Inline NodeKey construction and avoid AnyNodeRef

Co-authored-by: Micha Reiser <micha@reiser.io>

Cargo.lock

@@ -1953,7 +1953,6 @@ dependencies = [
  "red_knot_python_semantic",
  "red_knot_workspace",
  "ruff_db",
- "ruff_linter",
  "ruff_notebook",
  "ruff_python_ast",
  "ruff_source_file",
@@ -2742,7 +2741,7 @@ checksum = "e86697c916019a8588c99b5fac3cead74ec0b4b819707a682fd4d23fa0ce1ba1"
 [[package]]
 name = "salsa"
 version = "0.18.0"
-source = "git+https://github.com/MichaReiser/salsa.git?tag=red-knot-0.0.1#ece083e15b79f155f9e4368ec1318cec9a08d88b"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=f608ff8b24f07706492027199f51132244034f29#f608ff8b24f07706492027199f51132244034f29"
 dependencies = [
  "append-only-vec",
  "arc-swap",
@@ -2762,12 +2761,12 @@ dependencies = [
 [[package]]
 name = "salsa-macro-rules"
 version = "0.1.0"
-source = "git+https://github.com/MichaReiser/salsa.git?tag=red-knot-0.0.1#ece083e15b79f155f9e4368ec1318cec9a08d88b"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=f608ff8b24f07706492027199f51132244034f29#f608ff8b24f07706492027199f51132244034f29"
 
 [[package]]
 name = "salsa-macros"
 version = "0.18.0"
-source = "git+https://github.com/MichaReiser/salsa.git?tag=red-knot-0.0.1#ece083e15b79f155f9e4368ec1318cec9a08d88b"
+source = "git+https://github.com/salsa-rs/salsa.git?rev=f608ff8b24f07706492027199f51132244034f29#f608ff8b24f07706492027199f51132244034f29"
 dependencies = [
  "heck",
  "proc-macro2",

Cargo.toml

@@ -108,7 +108,7 @@ rand = { version = "0.8.5" }
 rayon = { version = "1.10.0" }
 regex = { version = "1.10.2" }
 rustc-hash = { version = "2.0.0" }
-salsa = { git = "https://github.com/MichaReiser/salsa.git", tag = "red-knot-0.0.1" }
+salsa = { git = "https://github.com/salsa-rs/salsa.git", rev = "f608ff8b24f07706492027199f51132244034f29" }
 schemars = { version = "0.8.16" }
 seahash = { version = "4.1.0" }
 serde = { version = "1.0.197", features = ["derive"] }

crates/red_knot/src/logging.rs

@@ -5,8 +5,8 @@ use colored::Colorize;
 use std::fmt;
 use std::fs::File;
 use std::io::BufWriter;
-use tracing::log::LevelFilter;
 use tracing::{Event, Subscriber};
+use tracing_subscriber::filter::LevelFilter;
 use tracing_subscriber::fmt::format::Writer;
 use tracing_subscriber::fmt::{FmtContext, FormatEvent, FormatFields};
 use tracing_subscriber::registry::LookupSpan;
@@ -60,10 +60,10 @@ pub(crate) enum VerbosityLevel {
 impl VerbosityLevel {
     const fn level_filter(self) -> LevelFilter {
         match self {
-            VerbosityLevel::Default => LevelFilter::Warn,
-            VerbosityLevel::Verbose => LevelFilter::Info,
-            VerbosityLevel::ExtraVerbose => LevelFilter::Debug,
-            VerbosityLevel::Trace => LevelFilter::Trace,
+            VerbosityLevel::Default => LevelFilter::WARN,
+            VerbosityLevel::Verbose => LevelFilter::INFO,
+            VerbosityLevel::ExtraVerbose => LevelFilter::DEBUG,
+            VerbosityLevel::Trace => LevelFilter::TRACE,
         }
     }
 
@@ -88,7 +88,7 @@ pub(crate) fn setup_tracing(level: VerbosityLevel) -> anyhow::Result<TracingGuar
         match level {
             VerbosityLevel::Default => {
                 // Show warning traces
-                EnvFilter::default().add_directive(tracing::level_filters::LevelFilter::WARN.into())
+                EnvFilter::default().add_directive(LevelFilter::WARN.into())
             }
             level => {
                 let level_filter = level.level_filter();

crates/red_knot/tests/file_watching.rs

@@ -127,7 +127,6 @@ impl TestCase {
     fn collect_package_files(&self, path: &SystemPath) -> Vec<File> {
         let package = self.db().workspace().package(self.db(), path).unwrap();
         let files = package.files(self.db());
-        let files = files.read();
         let mut collected: Vec<_> = files.into_iter().collect();
         collected.sort_unstable_by_key(|file| file.path(self.db()).as_system_path().unwrap());
         collected

crates/red_knot_python_semantic/src/db.rs

@@ -1,15 +1,18 @@
+use ruff_db::files::File;
 use ruff_db::{Db as SourceDb, Upcast};
 
 /// Database giving access to semantic information about a Python program.
 #[salsa::db]
-pub trait Db: SourceDb + Upcast<dyn SourceDb> {}
+pub trait Db: SourceDb + Upcast<dyn SourceDb> {
+    fn is_file_open(&self, file: File) -> bool;
+}
 
 #[cfg(test)]
 pub(crate) mod tests {
     use std::sync::Arc;
 
     use crate::module_resolver::vendored_typeshed_stubs;
-    use ruff_db::files::Files;
+    use ruff_db::files::{File, Files};
     use ruff_db::system::{DbWithTestSystem, System, TestSystem};
     use ruff_db::vendored::VendoredFileSystem;
     use ruff_db::{Db as SourceDb, Upcast};
@@ -91,7 +94,11 @@ pub(crate) mod tests {
     }
 
     #[salsa::db]
-    impl Db for TestDb {}
+    impl Db for TestDb {
+        fn is_file_open(&self, file: File) -> bool {
+            !file.path(self).is_vendored_path()
+        }
+    }
 
     #[salsa::db]
     impl salsa::Database for TestDb {

crates/red_knot_python_semantic/src/module_resolver/resolver.rs

@@ -41,7 +41,7 @@ pub(crate) fn resolve_module_query<'db>(
 
     let module = Module::new(name.clone(), kind, search_path, module_file);
 
-    tracing::debug!(
+    tracing::trace!(
         "Resolved module '{name}' to '{path}'.",
         path = module_file.path(db)
     );
@@ -172,11 +172,11 @@ impl SearchPaths {
             static_paths.push(search_path);
         }
 
-        tracing::debug!("Adding static search path '{src_root}'");
+        tracing::debug!("Adding first-party search path '{src_root}'");
         static_paths.push(SearchPath::first_party(system, src_root)?);
 
         static_paths.push(if let Some(custom_typeshed) = custom_typeshed {
-            tracing::debug!("Adding static custom-sdtlib search-path '{custom_typeshed}'");
+            tracing::debug!("Adding custom-stdlib search path '{custom_typeshed}'");
 
             let search_path = SearchPath::custom_stdlib(db, custom_typeshed)?;
             files.try_add_root(
@@ -192,7 +192,7 @@ impl SearchPaths {
         let mut site_packages: Vec<_> = Vec::with_capacity(site_packages_paths.len());
 
         for path in site_packages_paths {
-            tracing::debug!("Adding site-package path '{path}'");
+            tracing::debug!("Adding site-packages search path '{path}'");
             let search_path = SearchPath::site_packages(system, path)?;
             files.try_add_root(
                 db.upcast(),

crates/red_knot_python_semantic/src/node_key.rs

@@ -1,4 +1,4 @@
-use ruff_python_ast::{AnyNodeRef, NodeKind};
+use ruff_python_ast::{AnyNodeRef, AstNode, NodeKind};
 use ruff_text_size::{Ranged, TextRange};
 
 /// Compact key for a node for use in a hash map.
@@ -11,7 +11,19 @@ pub(super) struct NodeKey {
 }
 
 impl NodeKey {
-    pub(super) fn from_node<'a, N>(node: N) -> Self
+    #[inline]
+    pub(super) fn from_node<'a, N>(node: &N) -> Self
+    where
+        N: AstNode,
+    {
+        NodeKey {
+            kind: node.kind(),
+            range: node.range(),
+        }
+    }
+
+    #[inline]
+    pub(super) fn from_ref<'a, N>(node: N) -> Self
     where
         N: Into<AnyNodeRef<'a>>,
     {

crates/red_knot_python_semantic/src/program.rs

@@ -24,7 +24,7 @@ impl Program {
             search_paths,
         } = settings;
 
-        tracing::info!("Target version: {target_version}");
+        tracing::info!("Target version: Python {target_version}");
 
         let search_paths = SearchPaths::from_settings(db, search_paths)
             .with_context(|| "Invalid search path settings")?;

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -154,6 +154,10 @@ impl<'db> SemanticIndex<'db> {
         &self.scopes[id]
     }
 
+    pub(crate) fn scope_ids(&self) -> impl Iterator<Item = ScopeId> {
+        self.scope_ids_by_scope.iter().copied()
+    }
+
     /// Returns the id of the parent scope.
     pub(crate) fn parent_scope_id(&self, scope_id: FileScopeId) -> Option<FileScopeId> {
         let scope = self.scope(scope_id);

crates/red_knot_python_semantic/src/semantic_index/ast_ids.rs

@@ -197,12 +197,14 @@ pub(crate) mod node_key {
     pub(crate) struct ExpressionNodeKey(NodeKey);
 
     impl From<ast::ExpressionRef<'_>> for ExpressionNodeKey {
+        #[inline]
         fn from(value: ast::ExpressionRef<'_>) -> Self {
-            Self(NodeKey::from_node(value))
+            Self(NodeKey::from_ref(value))
         }
     }
 
     impl From<&ast::Expr> for ExpressionNodeKey {
+        #[inline]
         fn from(value: &ast::Expr) -> Self {
             Self(NodeKey::from_node(value))
         }

crates/red_knot_python_semantic/src/types.rs

@@ -5,21 +5,37 @@ use crate::builtins::builtins_scope;
 use crate::semantic_index::definition::Definition;
 use crate::semantic_index::symbol::{ScopeId, ScopedSymbolId};
 use crate::semantic_index::{
-    global_scope, symbol_table, use_def_map, DefinitionWithConstraints,
+    global_scope, semantic_index, symbol_table, use_def_map, DefinitionWithConstraints,
     DefinitionWithConstraintsIterator,
 };
 use crate::types::narrow::narrowing_constraint;
 use crate::{Db, FxOrderSet};
 
+pub(crate) use self::builder::{IntersectionBuilder, UnionBuilder};
+pub(crate) use self::diagnostic::TypeCheckDiagnostics;
+pub(crate) use self::infer::{
+    infer_definition_types, infer_expression_types, infer_scope_types, TypeInference,
+};
+
 mod builder;
+mod diagnostic;
 mod display;
 mod infer;
 mod narrow;
 
-pub(crate) use self::builder::{IntersectionBuilder, UnionBuilder};
-pub(crate) use self::infer::{
-    infer_definition_types, infer_expression_types, infer_scope_types, TypeInference,
-};
+pub fn check_types(db: &dyn Db, file: File) -> TypeCheckDiagnostics {
+    let _span = tracing::trace_span!("check_types", file=?file.path(db)).entered();
+
+    let index = semantic_index(db, file);
+    let mut diagnostics = TypeCheckDiagnostics::new();
+
+    for scope_id in index.scope_ids() {
+        let result = infer_scope_types(db, scope_id);
+        diagnostics.extend(result.diagnostics());
+    }
+
+    diagnostics
+}
 
 /// Infer the public type of a symbol (its type as seen from outside its scope).
 pub(crate) fn symbol_ty<'db>(
@@ -206,20 +222,42 @@ impl<'db> Type<'db> {
         }
     }
 
