red-knot: Symbol table (#11860)

Cargo.lock

@@ -2470,6 +2470,7 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "bitflags 2.5.0",
+ "hashbrown 0.14.5",
  "is-macro",
  "ruff_db",
  "ruff_index",
@@ -2480,6 +2481,7 @@ dependencies = [
  "ruff_text_size",
  "rustc-hash",
  "salsa-2022",
+ "smallvec",
  "smol_str",
  "tempfile",
  "tracing",

crates/ruff_db/src/parsed.rs

@@ -32,9 +32,7 @@ pub fn parsed_module(db: &dyn Db, file: VfsFile) -> ParsedModule {
         VfsPath::Vendored(_) => PySourceType::Stub,
     };
 
-    ParsedModule {
+    ParsedModule::new(parse_unchecked_source(&source, ty))
-        inner: Arc::new(parse_unchecked_source(&source, ty)),
-    }
 }
 
 /// Cheap cloneable wrapper around the parsed module.
@@ -44,6 +42,12 @@ pub struct ParsedModule {
 }
 
 impl ParsedModule {
+    pub fn new(parsed: Parsed<ModModule>) -> Self {
+        Self {
+            inner: Arc::new(parsed),
+        }
+    }
+
     /// Consumes `self` and returns the Arc storing the parsed module.
     pub fn into_arc(self) -> Arc<Parsed<ModModule>> {
         self.inner

crates/ruff_python_semantic/Cargo.toml

@@ -21,9 +21,11 @@ ruff_text_size = { workspace = true }
 bitflags = { workspace = true }
 is-macro = { workspace = true }
 salsa = { workspace = true, optional = true }
-smol_str = { workspace = true, optional = true }
+smallvec = { workspace = true, optional = true }
+smol_str = { workspace = true }
 tracing = { workspace = true, optional = true }
 rustc-hash = { workspace = true }
+hashbrown = { workspace = true, optional = true }
 
 [dev-dependencies]
 anyhow = { workspace = true }
@@ -34,4 +36,4 @@ tempfile = { workspace = true }
 workspace = true
 
 [features]
-red_knot = ["dep:salsa", "dep:smol_str", "dep:tracing"]
+red_knot = ["dep:salsa", "dep:tracing", "dep:hashbrown", "dep:smallvec"]

crates/ruff_python_semantic/src/db.rs

@@ -1,16 +1,25 @@
+use salsa::DbWithJar;
+
+use ruff_db::{Db as SourceDb, Upcast};
+
 use crate::module::resolver::{
     file_to_module, internal::ModuleNameIngredient, internal::ModuleResolverSearchPaths,
     resolve_module_query,
 };
-use ruff_db::{Db as SourceDb, Upcast};
+
-use salsa::DbWithJar;
+use crate::red_knot::semantic_index::symbol::ScopeId;
+use crate::red_knot::semantic_index::{scopes_map, semantic_index, symbol_table};
 
 #[salsa::jar(db=Db)]
 pub struct Jar(
     ModuleNameIngredient,
     ModuleResolverSearchPaths,
+    ScopeId,
+    symbol_table,
     resolve_module_query,
     file_to_module,
+    scopes_map,
+    semantic_index,
 );
 
 /// Database giving access to semantic information about a Python program.
@@ -18,12 +27,15 @@ pub trait Db: SourceDb + DbWithJar<Jar> + Upcast<dyn SourceDb> {}
 
 #[cfg(test)]
 pub(crate) mod tests {
-    use super::{Db, Jar};
+    use std::sync::Arc;
+
+    use salsa::DebugWithDb;
+
     use ruff_db::file_system::{FileSystem, MemoryFileSystem, OsFileSystem};
     use ruff_db::vfs::Vfs;
     use ruff_db::{Db as SourceDb, Jar as SourceJar, Upcast};
-    use salsa::DebugWithDb;
+
-    use std::sync::Arc;
+    use super::{Db, Jar};
 
     #[salsa::db(Jar, SourceJar)]
     pub(crate) struct TestDb {

crates/ruff_python_semantic/src/lib.rs

@@ -9,7 +9,10 @@ mod globals;
 mod model;
 #[cfg(feature = "red_knot")]
 pub mod module;
+pub mod name;
 mod nodes;
+#[cfg(feature = "red_knot")]
+pub mod red_knot;
 mod reference;
 mod scope;
 mod star_import;

crates/ruff_python_semantic/src/name.rs

@@ -0,0 +1,56 @@
+use std::ops::Deref;
+
+#[derive(Debug, Clone, Eq, PartialEq, Hash)]
+pub struct Name(smol_str::SmolStr);
+
+impl Name {
+    #[inline]
+    pub fn new(name: &str) -> Self {
+        Self(smol_str::SmolStr::new(name))
+    }
+
+    #[inline]
+    pub fn new_static(name: &'static str) -> Self {
+        Self(smol_str::SmolStr::new_static(name))
+    }
+
+    pub fn as_str(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
+impl Deref for Name {
+    type Target = str;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        self.as_str()
+    }
+}
+
+impl<T> From<T> for Name
+where
+    T: Into<smol_str::SmolStr>,
+{
+    fn from(value: T) -> Self {
+        Self(value.into())
+    }
+}
+
+impl std::fmt::Display for Name {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+impl PartialEq<str> for Name {
+    fn eq(&self, other: &str) -> bool {
+        self.as_str() == other
+    }
+}
+
+impl PartialEq<Name> for str {
+    fn eq(&self, other: &Name) -> bool {
+        other == self
+    }
+}

