_exist.foo.bar import add  # error: [unresolved-import]
+
+stat = add(10, 15)
+```
+
+## Using `from` with a resolvable module but unresolvable item
+
+This ensures that diagnostics for an unresolvable item inside a resolvable import highlight the item
+and not the entire `from ... import ...` statement.
+
+`a.py`:
+
+```py
+does_exist1 = 1
+does_exist2 = 2
+```
+
+```py
+from a import does_exist1, does_not_exist, does_exist2  # error: [unresolved-import]
+```
+
+## An unresolvable import that does not use `from`
+
+This ensures that an unresolvable `import ...` statement highlights just the module name and not the
+entire statement.
+
+```py
+import does_not_exist  # error: [unresolved-import]
+
+x = does_not_exist.foo
+```

crates/red_knot_python_semantic/resources/mdtest/directives/assert_type.md

@@ -78,7 +78,7 @@ def _(a: type[Unknown], b: type[Any]):
 Tuple types with the same elements are the same.
 
 ```py
-from typing_extensions import assert_type
+from typing_extensions import Any, assert_type
 
 from knot_extensions import Unknown
 

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

@@ -124,42 +124,49 @@ def _(e: Exception | type[Exception] | None):
 ## Exception cause is not an exception
 
 ```py
-try:
-    raise EOFError() from GeneratorExit  # fine
-except:
-    ...
+def _():
+    try:
+        raise EOFError() from GeneratorExit  # fine
+    except:
+        ...
 
-try:
-    raise StopIteration from MemoryError()  # fine
-except:
-    ...
+def _():
+    try:
+        raise StopIteration from MemoryError()  # fine
+    except:
+        ...
 
-try:
-    raise BufferError() from None  # fine
-except:
-    ...
+def _():
+    try:
+        raise BufferError() from None  # fine
+    except:
+        ...
 
-try:
-    raise ZeroDivisionError from False  # error: [invalid-raise]
-except:
-    ...
+def _():
+    try:
+        raise ZeroDivisionError from False  # error: [invalid-raise]
+    except:
+        ...
 
-try:
-    raise SystemExit from bool()  # error: [invalid-raise]
-except:
-    ...
+def _():
+    try:
+        raise SystemExit from bool()  # error: [invalid-raise]
+    except:
+        ...
 
-try:
-    raise
-except KeyboardInterrupt as e:  # fine
-    reveal_type(e)  # revealed: KeyboardInterrupt
-    raise LookupError from e  # fine
+def _():
+    try:
+        raise
+    except KeyboardInterrupt as e:  # fine
+        reveal_type(e)  # revealed: KeyboardInterrupt
+        raise LookupError from e  # fine
 
-try:
-    raise
-except int as e:  # error: [invalid-exception-caught]
-    reveal_type(e)  # revealed: Unknown
-    raise KeyError from e
+def _():
+    try:
+        raise
+    except int as e:  # error: [invalid-exception-caught]
+        reveal_type(e)  # revealed: Unknown
+        raise KeyError from e
 
 def _(e: Exception | type[Exception]):
     raise ModuleNotFoundError from e  # fine

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

@@ -29,7 +29,7 @@ completing. The type of `x` at the beginning of the `except` suite in this examp
 `x = could_raise_returns_str()` redefinition, but we *also* could have jumped to the `except` suite
 *after* that redefinition.
 
-```py path=union_type_inferred.py
+```py
 def could_raise_returns_str() -> str:
     return "foo"
 
@@ -50,10 +50,7 @@ reveal_type(x)  # revealed: str | Literal[2]
 If `x` has the same type at the end of both branches, however, the branches unify and `x` is not
 inferred as having a union type following the `try`/`except` block:
 
-```py path=branches_unify_to_non_union_type.py
-def could_raise_returns_str() -> str:
-    return "foo"
-
+```py
 x = 1
 
 try:
@@ -133,7 +130,7 @@ the `except` suite:
 - At the end of `else`, `x == 3`
 - At the end of `except`, `x == 2`
 
-```py path=single_except.py
+```py
 def could_raise_returns_str() -> str:
     return "foo"
 
@@ -161,9 +158,6 @@ been executed in its entirety, or the `try` suite and the `else` suite must both
 in their entireties:
 
 ```py
-def could_raise_returns_str() -> str:
-    return "foo"
-
 x = 1
 
 try:
@@ -192,7 +186,7 @@ A `finally` suite is *always* executed. As such, if we reach the `reveal_type` c
 this example, we know that `x` *must* have been reassigned to `2` during the `finally` suite. The
 type of `x` at the end of the example is therefore `Literal[2]`:
 
-```py path=redef_in_finally.py
+```py
 def could_raise_returns_str() -> str:
     return "foo"
 
@@ -217,10 +211,7 @@ at this point than there were when we were inside the `finally` block.
 (Our current model does *not* correctly infer the types *inside* `finally` suites, however; this is
 still a TODO item for us.)
 
-```py path=no_redef_in_finally.py
-def could_raise_returns_str() -> str:
-    return "foo"
-
+```py
 x = 1
 
 try:
@@ -249,7 +240,7 @@ suites:
     exception raised in the `except` suite to cause us to jump to the `finally` suite before the
     `except` suite ran to completion
 
-```py path=redef_in_finally.py
+```py
 def could_raise_returns_str() -> str:
     return "foo"
 
@@ -286,16 +277,7 @@ itself. (In some control-flow possibilities, some exceptions were merely *suspen
 `finally` suite; these lead to the scope's termination following the conclusion of the `finally`
 suite.)
 
-```py path=no_redef_in_finally.py
-def could_raise_returns_str() -> str:
-    return "foo"
-
-def could_raise_returns_bytes() -> bytes:
-    return b"foo"
-
-def could_raise_returns_bool() -> bool:
-    return True
-
+```py
 x = 1
 
 try:
@@ -317,16 +299,7 @@ reveal_type(x)  # revealed: str | bool
 
 An example with multiple `except` branches and a `finally` branch:
 
-```py path=multiple_except_branches.py
-def could_raise_returns_str() -> str:
-    return "foo"
-
-def could_raise_returns_bytes() -> bytes:
-    return b"foo"
-
-def could_raise_returns_bool() -> bool:
-    return True
-
+```py
 def could_raise_returns_memoryview() -> memoryview:
     return memoryview(b"")
 
@@ -364,7 +337,7 @@ If the exception handler has an `else` branch, we must also take into account th
 control flow could have jumped to the `finally` suite from partway through the `else` suite due to
 an exception raised *there*.
 
-```py path=single_except_branch.py
+```py
 def could_raise_returns_str() -> str:
     return "foo"
 
@@ -407,22 +380,7 @@ reveal_type(x)  # revealed: bool | float
 
 The same again, this time with multiple `except` branches:
 
-```py path=multiple_except_branches.py
-def could_raise_returns_str() -> str:
-    return "foo"
-
-def could_raise_returns_bytes() -> bytes:
-    return b"foo"
-
-def could_raise_returns_bool() -> bool:
-    return True
-
-def could_raise_returns_memoryview() -> memoryview:
-    return memoryview(b"")
-
-def could_raise_returns_float() -> float:
-    return 3.14
-
+```py
 def could_raise_returns_range() -> range:
     return range(42)
 

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

@@ -54,7 +54,9 @@ reveal_type("x" or "y" and "")  # revealed: Literal["x"]
 
 ## Evaluates to builtin
 
-```py path=a.py
+`a.py`:
+
+```py
 redefined_builtin_bool: type[bool] = bool
 
 def my_bool(x) -> bool:

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

@@ -28,6 +28,8 @@ reveal_type(1 if 0 else 2)  # revealed: Literal[2]
 The test inside an if expression should not affect code outside of the expression.
 
 ```py
+from typing import Literal
+
 def _(flag: bool):
     x: Literal[42, "hello"] = 42 if flag else "hello"
 

crates/red_knot_python_semantic/resources/mdtest/generics.md

@@ -51,7 +51,7 @@ In type stubs, classes can reference themselves in their base class definitions.
 
 This should hold true even with generics at play.
 
-```py path=a.pyi
+```pyi
 class Seq[T]: ...
 
 # TODO not error on the subscripting

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

@@ -9,7 +9,9 @@ E = D
 reveal_type(E)  # revealed: Literal[C]
 ```
 
-```py path=b.py
+`b.py`:
+
+```py
 class C: ...
 ```
 
@@ -22,7 +24,9 @@ D = b.C
 reveal_type(D)  # revealed: Literal[C]
 ```
 
-```py path=b.py
+`b.py`:
+
+```py
 class C: ...
 ```
 
@@ -34,10 +38,14 @@ import a.b
 reveal_type(a.b.C)  # revealed: Literal[C]
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 class C: ...
 ```
 
@@ -49,13 +57,19 @@ import a.b.c
 reveal_type(a.b.c.C)  # revealed: Literal[C]
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b/__init__.py
+`a/b/__init__.py`:
+
+```py
 ```
 
-```py path=a/b/c.py
+`a/b/c.py`:
+
+```py
 class C: ...
 ```
 
@@ -67,10 +81,14 @@ import a.b as b
 reveal_type(b.C)  # revealed: Literal[C]
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 class C: ...
 ```
 
@@ -82,18 +100,34 @@ import a.b.c as c
 reveal_type(c.C)  # revealed: Literal[C]
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b/__init__.py
+`a/b/__init__.py`:
+
+```py
 ```
 
-```py path=a/b/c.py
+`a/b/c.py`:
+
+```py
 class C: ...
 ```
 
+## Unresolvable module import
+
+<!-- snapshot-diagnostics -->
+
+```py
+import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve import `zqzqzqzqzqzqzq`"
+```
+
 ## Unresolvable submodule imports
 
+<!-- snapshot-diagnostics -->
+
 ```py
 # Topmost component resolvable, submodule not resolvable:
 import a.foo  # error: [unresolved-import] "Cannot resolve import `a.foo`"
@@ -102,5 +136,7 @@ import a.foo  # error: [unresolved-import] "Cannot resolve import `a.foo`"
 import b.foo  # error: [unresolved-import] "Cannot resolve import `b.foo`"
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```

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

@@ -29,13 +29,17 @@ builtins from the "actual" vendored typeshed:
 typeshed = "/typeshed"
 ```
 
-```pyi path=/typeshed/stdlib/builtins.pyi
+`/typeshed/stdlib/builtins.pyi`:
+
+```pyi
 class Custom: ...
 
 custom_builtin: Custom
 ```
 
-```pyi path=/typeshed/stdlib/typing_extensions.pyi
+`/typeshed/stdlib/typing_extensions.pyi`:
+
+```pyi
 def reveal_type(obj, /): ...
 ```
 
@@ -56,12 +60,16 @@ that point:
 typeshed = "/typeshed"
 ```
 
-```pyi path=/typeshed/stdlib/builtins.pyi
+`/typeshed/stdlib/builtins.pyi`:
+
+```pyi
 foo = bar
 bar = 1
 ```
 
-```pyi path=/typeshed/stdlib/typing_extensions.pyi
+`/typeshed/stdlib/typing_extensions.pyi`:
+
+```pyi
 def reveal_type(obj, /): ...
 ```
 

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

@@ -2,7 +2,9 @@
 
 ## Maybe unbound
 
-```py path=maybe_unbound.py
+`maybe_unbound.py`:
+
+```py
 def coinflip() -> bool:
     return True
 
@@ -29,7 +31,9 @@ reveal_type(y)  # revealed: Unknown | Literal[3]
 
 ## Maybe unbound annotated
 
-```py path=maybe_unbound_annotated.py
+`maybe_unbound_annotated.py`:
+
+```py
 def coinflip() -> bool:
     return True
 
@@ -60,7 +64,9 @@ reveal_type(y)  # revealed: int
 
 Importing a possibly undeclared name still gives us its declared type:
 
-```py path=maybe_undeclared.py
+`maybe_undeclared.py`:
+
+```py
 def coinflip() -> bool:
     return True
 
@@ -76,11 +82,15 @@ reveal_type(x)  # revealed: int
 
 ## Reimport
 
-```py path=c.py
+`c.py`:
+
+```py
 def f(): ...
 ```
 
-```py path=b.py
+`b.py`:
+
+```py
 def coinflip() -> bool:
     return True
 
@@ -102,11 +112,15 @@ reveal_type(f)  # revealed: Literal[f, f]
 When we have a declared type in one path and only an inferred-from-definition type in the other, we
 should still be able to unify those:
 
-```py path=c.pyi
+`c.pyi`:
+
+```pyi
 x: int
 ```
 
-```py path=b.py
+`b.py`:
+
+```py
 def coinflip() -> bool:
     return True
 

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

@@ -8,11 +8,15 @@ import a.b
 reveal_type(a.b)  # revealed: <module 'a.b'>
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 b: int = 42
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 ```
 
 ## Via from/import
@@ -23,11 +27,15 @@ from a import b
 reveal_type(b)  # revealed: int
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 b: int = 42
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 ```
 
 ## Via both
@@ -40,11 +48,15 @@ reveal_type(b)  # revealed: <module 'a.b'>
 reveal_type(a.b)  # revealed: <module 'a.b'>
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 b: int = 42
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 ```
 
 ## Via both (backwards)
@@ -65,11 +77,15 @@ reveal_type(b)  # revealed: <module 'a.b'>
 reveal_type(a.b)  # revealed: <module 'a.b'>
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 b: int = 42
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 ```
 
 [from-import]: https://docs.python.org/3/reference/simple_stmts.html#the-import-statement

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

@@ -18,7 +18,9 @@ reveal_type(baz)  # revealed: Unknown
 
 ## Unresolved import from resolved module
 
-```py path=a.py
+`a.py`:
+
+```py
 ```
 
 ```py
@@ -29,7 +31,9 @@ reveal_type(thing)  # revealed: Unknown
 
 ## Resolved import of symbol from unresolved import
 
-```py path=a.py
+`a.py`:
+
+```py
 import foo as foo  # error: "Cannot resolve import `foo`"
 
 reveal_type(foo)  # revealed: Unknown
@@ -46,7 +50,9 @@ reveal_type(foo)  # revealed: Unknown
 
 ## No implicit shadowing
 
-```py path=b.py
+`b.py`:
+
+```py
 x: int
 ```
 
@@ -58,7 +64,9 @@ x = "foo"  # error: [invalid-assignment] "Object of type `Literal["foo"]"
 
 ## Import cycle
 
-```py path=a.py
+`a.py`:
+
+```py
 class A: ...
 
 reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Literal[object]]
@@ -69,7 +77,9 @@ class C(b.B): ...
 reveal_type(C.__mro__)  # revealed: tuple[Literal[C], Literal[B], Literal[A], Literal[object]]
 ```
 
-```py path=b.py
+`b.py`:
+
+```py
 from a import A
 
 class B(A): ...

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

@@ -23,9 +23,13 @@ reveal_type(b)  # revealed: <module 'a.b'>
 reveal_type(b.c)  # revealed: int
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 c: int = 1
 ```

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

@@ -2,10 +2,14 @@
 
 ## Non-existent
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from .foo import X  # error: [unresolved-import]
 
 reveal_type(X)  # revealed: Unknown
@@ -13,14 +17,20 @@ reveal_type(X)  # revealed: Unknown
 
 ## Simple
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/foo.py
+`package/foo.py`:
+
+```py
 X: int = 42
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from .foo import X
 
 reveal_type(X)  # revealed: int
@@ -28,14 +38,20 @@ reveal_type(X)  # revealed: int
 
 ## Dotted
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/foo/bar/baz.py
+`package/foo/bar/baz.py`:
+
+```py
 X: int = 42
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from .foo.bar.baz import X
 
 reveal_type(X)  # revealed: int
@@ -43,11 +59,15 @@ reveal_type(X)  # revealed: int
 
 ## Bare to package
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 X: int = 42
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from . import X
 
 reveal_type(X)  # revealed: int
@@ -55,7 +75,9 @@ reveal_type(X)  # revealed: int
 
 ## Non-existent + bare to package
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from . import X  # error: [unresolved-import]
 
 reveal_type(X)  # revealed: Unknown
@@ -63,19 +85,25 @@ reveal_type(X)  # revealed: Unknown
 
 ## Dunder init
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 from .foo import X
 
 reveal_type(X)  # revealed: int
 ```
 
-```py path=package/foo.py
+`package/foo.py`:
+
+```py
 X: int = 42
 ```
 
 ## Non-existent + dunder init
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 from .foo import X  # error: [unresolved-import]
 
 reveal_type(X)  # revealed: Unknown
@@ -83,14 +111,20 @@ reveal_type(X)  # revealed: Unknown
 
 ## Long relative import
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/foo.py
+`package/foo.py`:
+
+```py
 X: int = 42
 ```
 
-```py path=package/subpackage/subsubpackage/bar.py
+`package/subpackage/subsubpackage/bar.py`:
+
+```py
 from ...foo import X
 
 reveal_type(X)  # revealed: int
@@ -98,14 +132,20 @@ reveal_type(X)  # revealed: int
 
 ## Unbound symbol
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/foo.py
+`package/foo.py`:
+
+```py
 x  # error: [unresolved-reference]
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from .foo import x  # error: [unresolved-import]
 
 reveal_type(x)  # revealed: Unknown
@@ -113,14 +153,20 @@ reveal_type(x)  # revealed: Unknown
 
 ## Bare to module
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/foo.py
+`package/foo.py`:
+
+```py
 X: int = 42
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from . import foo
 
 reveal_type(foo.X)  # revealed: int
@@ -131,10 +177,14 @@ reveal_type(foo.X)  # revealed: int
 This test verifies that we emit an error when we try to import a symbol that is neither a submodule
 nor an attribute of `package`.
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from . import foo  # error: [unresolved-import]
 
 reveal_type(foo)  # revealed: Unknown
@@ -148,17 +198,53 @@ submodule when that submodule name appears in the `imported_modules` set. That m
 that are imported via `from...import` are not visible to our type inference if you also access that
 submodule via the attribute on its parent package.
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/foo.py
+`package/foo.py`:
+
+```py
 X: int = 42
 ```
 
-```py path=package/bar.py
+`package/bar.py`:
+
+```py
 from . import foo
 import package
 