+    /// Resolve a member access of a type.
+    ///
+    /// For example, if `foo` is `Type::Instance(<Bar>)`,
+    /// `foo.member(&db, "baz")` returns the type of `baz` attributes
+    /// as accessed from instances of the `Bar` class.
+    ///
+    /// TODO: use of this method currently requires manually checking
+    /// whether the returned type is `Unknown`/`Unbound`
+    /// (or a union with `Unknown`/`Unbound`) in many places.
+    /// Ideally we'd use a more type-safe pattern, such as returning
+    /// an `Option` or a `Result` from this method, which would force
+    /// us to explicitly consider whether to handle an error or propagate
+    /// it up the call stack.
     #[must_use]
     pub fn member(&self, db: &'db dyn Db, name: &Name) -> Type<'db> {
         match self {
             Type::Any => Type::Any,
-            Type::Never => todo!("attribute lookup on Never type"),
+            Type::Never => {
+                // TODO: attribute lookup on Never type
+                Type::Unknown
+            }
             Type::Unknown => Type::Unknown,
             Type::Unbound => Type::Unbound,
-            Type::None => todo!("attribute lookup on None type"),
-            Type::Function(_) => todo!("attribute lookup on Function type"),
+            Type::None => {
+                // TODO: attribute lookup on None type
+                Type::Unknown
+            }
+            Type::Function(_) => {
+                // TODO: attribute lookup on function type
+                Type::Unknown
+            }
             Type::Module(file) => global_symbol_ty_by_name(db, *file, name),
             Type::Class(class) => class.class_member(db, name),
             Type::Instance(_) => {
                 // TODO MRO? get_own_instance_member, get_instance_member
-                todo!("attribute lookup on Instance type")
+                Type::Unknown
             }
             Type::Union(union) => union
                 .elements(db)
@@ -231,7 +269,7 @@ impl<'db> Type<'db> {
             Type::Intersection(_) => {
                 // TODO perform the get_member on each type in the intersection
                 // TODO return the intersection of those results
-                todo!("attribute lookup on Intersection type")
+                Type::Unknown
             }
             Type::IntLiteral(_) => {
                 // TODO raise error
@@ -333,3 +371,115 @@ pub struct IntersectionType<'db> {
     /// directly in intersections rather than as a separate type.
     negative: FxOrderSet<Type<'db>>,
 }
+
+#[cfg(test)]
+mod tests {
+    use anyhow::Context;
+
+    use ruff_db::files::system_path_to_file;
+    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
+
+    use crate::db::tests::TestDb;
+    use crate::{Program, ProgramSettings, PythonVersion, SearchPathSettings};
+
+    use super::TypeCheckDiagnostics;
+
+    fn setup_db() -> TestDb {
+        let db = TestDb::new();
+        db.memory_file_system()
+            .create_directory_all("/src")
+            .unwrap();
+
+        Program::from_settings(
+            &db,
+            ProgramSettings {
+                target_version: PythonVersion::default(),
+                search_paths: SearchPathSettings {
+                    extra_paths: Vec::new(),
+                    src_root: SystemPathBuf::from("/src"),
+                    site_packages: vec![],
+                    custom_typeshed: None,
+                },
+            },
+        )
+        .expect("Valid search path settings");
+
+        db
+    }
+
+    fn assert_diagnostic_messages(diagnostics: &TypeCheckDiagnostics, expected: &[&str]) {
+        let messages: Vec<&str> = diagnostics
+            .iter()
+            .map(|diagnostic| diagnostic.message())
+            .collect();
+        assert_eq!(&messages, expected);
+    }
+
+    #[test]
+    fn unresolved_import_statement() -> anyhow::Result<()> {
+        let mut db = setup_db();
+
+        db.write_file("src/foo.py", "import bar\n")
+            .context("Failed to write foo.py")?;
+
+        let foo = system_path_to_file(&db, "src/foo.py").context("Failed to resolve foo.py")?;
+
+        let diagnostics = super::check_types(&db, foo);
+        assert_diagnostic_messages(&diagnostics, &["Import 'bar' could not be resolved."]);
+
+        Ok(())
+    }
+
+    #[test]
+    fn unresolved_import_from_statement() {
+        let mut db = setup_db();
+
+        db.write_file("src/foo.py", "from bar import baz\n")
+            .unwrap();
+        let foo = system_path_to_file(&db, "src/foo.py").unwrap();
+        let diagnostics = super::check_types(&db, foo);
+        assert_diagnostic_messages(&diagnostics, &["Import 'bar' could not be resolved."]);
+    }
+
+    #[test]
+    fn unresolved_import_from_resolved_module() {
+        let mut db = setup_db();
+
+        db.write_files([("/src/a.py", ""), ("/src/b.py", "from a import thing")])
+            .unwrap();
+
+        let b_file = system_path_to_file(&db, "/src/b.py").unwrap();
+        let b_file_diagnostics = super::check_types(&db, b_file);
+        assert_diagnostic_messages(
+            &b_file_diagnostics,
+            &["Could not resolve import of 'thing' from 'a'"],
+        );
+    }
+
+    #[ignore = "\
+A spurious second 'Unresolved import' diagnostic message is emitted on `b.py`, \
+despite the symbol existing in the symbol table for `a.py`"]
+    #[test]
+    fn resolved_import_of_symbol_from_unresolved_import() {
+        let mut db = setup_db();
+
+        db.write_files([
+            ("/src/a.py", "import foo as foo"),
+            ("/src/b.py", "from a import foo"),
+        ])
+        .unwrap();
+
+        let a_file = system_path_to_file(&db, "/src/a.py").unwrap();
+        let a_file_diagnostics = super::check_types(&db, a_file);
+        assert_diagnostic_messages(
+            &a_file_diagnostics,
+            &["Import 'foo' could not be resolved."],
+        );
+
+        // Importing the unresolved import into a second first-party file should not trigger
+        // an additional "unresolved import" violation
+        let b_file = system_path_to_file(&db, "/src/b.py").unwrap();
+        let b_file_diagnostics = super::check_types(&db, b_file);
+        assert_eq!(&*b_file_diagnostics, &[]);
+    }
+}

crates/red_knot_python_semantic/src/types/diagnostic.rs

@@ -0,0 +1,111 @@
+use ruff_db::files::File;
+use ruff_text_size::{Ranged, TextRange};
+use std::fmt::Formatter;
+use std::ops::Deref;
+use std::sync::Arc;
+
+#[derive(Debug, Eq, PartialEq)]
+pub struct TypeCheckDiagnostic {
+    // TODO: Don't use string keys for rules
+    pub(super) rule: String,
+    pub(super) message: String,
+    pub(super) range: TextRange,
+    pub(super) file: File,
+}
+
+impl TypeCheckDiagnostic {
+    pub fn rule(&self) -> &str {
+        &self.rule
+    }
+
+    pub fn message(&self) -> &str {
+        &self.message
+    }
+
+    pub fn file(&self) -> File {
+        self.file
+    }
+}
+
+impl Ranged for TypeCheckDiagnostic {
+    fn range(&self) -> TextRange {
+        self.range
+    }
+}
+
+/// A collection of type check diagnostics.
+///
+/// The diagnostics are wrapped in an `Arc` because they need to be cloned multiple times
+/// when going from `infer_expression` to `check_file`. We could consider
+/// making [`TypeCheckDiagnostic`] a Salsa struct to have them Arena-allocated (once the Tables refactor is done).
+/// Using Salsa struct does have the downside that it leaks the Salsa dependency into diagnostics and
+/// each Salsa-struct comes with an overhead.
+#[derive(Default, Eq, PartialEq)]
+pub struct TypeCheckDiagnostics {
+    inner: Vec<std::sync::Arc<TypeCheckDiagnostic>>,
+}
+
+impl TypeCheckDiagnostics {
+    pub fn new() -> Self {
+        Self { inner: Vec::new() }
+    }
+
+    pub(super) fn push(&mut self, diagnostic: TypeCheckDiagnostic) {
+        self.inner.push(Arc::new(diagnostic));
+    }
+
+    pub(crate) fn shrink_to_fit(&mut self) {
+        self.inner.shrink_to_fit();
+    }
+}
+
+impl Extend<TypeCheckDiagnostic> for TypeCheckDiagnostics {
+    fn extend<T: IntoIterator<Item = TypeCheckDiagnostic>>(&mut self, iter: T) {
+        self.inner.extend(iter.into_iter().map(std::sync::Arc::new));
+    }
+}
+
+impl Extend<std::sync::Arc<TypeCheckDiagnostic>> for TypeCheckDiagnostics {
+    fn extend<T: IntoIterator<Item = Arc<TypeCheckDiagnostic>>>(&mut self, iter: T) {
+        self.inner.extend(iter);
+    }
+}
+
+impl<'a> Extend<&'a std::sync::Arc<TypeCheckDiagnostic>> for TypeCheckDiagnostics {
+    fn extend<T: IntoIterator<Item = &'a Arc<TypeCheckDiagnostic>>>(&mut self, iter: T) {
+        self.inner
+            .extend(iter.into_iter().map(std::sync::Arc::clone));
+    }
+}
+
+impl std::fmt::Debug for TypeCheckDiagnostics {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        self.inner.fmt(f)
+    }
+}
+
+impl Deref for TypeCheckDiagnostics {
+    type Target = [std::sync::Arc<TypeCheckDiagnostic>];
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl IntoIterator for TypeCheckDiagnostics {
+    type Item = Arc<TypeCheckDiagnostic>;
+    type IntoIter = std::vec::IntoIter<std::sync::Arc<TypeCheckDiagnostic>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.inner.into_iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a TypeCheckDiagnostics {
+    type Item = &'a Arc<TypeCheckDiagnostic>;
+    type IntoIter = std::slice::Iter<'a, std::sync::Arc<TypeCheckDiagnostic>>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.inner.iter()
+    }
+}

crates/red_knot_python_semantic/src/types/infer.rs

@@ -29,7 +29,8 @@ use salsa::plumbing::AsId;
 use ruff_db::files::File;
 use ruff_db::parsed::parsed_module;
 use ruff_python_ast as ast;
-use ruff_python_ast::{Expr, ExprContext};
+use ruff_python_ast::{AnyNodeRef, ExprContext};
+use ruff_text_size::Ranged;
 
 use crate::builtins::builtins_scope;
 use crate::module_name::ModuleName;
@@ -40,6 +41,7 @@ use crate::semantic_index::expression::Expression;
 use crate::semantic_index::semantic_index;
 use crate::semantic_index::symbol::{FileScopeId, NodeWithScopeKind, NodeWithScopeRef, ScopeId};
 use crate::semantic_index::SemanticIndex;
+use crate::types::diagnostic::{TypeCheckDiagnostic, TypeCheckDiagnostics};
 use crate::types::{
     builtins_symbol_ty_by_name, definitions_ty, global_symbol_ty_by_name, ClassType, FunctionType,
     Name, Type, UnionBuilder,
@@ -123,13 +125,16 @@ pub(crate) enum InferenceRegion<'db> {
 }
 
 /// The inferred types for a single region.
-#[derive(Debug, Eq, PartialEq, Default, Clone)]
+#[derive(Debug, Eq, PartialEq, Default)]
 pub(crate) struct TypeInference<'db> {
     /// The types of every expression in this region.
     expressions: FxHashMap<ScopedExpressionId, Type<'db>>,
 
     /// The types of every definition in this region.
     definitions: FxHashMap<Definition<'db>, Type<'db>>,
+
+    /// The diagnostics for this region.
+    diagnostics: TypeCheckDiagnostics,
 }
 
 impl<'db> TypeInference<'db> {
@@ -142,9 +147,14 @@ impl<'db> TypeInference<'db> {
         self.definitions[&definition]
     }
 