crates/ruff_python_semantic/src/red_knot/ast_node_ref.rs

@@ -0,0 +1,162 @@
+use std::hash::Hash;
+use std::ops::Deref;
+
+use ruff_db::parsed::ParsedModule;
+
+/// Ref-counted owned reference to an AST node.
+///
+/// The type holds an owned reference to the node's ref-counted [`ParsedModule`].
+/// Holding on to the node's [`ParsedModule`] guarantees that the reference to the
+/// node must still be valid.
+///
+/// Holding on to any [`AstNodeRef`] prevents the [`ParsedModule`] from being released.
+///
+/// ## Equality
+/// Two `AstNodeRef` are considered equal if their wrapped nodes are equal.
+#[derive(Clone)]
+pub struct AstNodeRef<T> {
+    /// Owned reference to the node's [`ParsedModule`].
+    ///
+    /// The node's reference is guaranteed to remain valid as long as it's enclosing
+    /// [`ParsedModule`] is alive.
+    _parsed: ParsedModule,
+
+    /// Pointer to the referenced node.
+    node: std::ptr::NonNull<T>,
+}
+
+#[allow(unsafe_code)]
+impl<T> AstNodeRef<T> {
+    /// Creates a new `AstNodeRef` that reference `node`. The `parsed` is the [`ParsedModule`] to which
+    /// the `AstNodeRef` belongs.
+    ///
+    /// ## Safety
+    /// Dereferencing the `node` can result in undefined behavior if `parsed` isn't the [`ParsedModule`] to
+    /// which `node` belongs. It's the caller's responsibility to ensure that the invariant `node belongs to parsed` is upheld.
+
+    pub(super) unsafe fn new(parsed: ParsedModule, node: &T) -> Self {
+        Self {
+            _parsed: parsed,
+            node: std::ptr::NonNull::from(node),
+        }
+    }
+
+    /// Returns a reference to the wrapped node.
+    pub fn node(&self) -> &T {
+        // SAFETY: Holding on to `parsed` ensures that the AST to which `node` belongs is still alive
+        // and not moved.
+        unsafe { self.node.as_ref() }
+    }
+}
+
+impl<T> Deref for AstNodeRef<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        self.node()
+    }
+}
+
+impl<T> std::fmt::Debug for AstNodeRef<T>
+where
+    T: std::fmt::Debug,
+{
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_tuple("AstNodeRef").field(&self.node()).finish()
+    }
+}
+
+impl<T> PartialEq for AstNodeRef<T>
+where
+    T: PartialEq,
+{
+    fn eq(&self, other: &Self) -> bool {
+        self.node().eq(other.node())
+    }
+}
+
+impl<T> Eq for AstNodeRef<T> where T: Eq {}
+
+impl<T> Hash for AstNodeRef<T>
+where
+    T: Hash,
+{
+    fn hash<H: std::hash::Hasher>(&self, state: &mut H) {
+        self.node().hash(state);
+    }
+}
+
+#[allow(unsafe_code)]
+unsafe impl<T> Send for AstNodeRef<T> where T: Send {}
+#[allow(unsafe_code)]
+unsafe impl<T> Sync for AstNodeRef<T> where T: Sync {}
+
+#[cfg(test)]
+mod tests {
+    use crate::red_knot::ast_node_ref::AstNodeRef;
+    use ruff_db::parsed::ParsedModule;
+    use ruff_python_ast::PySourceType;
+    use ruff_python_parser::parse_unchecked_source;
+
+    #[test]
+    #[allow(unsafe_code)]
+    fn equality() {
+        let parsed_raw = parse_unchecked_source("1 + 2", PySourceType::Python);
+        let parsed = ParsedModule::new(parsed_raw.clone());
+
+        let stmt = &parsed.syntax().body[0];
+
+        let node1 = unsafe { AstNodeRef::new(parsed.clone(), stmt) };
+        let node2 = unsafe { AstNodeRef::new(parsed.clone(), stmt) };
+
+        assert_eq!(node1, node2);
+
+        // Compare from different trees
+        let cloned = ParsedModule::new(parsed_raw);
+        let stmt_cloned = &cloned.syntax().body[0];
+        let cloned_node = unsafe { AstNodeRef::new(cloned.clone(), stmt_cloned) };
+
+        assert_eq!(node1, cloned_node);
+
+        let other_raw = parse_unchecked_source("2 + 2", PySourceType::Python);
+        let other = ParsedModule::new(other_raw);
+
+        let other_stmt = &other.syntax().body[0];
+        let other_node = unsafe { AstNodeRef::new(other.clone(), other_stmt) };
+
+        assert_ne!(node1, other_node);
+    }
+
+    #[allow(unsafe_code)]
+    #[test]
+    fn inequality() {
+        let parsed_raw = parse_unchecked_source("1 + 2", PySourceType::Python);
+        let parsed = ParsedModule::new(parsed_raw.clone());
+
+        let stmt = &parsed.syntax().body[0];
+        let node = unsafe { AstNodeRef::new(parsed.clone(), stmt) };
+
+        let other_raw = parse_unchecked_source("2 + 2", PySourceType::Python);
+        let other = ParsedModule::new(other_raw);
+
+        let other_stmt = &other.syntax().body[0];
+        let other_node = unsafe { AstNodeRef::new(other.clone(), other_stmt) };
+
+        assert_ne!(node, other_node);
+    }
+
+    #[test]
+    #[allow(unsafe_code)]
+    fn debug() {
+        let parsed_raw = parse_unchecked_source("1 + 2", PySourceType::Python);
+        let parsed = ParsedModule::new(parsed_raw.clone());
+
+        let stmt = &parsed.syntax().body[0];
+
+        let stmt_node = unsafe { AstNodeRef::new(parsed.clone(), stmt) };
+
+        let debug = format!("{stmt_node:?}");
+
+        assert_eq!(debug, format!("AstNodeRef({stmt:?})"));
+    }
+}

crates/ruff_python_semantic/src/red_knot/mod.rs

@@ -0,0 +1,3 @@
+pub mod ast_node_ref;
+mod node_key;
+pub mod semantic_index;

crates/ruff_python_semantic/src/red_knot/node_key.rs

@@ -0,0 +1,24 @@
+use ruff_python_ast::{AnyNodeRef, NodeKind};
+use ruff_text_size::{Ranged, TextRange};
+
+/// Compact key for a node for use in a hash map.
+///
+/// Compares two nodes by their kind and text range.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
+pub(super) struct NodeKey {
+    kind: NodeKind,
+    range: TextRange,
+}
+
+impl NodeKey {
+    pub(super) fn from_node<'a, N>(node: N) -> Self
+    where
+        N: Into<AnyNodeRef<'a>>,
+    {
+        let node = node.into();
+        NodeKey {
+            kind: node.kind(),
+            range: node.range(),
+        }
+    }
+}