 # error: [unresolved-attribute] "Type `<module 'package'>` has no attribute `foo`"
 reveal_type(package.foo.X)  # revealed: Unknown
 ```
+
+## In the src-root
+
+`parser.py`:
+
+```py
+X: int = 42
+```
+
+`__main__.py`:
+
+```py
+from .parser import X
+
+reveal_type(X)  # revealed: int
+```
+
+## Beyond the src-root
+
+`parser.py`:
+
+```py
+X: int = 42
+```
+
+`__main__.py`:
+
+```py
+from ..parser import X  # error: [unresolved-import]
+```

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

@@ -9,7 +9,9 @@ y = x
 reveal_type(y)  # revealed: int
 ```
 
-```py path=b.pyi
+`b.pyi`:
+
+```pyi
 x: int
 ```
 
@@ -22,6 +24,8 @@ y = x
 reveal_type(y)  # revealed: int
 ```
 
-```py path=b.py
+`b.py`:
+
+```py
 x: int = 1
 ```

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

@@ -32,10 +32,14 @@ reveal_type(a.b.C)  # revealed: Literal[C]
 import a.b
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 class C: ...
 ```
 
@@ -55,14 +59,20 @@ reveal_type(a.b)  # revealed: <module 'a.b'>
 reveal_type(a.b.C)  # revealed: Literal[C]
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 class C: ...
 ```
 
-```py path=q.py
+`q.py`:
+
+```py
 import a as a
 import a.b as b
 ```
@@ -83,18 +93,26 @@ reveal_type(sub.b)  # revealed: <module 'sub.b'>
 reveal_type(attr.b)  # revealed: <module 'attr.b'>
 ```
 
-```py path=sub/__init__.py
+`sub/__init__.py`:
+
+```py
 b = 1
 ```
 
-```py path=sub/b.py
+`sub/b.py`:
+
+```py
 ```
 
-```py path=attr/__init__.py
+`attr/__init__.py`:
+
+```py
 from . import b as _
 
 b = 1
 ```
 
-```py path=attr/b.py
+`attr/b.py`:
+
+```py
 ```

crates/red_knot_python_semantic/resources/mdtest/intersection_types.md

@@ -808,6 +808,7 @@ Dynamic types do not cancel each other out. Intersecting an unknown set of value
 of another unknown set of values is not necessarily empty, so we keep the positive contribution:
 
 ```py
+from typing import Any
 from knot_extensions import Intersection, Not, Unknown
 
 def any(
@@ -830,6 +831,7 @@ def unknown(
 We currently do not simplify mixed dynamic types, but might consider doing so in the future:
 
 ```py
+from typing import Any
 from knot_extensions import Intersection, Not, Unknown
 
 def mixed(

crates/red_knot_python_semantic/resources/mdtest/known_constants.md

@@ -31,7 +31,9 @@ reveal_type(TC)  # revealed: Literal[True]
 Make sure we only use our special handling for `typing.TYPE_CHECKING` and not for other constants
 with the same name:
 
-```py path=constants.py
+`constants.py`:
+
+```py
 TYPE_CHECKING: bool = False
 ```
 

crates/red_knot_python_semantic/resources/mdtest/mdtest_config.md

@@ -13,6 +13,8 @@ python-version = "3.10"
 Here, we simply make sure that we pick up the global configuration from the root section:
 
 ```py
+import sys
+
 reveal_type(sys.version_info[:2] == (3, 10))  # revealed: Literal[True]
 ```
 
@@ -25,6 +27,8 @@ reveal_type(sys.version_info[:2] == (3, 10))  # revealed: Literal[True]
 The same should work for arbitrarily nested sections:
 
 ```py
+import sys
+
 reveal_type(sys.version_info[:2] == (3, 10))  # revealed: Literal[True]
 ```
 
@@ -38,6 +42,8 @@ python-version = "3.11"
 ```
 
 ```py
+import sys
+
 reveal_type(sys.version_info[:2] == (3, 11))  # revealed: Literal[True]
 ```
 
@@ -46,6 +52,8 @@ reveal_type(sys.version_info[:2] == (3, 11))  # revealed: Literal[True]
 There is no global state. This section should again use the root configuration:
 
 ```py
+import sys
+
 reveal_type(sys.version_info[:2] == (3, 10))  # revealed: Literal[True]
 ```
 
@@ -63,5 +71,7 @@ python-version = "3.12"
 ### Grandchild
 
 ```py
+import sys
+
 reveal_type(sys.version_info[:2] == (3, 12))  # revealed: Literal[True]
 ```

crates/red_knot_python_semantic/resources/mdtest/mdtest_custom_typeshed.md

@@ -19,13 +19,17 @@ typeshed = "/typeshed"
 
 We can then place custom stub files in `/typeshed/stdlib`, for example:
 
-```pyi path=/typeshed/stdlib/builtins.pyi
+`/typeshed/stdlib/builtins.pyi`:
+
+```pyi
 class BuiltinClass: ...
 
 builtin_symbol: BuiltinClass
 ```
 
-```pyi path=/typeshed/stdlib/sys/__init__.pyi
+`/typeshed/stdlib/sys/__init__.pyi`:
+
+```pyi
 version = "my custom Python"
 ```
 
@@ -54,15 +58,21 @@ python-version = "3.10"
 typeshed = "/typeshed"
 ```
 
-```pyi path=/typeshed/stdlib/old_module.pyi
+`/typeshed/stdlib/old_module.pyi`:
+
+```pyi
 class OldClass: ...
 ```
 
-```pyi path=/typeshed/stdlib/new_module.pyi
+`/typeshed/stdlib/new_module.pyi`:
+
+```pyi
 class NewClass: ...
 ```
 
-```text path=/typeshed/stdlib/VERSIONS
+`/typeshed/stdlib/VERSIONS`:
+
+```text
 old_module: 3.0-
 new_module: 3.11-
 ```
@@ -86,7 +96,9 @@ simple untyped definition is enough to make `reveal_type` work in tests:
 typeshed = "/typeshed"
 ```
 
-```pyi path=/typeshed/stdlib/typing_extensions.pyi
+`/typeshed/stdlib/typing_extensions.pyi`:
+
+```pyi
 def reveal_type(obj, /): ...
 ```
 

crates/red_knot_python_semantic/resources/mdtest/metaclass.md

@@ -205,7 +205,7 @@ reveal_type(D.__class__)  # revealed: Literal[SignatureMismatch]
 
 Retrieving the metaclass of a cyclically defined class should not cause an infinite loop.
 
-```py path=a.pyi
+```pyi
 class A(B): ...  # error: [cyclic-class-definition]
 class B(C): ...  # error: [cyclic-class-definition]
 class C(A): ...  # error: [cyclic-class-definition]

crates/red_knot_python_semantic/resources/mdtest/mro.md

@@ -347,7 +347,7 @@ reveal_type(unknown_object.__mro__)  # revealed: Unknown
 
 These are invalid, but we need to be able to handle them gracefully without panicking.
 
-```py path=a.pyi
+```pyi
 class Foo(Foo): ...  # error: [cyclic-class-definition]
 
 reveal_type(Foo)  # revealed: Literal[Foo]
@@ -365,7 +365,7 @@ reveal_type(Boz.__mro__)  # revealed: tuple[Literal[Boz], Unknown, Literal[objec
 
 These are similarly unlikely, but we still shouldn't crash:
 
-```py path=a.pyi
+```pyi
 class Foo(Bar): ...  # error: [cyclic-class-definition]
 class Bar(Baz): ...  # error: [cyclic-class-definition]
 class Baz(Foo): ...  # error: [cyclic-class-definition]
@@ -377,7 +377,7 @@ reveal_type(Baz.__mro__)  # revealed: tuple[Literal[Baz], Unknown, Literal[objec
 
 ## Classes with cycles in their MROs, and multiple inheritance
 
-```py path=a.pyi
+```pyi
 class Spam: ...
 class Foo(Bar): ...  # error: [cyclic-class-definition]
 class Bar(Baz): ...  # error: [cyclic-class-definition]
@@ -390,7 +390,7 @@ reveal_type(Baz.__mro__)  # revealed: tuple[Literal[Baz], Unknown, Literal[objec
 
 ## Classes with cycles in their MRO, and a sub-graph
 
-```py path=a.pyi
+```pyi
 class FooCycle(BarCycle): ...  # error: [cyclic-class-definition]
 class Foo: ...
 class BarCycle(FooCycle): ...  # error: [cyclic-class-definition]

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

@@ -57,6 +57,8 @@ def _(flag1: bool, flag2: bool, flag3: bool, flag4: bool):
 ## Multiple predicates
 
 ```py
+from typing import Literal
+
 def _(flag1: bool, flag2: bool):
     class A: ...
     x: A | None | Literal[1] = A() if flag1 else None if flag2 else 1
@@ -67,6 +69,8 @@ def _(flag1: bool, flag2: bool):
 ## Mix of `and` and `or`
 
 ```py
+from typing import Literal
+
 def _(flag1: bool, flag2: bool):
     class A: ...
     x: A | None | Literal[1] = A() if flag1 else None if flag2 else 1

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

@@ -3,6 +3,8 @@
 ## Value Literals
 
 ```py
+from typing import Literal
+
 def foo() -> Literal[0, -1, True, False, "", "foo", b"", b"bar", None] | tuple[()]:
     return 0
 
@@ -123,6 +125,8 @@ always returns a fixed value.
 These types can always be fully narrowed in boolean contexts, as shown below:
 
 ```py
+from typing import Literal
+
 class T:
     def __bool__(self) -> Literal[True]:
         return True
@@ -149,6 +153,8 @@ else:
 ## Narrowing Complex Intersection and Union
 
 ```py
+from typing import Literal
+
 class A: ...
 class B: ...
 
@@ -181,6 +187,8 @@ if isinstance(x, str) and not isinstance(x, B):
 ## Narrowing Multiple Variables
 
 ```py
+from typing import Literal
+
 def f(x: Literal[0, 1], y: Literal["", "hello"]):
     if x and y and not x and not y:
         reveal_type(x)  # revealed: Never
@@ -222,6 +230,8 @@ reveal_type(y)  # revealed: A
 ## Truthiness of classes
 
 ```py
+from typing import Literal
+
 class MetaAmbiguous(type):
     def __bool__(self) -> bool: ...
 

crates/red_knot_python_semantic/resources/mdtest/regression/14334_diagnostics_in_wrong_file.md

@@ -2,12 +2,16 @@
 
 Regression test for [this issue](https://github.com/astral-sh/ruff/issues/14334).
 
-```py path=base.py
+`base.py`:
+
+```py
 # error: [invalid-base]
 class Base(2): ...
 ```
 
-```py path=a.py
+`a.py`:
+
+```py
 # No error here
 from base import Base
 ```

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

@@ -13,7 +13,7 @@ if returns_bool():
     chr: int = 1
 
 def f():
-    reveal_type(chr)  # revealed: Literal[chr] | int
+    reveal_type(chr)  # revealed: int | Literal[chr]
 ```
 
 ## Conditionally global or builtin, with annotation
@@ -28,5 +28,5 @@ if returns_bool():
     chr: int = 1
 
 def f():
-    reveal_type(chr)  # revealed: Literal[chr] | int
+    reveal_type(chr)  # revealed: int | Literal[chr]
 ```

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

@@ -29,7 +29,7 @@ def foo():
 However, three attributes on `types.ModuleType` are not present as implicit module globals; these
 are excluded:
 
-```py path=unbound_dunders.py
+```py
 # error: [unresolved-reference]
 # revealed: Unknown
 reveal_type(__getattr__)
@@ -54,10 +54,10 @@ inside the module:
 import typing
 
 reveal_type(typing.__name__)  # revealed: str
-reveal_type(typing.__init__)  # revealed: Literal[__init__]
+reveal_type(typing.__init__)  # revealed: @Todo(bound method)
 
 # These come from `builtins.object`, not `types.ModuleType`:
-reveal_type(typing.__eq__)  # revealed: Literal[__eq__]
+reveal_type(typing.__eq__)  # revealed: @Todo(bound method)
 
 reveal_type(typing.__class__)  # revealed: Literal[ModuleType]
 
@@ -70,9 +70,7 @@ Typeshed includes a fake `__getattr__` method in the stub for `types.ModuleType`
 dynamic imports; but we ignore that for module-literal types where we know exactly which module
 we're dealing with:
 
-```py path=__getattr__.py
-import typing
-
+```py
 # error: [unresolved-attribute]
 reveal_type(typing.__getattr__)  # revealed: Unknown
 ```
@@ -83,13 +81,17 @@ It's impossible to override the `__dict__` attribute of `types.ModuleType` insta
 module; we should prioritise the attribute in the `types.ModuleType` stub over a variable named
 `__dict__` in the module's global namespace:
 
-```py path=foo.py
+`foo.py`:
+
+```py
 __dict__ = "foo"
 
 reveal_type(__dict__)  # revealed: Literal["foo"]
 ```
 
-```py path=bar.py
+`bar.py`:
+
+```py
 import foo
 from foo import __dict__ as foo_dict
 

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

@@ -5,14 +5,14 @@
 Parameter `x` of type `str` is shadowed and reassigned with a new `int` value inside the function.
 No diagnostics should be generated.
 
-```py path=a.py
+```py
 def f(x: str):
     x: int = int(x)
 ```
 
 ## Implicit error
 
-```py path=a.py
+```py
 def f(): ...
 
 f = 1  # error: "Implicit shadowing of function `f`; annotate to make it explicit if this is intentional"
@@ -20,7 +20,7 @@ f = 1  # error: "Implicit shadowing of function `f`; annotate to make it explici
 
 ## Explicit shadowing
 
-```py path=a.py
+```py
 def f(): ...
 
 f: int = 1

crates/red_knot_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Unresolvable_module_import.snap

@@ -0,0 +1,28 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Unresolvable module import
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve import `zqzqzqzqzqzqzq`"
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:1:8
+  |
+1 | import zqzqzqzqzqzqzq  # error: [unresolved-import] "Cannot resolve import `zqzqzqzqzqzqzq`"
+  |        ^^^^^^^^^^^^^^ Cannot resolve import `zqzqzqzqzqzqzq`
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/basic.md_-_Structures_-_Unresolvable_submodule_imports.snap

@@ -0,0 +1,51 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: basic.md - Structures - Unresolvable submodule imports
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/import/basic.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | # Topmost component resolvable, submodule not resolvable:
+2 | import a.foo  # error: [unresolved-import] "Cannot resolve import `a.foo`"
+3 | 
+4 | # Topmost component unresolvable:
+5 | import b.foo  # error: [unresolved-import] "Cannot resolve import `b.foo`"
+```
+
+## a/__init__.py
+
+```
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:2:8
+  |
+1 | # Topmost component resolvable, submodule not resolvable:
+2 | import a.foo  # error: [unresolved-import] "Cannot resolve import `a.foo`"
+  |        ^^^^^ Cannot resolve import `a.foo`
+3 |
+4 | # Topmost component unresolvable:
+  |
+
+```
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:5:8
+  |
+4 | # Topmost component unresolvable:
+5 | import b.foo  # error: [unresolved-import] "Cannot resolve import `b.foo`"
+  |        ^^^^^ Cannot resolve import `b.foo`
+  |
+
+```

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

@@ -0,0 +1,28 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unpacking.md - Unpacking - Right hand side not iterable
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unpacking.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | a, b = 1  # error: [not-iterable]
+```
+
+# Diagnostics
+
+```
+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
+  |
+
+```

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

@@ -0,0 +1,28 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unpacking.md - Unpacking - Too few values to unpack
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unpacking.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | a, b = (1,)  # error: [invalid-assignment]
+```
+
+# Diagnostics
+
+```
+error: lint:invalid-assignment
+ --> /src/mdtest_snippet.py:1:1
+  |
+1 | a, b = (1,)  # error: [invalid-assignment]
+  | ^^^^ Not enough values to unpack (expected 2, got 1)
+  |
+
+```

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

@@ -0,0 +1,28 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unpacking.md - Unpacking - Too many values to unpack
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unpacking.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | a, b = (1, 2, 3)  # error: [invalid-assignment]
+```
+
+# Diagnostics
+
+```
+error: lint:invalid-assignment
+ --> /src/mdtest_snippet.py:1:1
+  |
+1 | a, b = (1, 2, 3)  # error: [invalid-assignment]
+  | ^^^^ Too many values to unpack (expected 2, got 3)
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_diagnostics_-_An_unresolvable_import_that_does_not_use_`from`.snap

@@ -0,0 +1,32 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unresolved_import.md - Unresolved import diagnostics - An unresolvable import that does not use `from`
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unresolved_import.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | import does_not_exist  # error: [unresolved-import]
+2 | 
+3 | x = does_not_exist.foo
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:1:8
+  |
+1 | import does_not_exist  # error: [unresolved-import]
+  |        ^^^^^^^^^^^^^^ Cannot resolve import `does_not_exist`
+2 |
+3 | x = does_not_exist.foo
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_diagnostics_-_Using_`from`_with_a_resolvable_module_but_unresolvable_item.snap

@@ -0,0 +1,35 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unresolved_import.md - Unresolved import diagnostics - Using `from` with a resolvable module but unresolvable item
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unresolved_import.md
+---
+
+# Python source files
+
+## a.py
+
+```
+1 | does_exist1 = 1
+2 | does_exist2 = 2
+```
+
+## mdtest_snippet.py
+
+```
+1 | from a import does_exist1, does_not_exist, does_exist2  # error: [unresolved-import]
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:1:28
+  |
+1 | from a import does_exist1, does_not_exist, does_exist2  # error: [unresolved-import]
+  |                            ^^^^^^^^^^^^^^ Module `a` has no member `does_not_exist`
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_diagnostics_-_Using_`from`_with_an_unknown_current_module.snap

@@ -0,0 +1,32 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unresolved_import.md - Unresolved import diagnostics - Using `from` with an unknown current module
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unresolved_import.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from .does_not_exist import add  # error: [unresolved-import]
+2 | 
+3 | stat = add(10, 15)
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:1:7
+  |
+1 | from .does_not_exist import add  # error: [unresolved-import]
+  |       ^^^^^^^^^^^^^^ Cannot resolve import `.does_not_exist`
+2 |
+3 | stat = add(10, 15)
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_diagnostics_-_Using_`from`_with_an_unknown_nested_module.snap

@@ -0,0 +1,32 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unresolved_import.md - Unresolved import diagnostics - Using `from` with an unknown nested module
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unresolved_import.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from .does_not_exist.foo.bar import add  # error: [unresolved-import]
+2 | 
+3 | stat = add(10, 15)
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:1:7
+  |
+1 | from .does_not_exist.foo.bar import add  # error: [unresolved-import]
+  |       ^^^^^^^^^^^^^^^^^^^^^^ Cannot resolve import `.does_not_exist.foo.bar`
+2 |
+3 | stat = add(10, 15)
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_diagnostics_-_Using_`from`_with_an_unresolvable_module.snap

@@ -0,0 +1,32 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unresolved_import.md - Unresolved import diagnostics - Using `from` with an unresolvable module
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unresolved_import.md
+---
+
+# Python source files
+
+## mdtest_snippet.py
+
+```
+1 | from does_not_exist import add  # error: [unresolved-import]
+2 | 
+3 | stat = add(10, 15)
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/mdtest_snippet.py:1:6
+  |
+1 | from does_not_exist import add  # error: [unresolved-import]
+  |      ^^^^^^^^^^^^^^ Cannot resolve import `does_not_exist`
+2 |
+3 | stat = add(10, 15)
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_diagnostics_-_Using_`from`_with_too_many_leading_dots.snap

@@ -0,0 +1,44 @@
+---
+source: crates/red_knot_test/src/lib.rs
+expression: snapshot
+---
+---
+mdtest name: unresolved_import.md - Unresolved import diagnostics - Using `from` with too many leading dots
+mdtest path: crates/red_knot_python_semantic/resources/mdtest/diagnostics/unresolved_import.md
+---
+
+# Python source files
+
+## package/__init__.py
+
+```
+```
+
+## package/foo.py
+
+```
+1 | def add(x, y):
+2 |     return x + y
+```
+
+## package/subpackage/subsubpackage/__init__.py
+
+```
+1 | from ....foo import add  # error: [unresolved-import]
+2 | 
+3 | stat = add(10, 15)
+```
+
+# Diagnostics
+
+```
+error: lint:unresolved-import
+ --> /src/package/subpackage/subsubpackage/__init__.py:1:10
+  |
+1 | from ....foo import add  # error: [unresolved-import]
+  |          ^^^ Cannot resolve import `....foo`
+2 |
+3 | stat = add(10, 15)
+  |
+
+```

crates/red_knot_python_semantic/resources/mdtest/statically_known_branches.md

@@ -7,35 +7,36 @@ branches whose conditions we can statically determine to be always true or alway
 useful for `sys.version_info` branches, which can make new features available based on the Python
 version:
 
-```py path=module1.py
-import sys
-
-if sys.version_info >= (3, 9):
-    SomeFeature: str = "available"
-```
-
 If we can statically determine that the condition is always true, then we can also understand that
 `SomeFeature` is always bound, without raising any errors:
 
-```py path=test1.py
-from module1 import SomeFeature
+```py
+import sys
 
-# SomeFeature is unconditionally available here, because we are on Python 3.9 or newer:
-reveal_type(SomeFeature)  # revealed: str
+class C:
+    if sys.version_info >= (3, 9):
+        SomeFeature: str = "available"
+
+# C.SomeFeature is unconditionally available here, because we are on Python 3.9 or newer:
+reveal_type(C.SomeFeature)  # revealed: str
 ```
 