+    pub(crate) fn diagnostics(&self) -> &[std::sync::Arc<TypeCheckDiagnostic>] {
+        &self.diagnostics
+    }
+
     fn shrink_to_fit(&mut self) {
         self.expressions.shrink_to_fit();
         self.definitions.shrink_to_fit();
+        self.diagnostics.shrink_to_fit();
     }
 }
 
@@ -235,6 +245,7 @@ impl<'db> TypeInferenceBuilder<'db> {
     fn extend(&mut self, inference: &TypeInference<'db>) {
         self.types.definitions.extend(inference.definitions.iter());
         self.types.expressions.extend(inference.expressions.iter());
+        self.types.diagnostics.extend(&inference.diagnostics);
     }
 
     /// Infers types in the given [`InferenceRegion`].
@@ -855,7 +866,26 @@ impl<'db> TypeInferenceBuilder<'db> {
             asname: _,
         } = alias;
 
-        let module_ty = self.module_ty_from_name(ModuleName::new(name));
+        let module_ty = ModuleName::new(name)
+            .ok_or(ModuleResolutionError::InvalidSyntax)
+            .and_then(|module_name| self.module_ty_from_name(module_name));
+
+        let module_ty = match module_ty {
+            Ok(ty) => ty,
+            Err(ModuleResolutionError::InvalidSyntax) => {
+                tracing::debug!("Failed to resolve import due to invalid syntax");
+                Type::Unknown
+            }
+            Err(ModuleResolutionError::UnresolvedModule) => {
+                self.add_diagnostic(
+                    AnyNodeRef::Alias(alias),
+                    "unresolved-import",
+                    format_args!("Import '{name}' could not be resolved."),
+                );
+                Type::Unknown
+            }
+        };
+
         self.types.definitions.insert(definition, module_ty);
     }
 
@@ -903,10 +933,18 @@ impl<'db> TypeInferenceBuilder<'db> {
     /// - `tail` is the relative module name stripped of all leading dots:
     ///   - `from .foo import bar` => `tail == "foo"`
     ///   - `from ..foo.bar import baz` => `tail == "foo.bar"`
-    fn relative_module_name(&self, tail: Option<&str>, level: NonZeroU32) -> Option<ModuleName> {
+    fn relative_module_name(
+        &self,
+        tail: Option<&str>,
+        level: NonZeroU32,
+    ) -> Result<ModuleName, ModuleResolutionError> {
         let Some(module) = file_to_module(self.db, self.file) else {
-            tracing::debug!("Failed to resolve file {:?} to a module", self.file);
-            return None;
+            tracing::debug!(
+                "Relative module resolution '{}' failed; could not resolve file '{}' to a module",
+                format_import_from_module(level.get(), tail),
+                self.file.path(self.db)
+            );
+            return Err(ModuleResolutionError::UnresolvedModule);
         };
         let mut level = level.get();
         if module.kind().is_package() {
@@ -914,17 +952,19 @@ impl<'db> TypeInferenceBuilder<'db> {
         }
         let mut module_name = module.name().to_owned();
         for _ in 0..level {
-            module_name = module_name.parent()?;
+            module_name = module_name
+                .parent()
+                .ok_or(ModuleResolutionError::UnresolvedModule)?;
         }
         if let Some(tail) = tail {
             if let Some(valid_tail) = ModuleName::new(tail) {
                 module_name.extend(&valid_tail);
             } else {
-                tracing::debug!("Failed to resolve relative import due to invalid syntax");
-                return None;
+                tracing::debug!("Relative module resolution failed: invalid syntax");
+                return Err(ModuleResolutionError::InvalidSyntax);
             }
         }
-        Some(module_name)
+        Ok(module_name)
     }
 
     fn infer_import_from_definition(
@@ -944,16 +984,27 @@ impl<'db> TypeInferenceBuilder<'db> {
         // `follow_nonexistent_import_bare_to_module()`.
         let ast::StmtImportFrom { module, level, .. } = import_from;
         tracing::trace!("Resolving imported object {alias:?} from statement {import_from:?}");
+        let module = module.as_deref();
         let module_name = if let Some(level) = NonZeroU32::new(*level) {
-            self.relative_module_name(module.as_deref(), level)
+            tracing::trace!(
+                "Resolving imported object '{}' from module '{}' relative to file '{}'",
+                alias.name,
+                format_import_from_module(level.get(), module),
+                self.file.path(self.db),
+            );
+            self.relative_module_name(module, level)
         } else {
-            let module_name = module
-                .as_ref()
-                .expect("Non-relative import should always have a non-None `module`!");
-            ModuleName::new(module_name)
+            tracing::trace!(
+                "Resolving imported object '{}' from module '{}'",
+                alias.name,
+                format_import_from_module(*level, module),
+            );
+            module
+                .and_then(ModuleName::new)
+                .ok_or(ModuleResolutionError::InvalidSyntax)
         };
 
-        let module_ty = self.module_ty_from_name(module_name);
+        let module_ty = module_name.and_then(|module_name| self.module_ty_from_name(module_name));
 
         let ast::Alias {
             range: _,
@@ -966,11 +1017,34 @@ impl<'db> TypeInferenceBuilder<'db> {
         // the runtime error will occur immediately (rather than when the symbol is *used*,
         // as would be the case for a symbol with type `Unbound`), so it's appropriate to
         // think of the type of the imported symbol as `Unknown` rather than `Unbound`
-        let ty = module_ty
+        let member_ty = module_ty
+            .unwrap_or(Type::Unbound)
             .member(self.db, &Name::new(&name.id))
             .replace_unbound_with(self.db, Type::Unknown);
 
-        self.types.definitions.insert(definition, ty);
+        if matches!(module_ty, Err(ModuleResolutionError::UnresolvedModule)) {
+            self.add_diagnostic(
+                AnyNodeRef::StmtImportFrom(import_from),
+                "unresolved-import",
+                format_args!(
+                    "Import '{}{}' could not be resolved.",
+                    ".".repeat(*level as usize),
+                    module.unwrap_or_default()
+                ),
+            );
+        } else if module_ty.is_ok() && member_ty.is_unknown() {
+            self.add_diagnostic(
+                AnyNodeRef::Alias(alias),
+                "unresolved-import",
+                format_args!(
+                    "Could not resolve import of '{name}' from '{}{}'",
+                    ".".repeat(*level as usize),
+                    module.unwrap_or_default()
+                ),
+            );
+        }
+
+        self.types.definitions.insert(definition, member_ty);
     }
 
     fn infer_return_statement(&mut self, ret: &ast::StmtReturn) {
@@ -984,10 +1058,13 @@ impl<'db> TypeInferenceBuilder<'db> {
         }
     }
 
-    fn module_ty_from_name(&self, module_name: Option<ModuleName>) -> Type<'db> {
-        module_name
-            .and_then(|module_name| resolve_module(self.db, module_name))
-            .map_or(Type::Unknown, |module| Type::Module(module.file()))
+    fn module_ty_from_name(
+        &self,
+        module_name: ModuleName,
+    ) -> Result<Type<'db>, ModuleResolutionError> {
+        resolve_module(self.db, module_name)
+            .map(|module| Type::Module(module.file()))
+            .ok_or(ModuleResolutionError::UnresolvedModule)
     }
 
     fn infer_decorator(&mut self, decorator: &ast::Decorator) -> Type<'db> {
@@ -1059,7 +1136,7 @@ impl<'db> TypeInferenceBuilder<'db> {
             ast::Expr::Yield(yield_expression) => self.infer_yield_expression(yield_expression),
             ast::Expr::YieldFrom(yield_from) => self.infer_yield_from_expression(yield_from),
             ast::Expr::Await(await_expression) => self.infer_await_expression(await_expression),
-            Expr::IpyEscapeCommand(_) => todo!("Implement Ipy escape command support"),
+            ast::Expr::IpyEscapeCommand(_) => todo!("Implement Ipy escape command support"),
         };
 
         let expr_id = expression.scoped_ast_id(self.db, self.scope);
@@ -1117,21 +1194,7 @@ impl<'db> TypeInferenceBuilder<'db> {
                         flags: _,
                     } = fstring;
                     for element in elements {
-                        match element {
-                            ast::FStringElement::Literal(_) => {
-                                // TODO string literal type
-                            }
-                            ast::FStringElement::Expression(expr_element) => {
-                                let ast::FStringExpressionElement {
-                                    range: _,
-                                    expression,
-                                    debug_text: _,
-                                    conversion: _,
-                                    format_spec: _,
-                                } = expr_element;
-                                self.infer_expression(expression);
-                            }
-                        }
+                        self.infer_fstring_element(element);
                     }
                 }
             }
@@ -1141,6 +1204,30 @@ impl<'db> TypeInferenceBuilder<'db> {
         Type::Unknown
     }
 
+    fn infer_fstring_element(&mut self, element: &ast::FStringElement) {
+        match element {
+            ast::FStringElement::Literal(_) => {
+                // TODO string literal type
+            }
+            ast::FStringElement::Expression(expr_element) => {
+                let ast::FStringExpressionElement {
+                    range: _,
+                    expression,
+                    debug_text: _,
+                    conversion: _,
+                    format_spec,
+                } = expr_element;
+                self.infer_expression(expression);
+
+                if let Some(format_spec) = format_spec {
+                    for spec_element in &format_spec.elements {
+                        self.infer_fstring_element(spec_element);
+                    }
+                }
+            }
+        }
+    }
+
     #[allow(clippy::unused_self)]
     fn infer_ellipsis_literal_expression(
         &mut self,
@@ -1706,6 +1793,28 @@ impl<'db> TypeInferenceBuilder<'db> {
         }
     }
 
+    /// Adds a new diagnostic.
+    ///
+    /// The diagnostic does not get added if the rule isn't enabled for this file.
+    fn add_diagnostic(&mut self, node: AnyNodeRef, rule: &str, message: std::fmt::Arguments) {
+        if !self.db.is_file_open(self.file) {
+            return;
+        }
+
+        // TODO: Don't emit the diagnostic if:
+        // * The enclosing node contains any syntax errors
+        // * The rule is disabled for this file. We probably want to introduce a new query that
+        //   returns a rule selector for a given file that respects the package's settings,
+        //   any global pragma comments in the file, and any per-file-ignores.
+
+        self.types.diagnostics.push(TypeCheckDiagnostic {
+            file: self.file,
+            rule: rule.to_string(),
+            message: message.to_string(),
+            range: node.range(),
+        });
+    }
+
     pub(super) fn finish(mut self) -> TypeInference<'db> {
         self.infer_region();
         self.types.shrink_to_fit();
@@ -1713,6 +1822,20 @@ impl<'db> TypeInferenceBuilder<'db> {
     }
 }
 
+fn format_import_from_module(level: u32, module: Option<&str>) -> String {
+    format!(
+        "{}{}",
+        ".".repeat(level as usize),
+        module.unwrap_or_default()
+    )
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+enum ModuleResolutionError {
+    InvalidSyntax,
+    UnresolvedModule,
+}
+
 #[cfg(test)]
 mod tests {
     use anyhow::Context;
@@ -1966,6 +2089,16 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn from_import_with_no_module_name() -> anyhow::Result<()> {
+        // This test checks that invalid syntax in a `StmtImportFrom` node
+        // leads to the type being inferred as `Unknown`
+        let mut db = setup_db();
+        db.write_file("src/foo.py", "from import bar")?;
+        assert_public_ty(&db, "src/foo.py", "bar", "Unknown");
+        Ok(())
+    }
+
     #[test]
     fn resolve_base_class_by_name() -> anyhow::Result<()> {
         let mut db = setup_db();

crates/red_knot_server/Cargo.toml

@@ -14,7 +14,6 @@ license = { workspace = true }
 red_knot_python_semantic = { workspace = true }
 red_knot_workspace = { workspace = true }
 ruff_db = { workspace = true }
-ruff_linter = { workspace = true }
 ruff_notebook = { workspace = true }
 ruff_python_ast = { workspace = true }
 ruff_source_file = { workspace = true }

crates/red_knot_server/src/session/capabilities.rs

@@ -1,5 +1,4 @@
 use lsp_types::ClientCapabilities;
-use ruff_linter::display_settings;
 