crates/ruff_python_semantic/src/red_knot/semantic_index.rs

@@ -0,0 +1,655 @@
+use std::iter::FusedIterator;
+use std::sync::Arc;
+
+use rustc_hash::FxHashMap;
+
+use ruff_db::parsed::parsed_module;
+use ruff_db::vfs::VfsFile;
+use ruff_index::{IndexSlice, IndexVec};
+use ruff_python_ast as ast;
+
+use crate::red_knot::node_key::NodeKey;
+use crate::red_knot::semantic_index::ast_ids::AstIds;
+use crate::red_knot::semantic_index::builder::SemanticIndexBuilder;
+use crate::red_knot::semantic_index::symbol::{
+    FileScopeId, PublicSymbolId, Scope, ScopeId, ScopeSymbolId, ScopesMap, SymbolTable,
+};
+use crate::Db;
+
+pub mod ast_ids;
+mod builder;
+pub mod definition;
+pub mod symbol;
+
+type SymbolMap = hashbrown::HashMap<ScopeSymbolId, (), ()>;
+
+/// Returns the semantic index for `file`.
+///
+/// Prefer using [`symbol_table`] when working with symbols from a single scope.
+#[salsa::tracked(return_ref, no_eq)]
+pub(crate) fn semantic_index(db: &dyn Db, file: VfsFile) -> SemanticIndex {
+    let parsed = parsed_module(db.upcast(), file);
+
+    SemanticIndexBuilder::new(parsed).build()
+}
+
+/// Returns the symbol table for a specific `scope`.
+///
+/// Using [`symbol_table`] over [`semantic_index`] has the advantage that
+/// Salsa can avoid invalidating dependent queries if this scope's symbol table
+/// is unchanged.
+#[salsa::tracked]
+pub(crate) fn symbol_table(db: &dyn Db, scope: ScopeId) -> Arc<SymbolTable> {
+    let index = semantic_index(db, scope.file(db));
+
+    index.symbol_table(scope.scope_id(db))
+}
+
+/// Returns a mapping from file specific [`FileScopeId`] to a program-wide unique [`ScopeId`].
+#[salsa::tracked(return_ref)]
+pub(crate) fn scopes_map(db: &dyn Db, file: VfsFile) -> ScopesMap {
+    let index = semantic_index(db, file);
+
+    let scopes: IndexVec<_, _> = index
+        .scopes
+        .indices()
+        .map(|id| ScopeId::new(db, file, id))
+        .collect();
+
+    ScopesMap::new(scopes)
+}
+
+/// Returns the root scope of `file`.
+pub fn root_scope(db: &dyn Db, file: VfsFile) -> ScopeId {
+    FileScopeId::root().to_scope_id(db, file)
+}
+
+/// Returns the symbol with the given name in `file`'s public scope or `None` if
+/// no symbol with the given name exists.
+pub fn global_symbol(db: &dyn Db, file: VfsFile, name: &str) -> Option<PublicSymbolId> {
+    let root_scope = root_scope(db, file);
+    root_scope.symbol(db, name)
+}
+
+/// The symbol tables for an entire file.
+#[derive(Debug)]
+pub struct SemanticIndex {
+    /// List of all symbol tables in this file, indexed by scope.
+    symbol_tables: IndexVec<FileScopeId, Arc<SymbolTable>>,
+
+    /// List of all scopes in this file.
+    scopes: IndexVec<FileScopeId, Scope>,
+
+    /// Maps expressions to their corresponding scope.
+    /// We can't use [`ExpressionId`] here, because the challenge is how to get from
+    /// an [`ast::Expr`] to an [`ExpressionId`] (which requires knowing the scope).
+    expression_scopes: FxHashMap<NodeKey, FileScopeId>,
+
+    /// Lookup table to map between node ids and ast nodes.
+    ///
+    /// Note: We should not depend on this map when analysing other files or
+    /// changing a file invalidates all dependents.
+    ast_ids: IndexVec<FileScopeId, AstIds>,
+}
+
+impl SemanticIndex {
+    /// Returns the symbol table for a specific scope.
+    ///
+    /// Use the Salsa cached [`symbol_table`] query if you only need the
+    /// symbol table for a single scope.
+    fn symbol_table(&self, scope_id: FileScopeId) -> Arc<SymbolTable> {
+        self.symbol_tables[scope_id].clone()
+    }
+
+    pub(crate) fn ast_ids(&self, scope_id: FileScopeId) -> &AstIds {
+        &self.ast_ids[scope_id]
+    }
+
+    /// Returns the ID of the `expression`'s enclosing scope.
+    #[allow(unused)]
+    pub(crate) fn expression_scope_id(&self, expression: &ast::Expr) -> FileScopeId {
+        self.expression_scopes[&NodeKey::from_node(expression)]
+    }
+
+    /// Returns the [`Scope`] of the `expression`'s enclosing scope.
+    #[allow(unused)]
+    pub(crate) fn expression_scope(&self, expression: &ast::Expr) -> &Scope {
+        &self.scopes[self.expression_scope_id(expression)]
+    }
+
+    /// Returns the [`Scope`] with the given id.
+    #[allow(unused)]
+    pub(crate) fn scope(&self, id: FileScopeId) -> &Scope {
+        &self.scopes[id]
+    }
+
+    /// 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);
+        scope.parent
+    }
+
+    /// Returns the parent scope of `scope_id`.
+    #[allow(unused)]
+    pub(crate) fn parent_scope(&self, scope_id: FileScopeId) -> Option<&Scope> {
+        Some(&self.scopes[self.parent_scope_id(scope_id)?])
+    }
+
+    /// Returns an iterator over the descendent scopes of `scope`.
+    #[allow(unused)]
+    pub(crate) fn descendent_scopes(&self, scope: FileScopeId) -> DescendentsIter {
+        DescendentsIter::new(self, scope)
+    }
+
+    /// Returns an iterator over the direct child scopes of `scope`.
+    #[allow(unused)]
+    pub(crate) fn child_scopes(&self, scope: FileScopeId) -> ChildrenIter {
+        ChildrenIter::new(self, scope)
+    }
+
+    /// Returns an iterator over all ancestors of `scope`, starting with `scope` itself.
+    #[allow(unused)]
+    pub(crate) fn ancestor_scopes(&self, scope: FileScopeId) -> AncestorsIter {
+        AncestorsIter::new(self, scope)
+    }
+}
+
+/// ID that uniquely identifies an expression inside a [`Scope`].
+
+pub struct AncestorsIter<'a> {
+    scopes: &'a IndexSlice<FileScopeId, Scope>,
+    next_id: Option<FileScopeId>,
+}
+
+impl<'a> AncestorsIter<'a> {
+    fn new(module_symbol_table: &'a SemanticIndex, start: FileScopeId) -> Self {
+        Self {
+            scopes: &module_symbol_table.scopes,
+            next_id: Some(start),
+        }
+    }
+}
+
+impl<'a> Iterator for AncestorsIter<'a> {
+    type Item = (FileScopeId, &'a Scope);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current_id = self.next_id?;
+        let current = &self.scopes[current_id];
+        self.next_id = current.parent;
+
+        Some((current_id, current))
+    }
+}
+
+impl FusedIterator for AncestorsIter<'_> {}
+
+pub struct DescendentsIter<'a> {
+    next_id: FileScopeId,
+    descendents: std::slice::Iter<'a, Scope>,
+}
+
+impl<'a> DescendentsIter<'a> {
+    fn new(symbol_table: &'a SemanticIndex, scope_id: FileScopeId) -> Self {
+        let scope = &symbol_table.scopes[scope_id];
+        let scopes = &symbol_table.scopes[scope.descendents.clone()];
+
+        Self {
+            next_id: scope_id + 1,
+            descendents: scopes.iter(),
+        }
+    }
+}
+
+impl<'a> Iterator for DescendentsIter<'a> {
+    type Item = (FileScopeId, &'a Scope);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let descendent = self.descendents.next()?;