 Another scenario where this is useful is for `typing.TYPE_CHECKING` branches, which are often used
 for conditional imports:
 
-```py path=module2.py
+`module.py`:
+
+```py
 class SomeType: ...
 ```
 
-```py path=test2.py
+`main.py`:
+
+```py
 import typing
 
 if typing.TYPE_CHECKING:
-    from module2 import SomeType
+    from module import SomeType
 
 # `SomeType` is unconditionally available here for type checkers:
 def f(s: SomeType) -> None: ...
@@ -167,7 +168,11 @@ statically known conditions, but here, we show that the results are truly based
 not some special handling of specific conditions in semantic index building. We use two modules to
 demonstrate this, since semantic index building is inherently single-module:
 
-```py path=module.py
+`module.py`:
+
+```py
+from typing import Literal
+
 class AlwaysTrue:
     def __bool__(self) -> Literal[True]:
         return True
@@ -1424,7 +1429,9 @@ def f():
 
 #### Always false, unbound
 
-```py path=module.py
+`module.py`:
+
+```py
 if False:
     symbol = 1
 ```
@@ -1436,7 +1443,9 @@ from module import symbol
 
 #### Always true, bound
 
-```py path=module.py
+`module.py`:
+
+```py
 if True:
     symbol = 1
 ```
@@ -1448,7 +1457,9 @@ from module import symbol
 
 #### Ambiguous, possibly unbound
 
-```py path=module.py
+`module.py`:
+
+```py
 def flag() -> bool:
     return True
 
@@ -1463,7 +1474,9 @@ from module import symbol
 
 #### Always false, undeclared
 
-```py path=module.py
+`module.py`:
+
+```py
 if False:
     symbol: int
 ```
@@ -1477,7 +1490,9 @@ reveal_type(symbol)  # revealed: Unknown
 
 #### Always true, declared
 
-```py path=module.py
+`module.py`:
+
+```py
 if True:
     symbol: int
 ```
@@ -1487,37 +1502,6 @@ if True:
 from module import symbol
 ```
 
-## Known limitations
-
-We currently have a limitation in the complexity (depth) of the visibility constraints that are
-supported. This is to avoid pathological cases that would require us to recurse deeply.
-
-```py
-x = 1
-
-False or False or False or False or \
-    False or False or False or False or \
-    False or False or False or False or \
-    False or False or False or False or \
-    False or False or False or False or \
-    False or False or (x := 2)  # fmt: skip
-
-# This still works fine:
-reveal_type(x)  # revealed: Literal[2]
-
-y = 1
-
-False or False or False or False or \
-    False or False or False or False or \
-    False or False or False or False or \
-    False or False or False or False or \
-    False or False or False or False or \
-    False or False or False or (y := 2)  # fmt: skip
-
-# TODO: This should ideally be `Literal[2]` as well:
-reveal_type(y)  # revealed: Literal[1, 2]
-```
-
 ## Unsupported features
 
 We do not support full unreachable code analysis yet. We also raise diagnostics from

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

@@ -5,7 +5,7 @@
 In type stubs, classes can reference themselves in their base class definitions. For example, in
 `typeshed`, we have `class str(Sequence[str]): ...`.
 
-```py path=a.pyi
+```pyi
 class Foo[T]: ...
 
 # TODO: actually is subscriptable

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

@@ -5,7 +5,7 @@
 The ellipsis literal `...` can be used as a placeholder default value for a function parameter, in a
 stub file only, regardless of the type of the parameter.
 
-```py path=test.pyi
+```pyi
 def f(x: int = ...) -> None:
     reveal_type(x)  # revealed: int
 
@@ -18,7 +18,7 @@ def f2(x: str = ...) -> None:
 The ellipsis literal can be assigned to a class or module symbol, regardless of its declared type,
 in a stub file only.
 
-```py path=test.pyi
+```pyi
 y: bytes = ...
 reveal_type(y)  # revealed: bytes
 x = ...
@@ -35,7 +35,7 @@ reveal_type(Foo.y)  # revealed: int
 No diagnostic is emitted if an ellipsis literal is "unpacked" in a stub file as part of an
 assignment statement:
 
-```py path=test.pyi
+```pyi
 x, y = ...
 reveal_type(x)  # revealed: Unknown
 reveal_type(y)  # revealed: Unknown
@@ -46,7 +46,7 @@ reveal_type(y)  # revealed: Unknown
 Iterating over an ellipsis literal as part of a `for` loop in a stub is invalid, however, and
 results in a diagnostic:
 
-```py path=test.pyi
+```pyi
 # error: [not-iterable] "Object of type `ellipsis` is not iterable"
 for a, b in ...:
     reveal_type(a)  # revealed: Unknown
@@ -72,7 +72,7 @@ reveal_type(b)  # revealed: ellipsis
 
 There is no special treatment of the builtin name `Ellipsis` in stubs, only of `...` literals.
 
-```py path=test.pyi
+```pyi
 # error: 7 [invalid-parameter-default] "Default value of type `ellipsis` is not assignable to annotated parameter type `int`"
 def f(x: int = Ellipsis) -> None: ...
 ```

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

@@ -97,7 +97,7 @@ reveal_type(A.__mro__)  # revealed: tuple[Literal[A], Unknown, Literal[object]]
 `typing.Tuple` can be used interchangeably with `tuple`:
 
 ```py
-from typing import Tuple
+from typing import Any, Tuple
 
 class A: ...
 

crates/red_knot_python_semantic/resources/mdtest/suppressions/knot_ignore.md

@@ -77,7 +77,8 @@ def test(a: f"f-string type annotation", b: b"byte-string-type-annotation"): ...
 ```py
 # error: [invalid-syntax]
 # error: [unused-ignore-comment]
-def test(  # knot: ignore
+def test($):  # knot: ignore
+    pass
 ```
 
 <!-- blacken-docs:on -->
@@ -180,3 +181,11 @@ a = 4 / 0  # error: [division-by-zero]
 # error: [unknown-rule] "Unknown rule `is-equal-14`"
 a = 10 + 4  # knot: ignore[is-equal-14]
 ```
+
+## Code with `lint:` prefix
+
+```py
+# error:[unknown-rule] "Unknown rule `lint:division-by-zero`. Did you mean `division-by-zero`?"
+# error: [division-by-zero]
+a = 10 / 0  # knot: ignore[lint:division-by-zero]
+```

crates/red_knot_python_semantic/resources/mdtest/suppressions/type_ignore.md

@@ -37,7 +37,7 @@ child expression now suppresses errors in the outer expression.
 For example, the `type: ignore` comment in this example suppresses the error of adding `2` to
 `"test"` and adding `"other"` to the result of the cast.
 
-```py path=nested.py
+```py
 # fmt: off
 from typing import cast
 
@@ -151,7 +151,7 @@ b = a / 0
 ```py
 """
 File level suppressions must come before any non-trivia token,
-including module docstrings. 
+including module docstrings.
 """
 
 # error: [unused-ignore-comment] "Unused blanket `type: ignore` directive"

crates/red_knot_python_semantic/resources/mdtest/sys_version_info.md

@@ -86,14 +86,20 @@ reveal_type(bar >= (3, 9))  # revealed: Literal[True]
 
 Only comparisons with the symbol `version_info` from the `sys` module produce literal types:
 
-```py path=package/__init__.py
+`package/__init__.py`:
+
+```py
 ```
 
-```py path=package/sys.py
+`package/sys.py`:
+
+```py
 version_info: tuple[int, int] = (4, 2)
 ```
 
-```py path=package/script.py
+`package/script.py`:
+
+```py
 from .sys import version_info
 
 reveal_type(version_info >= (3, 9))  # revealed: bool
@@ -103,7 +109,7 @@ reveal_type(version_info >= (3, 9))  # revealed: bool
 
 The fields of `sys.version_info` can be accessed by name:
 
-```py path=a.py
+```py
 import sys
 
 reveal_type(sys.version_info.major >= 3)  # revealed: Literal[True]
@@ -114,9 +120,7 @@ reveal_type(sys.version_info.minor >= 10)  # revealed: Literal[False]
 But the `micro`, `releaselevel` and `serial` fields are inferred as `@Todo` until we support
 properties on instance types:
 
-```py path=b.py
-import sys
-
+```py
 reveal_type(sys.version_info.micro)  # revealed: @Todo(@property)
 reveal_type(sys.version_info.releaselevel)  # revealed: @Todo(@property)
 reveal_type(sys.version_info.serial)  # revealed: @Todo(@property)