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
 #[allow(clippy::struct_excessive_bools)]
@@ -66,20 +65,3 @@ impl ResolvedClientCapabilities {
         }
     }
 }
-
-impl std::fmt::Display for ResolvedClientCapabilities {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        display_settings! {
-            formatter = f,
-            namespace = "capabilities",
-            fields = [
-                self.code_action_deferred_edit_resolution,
-                self.apply_edit,
-                self.document_changes,
-                self.workspace_refresh,
-                self.pull_diagnostics,
-            ]
-        };
-        Ok(())
-    }
-}

crates/red_knot_server/src/session/index.rs

@@ -278,18 +278,6 @@ impl DocumentQuery {
         }
     }
 
-    /// Generate a source kind used by the linter.
-    pub(crate) fn make_source_kind(&self) -> ruff_linter::source_kind::SourceKind {
-        match self {
-            Self::Text { document, .. } => {
-                ruff_linter::source_kind::SourceKind::Python(document.contents().to_string())
-            }
-            Self::Notebook { notebook, .. } => {
-                ruff_linter::source_kind::SourceKind::IpyNotebook(notebook.make_ruff_notebook())
-            }
-        }
-    }
-
     /// Attempts to access the underlying notebook document that this query is selecting.
     pub fn as_notebook(&self) -> Option<&NotebookDocument> {
         match self {

crates/red_knot_wasm/src/lib.rs

@@ -109,7 +109,7 @@ impl Workspace {
     pub fn check_file(&self, file_id: &FileHandle) -> Result<Vec<String>, Error> {
         let result = self.db.check_file(file_id.file).map_err(into_error)?;
 
-        Ok(result.to_vec())
+        Ok(result.clone())
     }
 
     /// Checks all open files

crates/red_knot_wasm/tests/api.rs

@@ -17,5 +17,8 @@ fn check() {
 
     let result = workspace.check_file(&test).expect("Check to succeed");
 
-    assert_eq!(result, vec!["/test.py:1:8: Unresolved import 'random22'"]);
+    assert_eq!(
+        result,
+        vec!["/test.py:1:8: Import 'random22' could not be resolved.",]
+    );
 }

crates/red_knot_workspace/src/db.rs

@@ -11,7 +11,6 @@ use ruff_db::{Db as SourceDb, Upcast};
 use salsa::plumbing::ZalsaDatabase;
 use salsa::{Cancelled, Event};
 
-use crate::lint::Diagnostics;
 use crate::workspace::{check_file, Workspace, WorkspaceMetadata};
 
 mod changes;
@@ -61,7 +60,7 @@ impl RootDatabase {
         self.with_db(|db| db.workspace().check(db))
     }
 
-    pub fn check_file(&self, file: File) -> Result<Diagnostics, Cancelled> {
+    pub fn check_file(&self, file: File) -> Result<Vec<String>, Cancelled> {
         self.with_db(|db| check_file(db, file))
     }
 
@@ -115,7 +114,15 @@ impl Upcast<dyn SourceDb> for RootDatabase {
 }
 
 #[salsa::db]
-impl SemanticDb for RootDatabase {}
+impl SemanticDb for RootDatabase {
+    fn is_file_open(&self, file: File) -> bool {
+        let Some(workspace) = &self.workspace else {
+            return false;
+        };
+
+        workspace.is_file_open(self, file)
+    }
+}
 
 #[salsa::db]
 impl SourceDb for RootDatabase {
@@ -242,7 +249,12 @@ pub(crate) mod tests {
     }
 
     #[salsa::db]
-    impl red_knot_python_semantic::Db for TestDb {}
+    impl red_knot_python_semantic::Db for TestDb {
+        fn is_file_open(&self, file: ruff_db::files::File) -> bool {
+            !file.path(self).is_vendored_path()
+        }
+    }
+
     #[salsa::db]
     impl Db for TestDb {}
 

crates/red_knot_workspace/src/db/changes.rs

@@ -120,7 +120,7 @@ impl RootDatabase {
         if workspace_change {
             match WorkspaceMetadata::from_path(&workspace_path, self.system()) {
                 Ok(metadata) => {
-                    tracing::debug!("Reload workspace after structural change.");
+                    tracing::debug!("Reloading workspace after structural change.");
                     // TODO: Handle changes in the program settings.
                     workspace.reload(self, metadata);
                 }

crates/red_knot_workspace/src/lint.rs

@@ -1,5 +1,4 @@
 use std::cell::RefCell;
-use std::ops::Deref;
 use std::time::Duration;
 
 use tracing::debug_span;
@@ -22,7 +21,7 @@ use crate::db::Db;
 pub(crate) fn unwind_if_cancelled(db: &dyn Db) {}
 
 #[salsa::tracked(return_ref)]
-pub(crate) fn lint_syntax(db: &dyn Db, file_id: File) -> Diagnostics {
+pub(crate) fn lint_syntax(db: &dyn Db, file_id: File) -> Vec<String> {
     #[allow(clippy::print_stdout)]
     if std::env::var("RED_KNOT_SLOW_LINT").is_ok() {
         for i in 0..10 {
@@ -64,7 +63,7 @@ pub(crate) fn lint_syntax(db: &dyn Db, file_id: File) -> Diagnostics {
         }));
     }
 
-    Diagnostics::from(diagnostics)
+    diagnostics
 }
 
 fn lint_lines(source: &str, diagnostics: &mut Vec<String>) {
@@ -86,7 +85,7 @@ fn lint_lines(source: &str, diagnostics: &mut Vec<String>) {
 
 #[allow(unreachable_pub)]
 #[salsa::tracked(return_ref)]
-pub fn lint_semantic(db: &dyn Db, file_id: File) -> Diagnostics {
+pub fn lint_semantic(db: &dyn Db, file_id: File) -> Vec<String> {
     let _span = debug_span!("lint_semantic", file=%file_id.path(db)).entered();
 
     let source = source_text(db.upcast(), file_id);
@@ -94,7 +93,7 @@ pub fn lint_semantic(db: &dyn Db, file_id: File) -> Diagnostics {
     let semantic = SemanticModel::new(db.upcast(), file_id);
 
     if !parsed.is_valid() {
-        return Diagnostics::Empty;
+        return vec![];
     }
 
     let context = SemanticLintContext {
@@ -106,7 +105,7 @@ pub fn lint_semantic(db: &dyn Db, file_id: File) -> Diagnostics {
 
     SemanticVisitor { context: &context }.visit_body(parsed.suite());
 
-    Diagnostics::from(context.diagnostics.take())
+    context.diagnostics.take()
 }
 
 fn format_diagnostic(context: &SemanticLintContext, message: &str, start: TextSize) -> String {
@@ -116,48 +115,13 @@ fn format_diagnostic(context: &SemanticLintContext, message: &str, start: TextSi
         .source_location(start, context.source_text());
     format!(
         "{}:{}:{}: {}",
-        context.semantic.file_path().as_str(),
+        context.semantic.file_path(),
         source_location.row,
         source_location.column,
         message,
     )
 }
 
-fn lint_unresolved_imports(context: &SemanticLintContext, import: AnyImportRef) {
-    // TODO: this treats any symbol with `Type::Unknown` as an unresolved import,
-    // which isn't really correct: if it exists but has `Type::Unknown` in the
-    // module we're importing it from, we shouldn't really emit a diagnostic here,
-    // but currently do.
-    match import {
-        AnyImportRef::Import(import) => {
-            for alias in &import.names {
-                let ty = alias.ty(&context.semantic);
-
-                if ty.is_unknown() {
-                    context.push_diagnostic(format_diagnostic(
-                        context,
-                        &format!("Unresolved import '{}'", &alias.name),
-                        alias.start(),
-                    ));
-                }
-            }
-        }
-        AnyImportRef::ImportFrom(import) => {
-            for alias in &import.names {
-                let ty = alias.ty(&context.semantic);
-
-                if ty.is_unknown() {
-                    context.push_diagnostic(format_diagnostic(
-                        context,
-                        &format!("Unresolved import '{}'", &alias.name),
-                        alias.start(),
-                    ));
-                }
-            }
-        }
-    }
-}
-
 fn lint_maybe_undefined(context: &SemanticLintContext, name: &ast::ExprName) {
     if !matches!(name.ctx, ast::ExprContext::Load) {
         return;
@@ -280,17 +244,8 @@ struct SemanticVisitor<'a> {
 
 impl Visitor<'_> for SemanticVisitor<'_> {
     fn visit_stmt(&mut self, stmt: &ast::Stmt) {
-        match stmt {
-            ast::Stmt::ClassDef(class) => {
-                lint_bad_override(self.context, class);
-            }
-            ast::Stmt::Import(import) => {
-                lint_unresolved_imports(self.context, AnyImportRef::Import(import));
-            }
-            ast::Stmt::ImportFrom(import) => {
-                lint_unresolved_imports(self.context, AnyImportRef::ImportFrom(import));
-            }
-            _ => {}
+        if let ast::Stmt::ClassDef(class) = stmt {
+            lint_bad_override(self.context, class);
         }
 
         walk_stmt(self, stmt);
@@ -308,53 +263,6 @@ impl Visitor<'_> for SemanticVisitor<'_> {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub enum Diagnostics {
-    Empty,
-    List(Vec<String>),
-}
-
-impl Diagnostics {
-    pub fn as_slice(&self) -> &[String] {
-        match self {
-            Diagnostics::Empty => &[],
-            Diagnostics::List(list) => list.as_slice(),
-        }
-    }
-}
-
-impl Deref for Diagnostics {
-    type Target = [String];
-    fn deref(&self) -> &Self::Target {
-        self.as_slice()
-    }
-}
-
-impl From<Vec<String>> for Diagnostics {
-    fn from(value: Vec<String>) -> Self {
-        if value.is_empty() {
-            Diagnostics::Empty
-        } else {
-            Diagnostics::List(value)
-        }
-    }
-}
-
-#[derive(Copy, Clone, Debug)]
-enum AnyImportRef<'a> {
-    Import(&'a ast::StmtImport),
-    ImportFrom(&'a ast::StmtImportFrom),
-}
-
-impl Ranged for AnyImportRef<'_> {
-    fn range(&self) -> ruff_text_size::TextRange {
-        match self {
-            AnyImportRef::Import(import) => import.range(),
-            AnyImportRef::ImportFrom(import) => import.range(),
-        }
-    }
-}
-
 #[cfg(test)]
 mod tests {
     use red_knot_python_semantic::{Program, ProgramSettings, PythonVersion, SearchPathSettings};
@@ -363,7 +271,7 @@ mod tests {
 
     use crate::db::tests::TestDb;
 