+        let id = self.next_id;
+        self.next_id = self.next_id + 1;
+
+        Some((id, descendent))
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.descendents.size_hint()
+    }
+}
+
+impl FusedIterator for DescendentsIter<'_> {}
+
+impl ExactSizeIterator for DescendentsIter<'_> {}
+
+pub struct ChildrenIter<'a> {
+    parent: FileScopeId,
+    descendents: DescendentsIter<'a>,
+}
+
+impl<'a> ChildrenIter<'a> {
+    fn new(module_symbol_table: &'a SemanticIndex, parent: FileScopeId) -> Self {
+        let descendents = DescendentsIter::new(module_symbol_table, parent);
+
+        Self {
+            parent,
+            descendents,
+        }
+    }
+}
+
+impl<'a> Iterator for ChildrenIter<'a> {
+    type Item = (FileScopeId, &'a Scope);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.descendents
+            .find(|(_, scope)| scope.parent == Some(self.parent))
+    }
+}
+
+impl FusedIterator for ChildrenIter<'_> {}
+
+#[cfg(test)]
+mod tests {
+    use ruff_db::parsed::parsed_module;
+    use ruff_db::vfs::{system_path_to_file, VfsFile};
+
+    use crate::db::tests::TestDb;
+    use crate::red_knot::semantic_index::symbol::{FileScopeId, ScopeKind, SymbolTable};
+    use crate::red_knot::semantic_index::{root_scope, semantic_index, symbol_table};
+
+    struct TestCase {
+        db: TestDb,
+        file: VfsFile,
+    }
+
+    fn test_case(content: impl ToString) -> TestCase {
+        let db = TestDb::new();
+        db.memory_file_system()
+            .write_file("test.py", content)
+            .unwrap();
+
+        let file = system_path_to_file(&db, "test.py").unwrap();
+
+        TestCase { db, file }
+    }
+
+    fn names(table: &SymbolTable) -> Vec<&str> {
+        table
+            .symbols()
+            .map(|symbol| symbol.name().as_str())
+            .collect()
+    }
+
+    #[test]
+    fn empty() {
+        let TestCase { db, file } = test_case("");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), Vec::<&str>::new());
+    }
+
+    #[test]
+    fn simple() {
+        let TestCase { db, file } = test_case("x");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["x"]);
+    }
+
+    #[test]
+    fn annotation_only() {
+        let TestCase { db, file } = test_case("x: int");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["int", "x"]);
+        // TODO record definition
+    }
+
+    #[test]
+    fn import() {
+        let TestCase { db, file } = test_case("import foo");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["foo"]);
+        let foo = root_table.symbol_by_name("foo").unwrap();
+
+        assert_eq!(foo.definitions().len(), 1);
+    }
+
+    #[test]
+    fn import_sub() {
+        let TestCase { db, file } = test_case("import foo.bar");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["foo"]);
+    }
+
+    #[test]
+    fn import_as() {
+        let TestCase { db, file } = test_case("import foo.bar as baz");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["baz"]);
+    }
+
+    #[test]
+    fn import_from() {
+        let TestCase { db, file } = test_case("from bar import foo");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["foo"]);
+        assert_eq!(
+            root_table
+                .symbol_by_name("foo")
+                .unwrap()
+                .definitions()
+                .len(),
+            1
+        );
+        assert!(
+            root_table
+                .symbol_by_name("foo")
+                .is_some_and(|symbol| { symbol.is_defined() || !symbol.is_used() }),
+            "symbols that are defined get the defined flag"
+        );
+    }
+
+    #[test]
+    fn assign() {
+        let TestCase { db, file } = test_case("x = foo");
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["foo", "x"]);
+        assert_eq!(
+            root_table.symbol_by_name("x").unwrap().definitions().len(),
+            1
+        );
+        assert!(
+            root_table
+                .symbol_by_name("foo")
+                .is_some_and(|symbol| { !symbol.is_defined() && symbol.is_used() }),
+            "a symbol used but not defined in a scope should have only the used flag"
+        );
+    }
+
+    #[test]
+    fn class_scope() {
+        let TestCase { db, file } = test_case(
+            "
+class C:
+    x = 1
+y = 2
+",
+        );
+        let root_table = symbol_table(&db, root_scope(&db, file));
+
+        assert_eq!(names(&root_table), vec!["C", "y"]);
+
+        let index = semantic_index(&db, file);
+
+        let scopes: Vec<_> = index.child_scopes(FileScopeId::root()).collect();
+        assert_eq!(scopes.len(), 1);
+
+        let (class_scope_id, class_scope) = scopes[0];
+        assert_eq!(class_scope.kind(), ScopeKind::Class);
+        assert_eq!(class_scope.name(), "C");
+
+        let class_table = index.symbol_table(class_scope_id);
+        assert_eq!(names(&class_table), vec!["x"]);
+        assert_eq!(
+            class_table.symbol_by_name("x").unwrap().definitions().len(),
+            1
+        );
+    }
+
+    #[test]
+    fn function_scope() {
+        let TestCase { db, file } = test_case(
+            "
+def func():
+    x = 1
+y = 2
+",
+        );
+        let index = semantic_index(&db, file);
+        let root_table = index.symbol_table(FileScopeId::root());
+
+        assert_eq!(names(&root_table), vec!["func", "y"]);
+
+        let scopes = index.child_scopes(FileScopeId::root()).collect::<Vec<_>>();
+        assert_eq!(scopes.len(), 1);
+
+        let (function_scope_id, function_scope) = scopes[0];
+        assert_eq!(function_scope.kind(), ScopeKind::Function);
+        assert_eq!(function_scope.name(), "func");
+
+        let function_table = index.symbol_table(function_scope_id);
+        assert_eq!(names(&function_table), vec!["x"]);
+        assert_eq!(
+            function_table
+                .symbol_by_name("x")
+                .unwrap()
+                .definitions()
+                .len(),
+            1
+        );
+    }
+
+    #[test]
+    fn dupes() {
+        let TestCase { db, file } = test_case(
+            "
+def func():
+    x = 1
+def func():
+    y = 2
+",
+        );
+        let index = semantic_index(&db, file);
+        let root_table = index.symbol_table(FileScopeId::root());
+
+        assert_eq!(names(&root_table), vec!["func"]);
+        let scopes: Vec<_> = index.child_scopes(FileScopeId::root()).collect();
+        assert_eq!(scopes.len(), 2);
+
+        let (func_scope1_id, func_scope_1) = scopes[0];
+        let (func_scope2_id, func_scope_2) = scopes[1];
+
+        assert_eq!(func_scope_1.kind(), ScopeKind::Function);
+        assert_eq!(func_scope_1.name(), "func");
+        assert_eq!(func_scope_2.kind(), ScopeKind::Function);