crates/red_knot_python_semantic/resources/mdtest/terminal_statements.md

@@ -0,0 +1,663 @@
+# Terminal statements
+
+## Introduction
+
+Terminal statements complicate a naive control-flow analysis.
+
+As a simple example:
+
+```py
+def f(cond: bool) -> str:
+    if cond:
+        x = "test"
+    else:
+        raise ValueError
+    return x
+
+def g(cond: bool):
+    if cond:
+        x = "test"
+        reveal_type(x)  # revealed: Literal["test"]
+    else:
+        x = "terminal"
+        reveal_type(x)  # revealed: Literal["terminal"]
+        raise ValueError
+    reveal_type(x)  # revealed: Literal["test"]
+```
+
+In `f`, we should be able to determine that the `else` branch ends in a terminal statement, and that
+the `return` statement can only be executed when the condition is true. We should therefore consider
+the reference always bound, even though `x` is only bound in the true branch.
+
+Similarly, in `g`, we should see that the assignment of the value `"terminal"` can never be seen by
+the final `reveal_type`.
+
+## `return`
+
+A `return` statement is terminal; bindings that occur before it are not visible after it.
+
+```py
+def resolved_reference(cond: bool) -> str:
+    if cond:
+        x = "test"
+    else:
+        return "early"
+    return x  # no possibly-unresolved-reference diagnostic!
+
+def return_in_then_branch(cond: bool):
+    if cond:
+        x = "terminal"
+        reveal_type(x)  # revealed: Literal["terminal"]
+        return
+    else:
+        x = "test"
+        reveal_type(x)  # revealed: Literal["test"]
+    reveal_type(x)  # revealed: Literal["test"]
+
+def return_in_else_branch(cond: bool):
+    if cond:
+        x = "test"
+        reveal_type(x)  # revealed: Literal["test"]
+    else:
+        x = "terminal"
+        reveal_type(x)  # revealed: Literal["terminal"]
+        return
+    reveal_type(x)  # revealed: Literal["test"]
+
+def return_in_both_branches(cond: bool):
+    if cond:
+        x = "terminal1"
+        reveal_type(x)  # revealed: Literal["terminal1"]
+        return
+    else:
+        x = "terminal2"
+        reveal_type(x)  # revealed: Literal["terminal2"]
+        return
+
+def return_in_try(cond: bool):
+    x = "before"
+    try:
+        if cond:
+            x = "test"
+            return
+    except:
+        # TODO: Literal["before"]
+        reveal_type(x)  # revealed: Literal["before", "test"]
+    else:
+        reveal_type(x)  # revealed: Literal["before"]
+    finally:
+        reveal_type(x)  # revealed: Literal["before", "test"]
+    reveal_type(x)  # revealed: Literal["before", "test"]
+
+def return_in_nested_then_branch(cond1: bool, cond2: bool):
+    if cond1:
+        x = "test1"
+        reveal_type(x)  # revealed: Literal["test1"]
+    else:
+        if cond2:
+            x = "terminal"
+            reveal_type(x)  # revealed: Literal["terminal"]
+            return
+        else:
+            x = "test2"
+            reveal_type(x)  # revealed: Literal["test2"]
+        reveal_type(x)  # revealed: Literal["test2"]
+    reveal_type(x)  # revealed: Literal["test1", "test2"]
+
+def return_in_nested_else_branch(cond1: bool, cond2: bool):
+    if cond1:
+        x = "test1"
+        reveal_type(x)  # revealed: Literal["test1"]
+    else:
+        if cond2:
+            x = "test2"
+            reveal_type(x)  # revealed: Literal["test2"]
+        else:
+            x = "terminal"
+            reveal_type(x)  # revealed: Literal["terminal"]
+            return
+        reveal_type(x)  # revealed: Literal["test2"]
+    reveal_type(x)  # revealed: Literal["test1", "test2"]
+
+def return_in_both_nested_branches(cond1: bool, cond2: bool):
+    if cond1:
+        x = "test"
+        reveal_type(x)  # revealed: Literal["test"]
+    else:
+        x = "terminal0"
+        if cond2:
+            x = "terminal1"
+            reveal_type(x)  # revealed: Literal["terminal1"]
+            return
+        else:
+            x = "terminal2"
+            reveal_type(x)  # revealed: Literal["terminal2"]
+            return
+    reveal_type(x)  # revealed: Literal["test"]
+```
+
+## `continue`
+
+A `continue` statement jumps back to the top of the innermost loop. This makes it terminal within
+the loop body: definitions before it are not visible after it within the rest of the loop body. They
+are likely visible after the loop body, since loops do not introduce new scopes. (Statically known
+infinite loops are one exception — if control never leaves the loop body, bindings inside of the
+loop are not visible outside of it.)
+
+TODO: We are not currently modeling the cyclic control flow for loops, pending fixpoint support in
+Salsa. The false positives in this section are because of that, and not our terminal statement
+support. See [ruff#14160](https://github.com/astral-sh/ruff/issues/14160) for more details.
+
+```py
+def resolved_reference(cond: bool) -> str:
+    while True:
+        if cond:
+            x = "test"
+        else:
+            continue
+        return x
+
+def continue_in_then_branch(cond: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond:
+            x = "continue"
+            reveal_type(x)  # revealed: Literal["continue"]
+            continue
+        else:
+            x = "loop"
+            reveal_type(x)  # revealed: Literal["loop"]
+        reveal_type(x)  # revealed: Literal["loop"]
+    # TODO: Should be Literal["before", "loop", "continue"]
+    reveal_type(x)  # revealed: Literal["before", "loop"]
+
+def continue_in_else_branch(cond: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond:
+            x = "loop"
+            reveal_type(x)  # revealed: Literal["loop"]
+        else:
+            x = "continue"
+            reveal_type(x)  # revealed: Literal["continue"]
+            continue
+        reveal_type(x)  # revealed: Literal["loop"]
+    # TODO: Should be Literal["before", "loop", "continue"]
+    reveal_type(x)  # revealed: Literal["before", "loop"]
+
+def continue_in_both_branches(cond: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond:
+            x = "continue1"
+            reveal_type(x)  # revealed: Literal["continue1"]
+            continue
+        else:
+            x = "continue2"
+            reveal_type(x)  # revealed: Literal["continue2"]
+            continue
+    # TODO: Should be Literal["before", "continue1", "continue2"]
+    reveal_type(x)  # revealed: Literal["before"]
+
+def continue_in_nested_then_branch(cond1: bool, cond2: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond1:
+            x = "loop1"
+            reveal_type(x)  # revealed: Literal["loop1"]
+        else:
+            if cond2:
+                x = "continue"
+                reveal_type(x)  # revealed: Literal["continue"]
+                continue
+            else:
+                x = "loop2"
+                reveal_type(x)  # revealed: Literal["loop2"]
+            reveal_type(x)  # revealed: Literal["loop2"]
+        reveal_type(x)  # revealed: Literal["loop1", "loop2"]
+    # TODO: Should be Literal["before", "loop1", "loop2", "continue"]
+    reveal_type(x)  # revealed: Literal["before", "loop1", "loop2"]
+
+def continue_in_nested_else_branch(cond1: bool, cond2: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond1:
+            x = "loop1"
+            reveal_type(x)  # revealed: Literal["loop1"]
+        else:
+            if cond2:
+                x = "loop2"
+                reveal_type(x)  # revealed: Literal["loop2"]
+            else:
+                x = "continue"
+                reveal_type(x)  # revealed: Literal["continue"]
+                continue
+            reveal_type(x)  # revealed: Literal["loop2"]
+        reveal_type(x)  # revealed: Literal["loop1", "loop2"]
+    # TODO: Should be Literal["before", "loop1", "loop2", "continue"]
+    reveal_type(x)  # revealed: Literal["before", "loop1", "loop2"]
+
+def continue_in_both_nested_branches(cond1: bool, cond2: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond1:
+            x = "loop"
+            reveal_type(x)  # revealed: Literal["loop"]
+        else:
+            if cond2:
+                x = "continue1"
+                reveal_type(x)  # revealed: Literal["continue1"]
+                continue
+            else:
+                x = "continue2"
+                reveal_type(x)  # revealed: Literal["continue2"]
+                continue
+        reveal_type(x)  # revealed: Literal["loop"]
+    # TODO: Should be Literal["before", "loop", "continue1", "continue2"]
+    reveal_type(x)  # revealed: Literal["before", "loop"]
+```
+
+## `break`
+
+A `break` statement jumps to the end of the innermost loop. This makes it terminal within the loop
+body: definitions before it are not visible after it within the rest of the loop body. They are
+likely visible after the loop body, since loops do not introduce new scopes. (Statically known
+infinite loops are one exception — if control never leaves the loop body, bindings inside of the
+loop are not visible outside of it.)
+
+```py
+def resolved_reference(cond: bool) -> str:
+    while True:
+        if cond:
+            x = "test"
+        else:
+            break
+        return x
+    return x  # error: [unresolved-reference]
+
+def break_in_then_branch(cond: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond:
+            x = "break"
+            reveal_type(x)  # revealed: Literal["break"]
+            break
+        else:
+            x = "loop"
+            reveal_type(x)  # revealed: Literal["loop"]
+        reveal_type(x)  # revealed: Literal["loop"]
+    reveal_type(x)  # revealed: Literal["before", "break", "loop"]
+
+def break_in_else_branch(cond: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond:
+            x = "loop"
+            reveal_type(x)  # revealed: Literal["loop"]
+        else:
+            x = "break"
+            reveal_type(x)  # revealed: Literal["break"]
+            break
+        reveal_type(x)  # revealed: Literal["loop"]
+    reveal_type(x)  # revealed: Literal["before", "loop", "break"]
+
+def break_in_both_branches(cond: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond:
+            x = "break1"
+            reveal_type(x)  # revealed: Literal["break1"]
+            break
+        else:
+            x = "break2"
+            reveal_type(x)  # revealed: Literal["break2"]
+            break
+    reveal_type(x)  # revealed: Literal["before", "break1", "break2"]
+
+def break_in_nested_then_branch(cond1: bool, cond2: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond1:
+            x = "loop1"
+            reveal_type(x)  # revealed: Literal["loop1"]
+        else:
+            if cond2:
+                x = "break"
+                reveal_type(x)  # revealed: Literal["break"]
+                break
+            else:
+                x = "loop2"
+                reveal_type(x)  # revealed: Literal["loop2"]
+            reveal_type(x)  # revealed: Literal["loop2"]
+        reveal_type(x)  # revealed: Literal["loop1", "loop2"]
+    reveal_type(x)  # revealed: Literal["before", "loop1", "break", "loop2"]
+
+def break_in_nested_else_branch(cond1: bool, cond2: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond1:
+            x = "loop1"
+            reveal_type(x)  # revealed: Literal["loop1"]
+        else:
+            if cond2:
+                x = "loop2"
+                reveal_type(x)  # revealed: Literal["loop2"]
+            else:
+                x = "break"
+                reveal_type(x)  # revealed: Literal["break"]
+                break
+            reveal_type(x)  # revealed: Literal["loop2"]
+        reveal_type(x)  # revealed: Literal["loop1", "loop2"]
+    reveal_type(x)  # revealed: Literal["before", "loop1", "loop2", "break"]
+
+def break_in_both_nested_branches(cond1: bool, cond2: bool, i: int):
+    x = "before"
+    for _ in range(i):
+        if cond1:
+            x = "loop"
+            reveal_type(x)  # revealed: Literal["loop"]
+        else:
+            if cond2:
+                x = "break1"
+                reveal_type(x)  # revealed: Literal["break1"]
+                break
+            else:
+                x = "break2"
+                reveal_type(x)  # revealed: Literal["break2"]
+                break
+        reveal_type(x)  # revealed: Literal["loop"]
+    reveal_type(x)  # revealed: Literal["before", "loop", "break1", "break2"]
+```
+
+## `raise`
+
+A `raise` statement is terminal. If it occurs in a lexically containing `try` statement, it will
+jump to one of the `except` clauses (if it matches the value being raised), or to the `else` clause
+(if none match). Currently, we assume definitions from before the `raise` are visible in all
+`except` and `else` clauses. (In the future, we might analyze the `except` clauses to see which ones
+match the value being raised, and limit visibility to those clauses.) Definitions from before the
+`raise` are not visible in any `else` clause, but are visible in `except` clauses or after the
+containing `try` statement (since control flow may have passed through an `except`).
+
+Currently we assume that an exception could be raised anywhere within a `try` block. We may want to
+implement a more precise understanding of where exceptions (barring `KeyboardInterrupt` and
+`MemoryError`) can and cannot actually be raised.
+
+```py
+def raise_in_then_branch(cond: bool):
+    x = "before"
+    try:
+        if cond:
+            x = "raise"
+            reveal_type(x)  # revealed: Literal["raise"]
+            raise ValueError
+        else:
+            x = "else"
+            reveal_type(x)  # revealed: Literal["else"]
+        reveal_type(x)  # revealed: Literal["else"]
+    except ValueError:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "raise", "else"]
+    except:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "raise", "else"]
+    else:
+        reveal_type(x)  # revealed: Literal["else"]
+    finally:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "raise", "else"]
+    # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+    reveal_type(x)  # revealed: Literal["before", "raise", "else"]
+
+def raise_in_else_branch(cond: bool):
+    x = "before"
+    try:
+        if cond:
+            x = "else"
+            reveal_type(x)  # revealed: Literal["else"]
+        else:
+            x = "raise"
+            reveal_type(x)  # revealed: Literal["raise"]
+            raise ValueError
+        reveal_type(x)  # revealed: Literal["else"]
+    except ValueError:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else", "raise"]
+    except:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else", "raise"]
+    else:
+        reveal_type(x)  # revealed: Literal["else"]
+    finally:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else", "raise"]
+    # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+    reveal_type(x)  # revealed: Literal["before", "else", "raise"]
+
+def raise_in_both_branches(cond: bool):
+    x = "before"
+    try:
+        if cond:
+            x = "raise1"
+            reveal_type(x)  # revealed: Literal["raise1"]
+            raise ValueError
+        else:
+            x = "raise2"
+            reveal_type(x)  # revealed: Literal["raise2"]
+            raise ValueError
+    except ValueError:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "raise1", "raise2"]
+    except:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "raise1", "raise2"]
+    else:
+        # This branch is unreachable, since all control flows in the `try` clause raise exceptions.
+        # As a result, this binding should never be reachable, since new bindings are visible only
+        # when they are reachable.
+        x = "unreachable"
+    finally:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "raise1", "raise2"]
+    # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+    reveal_type(x)  # revealed: Literal["before", "raise1", "raise2"]
+
+def raise_in_nested_then_branch(cond1: bool, cond2: bool):
+    x = "before"
+    try:
+        if cond1:
+            x = "else1"
+            reveal_type(x)  # revealed: Literal["else1"]
+        else:
+            if cond2:
+                x = "raise"
+                reveal_type(x)  # revealed: Literal["raise"]
+                raise ValueError
+            else:
+                x = "else2"
+                reveal_type(x)  # revealed: Literal["else2"]
+            reveal_type(x)  # revealed: Literal["else2"]
+        reveal_type(x)  # revealed: Literal["else1", "else2"]
+    except ValueError:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else1", "raise", "else2"]
+    except:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else1", "raise", "else2"]
+    else:
+        reveal_type(x)  # revealed: Literal["else1", "else2"]
+    finally:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else1", "raise", "else2"]
+    # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+    reveal_type(x)  # revealed: Literal["before", "else1", "raise", "else2"]
+
+def raise_in_nested_else_branch(cond1: bool, cond2: bool):
+    x = "before"
+    try:
+        if cond1:
+            x = "else1"
+            reveal_type(x)  # revealed: Literal["else1"]
+        else:
+            if cond2:
+                x = "else2"
+                reveal_type(x)  # revealed: Literal["else2"]
+            else:
+                x = "raise"
+                reveal_type(x)  # revealed: Literal["raise"]
+                raise ValueError
+            reveal_type(x)  # revealed: Literal["else2"]
+        reveal_type(x)  # revealed: Literal["else1", "else2"]
+    except ValueError:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else1", "else2", "raise"]
+    except:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else1", "else2", "raise"]
+    else:
+        reveal_type(x)  # revealed: Literal["else1", "else2"]
+    finally:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else1", "else2", "raise"]
+    # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+    reveal_type(x)  # revealed: Literal["before", "else1", "else2", "raise"]
+
+def raise_in_both_nested_branches(cond1: bool, cond2: bool):
+    x = "before"
+    try:
+        if cond1:
+            x = "else"
+            reveal_type(x)  # revealed: Literal["else"]
+        else:
+            if cond2:
+                x = "raise1"
+                reveal_type(x)  # revealed: Literal["raise1"]
+                raise ValueError
+            else:
+                x = "raise2"
+                reveal_type(x)  # revealed: Literal["raise2"]
+                raise ValueError
+        reveal_type(x)  # revealed: Literal["else"]
+    except ValueError:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else", "raise1", "raise2"]
+    except:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else", "raise1", "raise2"]
+    else:
+        reveal_type(x)  # revealed: Literal["else"]
+    finally:
+        # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+        reveal_type(x)  # revealed: Literal["before", "else", "raise1", "raise2"]
+    # Exceptions can occur anywhere, so "before" and "raise" are valid possibilities
+    reveal_type(x)  # revealed: Literal["before", "else", "raise1", "raise2"]
+```
+
+## Terminal in `try` with `finally` clause
+
+TODO: we don't yet model that a `break` or `continue` in a `try` block will jump to a `finally`
+clause before it jumps to end/start of the loop.
+
+```py
+def f():
+    x = 1
+    while True:
+        try:
+            break
+        finally:
+            x = 2
+    # TODO: should be Literal[2]
+    reveal_type(x)  # revealed: Literal[1]
+```
+
+## Nested functions
+
+Free references inside of a function body refer to variables defined in the containing scope.
+Function bodies are _lazy scopes_: at runtime, these references are not resolved immediately at the
+point of the function definition. Instead, they are resolved _at the time of the call_, which means
+that their values (and types) can be different for different invocations. For simplicity, we instead
+resolve free references _at the end of the containing scope_. That means that in the examples below,
+all of the `x` bindings should be visible to the `reveal_type`, regardless of where we place the
+`return` statements.
+
+TODO: These currently produce the wrong results, but not because of our terminal statement support.
+See [ruff#15777](https://github.com/astral-sh/ruff/issues/15777) for more details.
+
+```py
+def top_level_return(cond1: bool, cond2: bool):
+    x = 1
+
+    def g():
+        # TODO eliminate Unknown
+        reveal_type(x)  # revealed: Unknown | Literal[1, 2, 3]
+    if cond1:
+        if cond2:
+            x = 2
+        else:
+            x = 3
+    return
+
+def return_from_if(cond1: bool, cond2: bool):
+    x = 1
+
+    def g():
+        # TODO: Literal[1, 2, 3]
+        reveal_type(x)  # revealed: Unknown | Literal[1]
+    if cond1:
+        if cond2:
+            x = 2
+        else:
+            x = 3
+        return
+
+def return_from_nested_if(cond1: bool, cond2: bool):
+    x = 1
+
+    def g():
+        # TODO: Literal[1, 2, 3]
+        reveal_type(x)  # revealed: Unknown | Literal[1, 3]
+    if cond1:
+        if cond2:
+            x = 2
+            return
+        else:
+            x = 3
+```
+
+## Statically known terminal statements
+
+We model reachability using the same visibility constraints that we use to model statically known
+bounds. In this example, we see that the `return` statement is always executed, and therefore that
+the `"b"` assignment is not visible to the `reveal_type`.
+
+```py
+def _(cond: bool):
+    x = "a"
+    if cond:
+        x = "b"
+        if True:
+            return
+
+    reveal_type(x)  # revealed: Literal["a"]
+```
+
+## Bindings after a terminal statement are unreachable
+
+Any bindings introduced after a terminal statement are unreachable, and are currently considered not
+visible. We [anticipate](https://github.com/astral-sh/ruff/issues/15797) that we want to provide a
+more useful analysis for code after terminal statements.
+
+```py
+def f(cond: bool) -> str:
+    x = "before"
+    if cond:
+        reveal_type(x)  # revealed: Literal["before"]
+        return
+        x = "after-return"
+        # TODO: no unresolved-reference error
+        # error: [unresolved-reference]
+        reveal_type(x)  # revealed: Unknown
+    else:
+        x = "else"
+    reveal_type(x)  # revealed: Literal["else"]
+```

crates/red_knot_python_semantic/resources/mdtest/type_api.md

@@ -15,6 +15,7 @@ directly.
 ### Negation
 
 ```py
+from typing import Literal
 from knot_extensions import Not, static_assert
 
 def negate(n1: Not[int], n2: Not[Not[int]], n3: Not[Not[Not[int]]]) -> None:
@@ -34,7 +35,7 @@ n: Not[int, str]
 
 ```py
 from knot_extensions import Intersection, Not, is_subtype_of, static_assert
-from typing_extensions import Never
+from typing_extensions import Literal, Never
 
 class S: ...
 class T: ...
@@ -83,8 +84,11 @@ def explicit_unknown(x: Unknown, y: tuple[str, Unknown], z: Unknown = 1) -> None
     reveal_type(x)  # revealed: Unknown
     reveal_type(y)  # revealed: tuple[str, Unknown]
     reveal_type(z)  # revealed: Unknown | Literal[1]
+```
 
-# Unknown can be subclassed, just like Any
+`Unknown` can be subclassed, just like `Any`:
+
+```py
 class C(Unknown): ...
 
 # revealed: tuple[Literal[C], Unknown, Literal[object]]
@@ -237,9 +241,12 @@ error_message = "A custom message "
 error_message += "constructed from multiple string literals"
 # error: "Static assertion error: A custom message constructed from multiple string literals"
 static_assert(False, error_message)
+```
 
-# There are limitations to what we can still infer as a string literal. In those cases,
-# we simply fall back to the default message.
+There are limitations to what we can still infer as a string literal. In those cases, we simply fall
+back to the default message:
+
+```py
 shouted_message = "A custom message".upper()
 # error: "Static assertion error: argument evaluates to `False`"
 static_assert(False, shouted_message)
@@ -304,6 +311,7 @@ static_assert(not is_assignable_to(int, str))
 
 ```py
 from knot_extensions import is_disjoint_from, static_assert
+from typing import Literal
 
 static_assert(is_disjoint_from(None, int))
 static_assert(not is_disjoint_from(Literal[2] | str, int))
@@ -326,6 +334,7 @@ static_assert(not is_fully_static(type[Any]))
 
 ```py
 from knot_extensions import is_singleton, static_assert
+from typing import Literal
 
 static_assert(is_singleton(None))
 static_assert(is_singleton(Literal[True]))
@@ -338,6 +347,7 @@ static_assert(not is_singleton(Literal["a"]))
 
 ```py
 from knot_extensions import is_single_valued, static_assert
+from typing import Literal
 
 static_assert(is_single_valued(None))
 static_assert(is_single_valued(Literal[True]))
@@ -367,8 +377,11 @@ static_assert(is_subtype_of(TypeOf[str], type[str]))
 
 class Base: ...
 class Derived(Base): ...
+```
 
-# `TypeOf` can be used in annotations:
+`TypeOf` can also be used in annotations:
+
+```py
 def type_of_annotation() -> None:
     t1: TypeOf[Base] = Base
     t2: TypeOf[Base] = Derived  # error: [invalid-assignment]

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

@@ -39,7 +39,9 @@ def f(c: type[A]):
     reveal_type(c)  # revealed: type[A]
 ```
 
-```py path=a.py
+`a.py`:
+
+```py
 class A: ...
 ```
 
@@ -52,23 +54,31 @@ def f(c: type[a.B]):
     reveal_type(c)  # revealed: type[B]
 ```
 
-```py path=a.py
+`a.py`:
+
+```py
 class B: ...
 ```
 
 ## Deeply qualified class literal from another module
 
-```py path=a/test.py
+`a/test.py`:
+
+```py
 import a.b
 
 def f(c: type[a.b.C]):
     reveal_type(c)  # revealed: type[C]
 ```
 
-```py path=a/__init__.py
+`a/__init__.py`:
+
+```py
 ```
 
-```py path=a/b.py
+`a/b.py`:
+
+```py
 class C: ...
 ```
 

crates/red_knot_python_semantic/resources/mdtest/type_of/dynamic.md

@@ -6,6 +6,8 @@ This file contains tests for non-fully-static `type[]` types, such as `type[Any]
 ## Simple
 
 ```py
+from typing import Any
+
 def f(x: type[Any], y: type[str]):
     reveal_type(x)  # revealed: type[Any]
     # TODO: could be `<object.__repr__ type> & Any`

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_assignable_to.md

@@ -41,7 +41,7 @@ static types can be assignable to gradual types):
 