-    use super::{lint_semantic, Diagnostics};
+    use super::lint_semantic;
 
     fn setup_db() -> TestDb {
         setup_db_with_root(SystemPathBuf::from("/src"))
@@ -409,9 +317,9 @@ mod tests {
         .unwrap();
 
         let file = system_path_to_file(&db, "/src/a.py").expect("file to exist");
-        let Diagnostics::List(messages) = lint_semantic(&db, file) else {
-            panic!("expected some diagnostics");
-        };
+        let messages = lint_semantic(&db, file);
+
+        assert_ne!(messages, &[] as &[String], "expected some diagnostics");
 
         assert_eq!(
             *messages,

crates/red_knot_workspace/src/site_packages.rs

@@ -55,7 +55,7 @@ impl VirtualEnvironment {
 
         let venv_path = SysPrefixPath::new(path, system)?;
         let pyvenv_cfg_path = venv_path.join("pyvenv.cfg");
-        tracing::debug!("Attempting to parse virtual environment metadata at {pyvenv_cfg_path}");
+        tracing::debug!("Attempting to parse virtual environment metadata at '{pyvenv_cfg_path}'");
 
         let pyvenv_cfg = system
             .read_to_string(&pyvenv_cfg_path)
@@ -191,7 +191,7 @@ impl VirtualEnvironment {
             } else {
                 tracing::warn!(
                     "Failed to resolve `sys.prefix` of the system Python installation \
-from the `home` value in the `pyvenv.cfg` file at {}. \
+from the `home` value in the `pyvenv.cfg` file at '{}'. \
 System site-packages will not be used for module resolution.",
                     venv_path.join("pyvenv.cfg")
                 );
@@ -425,7 +425,7 @@ impl Deref for SysPrefixPath {
 
 impl fmt::Display for SysPrefixPath {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "`sys.prefix` path {}", self.0)
+        write!(f, "`sys.prefix` path '{}'", self.0)
     }
 }
 
@@ -482,7 +482,7 @@ impl Deref for PythonHomePath {
 
 impl fmt::Display for PythonHomePath {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "`home` location {}", self.0)
+        write!(f, "`home` location '{}'", self.0)
     }
 }
 

crates/red_knot_workspace/src/watch/watcher.rs

@@ -109,7 +109,7 @@ struct WatcherInner {
 impl Watcher {
     /// Sets up file watching for `path`.
     pub fn watch(&mut self, path: &SystemPath) -> notify::Result<()> {
-        tracing::debug!("Watching path: {path}.");
+        tracing::debug!("Watching path: '{path}'.");
 
         self.inner_mut()
             .watcher
@@ -118,7 +118,7 @@ impl Watcher {
 
     /// Stops file watching for `path`.
     pub fn unwatch(&mut self, path: &SystemPath) -> notify::Result<()> {
-        tracing::debug!("Unwatching path: {path}.");
+        tracing::debug!("Unwatching path: '{path}'.");
 
         self.inner_mut().watcher.unwatch(path.as_std_path())
     }
@@ -351,7 +351,7 @@ impl Debouncer {
             }
 
             EventKind::Any => {
-                tracing::debug!("Skip any FS event for {path}.");
+                tracing::debug!("Skipping any FS event for '{path}'.");
                 return;
             }
         };

crates/red_knot_workspace/src/workspace.rs

@@ -4,17 +4,19 @@ use rustc_hash::{FxBuildHasher, FxHashSet};
 use salsa::{Durability, Setter as _};
 
 pub use metadata::{PackageMetadata, WorkspaceMetadata};
-use ruff_db::source::{source_text, SourceDiagnostic};
+use red_knot_python_semantic::types::check_types;
+use ruff_db::source::{line_index, source_text, SourceDiagnostic};
 use ruff_db::{
     files::{system_path_to_file, File},
     system::{walk_directory::WalkState, SystemPath, SystemPathBuf},
 };
 use ruff_python_ast::{name::Name, PySourceType};
+use ruff_text_size::Ranged;
 
-use crate::workspace::files::{Index, IndexedFiles, PackageFiles};
+use crate::workspace::files::{Index, Indexed, PackageFiles};
 use crate::{
     db::Db,
-    lint::{lint_semantic, lint_syntax, Diagnostics},
+    lint::{lint_semantic, lint_syntax},
 };
 
 mod files;
@@ -92,8 +94,8 @@ pub struct Package {
     root_buf: SystemPathBuf,
 
     /// The files that are part of this package.
-    #[return_ref]
     #[default]
+    #[return_ref]
     file_set: PackageFiles,
     // TODO: Add the loaded settings.
 }
@@ -141,9 +143,7 @@ impl Workspace {
             new_packages.insert(path, package);
         }
 
-        self.set_package_tree(db)
-            .with_durability(Durability::MEDIUM)
-            .to(new_packages);
+        self.set_package_tree(db).to(new_packages);
     }
 
     pub fn update_package(self, db: &mut dyn Db, metadata: PackageMetadata) -> anyhow::Result<()> {
@@ -197,7 +197,7 @@ impl Workspace {
     ///
     /// This changes the behavior of `check` to only check the open files rather than all files in the workspace.
     pub fn open_file(self, db: &mut dyn Db, file: File) {
-        tracing::debug!("Opening file {}", file.path(db));
+        tracing::debug!("Opening file '{}'", file.path(db));
 
         let mut open_files = self.take_open_files(db);
         open_files.insert(file);
@@ -206,7 +206,7 @@ impl Workspace {
 
     /// Closes a file in the workspace.
     pub fn close_file(self, db: &mut dyn Db, file: File) -> bool {
-        tracing::debug!("Closing file {}", file.path(db));
+        tracing::debug!("Closing file '{}'", file.path(db));
 
         let mut open_files = self.take_open_files(db);
         let removed = open_files.remove(&file);
@@ -249,6 +249,23 @@ impl Workspace {
             FxHashSet::default()
         }
     }
+
+    /// Returns `true` if the file is open in the workspace.
+    ///
+    /// A file is considered open when:
+    /// * explicitly set as an open file using [`open_file`](Self::open_file)
+    /// * It has a [`SystemPath`] and belongs to a package's `src` files
+    /// * It has a [`SystemVirtualPath`](ruff_db::system::SystemVirtualPath)
+    pub fn is_file_open(self, db: &dyn Db, file: File) -> bool {
+        if let Some(open_files) = self.open_files(db) {
+            open_files.contains(&file)
+        } else if let Some(system_path) = file.path(db).as_system_path() {
+            self.package(db, system_path)
+                .map_or(false, |package| package.contains_file(db, file))
+        } else {
+            file.path(db).is_system_virtual_path()
+        }
+    }
 }
 
 #[salsa::tracked]
@@ -259,13 +276,13 @@ impl Package {
 
     /// Returns `true` if `file` is a first-party file part of this package.
     pub fn contains_file(self, db: &dyn Db, file: File) -> bool {
-        self.files(db).read().contains(&file)
+        self.files(db).contains(&file)
     }
 
     #[tracing::instrument(level = "debug", skip(db))]
     pub fn remove_file(self, db: &mut dyn Db, file: File) {
         tracing::debug!(
-            "Remove file {} from package {}",
+            "Removing file '{}' from package '{}'",
             file.path(db),
             self.name(db)
         );
@@ -278,7 +295,11 @@ impl Package {
     }
 
     pub fn add_file(self, db: &mut dyn Db, file: File) {
-        tracing::debug!("Add file {} to package {}", file.path(db), self.name(db));
+        tracing::debug!(
+            "Adding file '{}' to package '{}'",
+            file.path(db),
+            self.name(db)
+        );
 
         let Some(mut index) = PackageFiles::indexed_mut(db, self) else {
             return;
@@ -289,10 +310,10 @@ impl Package {
 
     #[tracing::instrument(level = "debug", skip(db))]
     pub(crate) fn check(self, db: &dyn Db) -> Vec<String> {
-        tracing::debug!("Checking package {}", self.root(db));
+        tracing::debug!("Checking package '{}'", self.root(db));
 
         let mut result = Vec::new();
-        for file in &self.files(db).read() {
+        for file in &self.files(db) {
             let diagnostics = check_file(db, file);
             result.extend_from_slice(&diagnostics);
         }
@@ -301,15 +322,20 @@ impl Package {
     }
 
     /// Returns the files belonging to this package.
-    #[salsa::tracked]
-    pub fn files(self, db: &dyn Db) -> IndexedFiles {
-        let _entered = tracing::debug_span!("files").entered();
+    pub fn files(self, db: &dyn Db) -> Indexed<'_> {
         let files = self.file_set(db);
 
         let indexed = match files.get() {
             Index::Lazy(vacant) => {
-                tracing::debug!("Indexing files for package {}", self.name(db));
+                let _entered =
+                    tracing::debug_span!("index_package_files", package = %self.name(db)).entered();
+
                 let files = discover_package_files(db, self.root(db));
+                tracing::info!(
+                    "Indexed {} files for package '{}'",
+                    files.len(),
+                    self.name(db)
+                );
                 vacant.set(files)
             }
             Index::Indexed(indexed) => indexed,
@@ -330,14 +356,12 @@ impl Package {
         assert_eq!(root, metadata.root());
 
         if self.name(db) != metadata.name() {
-            self.set_name(db)
-                .with_durability(Durability::MEDIUM)
-                .to(metadata.name);
+            self.set_name(db).to(metadata.name);
         }
     }
 
     pub fn reload_files(self, db: &mut dyn Db) {
-        tracing::debug!("Reload files for package {}", self.name(db));
+        tracing::debug!("Reloading files for package '{}'", self.name(db));
 
         if !self.file_set(db).is_lazy() {
             // Force a re-index of the files in the next revision.
@@ -347,10 +371,10 @@ impl Package {
 }
 
 #[salsa::tracked]
-pub(super) fn check_file(db: &dyn Db, file: File) -> Diagnostics {
+pub(super) fn check_file(db: &dyn Db, file: File) -> Vec<String> {
     let path = file.path(db);
     let _span = tracing::debug_span!("check_file", file=%path).entered();
-    tracing::debug!("Checking file {path}");
+    tracing::debug!("Checking file '{path}'");
 
     let mut diagnostics = Vec::new();
 
@@ -363,13 +387,25 @@ pub(super) fn check_file(db: &dyn Db, file: File) -> Diagnostics {
     );
 
     // Abort checking if there are IO errors.
-    if source_text(db.upcast(), file).has_read_error() {
-        return Diagnostics::from(diagnostics);
+    let source = source_text(db.upcast(), file);
+
+    if source.has_read_error() {
+        return diagnostics;
+    }
+
+    for diagnostic in check_types(db.upcast(), file) {
+        let index = line_index(db.upcast(), diagnostic.file());
+        let location = index.source_location(diagnostic.start(), source.as_str());
+        diagnostics.push(format!(
+            "{path}:{location}: {message}",
+            path = file.path(db),
+            message = diagnostic.message()
+        ));
     }
 
     diagnostics.extend_from_slice(lint_syntax(db, file));
     diagnostics.extend_from_slice(lint_semantic(db, file));
-    Diagnostics::from(diagnostics)
+    diagnostics
 }
 
 fn discover_package_files(db: &dyn Db, path: &SystemPath) -> FxHashSet<File> {
@@ -423,7 +459,7 @@ mod tests {
     use ruff_db::testing::assert_function_query_was_not_run;
 
     use crate::db::tests::TestDb;
-    use crate::lint::{lint_syntax, Diagnostics};
+    use crate::lint::lint_syntax;
     use crate::workspace::check_file;
 
     #[test]
@@ -441,9 +477,7 @@ mod tests {
         assert_eq!(source_text(&db, file).as_str(), "");
         assert_eq!(
             check_file(&db, file),
-            Diagnostics::List(vec![
-                "Failed to read file: No such file or directory".to_string()
-            ])
+            vec!["Failed to read file: No such file or directory".to_string()]
         );
 
         let events = db.take_salsa_events();
@@ -454,7 +488,7 @@ mod tests {
         db.write_file(path, "").unwrap();
 
         assert_eq!(source_text(&db, file).as_str(), "");
-        assert_eq!(check_file(&db, file), Diagnostics::Empty);
+        assert_eq!(check_file(&db, file), vec![] as Vec<String>);
 
         Ok(())
     }

crates/red_knot_workspace/src/workspace/files.rs

@@ -1,4 +1,4 @@
-use std::iter::FusedIterator;
+use std::marker::PhantomData;
 use std::ops::Deref;
 use std::sync::Arc;
 
@@ -10,6 +10,9 @@ use ruff_db::files::File;
 use crate::db::Db;
 use crate::workspace::Package;
 