+        assert_eq!(func_scope_2.name(), "func");
+
+        let func1_table = index.symbol_table(func_scope1_id);
+        let func2_table = index.symbol_table(func_scope2_id);
+        assert_eq!(names(&func1_table), vec!["x"]);
+        assert_eq!(names(&func2_table), vec!["y"]);
+        assert_eq!(
+            root_table
+                .symbol_by_name("func")
+                .unwrap()
+                .definitions()
+                .len(),
+            2
+        );
+    }
+
+    #[test]
+    fn generic_function() {
+        let TestCase { db, file } = test_case(
+            "
+def func[T]():
+    x = 1
+",
+        );
+
+        let index = semantic_index(&db, file);
+        let root_table = index.symbol_table(FileScopeId::root());
+
+        assert_eq!(names(&root_table), vec!["func"]);
+
+        let scopes: Vec<_> = index.child_scopes(FileScopeId::root()).collect();
+        assert_eq!(scopes.len(), 1);
+        let (ann_scope_id, ann_scope) = scopes[0];
+
+        assert_eq!(ann_scope.kind(), ScopeKind::Annotation);
+        assert_eq!(ann_scope.name(), "func");
+        let ann_table = index.symbol_table(ann_scope_id);
+        assert_eq!(names(&ann_table), vec!["T"]);
+
+        let scopes: Vec<_> = index.child_scopes(ann_scope_id).collect();
+        assert_eq!(scopes.len(), 1);
+        let (func_scope_id, func_scope) = scopes[0];
+        assert_eq!(func_scope.kind(), ScopeKind::Function);
+        assert_eq!(func_scope.name(), "func");
+        let func_table = index.symbol_table(func_scope_id);
+        assert_eq!(names(&func_table), vec!["x"]);
+    }
+
+    #[test]
+    fn generic_class() {
+        let TestCase { db, file } = test_case(
+            "
+class C[T]:
+    x = 1
+",
+        );
+
+        let index = semantic_index(&db, file);
+        let root_table = index.symbol_table(FileScopeId::root());
+
+        assert_eq!(names(&root_table), vec!["C"]);
+
+        let scopes: Vec<_> = index.child_scopes(FileScopeId::root()).collect();
+
+        assert_eq!(scopes.len(), 1);
+        let (ann_scope_id, ann_scope) = scopes[0];
+        assert_eq!(ann_scope.kind(), ScopeKind::Annotation);
+        assert_eq!(ann_scope.name(), "C");
+        let ann_table = index.symbol_table(ann_scope_id);
+        assert_eq!(names(&ann_table), vec!["T"]);
+        assert!(
+            ann_table
+                .symbol_by_name("T")
+                .is_some_and(|s| s.is_defined() && !s.is_used()),
+            "type parameters are defined by the scope that introduces them"
+        );
+
+        let scopes: Vec<_> = index.child_scopes(ann_scope_id).collect();
+        assert_eq!(scopes.len(), 1);
+        let (func_scope_id, func_scope) = scopes[0];
+
+        assert_eq!(func_scope.kind(), ScopeKind::Class);
+        assert_eq!(func_scope.name(), "C");
+        assert_eq!(names(&index.symbol_table(func_scope_id)), vec!["x"]);
+    }
+
+    // TODO: After porting the control flow graph.
+    // #[test]
+    // fn reachability_trivial() {
+    //     let parsed = parse("x = 1; x");
+    //     let ast = parsed.syntax();
+    //     let index = SemanticIndex::from_ast(ast);
+    //     let table = &index.symbol_table;
+    //     let x_sym = table
+    //         .root_symbol_id_by_name("x")
+    //         .expect("x symbol should exist");
+    //     let ast::Stmt::Expr(ast::StmtExpr { value: x_use, .. }) = &ast.body[1] else {
+    //         panic!("should be an expr")
+    //     };
+    //     let x_defs: Vec<_> = index
+    //         .reachable_definitions(x_sym, x_use)
+    //         .map(|constrained_definition| constrained_definition.definition)
+    //         .collect();
+    //     assert_eq!(x_defs.len(), 1);
+    //     let Definition::Assignment(node_key) = &x_defs[0] else {
+    //         panic!("def should be an assignment")
+    //     };
+    //     let Some(def_node) = node_key.resolve(ast.into()) else {
+    //         panic!("node key should resolve")
+    //     };
+    //     let ast::Expr::NumberLiteral(ast::ExprNumberLiteral {
+    //                                      value: ast::Number::Int(num),
+    //                                      ..
+    //                                  }) = &*def_node.value
+    //     else {
+    //         panic!("should be a number literal")
+    //     };
+    //     assert_eq!(*num, 1);
+    // }
+
+    #[test]
+    fn expression_scope() {
+        let TestCase { db, file } = test_case("x = 1;\ndef test():\n  y = 4");
+
+        let index = semantic_index(&db, file);
+        let root_table = index.symbol_table(FileScopeId::root());
+        let parsed = parsed_module(&db, file);
+        let ast = parsed.syntax();
+
+        let x_sym = root_table
+            .symbol_by_name("x")
+            .expect("x symbol should exist");
+
+        let x_stmt = ast.body[0].as_assign_stmt().unwrap();
+        let x = &x_stmt.targets[0];
+
+        assert_eq!(index.expression_scope(x).kind(), ScopeKind::Module);
+        assert_eq!(index.expression_scope_id(x), x_sym.scope());
+
+        let def = ast.body[1].as_function_def_stmt().unwrap();
+        let y_stmt = def.body[0].as_assign_stmt().unwrap();
+        let y = &y_stmt.targets[0];
+
+        assert_eq!(index.expression_scope(y).kind(), ScopeKind::Function);
+    }
+
+    #[test]
+    fn scope_iterators() {
+        let TestCase { db, file } = test_case(
+            r#"
+class Test:
+    def foo():
+        def bar():
+            ...
+    def baz():
+        pass
+
+def x():
+    pass"#,
+        );
+
+        let index = semantic_index(&db, file);
+
+        let descendents: Vec<_> = index
+            .descendent_scopes(FileScopeId::root())
+            .map(|(_, scope)| scope.name().as_str())
+            .collect();
+        assert_eq!(descendents, vec!["Test", "foo", "bar", "baz", "x"]);
+
+        let children: Vec<_> = index
+            .child_scopes(FileScopeId::root())
+            .map(|(_, scope)| scope.name.as_str())
+            .collect();
+        assert_eq!(children, vec!["Test", "x"]);
+
+        let test_class = index.child_scopes(FileScopeId::root()).next().unwrap().0;
+        let test_child_scopes: Vec<_> = index
+            .child_scopes(test_class)
+            .map(|(_, scope)| scope.name.as_str())
+            .collect();
+        assert_eq!(test_child_scopes, vec!["foo", "baz"]);
+
+        let bar_scope = index
+            .descendent_scopes(FileScopeId::root())
+            .nth(2)
+            .unwrap()
+            .0;
+        let ancestors: Vec<_> = index
+            .ancestor_scopes(bar_scope)
+            .map(|(_, scope)| scope.name())
+            .collect();
+
+        assert_eq!(ancestors, vec!["bar", "foo", "Test", "<module>"]);
+    }
+}