 ```py
 from knot_extensions import static_assert, is_assignable_to, Unknown
-from typing import Any
+from typing import Any, Literal
 
 static_assert(is_assignable_to(Unknown, Literal[1]))
 static_assert(is_assignable_to(Any, Literal[1]))
@@ -333,7 +333,7 @@ assignable to any arbitrary type.
 
 ```py
 from knot_extensions import static_assert, is_assignable_to, Unknown
-from typing_extensions import Never, Any
+from typing_extensions import Never, Any, Literal
 
 static_assert(is_assignable_to(Never, str))
 static_assert(is_assignable_to(Never, Literal[1]))

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_disjoint_from.md

@@ -151,7 +151,7 @@ static_assert(is_disjoint_from(Never, object))
 ### `None`
 
 ```py
-from typing_extensions import Literal
+from typing_extensions import Literal, LiteralString
 from knot_extensions import is_disjoint_from, static_assert
 
 static_assert(is_disjoint_from(None, Literal[True]))
@@ -245,6 +245,7 @@ static_assert(not is_disjoint_from(TypeOf[f], object))
 
 ```py
 from knot_extensions import AlwaysFalsy, AlwaysTruthy, is_disjoint_from, static_assert
+from typing import Literal
 
 static_assert(is_disjoint_from(None, AlwaysTruthy))
 static_assert(not is_disjoint_from(None, AlwaysFalsy))

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_equivalent_to.md

@@ -84,4 +84,38 @@ static_assert(
 )
 ```
 
+## Unions containing tuples containing tuples containing unions (etc.)
+
+```py
+from knot_extensions import is_equivalent_to, static_assert, Intersection
+
+class P: ...
+class Q: ...
+
+static_assert(
+    is_equivalent_to(
+        tuple[tuple[tuple[P | Q]]] | P,
+        tuple[tuple[tuple[Q | P]]] | P,
+    )
+)
+static_assert(
+    is_equivalent_to(
+        tuple[tuple[tuple[tuple[tuple[Intersection[P, Q]]]]]],
+        tuple[tuple[tuple[tuple[tuple[Intersection[Q, P]]]]]],
+    )
+)
+```
+
+## Intersections containing tuples containing unions
+
+```py
+from knot_extensions import is_equivalent_to, static_assert, Intersection
+
+class P: ...
+class Q: ...
+class R: ...
+
+static_assert(is_equivalent_to(Intersection[tuple[P | Q], R], Intersection[tuple[Q | P], R]))
+```
+
 [the equivalence relation]: https://typing.readthedocs.io/en/latest/spec/glossary.html#term-equivalent

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_gradual_equivalent_to.md

@@ -54,6 +54,7 @@ static_assert(not is_gradual_equivalent_to(str | int | bytes, int | str | dict))
 
 ```py
 from knot_extensions import Unknown, is_gradual_equivalent_to, static_assert
+from typing import Any
 
 static_assert(is_gradual_equivalent_to(tuple[str, Any], tuple[str, Unknown]))
 

crates/red_knot_python_semantic/resources/mdtest/type_properties/is_subtype_of.md

@@ -148,6 +148,7 @@ static_assert(is_subtype_of(tuple[int], tuple))
 
 ```py
 from knot_extensions import is_subtype_of, static_assert
+from typing import Literal
 
 class A: ...
 class B1(A): ...
@@ -271,6 +272,7 @@ static_assert(is_subtype_of(Never, AlwaysFalsy))
 
 ```py
 from knot_extensions import AlwaysTruthy, AlwaysFalsy, is_subtype_of, static_assert
+from typing import Literal
 
 static_assert(is_subtype_of(Literal[1], AlwaysTruthy))
 static_assert(is_subtype_of(Literal[0], AlwaysFalsy))
@@ -309,7 +311,7 @@ static_assert(is_subtype_of(TypeOf[1:2:3], slice))
 ### Special forms
 
 ```py
-from typing import _SpecialForm
+from typing import _SpecialForm, Literal
 from knot_extensions import TypeOf, is_subtype_of, static_assert
 
 static_assert(is_subtype_of(TypeOf[Literal], _SpecialForm))

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

@@ -1,5 +1,7 @@
 # Truthiness
 
+## Literals
+
 ```py
 from typing_extensions import Literal, LiteralString
 from knot_extensions import AlwaysFalsy, AlwaysTruthy
@@ -45,3 +47,31 @@ def _(
     reveal_type(bool(c))  # revealed: bool
     reveal_type(bool(d))  # revealed: bool
 ```
+
+## Instances
+
+Checks that we don't get into a cycle if someone sets their `__bool__` method to the `bool` builtin:
+
+### __bool__ is bool
+
+```py
+class BoolIsBool:
+    __bool__ = bool
+
+reveal_type(bool(BoolIsBool()))  # revealed: bool
+```
+
+### Conditional __bool__ method
+
+```py
+def flag() -> bool:
+    return True
+
+class Boom:
+    if flag():
+        __bool__ = bool
+    else:
+        __bool__ = int
+
+reveal_type(bool(Boom()))  # revealed: bool
+```

crates/red_knot_python_semantic/resources/mdtest/type_properties/tuples_containing_never.md

@@ -19,11 +19,17 @@ static_assert(is_equivalent_to(Never, tuple[int, Never]))
 static_assert(is_equivalent_to(Never, tuple[int, Never, str]))
 static_assert(is_equivalent_to(Never, tuple[int, tuple[str, Never]]))
 static_assert(is_equivalent_to(Never, tuple[tuple[str, Never], int]))
+```
 
-# The empty tuple is *not* equivalent to Never!
+The empty `tuple` is *not* equivalent to `Never`!
+
+```py
 static_assert(not is_equivalent_to(Never, tuple[()]))
+```
 
-# NoReturn is just a different spelling of Never, so the same is true for NoReturn
+`NoReturn` is just a different spelling of `Never`, so the same is true for `NoReturn`:
+
+```py
 static_assert(is_equivalent_to(NoReturn, tuple[NoReturn]))
 static_assert(is_equivalent_to(NoReturn, tuple[NoReturn, int]))
 static_assert(is_equivalent_to(NoReturn, tuple[int, NoReturn]))

crates/red_knot_python_semantic/resources/mdtest/type_qualifiers/classvar.md

@@ -67,6 +67,8 @@ c.a = 2
 ## Too many arguments
 
 ```py
+from typing import ClassVar
+
 class C:
     # error: [invalid-type-form] "Type qualifier `typing.ClassVar` expects exactly one type parameter"
     x: ClassVar[int, str] = 1
@@ -75,6 +77,8 @@ class C:
 ## Illegal `ClassVar` in type expression
 
 ```py
+from typing import ClassVar
+
 class C:
     # error: [invalid-type-form] "Type qualifier `typing.ClassVar` is not allowed in type expressions (only in annotation expressions)"
     x: ClassVar | int
@@ -86,6 +90,8 @@ class C:
 ## Used outside of a class
 
 ```py
+from typing import ClassVar
+
 # TODO: this should be an error
 x: ClassVar[int] = 1
 ```

crates/red_knot_python_semantic/resources/mdtest/unary/not.md

@@ -28,7 +28,9 @@ reveal_type(not b)  # revealed: Literal[False]
 reveal_type(not warnings)  # revealed: Literal[False]
 ```
 
-```py path=b.py
+`b.py`:
+
+```py
 y = 1
 ```
 
@@ -123,6 +125,8 @@ classes without a `__bool__` method, with or without `__len__`, must be inferred
 truthiness.
 
 ```py
+from typing import Literal
+
 class AlwaysTrue:
     def __bool__(self) -> Literal[True]:
         return True
@@ -137,15 +141,6 @@ class AlwaysFalse:
 # revealed: Literal[True]
 reveal_type(not AlwaysFalse())
 
-# We don't get into a cycle if someone sets their `__bool__` method to the `bool` builtin:
-class BoolIsBool:
-    # TODO: The `type[bool]` declaration here is a workaround to avoid running into
-    # https://github.com/astral-sh/ruff/issues/15672
-    __bool__: type[bool] = bool
-
-# revealed: bool
-reveal_type(not BoolIsBool())
-
 # At runtime, no `__bool__` and no `__len__` means truthy, but we can't rely on that, because
 # a subclass could add a `__bool__` method.
 class NoBoolMethod: ...

crates/red_knot_python_semantic/resources/mdtest/unpacking.md

@@ -282,7 +282,7 @@ reveal_type(b)  # revealed: Unknown
 
 ```py
 # error: [invalid-assignment] "Not enough values to unpack (expected 2, got 1)"
-(a, b) = "\u9E6C"
+(a, b) = "\u9e6c"
 
 reveal_type(a)  # revealed: LiteralString
 reveal_type(b)  # revealed: Unknown
@@ -292,7 +292,7 @@ reveal_type(b)  # revealed: Unknown
 
 ```py
 # error: [invalid-assignment] "Not enough values to unpack (expected 2, got 1)"
-(a, b) = "\U0010FFFF"
+(a, b) = "\U0010ffff"
 
 reveal_type(a)  # revealed: LiteralString
 reveal_type(b)  # revealed: Unknown
@@ -301,7 +301,7 @@ reveal_type(b)  # revealed: Unknown
 ### Surrogates
 
 ```py
-(a, b) = "\uD800\uDFFF"
+(a, b) = "\ud800\udfff"
 
 reveal_type(a)  # revealed: LiteralString
 reveal_type(b)  # revealed: LiteralString
@@ -361,6 +361,8 @@ def _(arg: tuple[int, int, int] | tuple[int, str, bytes] | tuple[int, int, str])
 ### Nested
 
 ```py
+from typing import Literal
+
 def _(arg: tuple[int, tuple[str, bytes]] | tuple[tuple[int, bytes], Literal["ab"]]):
     a, (b, c) = arg
     reveal_type(a)  # revealed: int | tuple[int, bytes]

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

@@ -88,6 +88,8 @@ with Manager():
 ## Context manager with non-callable `__exit__` attribute
 
 ```py
+from typing_extensions import Self
+
 class Manager:
     def __enter__(self) -> Self: ...
 

crates/red_knot_python_semantic/src/lint.rs

@@ -1,5 +1,5 @@
 use itertools::Itertools;
-use ruff_db::diagnostic::{LintName, Severity};
+use ruff_db::diagnostic::{DiagnosticId, LintName, Severity};
 use rustc_hash::FxHashMap;
 use std::hash::Hasher;
 use thiserror::Error;
@@ -345,7 +345,18 @@ impl LintRegistry {
                 }
             }
             Some(LintEntry::Removed(lint)) => Err(GetLintError::Removed(lint.name())),
-            None => Err(GetLintError::Unknown(code.to_string())),
+            None => {
+                if let Some(without_prefix) = DiagnosticId::strip_category(code) {
+                    if let Some(entry) = self.by_name.get(without_prefix) {
+                        return Err(GetLintError::PrefixedWithCategory {
+                            prefixed: code.to_string(),
+                            suggestion: entry.id().name.to_string(),
+                        });
+                    }
+                }
+
+                Err(GetLintError::Unknown(code.to_string()))
+            }
         }
     }
 
@@ -382,12 +393,20 @@ impl LintRegistry {
 #[derive(Error, Debug, Clone, PartialEq, Eq)]
 pub enum GetLintError {
     /// The name maps to this removed lint.
-    #[error("lint {0} has been removed")]
+    #[error("lint `{0}` has been removed")]
     Removed(LintName),
 
     /// No lint with the given name is known.
-    #[error("unknown lint {0}")]
+    #[error("unknown lint `{0}`")]
     Unknown(String),
+
+    /// The name uses the full qualified diagnostic id `lint:<rule>` instead of just `rule`.
+    /// The String is the name without the `lint:` category prefix.
+    #[error("unknown lint `{prefixed}`. Did you mean `{suggestion}`?")]
+    PrefixedWithCategory {
+        prefixed: String,
+        suggestion: String,
+    },
 }
 
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
@@ -399,6 +418,16 @@ pub enum LintEntry {
     Alias(LintId),
 }
 
+impl LintEntry {
+    fn id(self) -> LintId {
+        match self {
+            LintEntry::Lint(id) => id,
+            LintEntry::Removed(id) => id,
+            LintEntry::Alias(id) => id,
+        }
+    }
+}
+
 impl From<&'static LintMetadata> for LintEntry {
     fn from(metadata: &'static LintMetadata) -> Self {
         if metadata.status.is_removed() {

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -11,6 +11,7 @@ use ruff_index::{IndexSlice, IndexVec};
 use crate::module_name::ModuleName;
 use crate::semantic_index::ast_ids::node_key::ExpressionNodeKey;
 use crate::semantic_index::ast_ids::AstIds;
+use crate::semantic_index::attribute_assignment::AttributeAssignments;
 use crate::semantic_index::builder::SemanticIndexBuilder;
 use crate::semantic_index::definition::{Definition, DefinitionNodeKey};
 use crate::semantic_index::expression::Expression;
@@ -21,6 +22,7 @@ use crate::semantic_index::use_def::UseDefMap;
 use crate::Db;
 
 pub mod ast_ids;
+pub mod attribute_assignment;
 mod builder;
 pub(crate) mod constraint;
 pub mod definition;
@@ -30,7 +32,7 @@ mod use_def;
 
 pub(crate) use self::use_def::{
     BindingWithConstraints, BindingWithConstraintsIterator, DeclarationWithConstraint,
-    DeclarationsIterator, ScopedVisibilityConstraintId,
+    DeclarationsIterator,
 };
 
 type SymbolMap = hashbrown::HashMap<ScopedSymbolId, (), FxBuildHasher>;
@@ -93,6 +95,25 @@ pub(crate) fn use_def_map<'db>(db: &'db dyn Db, scope: ScopeId<'db>) -> Arc<UseD
     index.use_def_map(scope.file_scope_id(db))
 }
 
+/// Returns all attribute assignments for a specific class body scope.
+///
+/// Using [`attribute_assignments`] over [`semantic_index`] has the advantage that
+/// Salsa can avoid invalidating dependent queries if this scope's instance attributes
+/// are unchanged.
+#[salsa::tracked]
+pub(crate) fn attribute_assignments<'db>(
+    db: &'db dyn Db,
+    class_body_scope: ScopeId<'db>,
+) -> Option<Arc<AttributeAssignments<'db>>> {
+    let file = class_body_scope.file(db);
+    let index = semantic_index(db, file);
+
+    index
+        .attribute_assignments
+        .get(&class_body_scope.file_scope_id(db))
+        .cloned()
+}
+
 /// Returns the module global scope of `file`.
 #[salsa::tracked]
 pub(crate) fn global_scope(db: &dyn Db, file: File) -> ScopeId<'_> {
@@ -139,6 +160,10 @@ pub(crate) struct SemanticIndex<'db> {
 
     /// Flags about the global scope (code usage impacting inference)
     has_future_annotations: bool,
+
+    /// Maps from class body scopes to attribute assignments that were found
+    /// in methods of that class.
+    attribute_assignments: FxHashMap<FileScopeId, Arc<AttributeAssignments<'db>>>,
 }
 
 impl<'db> SemanticIndex<'db> {

crates/red_knot_python_semantic/src/semantic_index/attribute_assignment.rs

@@ -0,0 +1,19 @@
+use crate::semantic_index::expression::Expression;
+
+use ruff_python_ast::name::Name;
+
+use rustc_hash::FxHashMap;
+
+/// Describes an (annotated) attribute assignment that we discovered in a method
+/// body, typically of the form `self.x: int`, `self.x: int = …` or `self.x = …`.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub(crate) enum AttributeAssignment<'db> {
+    /// An attribute assignment with an explicit type annotation, either
+    /// `self.x: <annotation>` or `self.x: <annotation> = …`.
+    Annotated { annotation: Expression<'db> },
+
+    /// An attribute assignment without a type annotation, e.g. `self.x = <value>`.
+    Unannotated { value: Expression<'db> },
+}
+
+pub(crate) type AttributeAssignments<'db> = FxHashMap<Name, Vec<AttributeAssignment<'db>>>;

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -14,22 +14,21 @@ use crate::ast_node_ref::AstNodeRef;
 use crate::module_name::ModuleName;
 use crate::semantic_index::ast_ids::node_key::ExpressionNodeKey;
 use crate::semantic_index::ast_ids::AstIdsBuilder;
+use crate::semantic_index::attribute_assignment::{AttributeAssignment, AttributeAssignments};
 use crate::semantic_index::constraint::PatternConstraintKind;
 use crate::semantic_index::definition::{
     AssignmentDefinitionNodeRef, ComprehensionDefinitionNodeRef, Definition, DefinitionNodeKey,
     DefinitionNodeRef, ForStmtDefinitionNodeRef, ImportFromDefinitionNodeRef,
 };
-use crate::semantic_index::expression::Expression;
+use crate::semantic_index::expression::{Expression, ExpressionKind};
 use crate::semantic_index::symbol::{
-    FileScopeId, NodeWithScopeKey, NodeWithScopeRef, Scope, ScopeId, ScopedSymbolId,
+    FileScopeId, NodeWithScopeKey, NodeWithScopeRef, Scope, ScopeId, ScopeKind, ScopedSymbolId,
     SymbolTableBuilder,
 };
-use crate::semantic_index::use_def::{
-    FlowSnapshot, ScopedConstraintId, ScopedVisibilityConstraintId, UseDefMapBuilder,
-};
+use crate::semantic_index::use_def::{FlowSnapshot, ScopedConstraintId, UseDefMapBuilder};
 use crate::semantic_index::SemanticIndex;
 use crate::unpack::{Unpack, UnpackValue};
-use crate::visibility_constraints::VisibilityConstraint;
+use crate::visibility_constraints::{ScopedVisibilityConstraintId, VisibilityConstraintsBuilder};
 use crate::Db;
 
 use super::constraint::{Constraint, ConstraintNode, PatternConstraint};
@@ -53,17 +52,27 @@ impl LoopState {
     }
 }
 