+/// Cheap cloneable hash set of files.
+type FileSet = Arc<FxHashSet<File>>;
+
 /// The indexed files of a package.
 ///
 /// The indexing happens lazily, but the files are then cached for subsequent reads.
@@ -18,7 +21,7 @@ use crate::workspace::Package;
 /// The implementation uses internal mutability to transition between the lazy and indexed state
 /// without triggering a new salsa revision. This is safe because the initial indexing happens on first access,
 /// so no query can be depending on the contents of the indexed files before that. All subsequent mutations to
-/// the indexed files must go through `IndexedFilesMut`, which uses the Salsa setter `package.set_file_set` to
+/// the indexed files must go through `IndexedMut`, which uses the Salsa setter `package.set_file_set` to
 /// ensure that Salsa always knows when the set of indexed files have changed.
 #[derive(Debug)]
 pub struct PackageFiles {
@@ -32,46 +35,67 @@ impl PackageFiles {
         }
     }
 
-    fn indexed(indexed_files: IndexedFiles) -> Self {
+    fn indexed(files: FileSet) -> Self {
         Self {
-            state: std::sync::Mutex::new(State::Indexed(indexed_files)),
+            state: std::sync::Mutex::new(State::Indexed(files)),
         }
     }
 
-    pub fn get(&self) -> Index {
+    pub(super) fn get(&self) -> Index {
         let state = self.state.lock().unwrap();
 
         match &*state {
             State::Lazy => Index::Lazy(LazyFiles { files: state }),
-            State::Indexed(files) => Index::Indexed(files.clone()),
+            State::Indexed(files) => Index::Indexed(Indexed {
+                files: Arc::clone(files),
+                _lifetime: PhantomData,
+            }),
         }
     }
 
-    pub fn is_lazy(&self) -> bool {
+    pub(super) fn is_lazy(&self) -> bool {
         matches!(*self.state.lock().unwrap(), State::Lazy)
     }
 
     /// Returns a mutable view on the index that allows cheap in-place mutations.
     ///
     /// The changes are automatically written back to the database once the view is dropped.
-    pub fn indexed_mut(db: &mut dyn Db, package: Package) -> Option<IndexedFilesMut> {
+    pub(super) fn indexed_mut(db: &mut dyn Db, package: Package) -> Option<IndexedMut> {
         // Calling `zalsa_mut` cancels all pending salsa queries. This ensures that there are no pending
         // reads to the file set.
         // TODO: Use a non-internal API instead https://salsa.zulipchat.com/#narrow/stream/333573-salsa-3.2E0/topic/Expose.20an.20API.20to.20cancel.20other.20queries
         let _ = db.as_dyn_database_mut().zalsa_mut();
 
-        let files = package.file_set(db);
-
-        let indexed = match &*files.state.lock().unwrap() {
-            State::Lazy => return None,
-            State::Indexed(indexed) => indexed.clone(),
+        // Replace the state with lazy. The `IndexedMut` guard restores the state
+        // to `State::Indexed`  or sets a new `PackageFiles` when it gets dropped to ensure the state
+        // is restored to how it has been before replacing the value.
+        //
+        // It isn't necessary to hold on to the lock after this point:
+        // * The above call to `zalsa_mut` guarantees that there's exactly **one** DB reference.
+        // * `Indexed` has a `'db` lifetime, and this method requires a `&mut db`.
+        //   This means that there can't be any pending reference to `Indexed` because Rust
+        //   doesn't allow borrowing `db` as mutable (to call this method) and immutable (`Indexed<'db>`) at the same time.
+        //   There can't be any other `Indexed<'db>` references created by clones of this DB because
+        //   all clones must have been dropped at this point and the `Indexed`
+        //   can't outlive the database (constrained by the `db` lifetime).
+        let state = {
+            let files = package.file_set(db);
+            let mut locked = files.state.lock().unwrap();
+            std::mem::replace(&mut *locked, State::Lazy)
         };
 
-        Some(IndexedFilesMut {
+        let indexed = match state {
+            // If it's already lazy, just return. We also don't need to restore anything because the
+            // replace above was a no-op.
+            State::Lazy => return None,
+            State::Indexed(indexed) => indexed,
+        };
+
+        Some(IndexedMut {
             db: Some(db),
             package,
-            new_revision: indexed.revision,
-            indexed,
+            files: indexed,
+            did_change: false,
         })
     }
 }
@@ -88,152 +112,93 @@ enum State {
     Lazy,
 
     /// The files are indexed. Stores the known files of a package.
-    Indexed(IndexedFiles),
+    Indexed(FileSet),
 }
 
-pub enum Index<'a> {
+pub(super) enum Index<'db> {
     /// The index has not yet been computed. Allows inserting the files.
-    Lazy(LazyFiles<'a>),
+    Lazy(LazyFiles<'db>),
 
-    Indexed(IndexedFiles),
+    Indexed(Indexed<'db>),
 }
 
 /// Package files that have not been indexed yet.
-pub struct LazyFiles<'a> {
-    files: std::sync::MutexGuard<'a, State>,
+pub(super) struct LazyFiles<'db> {
+    files: std::sync::MutexGuard<'db, State>,
 }
 
-impl<'a> LazyFiles<'a> {
+impl<'db> LazyFiles<'db> {
     /// Sets the indexed files of a package to `files`.
-    pub fn set(mut self, files: FxHashSet<File>) -> IndexedFiles {
-        let files = IndexedFiles::new(files);
-        *self.files = State::Indexed(files.clone());
+    pub(super) fn set(mut self, files: FxHashSet<File>) -> Indexed<'db> {
+        let files = Indexed {
+            files: Arc::new(files),
+            _lifetime: PhantomData,
+        };
+        *self.files = State::Indexed(Arc::clone(&files.files));
         files
     }
 }
 
 /// The indexed files of a package.
 ///
-/// # Salsa integration
-/// The type is cheap clonable and allows for in-place mutation of the files. The in-place mutation requires
-/// extra care because the type is used as the result of Salsa queries and Salsa relies on a type's equality
-/// to determine if the output has changed. This is accomplished by using a `revision` that gets incremented
-/// whenever the files are changed. The revision ensures that salsa's comparison of the
-/// previous [`IndexedFiles`] with the next [`IndexedFiles`] returns false even though they both
-/// point to the same underlying hash set.
-///
-/// # Equality
-/// Two [`IndexedFiles`] are only equal if they have the same revision and point to the **same** (identity) hash set.
-#[derive(Debug, Clone)]
-pub struct IndexedFiles {
-    revision: u64,
-    files: Arc<std::sync::Mutex<FxHashSet<File>>>,
+/// Note: This type is intentionally non-cloneable. Making it cloneable requires
+/// revisiting the locking behavior in [`PackageFiles::indexed_mut`].
+#[derive(Debug, PartialEq, Eq)]
+pub struct Indexed<'db> {
+    files: FileSet,
+    // Preserve the lifetime of `PackageFiles`.
+    _lifetime: PhantomData<&'db ()>,
 }
 
-impl IndexedFiles {
-    fn new(files: FxHashSet<File>) -> Self {
-        Self {
-            files: Arc::new(std::sync::Mutex::new(files)),
-            revision: 0,
-        }
-    }
-
-    /// Locks the file index for reading.
-    pub fn read(&self) -> IndexedFilesGuard {
-        IndexedFilesGuard {
-            guard: self.files.lock().unwrap(),
-        }
-    }
-}
-
-impl PartialEq for IndexedFiles {
-    fn eq(&self, other: &Self) -> bool {
-        self.revision == other.revision && Arc::ptr_eq(&self.files, &other.files)
-    }
-}
-
-impl Eq for IndexedFiles {}
-
-pub struct IndexedFilesGuard<'a> {
-    guard: std::sync::MutexGuard<'a, FxHashSet<File>>,
-}
-
-impl Deref for IndexedFilesGuard<'_> {
+impl Deref for Indexed<'_> {
     type Target = FxHashSet<File>;
 
     fn deref(&self) -> &Self::Target {
-        &self.guard
+        &self.files
     }
 }
 
-impl<'a> IntoIterator for &'a IndexedFilesGuard<'a> {
+impl<'a> IntoIterator for &'a Indexed<'_> {
     type Item = File;
-    type IntoIter = IndexedFilesIter<'a>;
+    type IntoIter = std::iter::Copied<std::collections::hash_set::Iter<'a, File>>;
 
     fn into_iter(self) -> Self::IntoIter {
-        IndexedFilesIter {
-            inner: self.guard.iter(),
-        }
+        self.files.iter().copied()
     }
 }
 
-/// Iterator over the indexed files.
-///
-/// # Locks
-/// Holding on to the iterator locks the file index for reading.
-pub struct IndexedFilesIter<'a> {
-    inner: std::collections::hash_set::Iter<'a, File>,
-}
-
-impl<'a> Iterator for IndexedFilesIter<'a> {
-    type Item = File;
-
-    fn next(&mut self) -> Option<Self::Item> {
-        self.inner.next().copied()
-    }
-
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        self.inner.size_hint()
-    }
-}
-
-impl FusedIterator for IndexedFilesIter<'_> {}
-
-impl ExactSizeIterator for IndexedFilesIter<'_> {}
-
 /// A Mutable view of a package's indexed files.
 ///
 /// Allows in-place mutation of the files without deep cloning the hash set.
 /// The changes are written back when the mutable view is dropped or by calling [`Self::set`] manually.
-pub struct IndexedFilesMut<'db> {
+pub(super) struct IndexedMut<'db> {
     db: Option<&'db mut dyn Db>,
     package: Package,
-    indexed: IndexedFiles,
-    new_revision: u64,
+    files: FileSet,
+    did_change: bool,
 }
 
-impl IndexedFilesMut<'_> {
-    pub fn insert(&mut self, file: File) -> bool {
-        if self.indexed.files.lock().unwrap().insert(file) {
-            self.new_revision += 1;
+impl IndexedMut<'_> {
+    pub(super) fn insert(&mut self, file: File) -> bool {
+        if self.files_mut().insert(file) {
+            self.did_change = true;
             true
         } else {
             false
         }
     }
 
-    pub fn remove(&mut self, file: File) -> bool {
-        if self.indexed.files.lock().unwrap().remove(&file) {
-            self.new_revision += 1;
+    pub(super) fn remove(&mut self, file: File) -> bool {
+        if self.files_mut().remove(&file) {
+            self.did_change = true;
             true
         } else {
             false
         }
     }
 
-    /// Writes the changes back to the database.
-    pub fn set(mut self) {
-        self.set_impl();
+    fn files_mut(&mut self) -> &mut FxHashSet<File> {
+        Arc::get_mut(&mut self.files).expect("All references to `FilesSet` to have been dropped")
     }
 
     fn set_impl(&mut self) {
@@ -241,19 +206,70 @@ impl IndexedFilesMut<'_> {
             return;
         };
 
-        if self.indexed.revision != self.new_revision {
+        let files = Arc::clone(&self.files);
+
+        if self.did_change {
+            // If there are changes, set the new file_set to trigger a salsa revision change.
             self.package
                 .set_file_set(db)
-                .to(PackageFiles::indexed(IndexedFiles {
-                    revision: self.new_revision,
-                    files: self.indexed.files.clone(),
-                }));
+                .to(PackageFiles::indexed(files));
+        } else {
+            // The `indexed_mut` replaced the `state` with Lazy. Restore it back to the indexed state.
+            *self.package.file_set(db).state.lock().unwrap() = State::Indexed(files);
         }
     }
 }
 