crates/ruff_python_semantic/src/red_knot/semantic_index/ast_ids.rs

@@ -0,0 +1,384 @@
+use rustc_hash::FxHashMap;
+
+use ruff_db::parsed::ParsedModule;
+use ruff_db::vfs::VfsFile;
+use ruff_index::{newtype_index, IndexVec};
+use ruff_python_ast as ast;
+use ruff_python_ast::AnyNodeRef;
+
+use crate::red_knot::ast_node_ref::AstNodeRef;
+use crate::red_knot::node_key::NodeKey;
+use crate::red_knot::semantic_index::semantic_index;
+use crate::red_knot::semantic_index::symbol::{FileScopeId, ScopeId};
+use crate::Db;
+
+/// AST ids for a single scope.
+///
+/// The motivation for building the AST ids per scope isn't about reducing invalidation because
+/// the struct changes whenever the parsed AST changes. Instead, it's mainly that we can
+/// build the AST ids struct when building the symbol table and also keep the property that
+/// IDs of outer scopes are unaffected by changes in inner scopes.
+///
+/// For example, we don't want that adding new statements to `foo` changes the statement id of `x = foo()` in:
+///
+/// ```python
+/// def foo():
+///     return 5
+///
+/// x = foo()
+/// ```
+pub(crate) struct AstIds {
+    /// Maps expression ids to their expressions.
+    expressions: IndexVec<ScopeExpressionId, AstNodeRef<ast::Expr>>,
+
+    /// Maps expressions to their expression id. Uses `NodeKey` because it avoids cloning [`Parsed`].
+    expressions_map: FxHashMap<NodeKey, ScopeExpressionId>,
+
+    statements: IndexVec<ScopeStatementId, AstNodeRef<ast::Stmt>>,
+
+    statements_map: FxHashMap<NodeKey, ScopeStatementId>,
+}
+
+impl AstIds {
+    fn statement_id<'a, N>(&self, node: N) -> ScopeStatementId
+    where
+        N: Into<AnyNodeRef<'a>>,
+    {
+        self.statements_map[&NodeKey::from_node(node.into())]
+    }
+
+    fn expression_id<'a, N>(&self, node: N) -> ScopeExpressionId
+    where
+        N: Into<AnyNodeRef<'a>>,
+    {
+        self.expressions_map[&NodeKey::from_node(node.into())]
+    }
+}
+
+#[allow(clippy::missing_fields_in_debug)]
+impl std::fmt::Debug for AstIds {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.debug_struct("AstIds")
+            .field("expressions", &self.expressions)
+            .field("statements", &self.statements)
+            .finish()
+    }
+}
+
+fn ast_ids(db: &dyn Db, scope: ScopeId) -> &AstIds {
+    semantic_index(db, scope.file(db)).ast_ids(scope.scope_id(db))
+}
+
+/// Node that can be uniquely identified by an id in a [`FileScopeId`].
+pub trait ScopeAstIdNode {
+    /// The type of the ID uniquely identifying the node.
+    type Id;
+
+    /// Returns the ID that uniquely identifies the node in `scope`.
+    ///
+    /// ## Panics
+    /// Panics if the node doesn't belong to `file` or is outside `scope`.
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, scope: FileScopeId) -> Self::Id;
+
+    /// Looks up the AST node by its ID.
+    ///
+    /// ## Panics
+    /// May panic if the `id` does not belong to the AST of `file`, or is outside `scope`.
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self
+    where
+        Self: Sized;
+}
+
+/// Extension trait for AST nodes that can be resolved by an `AstId`.
+pub trait AstIdNode {
+    type ScopeId;
+
+    /// Resolves the AST id of the node.
+    ///
+    /// ## Panics
+    /// May panic if the node does not belongs to `file`'s AST or is outside of `scope`. It may also
+    /// return an incorrect node if that's the case.
+
+    fn ast_id(&self, db: &dyn Db, file: VfsFile, scope: FileScopeId) -> AstId<Self::ScopeId>;
+
+    /// Resolves the AST node for `id`.
+    ///
+    /// ## Panics
+    /// May panic if the `id` does not belong to the AST of `file` or it returns an incorrect node.
+
+    fn lookup(db: &dyn Db, file: VfsFile, id: AstId<Self::ScopeId>) -> &Self
+    where
+        Self: Sized;
+}
+
+impl<T> AstIdNode for T
+where
+    T: ScopeAstIdNode,
+{
+    type ScopeId = T::Id;
+
+    fn ast_id(&self, db: &dyn Db, file: VfsFile, scope: FileScopeId) -> AstId<Self::ScopeId> {
+        let in_scope_id = self.scope_ast_id(db, file, scope);
+        AstId { scope, in_scope_id }
+    }
+
+    fn lookup(db: &dyn Db, file: VfsFile, id: AstId<Self::ScopeId>) -> &Self
+    where
+        Self: Sized,
+    {
+        let scope = id.scope;
+        Self::lookup_in_scope(db, file, scope, id.in_scope_id)
+    }
+}
+
+/// Uniquely identifies an AST node in a file.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
+pub struct AstId<L> {
+    /// The node's scope.
+    scope: FileScopeId,
+
+    /// The ID of the node inside [`Self::scope`].
+    in_scope_id: L,
+}
+
+/// Uniquely identifies an [`ast::Expr`] in a [`FileScopeId`].
+#[newtype_index]
+pub struct ScopeExpressionId;
+
+impl ScopeAstIdNode for ast::Expr {
+    type Id = ScopeExpressionId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ast_ids.expressions_map[&NodeKey::from_node(self)]
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, file_scope: FileScopeId, id: Self::Id) -> &Self {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ast_ids.expressions[id].node()
+    }
+}
+
+/// Uniquely identifies an [`ast::Stmt`] in a [`FileScopeId`].
+#[newtype_index]
+pub struct ScopeStatementId;
+
+impl ScopeAstIdNode for ast::Stmt {
+    type Id = ScopeStatementId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ast_ids.statement_id(self)
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, file_scope: FileScopeId, id: Self::Id) -> &Self {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+
+        ast_ids.statements[id].node()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct ScopeFunctionId(pub(super) ScopeStatementId);
+
+impl ScopeAstIdNode for ast::StmtFunctionDef {