+struct ScopeInfo {
+    file_scope_id: FileScopeId,
+    loop_state: LoopState,
+}
+
 pub(super) struct SemanticIndexBuilder<'db> {
     // Builder state
     db: &'db dyn Db,
     file: File,
+    // A shared clone of the path of the file being analyzed. We use this as a label for all of the
+    // metrics that we export, and this avoids cloning the path into a new string each time.
+    file_path: Arc<str>,
     module: &'db ParsedModule,
-    scope_stack: Vec<(FileScopeId, LoopState)>,
+    scope_stack: Vec<ScopeInfo>,
     /// The assignments we're currently visiting, with
     /// the most recent visit at the end of the Vec
     current_assignments: Vec<CurrentAssignment<'db>>,
     /// The match case we're currently visiting.
     current_match_case: Option<CurrentMatchCase<'db>>,
+    /// The name of the first function parameter of the innermost function that we're currently visiting.
+    current_first_parameter_name: Option<&'db str>,
 
     /// Flow states at each `break` in the current loop.
     loop_break_states: Vec<FlowSnapshot>,
@@ -84,17 +93,21 @@ pub(super) struct SemanticIndexBuilder<'db> {
     definitions_by_node: FxHashMap<DefinitionNodeKey, Definition<'db>>,
     expressions_by_node: FxHashMap<ExpressionNodeKey, Expression<'db>>,
     imported_modules: FxHashSet<ModuleName>,
+    attribute_assignments: FxHashMap<FileScopeId, AttributeAssignments<'db>>,
 }
 
 impl<'db> SemanticIndexBuilder<'db> {
     pub(super) fn new(db: &'db dyn Db, file: File, parsed: &'db ParsedModule) -> Self {
+        let file_path = Arc::from(file.path(db).as_str());
         let mut builder = Self {
             db,
             file,
+            file_path,
             module: parsed,
             scope_stack: Vec::new(),
             current_assignments: vec![],
             current_match_case: None,
+            current_first_parameter_name: None,
             loop_break_states: vec![],
             try_node_context_stack_manager: TryNodeContextStackManager::default(),
 
@@ -112,6 +125,8 @@ impl<'db> SemanticIndexBuilder<'db> {
             expressions_by_node: FxHashMap::default(),
 
             imported_modules: FxHashSet::default(),
+
+            attribute_assignments: FxHashMap::default(),
         };
 
         builder.push_scope_with_parent(NodeWithScopeRef::Module, None);
@@ -123,7 +138,7 @@ impl<'db> SemanticIndexBuilder<'db> {
         *self
             .scope_stack
             .last()
-            .map(|(scope, _)| scope)
+            .map(|ScopeInfo { file_scope_id, .. }| file_scope_id)
             .expect("Always to have a root scope")
     }
 
@@ -131,14 +146,32 @@ impl<'db> SemanticIndexBuilder<'db> {
         self.scope_stack
             .last()
             .expect("Always to have a root scope")
-            .1
+            .loop_state
+    }
+
+    /// Returns the scope ID of the surrounding class body scope if the current scope
+    /// is a method inside a class body. Returns `None` otherwise, e.g. if the current
+    /// scope is a function body outside of a class, or if the current scope is not a
+    /// function body.
+    fn is_method_of_class(&self) -> Option<FileScopeId> {
+        let mut scopes_rev = self.scope_stack.iter().rev();
+        let current = scopes_rev.next()?;
+        let parent = scopes_rev.next()?;
+
+        match (
+            self.scopes[current.file_scope_id].kind(),
+            self.scopes[parent.file_scope_id].kind(),
+        ) {
+            (ScopeKind::Function, ScopeKind::Class) => Some(parent.file_scope_id),
+            _ => None,
+        }
     }
 
     fn set_inside_loop(&mut self, state: LoopState) {
         self.scope_stack
             .last_mut()
             .expect("Always to have a root scope")
-            .1 = state;
+            .loop_state = state;
     }
 
     fn push_scope(&mut self, node: NodeWithScopeRef) {
@@ -158,6 +191,13 @@ impl<'db> SemanticIndexBuilder<'db> {
         };
         self.try_node_context_stack_manager.enter_nested_scope();
 
+        metrics::counter!(
+            "semantic_index.scope_count",
+            "file" => self.file_path.clone(),
+            "kind" => scope.kind().as_str(),
+        )
+        .increment(1);
+
         let file_scope_id = self.scopes.push(scope);
         self.symbol_tables.push(SymbolTableBuilder::default());
         self.use_def_maps.push(UseDefMapBuilder::default());
@@ -171,16 +211,20 @@ impl<'db> SemanticIndexBuilder<'db> {
 
         debug_assert_eq!(ast_id_scope, file_scope_id);
 
-        self.scope_stack.push((file_scope_id, LoopState::NotInLoop));
+        self.scope_stack.push(ScopeInfo {
+            file_scope_id,
+            loop_state: LoopState::NotInLoop,
+        });
     }
 
     fn pop_scope(&mut self) -> FileScopeId {
-        let (id, _) = self.scope_stack.pop().expect("Root scope to be present");
+        let ScopeInfo { file_scope_id, .. } =
+            self.scope_stack.pop().expect("Root scope to be present");
         let children_end = self.scopes.next_index();
-        let scope = &mut self.scopes[id];
+        let scope = &mut self.scopes[file_scope_id];
         scope.descendents = scope.descendents.start..children_end;
         self.try_node_context_stack_manager.exit_scope();
-        id
+        file_scope_id
     }
 
     fn current_symbol_table(&mut self) -> &mut SymbolTableBuilder {
@@ -198,6 +242,11 @@ impl<'db> SemanticIndexBuilder<'db> {
         &self.use_def_maps[scope_id]
     }
 
+    fn current_visibility_constraints_mut(&mut self) -> &mut VisibilityConstraintsBuilder<'db> {
+        let scope_id = self.current_scope();
+        &mut self.use_def_maps[scope_id].visibility_constraints
+    }
+
     fn current_ast_ids(&mut self) -> &mut AstIdsBuilder {
         let scope_id = self.current_scope();
         &mut self.ast_ids[scope_id]
@@ -333,21 +382,11 @@ impl<'db> SemanticIndexBuilder<'db> {
         id
     }
 
-    /// Adds a new visibility constraint, but does not record it. Returns the constraint ID
-    /// for later recording using [`SemanticIndexBuilder::record_visibility_constraint_id`].
-    fn add_visibility_constraint(
-        &mut self,
-        constraint: VisibilityConstraint<'db>,
-    ) -> ScopedVisibilityConstraintId {
-        self.current_use_def_map_mut()
-            .add_visibility_constraint(constraint)
-    }
-
     /// Records a previously added visibility constraint by applying it to all live bindings
     /// and declarations.
     fn record_visibility_constraint_id(&mut self, constraint: ScopedVisibilityConstraintId) {
         self.current_use_def_map_mut()
-            .record_visibility_constraint_id(constraint);
+            .record_visibility_constraint(constraint);
     }
 
     /// Negates the given visibility constraint and then adds it to all live bindings and declarations.
@@ -355,8 +394,11 @@ impl<'db> SemanticIndexBuilder<'db> {
         &mut self,
         constraint: ScopedVisibilityConstraintId,
     ) -> ScopedVisibilityConstraintId {
-        self.current_use_def_map_mut()
-            .record_visibility_constraint(VisibilityConstraint::VisibleIfNot(constraint))
+        let id = self
+            .current_visibility_constraints_mut()
+            .add_not_constraint(constraint);
+        self.record_visibility_constraint_id(id);
+        id
     }
 
     /// Records a visibility constraint by applying it to all live bindings and declarations.
@@ -364,14 +406,23 @@ impl<'db> SemanticIndexBuilder<'db> {
         &mut self,
         constraint: Constraint<'db>,
     ) -> ScopedVisibilityConstraintId {
-        self.current_use_def_map_mut()
-            .record_visibility_constraint(VisibilityConstraint::VisibleIf(constraint))
+        let id = self
+            .current_visibility_constraints_mut()
+            .add_atom(constraint, 0);
+        self.record_visibility_constraint_id(id);
+        id
     }
 
-    /// Records a [`VisibilityConstraint::Ambiguous`] constraint.
-    fn record_ambiguous_visibility(&mut self) -> ScopedVisibilityConstraintId {
+    /// Records that all remaining statements in the current block are unreachable, and therefore
+    /// not visible.
+    fn mark_unreachable(&mut self) {
+        self.current_use_def_map_mut().mark_unreachable();
+    }
+
+    /// Records a visibility constraint that always evaluates to "ambiguous".
+    fn record_ambiguous_visibility(&mut self) {
         self.current_use_def_map_mut()
-            .record_visibility_constraint(VisibilityConstraint::Ambiguous)
+            .record_visibility_constraint(ScopedVisibilityConstraintId::AMBIGUOUS);
     }
 
     /// Simplifies (resets) visibility constraints on all live bindings and declarations that did
@@ -398,6 +449,32 @@ impl<'db> SemanticIndexBuilder<'db> {
         self.current_assignments.last_mut()
     }
 
+    /// Records the fact that we saw an attribute assignment of the form
+    /// `object.attr: <annotation>( = …)` or `object.attr = <value>`.
+    fn register_attribute_assignment(
+        &mut self,
+        object: &ast::Expr,
+        attr: &'db ast::Identifier,
+        attribute_assignment: AttributeAssignment<'db>,
+    ) {
+        if let Some(class_body_scope) = self.is_method_of_class() {
+            // We only care about attribute assignments to the first parameter of a method,
+            // i.e. typically `self` or `cls`.
+            let accessed_object_refers_to_first_parameter =
+                object.as_name_expr().map(|name| name.id.as_str())
+                    == self.current_first_parameter_name;
+
+            if accessed_object_refers_to_first_parameter {
+                self.attribute_assignments
+                    .entry(class_body_scope)
+                    .or_default()
+                    .entry(attr.id().clone())
+                    .or_default()
+                    .push(attribute_assignment);
+            }
+        }
+    }
+
     fn add_pattern_constraint(
         &mut self,
         subject: Expression<'db>,
@@ -451,6 +528,20 @@ impl<'db> SemanticIndexBuilder<'db> {
     /// Record an expression that needs to be a Salsa ingredient, because we need to infer its type
     /// standalone (type narrowing tests, RHS of an assignment.)
     fn add_standalone_expression(&mut self, expression_node: &ast::Expr) -> Expression<'db> {
+        self.add_standalone_expression_impl(expression_node, ExpressionKind::Normal)
+    }
+
+    /// Same as [`SemanticIndexBuilder::add_standalone_expression`], but marks the expression as a
+    /// *type* expression, which makes sure that it will later be inferred as such.
+    fn add_standalone_type_expression(&mut self, expression_node: &ast::Expr) -> Expression<'db> {
+        self.add_standalone_expression_impl(expression_node, ExpressionKind::TypeExpression)
+    }
+
+    fn add_standalone_expression_impl(
+        &mut self,
+        expression_node: &ast::Expr,
+        expression_kind: ExpressionKind,
+    ) -> Expression<'db> {
         let expression = Expression::new(
             self.db,
             self.file,
@@ -459,6 +550,7 @@ impl<'db> SemanticIndexBuilder<'db> {
             unsafe {
                 AstNodeRef::new(self.module.clone(), expression_node)
             },
+            expression_kind,
             countme::Count::default(),
         );
         self.expressions_by_node
@@ -599,7 +691,7 @@ impl<'db> SemanticIndexBuilder<'db> {
     }
 
     fn declare_parameter(&mut self, parameter: &'db ast::ParameterWithDefault) {
-        let symbol = self.add_symbol(parameter.parameter.name.id().clone());
+        let symbol = self.add_symbol(parameter.name().id().clone());
 
         let definition = self.add_definition(symbol, parameter);
 
@@ -662,6 +754,11 @@ impl<'db> SemanticIndexBuilder<'db> {
             use_def_maps,
             imported_modules: Arc::new(self.imported_modules),
             has_future_annotations: self.has_future_annotations,
+            attribute_assignments: self
+                .attribute_assignments
+                .into_iter()
+                .map(|(k, v)| (k, Arc::new(v)))
+                .collect(),
         }
     }
 }
@@ -700,7 +797,38 @@ where
 
                         builder.declare_parameters(parameters);
 
-                        builder.visit_body(body);
+                        let mut first_parameter_name = parameters
+                            .iter_non_variadic_params()
+                            .next()
+                            .map(|first_param| first_param.parameter.name.id().as_str());
+                        std::mem::swap(
+                            &mut builder.current_first_parameter_name,
+                            &mut first_parameter_name,
+                        );
+
+                        // TODO: Fix how we determine the public types of symbols in a
+                        // function-like scope: https://github.com/astral-sh/ruff/issues/15777
+                        //
+                        // In the meantime, visit the function body, but treat the last statement
+                        // specially if it is a return. If it is, this would cause all definitions
+                        // in the function to be marked as non-visible with our current treatment
+                        // of terminal statements. Since we currently model the externally visible
+                        // definitions in a function scope as the set of bindings that are visible
+                        // at the end of the body, we then consider this function to have no
+                        // externally visible definitions. To get around this, we take a flow
+                        // snapshot just before processing the return statement, and use _that_ as
+                        // the "end-of-body" state that we resolve external references against.
+                        if let Some((last_stmt, first_stmts)) = body.split_last() {
+                            builder.visit_body(first_stmts);
+                            let pre_return_state = matches!(last_stmt, ast::Stmt::Return(_))
+                                .then(|| builder.flow_snapshot());
+                            builder.visit_stmt(last_stmt);
+                            if let Some(pre_return_state) = pre_return_state {
+                                builder.flow_restore(pre_return_state);
+                            }
+                        }
+
+                        builder.current_first_parameter_name = first_parameter_name;
                         builder.pop_scope()
                     },
                 );
@@ -834,6 +962,19 @@ where
                             unpack: None,
                             first: false,
                         }),
+                        ast::Expr::Attribute(ast::ExprAttribute {
+                            value: object,
+                            attr,
+                            ..
+                        }) => {
+                            self.register_attribute_assignment(
+                                object,
+                                attr,
+                                AttributeAssignment::Unannotated { value },
+                            );
+
+                            None
+                        }
                         _ => None,
                     };
 
@@ -852,6 +993,7 @@ where
             ast::Stmt::AnnAssign(node) => {
                 debug_assert_eq!(&self.current_assignments, &[]);
                 self.visit_expr(&node.annotation);
+                let annotation = self.add_standalone_type_expression(&node.annotation);
                 if let Some(value) = &node.value {
                     self.visit_expr(value);
                 }
@@ -863,6 +1005,20 @@ where
                 ) {
                     self.push_assignment(node.into());
                     self.visit_expr(&node.target);
+
+                    if let ast::Expr::Attribute(ast::ExprAttribute {
+                        value: object,
+                        attr,
+                        ..
+                    }) = &*node.target
+                    {
+                        self.register_attribute_assignment(
+                            object,
+                            attr,
+                            AttributeAssignment::Annotated { annotation },
+                        );
+                    }
+
                     self.pop_assignment();
                 } else {
                     self.visit_expr(&node.target);
@@ -964,6 +1120,16 @@ where
                 let pre_loop = self.flow_snapshot();
                 let constraint = self.record_expression_constraint(test);
 
+                // We need multiple copies of the visibility constraint for the while condition,
+                // since we need to model situations where the first evaluation of the condition
+                // returns True, but a later evaluation returns False.
+                let first_vis_constraint_id = self
+                    .current_visibility_constraints_mut()
+                    .add_atom(constraint, 0);
+                let later_vis_constraint_id = self
+                    .current_visibility_constraints_mut()
+                    .add_atom(constraint, 1);
+
                 // Save aside any break states from an outer loop
                 let saved_break_states = std::mem::take(&mut self.loop_break_states);
 
@@ -974,26 +1140,42 @@ where
                 self.visit_body(body);
                 self.set_inside_loop(outer_loop_state);
 
-                let vis_constraint_id = self.record_visibility_constraint(constraint);
+                // If the body is executed, we know that we've evaluated the condition at least
+                // once, and that the first evaluation was True. We might not have evaluated the
+                // condition more than once, so we can't assume that later evaluations were True.
+                // So the body's full visibility constraint is `first`.
+                let body_vis_constraint_id = first_vis_constraint_id;
+                self.record_visibility_constraint_id(body_vis_constraint_id);
 
                 // Get the break states from the body of this loop, and restore the saved outer
                 // ones.
                 let break_states =
                     std::mem::replace(&mut self.loop_break_states, saved_break_states);
 
-                // We may execute the `else` clause without ever executing the body, so merge in
-                // the pre-loop state before visiting `else`.
-                self.flow_merge(pre_loop.clone());
+                // We execute the `else` once the condition evaluates to false. This could happen
+                // without ever executing the body, if the condition is false the first time it's
+                // tested. So the starting flow state of the `else` clause is the union of:
+                //   - the pre-loop state with a visibility constraint that the first evaluation of
+                //     the while condition was false,
+                //   - the post-body state (which already has a visibility constraint that the
+                //     first evaluation was true) with a visibility constraint that a _later_
+                //     evaluation of the while condition was false.
+                // To model this correctly, we need two copies of the while condition constraint,
+                // since the first and later evaluations might produce different results.
+                let post_body = self.flow_snapshot();
+                self.flow_restore(pre_loop.clone());
+                self.record_negated_visibility_constraint(first_vis_constraint_id);
+                self.flow_merge(post_body);
                 self.record_negated_constraint(constraint);
                 self.visit_body(orelse);
-                self.record_negated_visibility_constraint(vis_constraint_id);
+                self.record_negated_visibility_constraint(later_vis_constraint_id);
 
                 // Breaking out of a while loop bypasses the `else` clause, so merge in the break
                 // states after visiting `else`.
                 for break_state in break_states {
                     let snapshot = self.flow_snapshot();
                     self.flow_restore(break_state);
-                    self.record_visibility_constraint(constraint);
+                    self.record_visibility_constraint_id(body_vis_constraint_id);
                     self.flow_merge(snapshot);
                 }
 