-impl Drop for IndexedFilesMut<'_> {
+impl Drop for IndexedMut<'_> {
     fn drop(&mut self) {
         self.set_impl();
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use rustc_hash::FxHashSet;
+
+    use ruff_db::files::system_path_to_file;
+    use ruff_db::system::{DbWithTestSystem, SystemPathBuf};
+    use ruff_python_ast::name::Name;
+
+    use crate::db::tests::TestDb;
+    use crate::workspace::files::Index;
+    use crate::workspace::Package;
+
+    #[test]
+    fn re_entrance() -> anyhow::Result<()> {
+        let mut db = TestDb::new();
+
+        db.write_file("test.py", "")?;
+
+        let package = Package::new(&db, Name::new("test"), SystemPathBuf::from("/test"));
+
+        let file = system_path_to_file(&db, "test.py").unwrap();
+
+        let files = match package.file_set(&db).get() {
+            Index::Lazy(lazy) => lazy.set(FxHashSet::from_iter([file])),
+            Index::Indexed(files) => files,
+        };
+
+        // Calling files a second time should not dead-lock.
+        // This can e.g. happen when `check_file` iterates over all files and
+        // `is_file_open` queries the open files.
+        let files_2 = package.file_set(&db).get();
+
+        match files_2 {
+            Index::Lazy(_) => {
+                panic!("Expected indexed files, got lazy files");
+            }
+            Index::Indexed(files_2) => {
+                assert_eq!(
+                    files_2.iter().collect::<Vec<_>>(),
+                    files.iter().collect::<Vec<_>>()
+                );
+            }
+        }
+
+        Ok(())
+    }
+}

crates/ruff_benchmark/benches/red_knot.rs

@@ -2,6 +2,7 @@
 
 use red_knot_python_semantic::{ProgramSettings, PythonVersion, SearchPathSettings};
 use red_knot_workspace::db::RootDatabase;
+use red_knot_workspace::watch::{ChangeEvent, ChangedKind};
 use red_knot_workspace::workspace::WorkspaceMetadata;
 use ruff_benchmark::criterion::{criterion_group, criterion_main, BatchSize, Criterion};
 use ruff_benchmark::TestFile;
@@ -12,13 +13,53 @@ use ruff_db::system::{MemoryFileSystem, SystemPath, TestSystem};
 struct Case {
     db: RootDatabase,
     fs: MemoryFileSystem,
-    parser: File,
     re: File,
     re_path: &'static SystemPath,
 }
 
 const TOMLLIB_312_URL: &str = "https://raw.githubusercontent.com/python/cpython/8e8a4baf652f6e1cee7acde9d78c4b6154539748/Lib/tomllib";
 
+// This first "unresolved import" is because we don't understand `*` imports yet.
+// The following "unresolved import" violations are because we can't distinguish currently from
+// "Symbol exists in the module but its type is unknown" and
+// "Symbol does not exist in the module"
+static EXPECTED_DIAGNOSTICS: &[&str] = &[
+    "/src/tomllib/_parser.py:7:29: Could not resolve import of 'Iterable' from 'collections.abc'",
+    "/src/tomllib/_parser.py:10:20: Could not resolve import of 'Any' from 'typing'",
+    "/src/tomllib/_parser.py:13:5: Could not resolve import of 'RE_DATETIME' from '._re'",
+    "/src/tomllib/_parser.py:14:5: Could not resolve import of 'RE_LOCALTIME' from '._re'",
+    "/src/tomllib/_parser.py:15:5: Could not resolve import of 'RE_NUMBER' from '._re'",
+    "/src/tomllib/_parser.py:20:21: Could not resolve import of 'Key' from '._types'",
+    "/src/tomllib/_parser.py:20:26: Could not resolve import of 'ParseFloat' from '._types'",
+    "Line 69 is too long (89 characters)",
+    "Use double quotes for strings",
+    "Use double quotes for strings",
+    "Use double quotes for strings",
+    "Use double quotes for strings",
+    "Use double quotes for strings",
+    "Use double quotes for strings",
+    "Use double quotes for strings",
+    "/src/tomllib/_parser.py:153:22: Name 'key' used when not defined.",
+    "/src/tomllib/_parser.py:153:27: Name 'flag' used when not defined.",
+    "/src/tomllib/_parser.py:159:16: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:161:25: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:168:16: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:169:22: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:170:25: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:180:16: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:182:31: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:206:16: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:207:22: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:208:25: Name 'k' used when not defined.",
+    "/src/tomllib/_parser.py:330:32: Name 'header' used when not defined.",
+    "/src/tomllib/_parser.py:330:41: Name 'key' used when not defined.",
+    "/src/tomllib/_parser.py:333:26: Name 'cont_key' used when not defined.",
+    "/src/tomllib/_parser.py:334:71: Name 'cont_key' used when not defined.",
+    "/src/tomllib/_parser.py:337:31: Name 'cont_key' used when not defined.",
+    "/src/tomllib/_parser.py:628:75: Name 'e' used when not defined.",
+    "/src/tomllib/_parser.py:686:23: Name 'parse_float' used when not defined.",
+];
+
 fn get_test_file(name: &str) -> TestFile {
     let path = format!("tomllib/{name}");
     let url = format!("{TOMLLIB_312_URL}/{name}");
@@ -28,15 +69,19 @@ fn get_test_file(name: &str) -> TestFile {
 fn setup_case() -> Case {
     let system = TestSystem::default();
     let fs = system.memory_file_system().clone();
-    let init_path = SystemPath::new("/src/tomllib/__init__.py");
     let parser_path = SystemPath::new("/src/tomllib/_parser.py");
     let re_path = SystemPath::new("/src/tomllib/_re.py");
-    let types_path = SystemPath::new("/src/tomllib/_types.py");
     fs.write_files([
-        (init_path, get_test_file("__init__.py").code()),
+        (
+            SystemPath::new("/src/tomllib/__init__.py"),
+            get_test_file("__init__.py").code(),
+        ),
         (parser_path, get_test_file("_parser.py").code()),
         (re_path, get_test_file("_re.py").code()),
-        (types_path, get_test_file("_types.py").code()),
+        (
+            SystemPath::new("/src/tomllib/_types.py"),
+            get_test_file("_types.py").code(),
+        ),
     ])
     .unwrap();
 
@@ -62,7 +107,6 @@ fn setup_case() -> Case {
     Case {
         db,
         fs,
-        parser,
         re,
         re_path,
     }
@@ -72,8 +116,8 @@ fn benchmark_incremental(criterion: &mut Criterion) {
     criterion.bench_function("red_knot_check_file[incremental]", |b| {
         b.iter_batched_ref(
             || {
-                let mut case = setup_case();
-                case.db.check_file(case.parser).unwrap();
+                let case = setup_case();
+                case.db.check().unwrap();
 
                 case.fs
                     .write_file(
@@ -82,14 +126,19 @@ fn benchmark_incremental(criterion: &mut Criterion) {
                     )
                     .unwrap();
 
-                case.re.sync(&mut case.db);
                 case
             },
             |case| {
-                let Case { db, parser, .. } = case;
-                let result = db.check_file(*parser).unwrap();
+                let Case { db, .. } = case;
 
-                assert_eq!(result.len(), 34);
+                db.apply_changes(vec![ChangeEvent::Changed {
+                    path: case.re_path.to_path_buf(),
+                    kind: ChangedKind::FileContent,
+                }]);
+
+                let result = db.check().unwrap();
+
+                assert_eq!(result, EXPECTED_DIAGNOSTICS);
             },
             BatchSize::SmallInput,
         );
@@ -101,10 +150,10 @@ fn benchmark_cold(criterion: &mut Criterion) {
         b.iter_batched_ref(
             setup_case,
             |case| {
-                let Case { db, parser, .. } = case;
-                let result = db.check_file(*parser).unwrap();
+                let Case { db, .. } = case;
+                let result = db.check().unwrap();
 
-                assert_eq!(result.len(), 34);
+                assert_eq!(result, EXPECTED_DIAGNOSTICS);
             },
             BatchSize::SmallInput,
         );

crates/ruff_db/src/files.rs

@@ -85,7 +85,7 @@ impl Files {
             .system_by_path
             .entry(absolute.clone())
             .or_insert_with(|| {
-                tracing::trace!("Adding file {path}");
+                tracing::trace!("Adding file '{path}'");
 
                 let metadata = db.system().path_metadata(path);
                 let durability = self
@@ -131,7 +131,7 @@ impl Files {
                     Err(_) => return Err(FileError::NotFound),
                 };
 
-                tracing::trace!("Adding vendored file {}", path);
+                tracing::trace!("Adding vendored file '{}'", path);
                 let file = File::builder(FilePath::Vendored(path.to_path_buf()))
                     .permissions(Some(0o444))
                     .revision(metadata.revision())
@@ -158,7 +158,7 @@ impl Files {
             Entry::Vacant(entry) => {
                 let metadata = db.system().virtual_path_metadata(path).ok()?;
 
-                tracing::trace!("Adding virtual file {}", path);
+                tracing::trace!("Adding virtual file '{}'", path);
 
                 let file = File::builder(FilePath::SystemVirtual(path.to_path_buf()))
                     .revision(metadata.revision())
@@ -211,7 +211,7 @@ impl Files {
     /// That's why [`File::sync_path`] and [`File::sync_path`] is preferred if it is known that the path is a file.
     pub fn sync_recursively(db: &mut dyn Db, path: &SystemPath) {
         let path = SystemPath::absolute(path, db.system().current_directory());
-        tracing::debug!("Syncing all files in {path}");
+        tracing::debug!("Syncing all files in '{path}'");
 
         let inner = Arc::clone(&db.files().inner);
         for entry in inner.system_by_path.iter_mut() {
@@ -224,9 +224,7 @@ impl Files {
 
         for root in roots.all() {
             if root.path(db).starts_with(&path) {
-                root.set_revision(db)
-                    .with_durability(Durability::HIGH)
-                    .to(FileRevision::now());
+                root.set_revision(db).to(FileRevision::now());
             }
         }
     }
@@ -249,9 +247,7 @@ impl Files {
         let roots = inner.roots.read().unwrap();
 
         for root in roots.all() {
-            root.set_revision(db)
-                .with_durability(Durability::HIGH)
-                .to(FileRevision::now());
+            root.set_revision(db).to(FileRevision::now());
         }
     }
 
@@ -381,23 +377,17 @@ impl File {
             return;
         };
         let metadata = db.system().path_metadata(path);
-        let durability = db.files().root(db, path).map(|root| root.durability(db));
-        Self::sync_impl(db, metadata, file, durability);
+        Self::sync_impl(db, metadata, file);
     }
 
     fn sync_system_virtual_path(db: &mut dyn Db, path: &SystemVirtualPath, file: File) {
         let metadata = db.system().virtual_path_metadata(path);
-        Self::sync_impl(db, metadata, file, None);
+        Self::sync_impl(db, metadata, file);
     }
 
     /// Private method providing the implementation for [`Self::sync_system_path`] and
     /// [`Self::sync_system_virtual_path`].
-    fn sync_impl(
-        db: &mut dyn Db,
-        metadata: crate::system::Result<Metadata>,
-        file: File,
-        durability: Option<Durability>,
-    ) {
+    fn sync_impl(db: &mut dyn Db, metadata: crate::system::Result<Metadata>, file: File) {
         let (status, revision, permission) = match metadata {
             Ok(metadata) if metadata.file_type().is_file() => (
                 FileStatus::Exists,
@@ -410,25 +400,19 @@ impl File {
             _ => (FileStatus::NotFound, FileRevision::zero(), None),
         };
 