+    type Id = ScopeFunctionId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ScopeFunctionId(ast_ids.statement_id(self))
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self {
+        ast::Stmt::lookup_in_scope(db, file, scope, id.0)
+            .as_function_def_stmt()
+            .unwrap()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct ScopeClassId(pub(super) ScopeStatementId);
+
+impl ScopeAstIdNode for ast::StmtClassDef {
+    type Id = ScopeClassId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ScopeClassId(ast_ids.statement_id(self))
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self {
+        let statement = ast::Stmt::lookup_in_scope(db, file, scope, id.0);
+        statement.as_class_def_stmt().unwrap()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct ScopeAssignmentId(pub(super) ScopeStatementId);
+
+impl ScopeAstIdNode for ast::StmtAssign {
+    type Id = ScopeAssignmentId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ScopeAssignmentId(ast_ids.statement_id(self))
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self {
+        let statement = ast::Stmt::lookup_in_scope(db, file, scope, id.0);
+        statement.as_assign_stmt().unwrap()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct ScopeAnnotatedAssignmentId(ScopeStatementId);
+
+impl ScopeAstIdNode for ast::StmtAnnAssign {
+    type Id = ScopeAnnotatedAssignmentId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ScopeAnnotatedAssignmentId(ast_ids.statement_id(self))
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self {
+        let statement = ast::Stmt::lookup_in_scope(db, file, scope, id.0);
+        statement.as_ann_assign_stmt().unwrap()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct ScopeImportId(pub(super) ScopeStatementId);
+
+impl ScopeAstIdNode for ast::StmtImport {
+    type Id = ScopeImportId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ScopeImportId(ast_ids.statement_id(self))
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self {
+        let statement = ast::Stmt::lookup_in_scope(db, file, scope, id.0);
+        statement.as_import_stmt().unwrap()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct ScopeImportFromId(pub(super) ScopeStatementId);
+
+impl ScopeAstIdNode for ast::StmtImportFrom {
+    type Id = ScopeImportFromId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ScopeImportFromId(ast_ids.statement_id(self))
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self {
+        let statement = ast::Stmt::lookup_in_scope(db, file, scope, id.0);
+        statement.as_import_from_stmt().unwrap()
+    }
+}
+
+#[derive(Debug, Eq, PartialEq, Copy, Clone, Hash)]
+pub struct ScopeNamedExprId(pub(super) ScopeExpressionId);
+
+impl ScopeAstIdNode for ast::ExprNamed {
+    type Id = ScopeNamedExprId;
+
+    fn scope_ast_id(&self, db: &dyn Db, file: VfsFile, file_scope: FileScopeId) -> Self::Id {
+        let scope = file_scope.to_scope_id(db, file);
+        let ast_ids = ast_ids(db, scope);
+        ScopeNamedExprId(ast_ids.expression_id(self))
+    }
+
+    fn lookup_in_scope(db: &dyn Db, file: VfsFile, scope: FileScopeId, id: Self::Id) -> &Self
+    where
+        Self: Sized,
+    {
+        let expression = ast::Expr::lookup_in_scope(db, file, scope, id.0);
+        expression.as_named_expr().unwrap()
+    }
+}
+
+#[derive(Debug)]
+pub(super) struct AstIdsBuilder {
+    expressions: IndexVec<ScopeExpressionId, AstNodeRef<ast::Expr>>,
+    expressions_map: FxHashMap<NodeKey, ScopeExpressionId>,
+    statements: IndexVec<ScopeStatementId, AstNodeRef<ast::Stmt>>,
+    statements_map: FxHashMap<NodeKey, ScopeStatementId>,
+}
+
+impl AstIdsBuilder {
+    pub(super) fn new() -> Self {
+        Self {
+            expressions: IndexVec::default(),
+            expressions_map: FxHashMap::default(),
+            statements: IndexVec::default(),
+            statements_map: FxHashMap::default(),
+        }
+    }
+
+    /// Adds `stmt` to the AST ids map and returns its id.
+    ///
+    /// ## Safety
+    /// The function is marked as unsafe because it calls [`AstNodeRef::new`] which requires
+    /// that `stmt` is a child of `parsed`.
+    #[allow(unsafe_code)]
+    pub(super) unsafe fn record_statement(
+        &mut self,
+        stmt: &ast::Stmt,
+        parsed: &ParsedModule,
+    ) -> ScopeStatementId {
+        let statement_id = self.statements.push(AstNodeRef::new(parsed.clone(), stmt));
+
+        self.statements_map
+            .insert(NodeKey::from_node(stmt), statement_id);
+
+        statement_id
+    }
+
+    /// Adds `expr` to the AST ids map and returns its id.
+    ///
+    /// ## Safety
+    /// The function is marked as unsafe because it calls [`AstNodeRef::new`] which requires
+    /// that `expr` is a child of `parsed`.
+    #[allow(unsafe_code)]
+    pub(super) unsafe fn record_expression(
+        &mut self,
+        expr: &ast::Expr,
+        parsed: &ParsedModule,
+    ) -> ScopeExpressionId {
+        let expression_id = self.expressions.push(AstNodeRef::new(parsed.clone(), expr));
+
+        self.expressions_map
+            .insert(NodeKey::from_node(expr), expression_id);
+
+        expression_id
+    }
+
+    pub(super) fn finish(mut self) -> AstIds {
+        self.expressions.shrink_to_fit();
+        self.expressions_map.shrink_to_fit();
+        self.statements.shrink_to_fit();
+        self.statements_map.shrink_to_fit();
+
+        AstIds {
+            expressions: self.expressions,
+            expressions_map: self.expressions_map,
+            statements: self.statements,
+            statements_map: self.statements_map,
+        }
+    }
+}