@@ -1019,11 +1201,6 @@ where
                 }
                 self.visit_body(body);
             }
-            ast::Stmt::Break(_) => {
-                if self.loop_state().is_inside() {
-                    self.loop_break_states.push(self.flow_snapshot());
-                }
-            }
 
             ast::Stmt::For(
                 for_stmt @ ast::StmtFor {
@@ -1270,6 +1447,21 @@ where
                 // - https://github.com/astral-sh/ruff/pull/13633#discussion_r1788626702
                 self.visit_body(finalbody);
             }
+
+            ast::Stmt::Raise(_) | ast::Stmt::Return(_) | ast::Stmt::Continue(_) => {
+                walk_stmt(self, stmt);
+                // Everything in the current block after a terminal statement is unreachable.
+                self.mark_unreachable();
+            }
+
+            ast::Stmt::Break(_) => {
+                if self.loop_state().is_inside() {
+                    self.loop_break_states.push(self.flow_snapshot());
+                }
+                // Everything in the current block after a terminal statement is unreachable.
+                self.mark_unreachable();
+            }
+
             _ => {
                 walk_stmt(self, stmt);
             }
@@ -1508,7 +1700,8 @@ where
                             ast::BoolOp::Or => self.add_negated_constraint(constraint),
                         };
                         let visibility_constraint = self
-                            .add_visibility_constraint(VisibilityConstraint::VisibleIf(constraint));
+                            .current_visibility_constraints_mut()
+                            .add_atom(constraint, 0);
 
                         let after_expr = self.flow_snapshot();
 

crates/red_knot_python_semantic/src/semantic_index/constraint.rs

@@ -5,20 +5,20 @@ use crate::db::Db;
 use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{FileScopeId, ScopeId};
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
 pub(crate) struct Constraint<'db> {
     pub(crate) node: ConstraintNode<'db>,
     pub(crate) is_positive: bool,
 }
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
 pub(crate) enum ConstraintNode<'db> {
     Expression(Expression<'db>),
     Pattern(PatternConstraint<'db>),
 }
 
 /// Pattern kinds for which we support type narrowing and/or static visibility analysis.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug, Clone, Hash, PartialEq)]
 pub(crate) enum PatternConstraintKind<'db> {
     Singleton(Singleton, Option<Expression<'db>>),
     Value(Expression<'db>, Option<Expression<'db>>),

crates/red_knot_python_semantic/src/semantic_index/expression.rs

@@ -5,6 +5,16 @@ use ruff_db::files::File;
 use ruff_python_ast as ast;
 use salsa;
 
+/// Whether or not this expression should be inferred as a normal expression or
+/// a type expression. For example, in `self.x: <annotation> = <value>`, the
+/// `<annotation>` is inferred as a type expression, while `<value>` is inferred
+/// as a normal expression.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub(crate) enum ExpressionKind {
+    Normal,
+    TypeExpression,
+}
+
 /// An independently type-inferable expression.
 ///
 /// Includes constraint expressions (e.g. if tests) and the RHS of an unpacking assignment.
@@ -35,6 +45,10 @@ pub(crate) struct Expression<'db> {
     #[return_ref]
     pub(crate) node_ref: AstNodeRef<ast::Expr>,
 
+    /// Should this expression be inferred as a normal expression or a type expression?
+    #[id]
+    pub(crate) kind: ExpressionKind,
+
     #[no_eq]
     count: countme::Count<Expression<'static>>,
 }

crates/red_knot_python_semantic/src/semantic_index/symbol.rs

@@ -119,6 +119,7 @@ impl<'db> ScopeId<'db> {
             self.node(db).scope_kind(),
             ScopeKind::Annotation
                 | ScopeKind::Function
+                | ScopeKind::Lambda
                 | ScopeKind::TypeAlias
                 | ScopeKind::Comprehension
         )
@@ -203,6 +204,7 @@ pub enum ScopeKind {
     Annotation,
     Class,
     Function,
+    Lambda,
     Comprehension,
     TypeAlias,
 }
@@ -211,6 +213,18 @@ impl ScopeKind {
     pub const fn is_comprehension(self) -> bool {
         matches!(self, ScopeKind::Comprehension)
     }
+
+    pub const fn as_str(self) -> &'static str {
+        match self {
+            Self::Module => "Module",
+            Self::Annotation => "Annotation",
+            Self::Class => "Class",
+            Self::Function => "Function",
+            Self::Lambda => "Lambda",
+            Self::Comprehension => "Comprehension",
+            Self::TypeAlias => "TypeAlias",
+        }
+    }
 }
 
 /// Symbol table for a specific [`Scope`].
@@ -443,7 +457,8 @@ impl NodeWithScopeKind {
         match self {
             Self::Module => ScopeKind::Module,
             Self::Class(_) => ScopeKind::Class,
-            Self::Function(_) | Self::Lambda(_) => ScopeKind::Function,
+            Self::Function(_) => ScopeKind::Function,
+            Self::Lambda(_) => ScopeKind::Lambda,
             Self::FunctionTypeParameters(_)
             | Self::ClassTypeParameters(_)
             | Self::TypeAliasTypeParameters(_) => ScopeKind::Annotation,

crates/red_knot_python_semantic/src/semantic_index/use_def.rs

@@ -255,16 +255,18 @@
 //! snapshot, and merging a snapshot into the current state. The logic using these methods lives in
 //! [`SemanticIndexBuilder`](crate::semantic_index::builder::SemanticIndexBuilder), e.g. where it
 //! visits a `StmtIf` node.
+pub(crate) use self::symbol_state::ScopedConstraintId;
 use self::symbol_state::{
     BindingIdWithConstraintsIterator, ConstraintIdIterator, DeclarationIdIterator,
     ScopedDefinitionId, SymbolBindings, SymbolDeclarations, SymbolState,
 };
-pub(crate) use self::symbol_state::{ScopedConstraintId, ScopedVisibilityConstraintId};
 use crate::semantic_index::ast_ids::ScopedUseId;
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::symbol::ScopedSymbolId;
 use crate::semantic_index::use_def::symbol_state::DeclarationIdWithConstraint;
-use crate::visibility_constraints::{VisibilityConstraint, VisibilityConstraints};
+use crate::visibility_constraints::{
+    ScopedVisibilityConstraintId, VisibilityConstraints, VisibilityConstraintsBuilder,
+};
 use ruff_index::IndexVec;
 use rustc_hash::FxHashMap;
 
@@ -285,7 +287,7 @@ pub(crate) struct UseDefMap<'db> {
     /// Array of [`Constraint`] in this scope.
     all_constraints: AllConstraints<'db>,
 
-    /// Array of [`VisibilityConstraint`]s in this scope.
+    /// Array of visibility constraints in this scope.
     visibility_constraints: VisibilityConstraints<'db>,
 
     /// [`SymbolBindings`] reaching a [`ScopedUseId`].
@@ -486,8 +488,8 @@ pub(super) struct UseDefMapBuilder<'db> {
     /// Append-only array of [`Constraint`].
     all_constraints: AllConstraints<'db>,
 
-    /// Append-only array of [`VisibilityConstraint`].
-    visibility_constraints: VisibilityConstraints<'db>,
+    /// Builder of visibility constraints.
+    pub(super) visibility_constraints: VisibilityConstraintsBuilder<'db>,
 
     /// A constraint which describes the visibility of the unbound/undeclared state, i.e.
     /// whether or not the start of the scope is visible. This is important for cases like
@@ -510,7 +512,7 @@ impl Default for UseDefMapBuilder<'_> {
         Self {
             all_definitions: IndexVec::from_iter([None]),
             all_constraints: IndexVec::new(),
-            visibility_constraints: VisibilityConstraints::default(),
+            visibility_constraints: VisibilityConstraintsBuilder::default(),
             scope_start_visibility: ScopedVisibilityConstraintId::ALWAYS_TRUE,
             bindings_by_use: IndexVec::new(),
             definitions_by_definition: FxHashMap::default(),
@@ -520,6 +522,10 @@ impl Default for UseDefMapBuilder<'_> {
 }
 
 impl<'db> UseDefMapBuilder<'db> {
+    pub(super) fn mark_unreachable(&mut self) {
+        self.record_visibility_constraint(ScopedVisibilityConstraintId::ALWAYS_FALSE);
+    }
+
     pub(super) fn add_symbol(&mut self, symbol: ScopedSymbolId) {
         let new_symbol = self
             .symbol_states
@@ -534,7 +540,7 @@ impl<'db> UseDefMapBuilder<'db> {
             binding,
             SymbolDefinitions::Declarations(symbol_state.declarations().clone()),
         );
-        symbol_state.record_binding(def_id);
+        symbol_state.record_binding(def_id, self.scope_start_visibility);
     }
 
     pub(super) fn add_constraint(&mut self, constraint: Constraint<'db>) -> ScopedConstraintId {
@@ -553,35 +559,18 @@ impl<'db> UseDefMapBuilder<'db> {
         new_constraint_id
     }
 
-    pub(super) fn add_visibility_constraint(
-        &mut self,
-        constraint: VisibilityConstraint<'db>,
-    ) -> ScopedVisibilityConstraintId {
-        self.visibility_constraints.add(constraint)
-    }
-
-    pub(super) fn record_visibility_constraint_id(
+    pub(super) fn record_visibility_constraint(
         &mut self,
         constraint: ScopedVisibilityConstraintId,
     ) {
         for state in &mut self.symbol_states {
             state.record_visibility_constraint(&mut self.visibility_constraints, constraint);
         }
-
         self.scope_start_visibility = self
             .visibility_constraints
             .add_and_constraint(self.scope_start_visibility, constraint);
     }
 
-    pub(super) fn record_visibility_constraint(
-        &mut self,
-        constraint: VisibilityConstraint<'db>,
-    ) -> ScopedVisibilityConstraintId {
-        let new_constraint_id = self.add_visibility_constraint(constraint);
-        self.record_visibility_constraint_id(new_constraint_id);
-        new_constraint_id
-    }
-
     /// This method resets the visibility constraints for all symbols to a previous state
     /// *if* there have been no new declarations or bindings since then. Consider the
     /// following example:
@@ -603,7 +592,11 @@ impl<'db> UseDefMapBuilder<'db> {
     pub(super) fn simplify_visibility_constraints(&mut self, snapshot: FlowSnapshot) {
         debug_assert!(self.symbol_states.len() >= snapshot.symbol_states.len());
 
-        self.scope_start_visibility = snapshot.scope_start_visibility;
+        // If there are any control flow paths that have become unreachable between `snapshot` and
+        // now, then it's not valid to simplify any visibility constraints to `snapshot`.
+        if self.scope_start_visibility != snapshot.scope_start_visibility {
+            return;
+        }
 
         // Note that this loop terminates when we reach a symbol not present in the snapshot.
         // This means we keep visibility constraints for all new symbols, which is intended,
@@ -639,7 +632,7 @@ impl<'db> UseDefMapBuilder<'db> {
         let def_id = self.all_definitions.push(Some(definition));
         let symbol_state = &mut self.symbol_states[symbol];
         symbol_state.record_declaration(def_id);
-        symbol_state.record_binding(def_id);
+        symbol_state.record_binding(def_id, self.scope_start_visibility);
     }
 
     pub(super) fn record_use(&mut self, symbol: ScopedSymbolId, use_id: ScopedUseId) {
@@ -684,6 +677,21 @@ impl<'db> UseDefMapBuilder<'db> {
     /// path to get here. The new state for each symbol should include definitions from both the
     /// prior state and the snapshot.
     pub(super) fn merge(&mut self, snapshot: FlowSnapshot) {
+        // As an optimization, if we know statically that either of the snapshots is always
+        // unreachable, we can leave it out of the merged result entirely. Note that we cannot
+        // perform any type inference at this point, so this is largely limited to unreachability
+        // via terminal statements. If a flow's reachability depends on an expression in the code,
+        // we will include the flow in the merged result; the visibility constraints of its
+        // bindings will include this reachability condition, so that later during type inference,
+        // we can determine whether any particular binding is non-visible due to unreachability.
+        if snapshot.scope_start_visibility == ScopedVisibilityConstraintId::ALWAYS_FALSE {
+            return;
+        }
+        if self.scope_start_visibility == ScopedVisibilityConstraintId::ALWAYS_FALSE {
+            self.restore(snapshot);
+            return;
+        }
+
         // We never remove symbols from `symbol_states` (it's an IndexVec, and the symbol
         // IDs must line up), so the current number of known symbols must always be equal to or
         // greater than the number of known symbols in a previously-taken snapshot.
@@ -717,7 +725,7 @@ impl<'db> UseDefMapBuilder<'db> {
         UseDefMap {
             all_definitions: self.all_definitions,
             all_constraints: self.all_constraints,
-            visibility_constraints: self.visibility_constraints,
+            visibility_constraints: self.visibility_constraints.build(),
             bindings_by_use: self.bindings_by_use,
             public_symbols: self.symbol_states,
             definitions_by_definition: self.definitions_by_definition,

crates/red_knot_python_semantic/src/semantic_index/use_def/symbol_state.rs

@@ -43,12 +43,15 @@
 //!
 //! Tracking live declarations is simpler, since constraints are not involved, but otherwise very
 //! similar to tracking live bindings.
-use crate::semantic_index::use_def::VisibilityConstraints;
 
-use super::bitset::{BitSet, BitSetIterator};
+use itertools::{EitherOrBoth, Itertools};
 use ruff_index::newtype_index;
 use smallvec::SmallVec;
 
+use crate::semantic_index::use_def::bitset::{BitSet, BitSetIterator};
+use crate::semantic_index::use_def::VisibilityConstraintsBuilder;
+use crate::visibility_constraints::ScopedVisibilityConstraintId;
+
 /// A newtype-index for a definition in a particular scope.
 #[newtype_index]
 pub(super) struct ScopedDefinitionId;
@@ -96,19 +99,6 @@ type ConstraintsPerBinding = SmallVec<InlineConstraintArray>;
 
 /// Iterate over all constraints for a single binding.
 type ConstraintsIterator<'a> = std::slice::Iter<'a, Constraints>;
-type ConstraintsIntoIterator = smallvec::IntoIter<InlineConstraintArray>;
-
-/// A newtype-index for a visibility constraint in a particular scope.
-#[newtype_index]
-pub(crate) struct ScopedVisibilityConstraintId;
-
-impl ScopedVisibilityConstraintId {
-    /// A special ID that is used for an "always true" / "always visible" constraint.
-    /// When we create a new [`VisibilityConstraints`] object, this constraint is always
-    /// present at index 0.
-    pub(crate) const ALWAYS_TRUE: ScopedVisibilityConstraintId =
-        ScopedVisibilityConstraintId::from_u32(0);
-}
 
 const INLINE_VISIBILITY_CONSTRAINTS: usize = 4;
 type InlineVisibilityConstraintsArray =
@@ -123,13 +113,18 @@ type VisibilityConstraintPerBinding = SmallVec<InlineVisibilityConstraintsArray>
 /// Iterator over the visibility constraints for all live bindings/declarations.
 type VisibilityConstraintsIterator<'a> = std::slice::Iter<'a, ScopedVisibilityConstraintId>;
 
-type VisibilityConstraintsIntoIterator = smallvec::IntoIter<InlineVisibilityConstraintsArray>;
-
 /// Live declarations for a single symbol at some point in control flow, with their
 /// corresponding visibility constraints.
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub(super) struct SymbolDeclarations {
     /// [`BitSet`]: which declarations (as [`ScopedDefinitionId`]) can reach the current location?
+    ///
+    /// Invariant: Because this is a `BitSet`, it can be viewed as a _sorted_ set of definition
+    /// IDs. The `visibility_constraints` field stores constraints for each definition. Therefore
+    /// those fields must always have the same `len()` as `live_declarations`, and the elements
+    /// must appear in the same order.  Effectively, this means that elements must always be added
+    /// in sorted order, or via a binary search that determines the correct place to insert new
+    /// constraints.
     pub(crate) live_declarations: Declarations,
 
     /// For each live declaration, which visibility constraint applies to it?
@@ -158,7 +153,7 @@ impl SymbolDeclarations {
     /// Add given visibility constraint to all live declarations.
     pub(super) fn record_visibility_constraint(
         &mut self,
-        visibility_constraints: &mut VisibilityConstraints,
+        visibility_constraints: &mut VisibilityConstraintsBuilder,
         constraint: ScopedVisibilityConstraintId,
     ) {
         for existing in &mut self.visibility_constraints {
@@ -173,13 +168,51 @@ impl SymbolDeclarations {
             visibility_constraints: self.visibility_constraints.iter(),
         }
     }
+
+    fn merge(&mut self, b: Self, visibility_constraints: &mut VisibilityConstraintsBuilder) {
+        let a = std::mem::take(self);
+        self.live_declarations = a.live_declarations.clone();
+        self.live_declarations.union(&b.live_declarations);
+
+        // Invariant: These zips are well-formed since we maintain an invariant that all of our
+        // fields are sets/vecs with the same length.
+        let a = (a.live_declarations.iter()).zip(a.visibility_constraints);
+        let b = (b.live_declarations.iter()).zip(b.visibility_constraints);
+
+        // Invariant: merge_join_by consumes the two iterators in sorted order, which ensures that
+        // the definition IDs and constraints line up correctly in the merged result. If a
+        // definition is found in both `a` and `b`, we compose the constraints from the two paths
+        // in an appropriate way (intersection for narrowing constraints; ternary OR for visibility
+        // constraints). If a definition is found in only one path, it is used as-is.
+        for zipped in a.merge_join_by(b, |(a_decl, _), (b_decl, _)| a_decl.cmp(b_decl)) {
+            match zipped {
+                EitherOrBoth::Both((_, a_vis_constraint), (_, b_vis_constraint)) => {
+                    let vis_constraint = visibility_constraints
+                        .add_or_constraint(a_vis_constraint, b_vis_constraint);
+                    self.visibility_constraints.push(vis_constraint);
+                }
+
+                EitherOrBoth::Left((_, vis_constraint))
+                | EitherOrBoth::Right((_, vis_constraint)) => {
+                    self.visibility_constraints.push(vis_constraint);
+                }
+            }
+        }
+    }
 }
 