-        let durability = durability.unwrap_or_default();
-
         if file.status(db) != status {
-            tracing::debug!("Updating the status of {}", file.path(db),);
-            file.set_status(db).with_durability(durability).to(status);
+            tracing::debug!("Updating the status of '{}'", file.path(db),);
+            file.set_status(db).to(status);
         }
 
         if file.revision(db) != revision {
-            tracing::debug!("Updating the revision of {}", file.path(db));
-            file.set_revision(db)
-                .with_durability(durability)
-                .to(revision);
+            tracing::debug!("Updating the revision of '{}'", file.path(db));
+            file.set_revision(db).to(revision);
         }
 
         if file.permissions(db) != permission {
-            tracing::debug!("Updating the permissions of {}", file.path(db),);
-            file.set_permissions(db)
-                .with_durability(durability)
-                .to(permission);
+            tracing::debug!("Updating the permissions of '{}'", file.path(db),);
+            file.set_permissions(db).to(permission);
         }
     }
 

crates/ruff_db/src/source.rs

@@ -22,7 +22,7 @@ pub fn source_text(db: &dyn Db, file: File) -> SourceText {
     let kind = if is_notebook(file.path(db)) {
         file.read_to_notebook(db)
             .unwrap_or_else(|error| {
-                tracing::debug!("Failed to read notebook {path}: {error}");
+                tracing::debug!("Failed to read notebook '{path}': {error}");
 
                 has_read_error = true;
                 SourceDiagnostic(Arc::new(SourceTextError::FailedToReadNotebook(error)))
@@ -33,7 +33,7 @@ pub fn source_text(db: &dyn Db, file: File) -> SourceText {
     } else {
         file.read_to_string(db)
             .unwrap_or_else(|error| {
-                tracing::debug!("Failed to read file {path}: {error}");
+                tracing::debug!("Failed to read file '{path}': {error}");
 
                 has_read_error = true;
                 SourceDiagnostic(Arc::new(SourceTextError::FailedToReadFile(error))).accumulate(db);

crates/ruff_db/src/testing.rs

@@ -31,10 +31,20 @@ pub fn assert_const_function_query_was_not_run<Db, Q, QDb, R>(
     Db: salsa::Database,
     Q: Fn(QDb) -> R,
 {
-    let (query_name, will_execute_event) = find_will_execute_event(db, query, (), events);
+    // Salsa now interns singleton ingredients. But we know that it is a singleton, so we can just search for
+    // any event of that ingredient.
+    let query_name = query_name(&query);
+
+    let event = events.iter().find(|event| {
+        if let salsa::EventKind::WillExecute { database_key } = event.kind {
+            db.ingredient_debug_name(database_key.ingredient_index()) == query_name
+        } else {
+            false
+        }
+    });
 
     db.attach(|_| {
-        if let Some(will_execute_event) = will_execute_event {
+        if let Some(will_execute_event) = event {
             panic!(
                 "Expected query {query_name}() not to have run but it did: {will_execute_event:?}"
             );

crates/ruff_db/src/vendored.rs

@@ -97,7 +97,16 @@ impl VendoredFileSystem {
         fn read_to_string(fs: &VendoredFileSystem, path: &VendoredPath) -> Result<String> {
             let mut archive = fs.lock_archive();
             let mut zip_file = archive.lookup_path(&NormalizedVendoredPath::from(path))?;
-            let mut buffer = String::new();
+
+            // Pre-allocate the buffer with the size specified in the ZIP file metadata
+            // because `read_to_string` passes `None` as the size hint.
+            // But let's not trust the zip file metadata (even though it's vendored)
+            // and limit it to a reasonable size.
+            let mut buffer = String::with_capacity(
+                usize::try_from(zip_file.size())
+                    .unwrap_or(usize::MAX)
+                    .min(10_000_000),
+            );
             zip_file.read_to_string(&mut buffer)?;
             Ok(buffer)
         }

crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI053.pyi

@@ -66,3 +66,6 @@ def not_warnings_dot_deprecated(
 def not_a_deprecated_function() -> None: ...
 
 fbaz: str = f"51 character {foo} stringgggggggggggggggggggggggggg"  # Error: PYI053
+
+# see https://github.com/astral-sh/ruff/issues/12995
+def foo(bar: typing.Literal["a", "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"]):...

crates/ruff_linter/src/rules/flake8_pyi/rules/string_or_bytes_too_long.rs

@@ -59,6 +59,10 @@ pub(crate) fn string_or_bytes_too_long(checker: &mut Checker, string: StringLike
         return;
     }
 
+    if semantic.in_annotation() {
+        return;
+    }
+
     let length = match string {
         StringLike::String(ast::ExprStringLiteral { value, .. }) => value.chars().count(),
         StringLike::Bytes(ast::ExprBytesLiteral { value, .. }) => value.len(),

crates/ruff_linter/src/rules/flake8_pyi/snapshots/ruff_linter__rules__flake8_pyi__tests__PYI053_PYI053.pyi.snap

@@ -152,6 +152,8 @@ PYI053.pyi:68:13: PYI053 [*] String and bytes literals longer than 50 characters
 67 | 
 68 | fbaz: str = f"51 character {foo} stringgggggggggggggggggggggggggg"  # Error: PYI053
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ PYI053
+69 | 
+70 | # see https://github.com/astral-sh/ruff/issues/12995
    |
    = help: Replace with `...`
 
@@ -161,3 +163,6 @@ PYI053.pyi:68:13: PYI053 [*] String and bytes literals longer than 50 characters
 67 67 | 
 68    |-fbaz: str = f"51 character {foo} stringgggggggggggggggggggggggggg"  # Error: PYI053
    68 |+fbaz: str = ...  # Error: PYI053
+69 69 | 
+70 70 | # see https://github.com/astral-sh/ruff/issues/12995
+71 71 | def foo(bar: typing.Literal["a", "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"]):...

crates/ruff_linter/src/rules/flake8_pytest_style/rules/fixture.rs

@@ -42,7 +42,7 @@ use super::helpers::{
 /// import pytest
 ///
 ///
-/// @pytest.fixture
+/// @pytest.fixture()
 /// def my_fixture(): ...
 /// ```
 ///
@@ -52,7 +52,7 @@ use super::helpers::{
 /// import pytest
 ///
 ///
-/// @pytest.fixture()
+/// @pytest.fixture
 /// def my_fixture(): ...
 /// ```
 ///

crates/ruff_linter/src/rules/pydocstyle/rules/not_missing.rs

@@ -218,6 +218,9 @@ impl Violation for UndocumentedPublicClass {
 ///             raise ValueError("Tried to greet an unhappy cat.")
 /// ```
 ///
+/// ## Options
+/// - `lint.pydocstyle.ignore-decorators`
+///
 /// ## References
 /// - [PEP 257 – Docstring Conventions](https://peps.python.org/pep-0257/)
 /// - [PEP 287 – reStructuredText Docstring Format](https://peps.python.org/pep-0287/)
@@ -305,6 +308,9 @@ impl Violation for UndocumentedPublicMethod {
 ///         raise FasterThanLightError from exc
 /// ```
 ///
+/// ## Options
+/// - `lint.pydocstyle.ignore-decorators`
+///
 /// ## References
 /// - [PEP 257 – Docstring Conventions](https://peps.python.org/pep-0257/)
 /// - [PEP 287 – reStructuredText Docstring Format](https://peps.python.org/pep-0287/)
@@ -402,6 +408,9 @@ impl Violation for UndocumentedPublicPackage {
 /// print(cat)  # "Cat: Dusty"
 /// ```
 ///
+/// ## Options
+/// - `lint.pydocstyle.ignore-decorators`
+///
 /// ## References
 /// - [PEP 257 – Docstring Conventions](https://peps.python.org/pep-0257/)
 /// - [PEP 287 – reStructuredText Docstring Format](https://peps.python.org/pep-0287/)
@@ -502,6 +511,9 @@ impl Violation for UndocumentedPublicNestedClass {
 ///         self.population: int = population
 /// ```
 ///
+/// ## Options
+/// - `lint.pydocstyle.ignore-decorators`
+///
 /// ## References
 /// - [PEP 257 – Docstring Conventions](https://peps.python.org/pep-0257/)
 /// - [PEP 287 – reStructuredText Docstring Format](https://peps.python.org/pep-0287/)

crates/ruff_server/src/session/index/ruff_settings.rs

@@ -3,6 +3,7 @@ use std::path::{Path, PathBuf};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 
+use anyhow::Context;
 use ignore::{WalkBuilder, WalkState};
 
 use ruff_linter::{
@@ -100,7 +101,7 @@ impl RuffSettings {
 
 impl RuffSettingsIndex {
     pub(super) fn new(root: &Path, editor_settings: &ResolvedEditorSettings) -> Self {
-        let mut error = false;
+        let mut has_error = false;
         let mut index = BTreeMap::default();
         let mut respect_gitignore = None;
 
@@ -127,20 +128,27 @@ impl RuffSettingsIndex {
                             );
                             break;
                         }
-                        Err(err) => {
+                        error => {
                             tracing::error!(
-                                "Error while resolving settings from {}: {err}",
-                                pyproject.display()
+                                "{:#}",
+                                error
+                                    .with_context(|| {
+                                        format!(
+                                            "Failed to resolve settings for {}",
+                                            pyproject.display()
+                                        )
+                                    })
+                                    .unwrap_err()
                             );
-                            error = true;
+                            has_error = true;
                             continue;
                         }
                     }
                 }
                 Ok(None) => continue,
                 Err(err) => {
-                    tracing::error!("{err}");
-                    error = true;
+                    tracing::error!("{err:#}");
+                    has_error = true;
                     continue;
                 }
             }
@@ -162,7 +170,7 @@ impl RuffSettingsIndex {
         let walker = builder.build_parallel();
 
         let index = std::sync::RwLock::new(index);
-        let error = AtomicBool::new(error);
+        let has_error = AtomicBool::new(has_error);
 
         walker.run(|| {
             Box::new(|result| {
@@ -224,19 +232,26 @@ impl RuffSettingsIndex {
                                     }),
                                 );
                             }
-                            Err(err) => {
+                            error => {
                                 tracing::error!(
-                                    "Error while resolving settings from {}: {err}",
-                                    pyproject.display()
+                                    "{:#}",
+                                    error
+                                        .with_context(|| {
+                                            format!(
+                                                "Failed to resolve settings for {}",
+                                                pyproject.display()
+                                            )
+                                        })
+                                        .unwrap_err()
                                 );
-                                error.store(true, Ordering::Relaxed);
+                                has_error.store(true, Ordering::Relaxed);
                             }
                         }
                     }
                     Ok(None) => {}
                     Err(err) => {
-                        tracing::error!("{err}");
-                        error.store(true, Ordering::Relaxed);
+                        tracing::error!("{err:#}");
+                        has_error.store(true, Ordering::Relaxed);
                     }
                 }
 
@@ -244,7 +259,7 @@ impl RuffSettingsIndex {
             })
         });
 
-        if error.load(Ordering::Relaxed) {
+        if has_error.load(Ordering::Relaxed) {
             let root = root.display();
             show_err_msg!(
                 "Error while resolving settings from workspace {root}. Please refer to the logs for more details.",

crates/ruff_source_file/src/lib.rs

@@ -254,6 +254,12 @@ impl Debug for SourceLocation {
     }
 }
 
+impl std::fmt::Display for SourceLocation {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{row}:{column}", row = self.row, column = self.column)
+    }
+}
+
 #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
 pub enum SourceRow {
     /// A row within a cell in a Jupyter Notebook.

docs/editors/setup.md

@@ -310,7 +310,12 @@ See [LSP Client documentation](https://docs.kde.org/stable5/en/kate/kate/kate-ap
 on how to configure the server from there.
 
 !!! important
-    Kate's LSP Client plugin does not support multiple servers for the same language.
+    Kate's LSP Client plugin does not support multiple servers for the same language. As a
+    workaround, you can use the [`python-lsp-server`](https://github.com/python-lsp/python-lsp-server)
+    along with the [`python-lsp-ruff`](https://github.com/python-lsp/python-lsp-ruff) plugin to
+    use Ruff alongside another language server. Note that this setup won't use the [server settings](settings.md)
+    because the [`python-lsp-ruff`](https://github.com/python-lsp/python-lsp-ruff) plugin uses the
+    `ruff` executable and not the language server.
 
 ## Sublime Text