 /// Live bindings for a single symbol at some point in control flow. Each live binding comes
 /// with a set of narrowing constraints and a visibility constraint.
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub(super) struct SymbolBindings {
     /// [`BitSet`]: which bindings (as [`ScopedDefinitionId`]) can reach the current location?
+    ///
+    /// Invariant: Because this is a `BitSet`, it can be viewed as a _sorted_ set of definition
+    /// IDs. The `constraints` and `visibility_constraints` field stores constraints for each
+    /// definition. Therefore those fields must always have the same `len()` as
+    /// `live_bindings`, and the elements must appear in the same order.  Effectively, this means
+    /// that elements must always be added in sorted order, or via a binary search that determines
+    /// the correct place to insert new constraints.
     live_bindings: Bindings,
 
     /// For each live binding, which [`ScopedConstraintId`] apply?
@@ -204,7 +237,11 @@ impl SymbolBindings {
     }
 
     /// Record a newly-encountered binding for this symbol.
-    pub(super) fn record_binding(&mut self, binding_id: ScopedDefinitionId) {
+    pub(super) fn record_binding(
+        &mut self,
+        binding_id: ScopedDefinitionId,
+        visibility_constraint: ScopedVisibilityConstraintId,
+    ) {
         // The new binding replaces all previous live bindings in this path, and has no
         // constraints.
         self.live_bindings = Bindings::with(binding_id.into());
@@ -212,8 +249,7 @@ impl SymbolBindings {
         self.constraints.push(Constraints::default());
 
         self.visibility_constraints = VisibilityConstraintPerBinding::with_capacity(1);
-        self.visibility_constraints
-            .push(ScopedVisibilityConstraintId::ALWAYS_TRUE);
+        self.visibility_constraints.push(visibility_constraint);
     }
 
     /// Add given constraint to all live bindings.
@@ -226,7 +262,7 @@ impl SymbolBindings {
     /// Add given visibility constraint to all live bindings.
     pub(super) fn record_visibility_constraint(
         &mut self,
-        visibility_constraints: &mut VisibilityConstraints,
+        visibility_constraints: &mut VisibilityConstraintsBuilder,
         constraint: ScopedVisibilityConstraintId,
     ) {
         for existing in &mut self.visibility_constraints {
@@ -242,6 +278,62 @@ impl SymbolBindings {
             visibility_constraints: self.visibility_constraints.iter(),
         }
     }
+
+    fn merge(&mut self, mut b: Self, visibility_constraints: &mut VisibilityConstraintsBuilder) {
+        let mut a = std::mem::take(self);
+        self.live_bindings = a.live_bindings.clone();
+        self.live_bindings.union(&b.live_bindings);
+
+        // Invariant: These zips are well-formed since we maintain an invariant that all of our
+        // fields are sets/vecs with the same length.
+        //
+        // Performance: We iterate over the `constraints` smallvecs via mut reference, because the
+        // individual elements are `BitSet`s (currently 24 bytes in size), and we don't want to
+        // move them by value multiple times during iteration. By iterating by reference, we only
+        // have to copy single pointers around.  In the loop below, the `std::mem::take` calls
+        // specify precisely where we want to move them into the merged `constraints` smallvec.
+        //
+        // We don't need a similar optimization for `visibility_constraints`, since those elements
+        // are 32-bit IndexVec IDs, and so are already cheap to move/copy.
+        let a = (a.live_bindings.iter())
+            .zip(a.constraints.iter_mut())
+            .zip(a.visibility_constraints);
+        let b = (b.live_bindings.iter())
+            .zip(b.constraints.iter_mut())
+            .zip(b.visibility_constraints);
+
+        // Invariant: merge_join_by consumes the two iterators in sorted order, which ensures that
+        // the definition IDs and constraints line up correctly in the merged result. If a
+        // definition is found in both `a` and `b`, we compose the constraints from the two paths
+        // in an appropriate way (intersection for narrowing constraints; ternary OR for visibility
+        // constraints). If a definition is found in only one path, it is used as-is.
+        for zipped in a.merge_join_by(b, |((a_def, _), _), ((b_def, _), _)| a_def.cmp(b_def)) {
+            match zipped {
+                EitherOrBoth::Both(
+                    ((_, a_constraints), a_vis_constraint),
+                    ((_, b_constraints), b_vis_constraint),
+                ) => {
+                    // If the same definition is visible through both paths, any constraint
+                    // that applies on only one path is irrelevant to the resulting type from
+                    // unioning the two paths, so we intersect the constraints.
+                    let constraints = a_constraints;
+                    constraints.intersect(b_constraints);
+                    self.constraints.push(std::mem::take(constraints));
+
+                    // For visibility constraints, we merge them using a ternary OR operation:
+                    let vis_constraint = visibility_constraints
+                        .add_or_constraint(a_vis_constraint, b_vis_constraint);
+                    self.visibility_constraints.push(vis_constraint);
+                }
+
+                EitherOrBoth::Left(((_, constraints), vis_constraint))
+                | EitherOrBoth::Right(((_, constraints), vis_constraint)) => {
+                    self.constraints.push(std::mem::take(constraints));
+                    self.visibility_constraints.push(vis_constraint);
+                }
+            }
+        }
+    }
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -260,9 +352,14 @@ impl SymbolState {
     }
 
     /// Record a newly-encountered binding for this symbol.
-    pub(super) fn record_binding(&mut self, binding_id: ScopedDefinitionId) {
+    pub(super) fn record_binding(
+        &mut self,
+        binding_id: ScopedDefinitionId,
+        visibility_constraint: ScopedVisibilityConstraintId,
+    ) {
         debug_assert_ne!(binding_id, ScopedDefinitionId::UNBOUND);
-        self.bindings.record_binding(binding_id);
+        self.bindings
+            .record_binding(binding_id, visibility_constraint);
     }
 
     /// Add given constraint to all live bindings.
@@ -273,7 +370,7 @@ impl SymbolState {
     /// Add given visibility constraint to all live bindings.
     pub(super) fn record_visibility_constraint(
         &mut self,
-        visibility_constraints: &mut VisibilityConstraints,
+        visibility_constraints: &mut VisibilityConstraintsBuilder,
         constraint: ScopedVisibilityConstraintId,
     ) {
         self.bindings
@@ -301,204 +398,11 @@ impl SymbolState {
     pub(super) fn merge(
         &mut self,
         b: SymbolState,
-        visibility_constraints: &mut VisibilityConstraints,
+        visibility_constraints: &mut VisibilityConstraintsBuilder,
     ) {
-        let mut a = Self {
-            bindings: SymbolBindings {
-                live_bindings: Bindings::default(),
-                constraints: ConstraintsPerBinding::default(),
-                visibility_constraints: VisibilityConstraintPerBinding::default(),
-            },
-            declarations: SymbolDeclarations {
-                live_declarations: self.declarations.live_declarations.clone(),
-                visibility_constraints: VisibilityConstraintPerDeclaration::default(),
-            },
-        };
-
-        std::mem::swap(&mut a, self);
+        self.bindings.merge(b.bindings, visibility_constraints);
         self.declarations
-            .live_declarations
-            .union(&b.declarations.live_declarations);
-
-        let mut a_defs_iter = a.bindings.live_bindings.iter();
-        let mut b_defs_iter = b.bindings.live_bindings.iter();
-        let mut a_constraints_iter = a.bindings.constraints.into_iter();
-        let mut b_constraints_iter = b.bindings.constraints.into_iter();
-        let mut a_vis_constraints_iter = a.bindings.visibility_constraints.into_iter();
-        let mut b_vis_constraints_iter = b.bindings.visibility_constraints.into_iter();
-
-        let mut opt_a_def: Option<u32> = a_defs_iter.next();
-        let mut opt_b_def: Option<u32> = b_defs_iter.next();
-
-        // Iterate through the definitions from `a` and `b`, always processing the lower definition
-        // ID first, and pushing each definition onto the merged `SymbolState` with its
-        // constraints. If a definition is found in both `a` and `b`, push it with the intersection
-        // of the constraints from the two paths; a constraint that applies from only one possible
-        // path is irrelevant.
-
-        // Helper to push `def`, with constraints in `constraints_iter`, onto `self`.
-        let push = |def,
-                    constraints_iter: &mut ConstraintsIntoIterator,
-                    visibility_constraints_iter: &mut VisibilityConstraintsIntoIterator,
-                    merged: &mut Self| {
-            merged.bindings.live_bindings.insert(def);
-            // SAFETY: we only ever create SymbolState using [`SymbolState::undefined`], which adds
-            // one "unbound" definition with corresponding narrowing and visibility constraints, or
-            // using [`SymbolState::record_binding`] or [`SymbolState::record_declaration`], which
-            // similarly add one definition with corresponding constraints. [`SymbolState::merge`]
-            // always pushes one definition and one constraint bitset and one visibility constraint
-            // together (just below), so the number of definitions and the number of constraints can
-            // never get out of sync.
-            // get out of sync.
-            let constraints = constraints_iter
-                .next()
-                .expect("definitions and constraints length mismatch");
-            let visibility_constraints = visibility_constraints_iter
-                .next()
-                .expect("definitions and visibility_constraints length mismatch");
-            merged.bindings.constraints.push(constraints);
-            merged
-                .bindings
-                .visibility_constraints
-                .push(visibility_constraints);
-        };
-
-        loop {
-            match (opt_a_def, opt_b_def) {
-                (Some(a_def), Some(b_def)) => match a_def.cmp(&b_def) {
-                    std::cmp::Ordering::Less => {
-                        // Next definition ID is only in `a`, push it to `self` and advance `a`.
-                        push(
-                            a_def,
-                            &mut a_constraints_iter,
-                            &mut a_vis_constraints_iter,
-                            self,
-                        );
-                        opt_a_def = a_defs_iter.next();
-                    }
-                    std::cmp::Ordering::Greater => {
-                        // Next definition ID is only in `b`, push it to `self` and advance `b`.
-                        push(
-                            b_def,
-                            &mut b_constraints_iter,
-                            &mut b_vis_constraints_iter,
-                            self,
-                        );
-                        opt_b_def = b_defs_iter.next();
-                    }
-                    std::cmp::Ordering::Equal => {
-                        // Next definition is in both; push to `self` and intersect constraints.
-                        push(
-                            a_def,
-                            &mut b_constraints_iter,
-                            &mut b_vis_constraints_iter,
-                            self,
-                        );
-
-                        // SAFETY: see comment in `push` above.
-                        let a_constraints = a_constraints_iter
-                            .next()
-                            .expect("definitions and constraints length mismatch");
-                        let current_constraints = self.bindings.constraints.last_mut().unwrap();
-
-                        // If the same definition is visible through both paths, any constraint
-                        // that applies on only one path is irrelevant to the resulting type from
-                        // unioning the two paths, so we intersect the constraints.
-                        current_constraints.intersect(&a_constraints);
-
-                        // For visibility constraints, we merge them using a ternary OR operation:
-                        let a_vis_constraint = a_vis_constraints_iter
-                            .next()
-                            .expect("visibility_constraints length mismatch");
-                        let current_vis_constraint =
-                            self.bindings.visibility_constraints.last_mut().unwrap();
-                        *current_vis_constraint = visibility_constraints
-                            .add_or_constraint(*current_vis_constraint, a_vis_constraint);
-
-                        opt_a_def = a_defs_iter.next();
-                        opt_b_def = b_defs_iter.next();
-                    }
-                },
-                (Some(a_def), None) => {
-                    // We've exhausted `b`, just push the def from `a` and move on to the next.
-                    push(
-                        a_def,
-                        &mut a_constraints_iter,
-                        &mut a_vis_constraints_iter,
-                        self,
-                    );
-                    opt_a_def = a_defs_iter.next();
-                }
-                (None, Some(b_def)) => {
-                    // We've exhausted `a`, just push the def from `b` and move on to the next.
-                    push(
-                        b_def,
-                        &mut b_constraints_iter,
-                        &mut b_vis_constraints_iter,
-                        self,
-                    );
-                    opt_b_def = b_defs_iter.next();
-                }
-                (None, None) => break,
-            }
-        }
-
-        // Same as above, but for declarations.
-        let mut a_decls_iter = a.declarations.live_declarations.iter();
-        let mut b_decls_iter = b.declarations.live_declarations.iter();
-        let mut a_vis_constraints_iter = a.declarations.visibility_constraints.into_iter();
-        let mut b_vis_constraints_iter = b.declarations.visibility_constraints.into_iter();
-
-        let mut opt_a_decl: Option<u32> = a_decls_iter.next();
-        let mut opt_b_decl: Option<u32> = b_decls_iter.next();
-
-        let push = |vis_constraints_iter: &mut VisibilityConstraintsIntoIterator,
-                    merged: &mut Self| {
-            let vis_constraints = vis_constraints_iter
-                .next()
-                .expect("declarations and visibility_constraints length mismatch");
-            merged
-                .declarations
-                .visibility_constraints
-                .push(vis_constraints);
-        };
-
-        loop {
-            match (opt_a_decl, opt_b_decl) {
-                (Some(a_decl), Some(b_decl)) => match a_decl.cmp(&b_decl) {
-                    std::cmp::Ordering::Less => {
-                        push(&mut a_vis_constraints_iter, self);
-                        opt_a_decl = a_decls_iter.next();
-                    }
-                    std::cmp::Ordering::Greater => {
-                        push(&mut b_vis_constraints_iter, self);
-                        opt_b_decl = b_decls_iter.next();
-                    }
-                    std::cmp::Ordering::Equal => {
-                        push(&mut b_vis_constraints_iter, self);
-
-                        let a_vis_constraint = a_vis_constraints_iter
-                            .next()
-                            .expect("declarations and visibility_constraints length mismatch");
-                        let current = self.declarations.visibility_constraints.last_mut().unwrap();
-                        *current =
-                            visibility_constraints.add_or_constraint(*current, a_vis_constraint);
-
-                        opt_a_decl = a_decls_iter.next();
-                        opt_b_decl = b_decls_iter.next();
-                    }
-                },
-                (Some(_), None) => {
-                    push(&mut a_vis_constraints_iter, self);
-                    opt_a_decl = a_decls_iter.next();
-                }
-                (None, Some(_)) => {
-                    push(&mut b_vis_constraints_iter, self);
-                    opt_b_decl = b_decls_iter.next();
-                }
-                (None, None) => break,
-            }
-        }
+            .merge(b.declarations, visibility_constraints);
     }
 
     pub(super) fn bindings(&self) -> &SymbolBindings {
@@ -661,7 +565,10 @@ mod tests {
     #[test]
     fn with() {
         let mut sym = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
-        sym.record_binding(ScopedDefinitionId::from_u32(1));
+        sym.record_binding(
+            ScopedDefinitionId::from_u32(1),
+            ScopedVisibilityConstraintId::ALWAYS_TRUE,
+        );
 
         assert_bindings(&sym, &["1<>"]);
     }
@@ -669,7 +576,10 @@ mod tests {
     #[test]
     fn record_constraint() {
         let mut sym = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
-        sym.record_binding(ScopedDefinitionId::from_u32(1));
+        sym.record_binding(
+            ScopedDefinitionId::from_u32(1),
+            ScopedVisibilityConstraintId::ALWAYS_TRUE,
+        );
         sym.record_constraint(ScopedConstraintId::from_u32(0));
 
         assert_bindings(&sym, &["1<0>"]);
@@ -677,15 +587,21 @@ mod tests {
 
     #[test]
     fn merge() {
-        let mut visibility_constraints = VisibilityConstraints::default();
+        let mut visibility_constraints = VisibilityConstraintsBuilder::default();
 
         // merging the same definition with the same constraint keeps the constraint
         let mut sym1a = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
-        sym1a.record_binding(ScopedDefinitionId::from_u32(1));
+        sym1a.record_binding(
+            ScopedDefinitionId::from_u32(1),
+            ScopedVisibilityConstraintId::ALWAYS_TRUE,
+        );
         sym1a.record_constraint(ScopedConstraintId::from_u32(0));
 
         let mut sym1b = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
-        sym1b.record_binding(ScopedDefinitionId::from_u32(1));
+        sym1b.record_binding(
+            ScopedDefinitionId::from_u32(1),
+            ScopedVisibilityConstraintId::ALWAYS_TRUE,
+        );
         sym1b.record_constraint(ScopedConstraintId::from_u32(0));
 
         sym1a.merge(sym1b, &mut visibility_constraints);
@@ -694,11 +610,17 @@ mod tests {
 
         // merging the same definition with differing constraints drops all constraints
         let mut sym2a = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
-        sym2a.record_binding(ScopedDefinitionId::from_u32(2));
+        sym2a.record_binding(
+            ScopedDefinitionId::from_u32(2),
+            ScopedVisibilityConstraintId::ALWAYS_TRUE,
+        );
         sym2a.record_constraint(ScopedConstraintId::from_u32(1));
 
         let mut sym1b = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
-        sym1b.record_binding(ScopedDefinitionId::from_u32(2));
+        sym1b.record_binding(
+            ScopedDefinitionId::from_u32(2),
+            ScopedVisibilityConstraintId::ALWAYS_TRUE,
+        );
         sym1b.record_constraint(ScopedConstraintId::from_u32(2));
 
         sym2a.merge(sym1b, &mut visibility_constraints);
@@ -707,7 +629,10 @@ mod tests {
 
         // merging a constrained definition with unbound keeps both
         let mut sym3a = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
-        sym3a.record_binding(ScopedDefinitionId::from_u32(3));
+        sym3a.record_binding(
+            ScopedDefinitionId::from_u32(3),
+            ScopedVisibilityConstraintId::ALWAYS_TRUE,
+        );
         sym3a.record_constraint(ScopedConstraintId::from_u32(3));
 
         let sym2b = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
@@ -748,7 +673,7 @@ mod tests {
 
     #[test]
     fn record_declaration_merge() {
-        let mut visibility_constraints = VisibilityConstraints::default();
+        let mut visibility_constraints = VisibilityConstraintsBuilder::default();
         let mut sym = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
         sym.record_declaration(ScopedDefinitionId::from_u32(1));
 
@@ -762,7 +687,7 @@ mod tests {
 
     #[test]
     fn record_declaration_merge_partial_undeclared() {
-        let mut visibility_constraints = VisibilityConstraints::default();
+        let mut visibility_constraints = VisibilityConstraintsBuilder::default();
         let mut sym = SymbolState::undefined(ScopedVisibilityConstraintId::ALWAYS_TRUE);
         sym.record_declaration(ScopedDefinitionId::from_u32(1));