crates/ruff_python_semantic/src/red_knot/semantic_index/builder.rs

@@ -0,0 +1,398 @@
+use std::sync::Arc;
+
+use rustc_hash::FxHashMap;
+
+use ruff_db::parsed::ParsedModule;
+use ruff_index::IndexVec;
+use ruff_python_ast as ast;
+use ruff_python_ast::visitor::{walk_expr, walk_stmt, Visitor};
+
+use crate::name::Name;
+use crate::red_knot::node_key::NodeKey;
+use crate::red_knot::semantic_index::ast_ids::{
+    AstIdsBuilder, ScopeAssignmentId, ScopeClassId, ScopeFunctionId, ScopeImportFromId,
+    ScopeImportId, ScopeNamedExprId,
+};
+use crate::red_knot::semantic_index::definition::{
+    Definition, ImportDefinition, ImportFromDefinition,
+};
+use crate::red_knot::semantic_index::symbol::{
+    FileScopeId, FileSymbolId, Scope, ScopeKind, ScopeSymbolId, SymbolFlags, SymbolTableBuilder,
+};
+use crate::red_knot::semantic_index::SemanticIndex;
+
+pub(super) struct SemanticIndexBuilder<'a> {
+    // Builder state
+    module: &'a ParsedModule,
+    scope_stack: Vec<FileScopeId>,
+    /// the definition whose target(s) we are currently walking
+    current_definition: Option<Definition>,
+
+    // Semantic Index fields
+    scopes: IndexVec<FileScopeId, Scope>,
+    symbol_tables: IndexVec<FileScopeId, SymbolTableBuilder>,
+    ast_ids: IndexVec<FileScopeId, AstIdsBuilder>,
+    expression_scopes: FxHashMap<NodeKey, FileScopeId>,
+}
+
+impl<'a> SemanticIndexBuilder<'a> {
+    pub(super) fn new(parsed: &'a ParsedModule) -> Self {
+        let mut builder = Self {
+            module: parsed,
+            scope_stack: Vec::new(),
+            current_definition: None,
+
+            scopes: IndexVec::new(),
+            symbol_tables: IndexVec::new(),
+            ast_ids: IndexVec::new(),
+            expression_scopes: FxHashMap::default(),
+        };
+
+        builder.push_scope_with_parent(
+            ScopeKind::Module,
+            &Name::new_static("<module>"),
+            None,
+            None,
+            None,
+        );
+
+        builder
+    }
+
+    fn current_scope(&self) -> FileScopeId {
+        *self
+            .scope_stack
+            .last()
+            .expect("Always to have a root scope")
+    }
+
+    fn push_scope(
+        &mut self,
+        scope_kind: ScopeKind,
+        name: &Name,
+        defining_symbol: Option<FileSymbolId>,
+        definition: Option<Definition>,
+    ) {
+        let parent = self.current_scope();
+        self.push_scope_with_parent(scope_kind, name, defining_symbol, definition, Some(parent));
+    }
+
+    fn push_scope_with_parent(
+        &mut self,
+        scope_kind: ScopeKind,
+        name: &Name,
+        defining_symbol: Option<FileSymbolId>,
+        definition: Option<Definition>,
+        parent: Option<FileScopeId>,
+    ) {
+        let children_start = self.scopes.next_index() + 1;
+
+        let scope = Scope {
+            name: name.clone(),
+            parent,
+            defining_symbol,
+            definition,
+            kind: scope_kind,
+            descendents: children_start..children_start,
+        };
+
+        let scope_id = self.scopes.push(scope);
+        self.symbol_tables.push(SymbolTableBuilder::new());
+        self.ast_ids.push(AstIdsBuilder::new());
+        self.scope_stack.push(scope_id);
+    }
+
+    fn pop_scope(&mut self) -> FileScopeId {
+        let id = self.scope_stack.pop().expect("Root scope to be present");
+        let children_end = self.scopes.next_index();
+        let scope = &mut self.scopes[id];
+        scope.descendents = scope.descendents.start..children_end;
+        id
+    }
+
+    fn current_symbol_table(&mut self) -> &mut SymbolTableBuilder {
+        let scope_id = self.current_scope();
+        &mut self.symbol_tables[scope_id]
+    }
+
+    fn current_ast_ids(&mut self) -> &mut AstIdsBuilder {
+        let scope_id = self.current_scope();
+        &mut self.ast_ids[scope_id]
+    }
+
+    fn add_or_update_symbol(&mut self, name: Name, flags: SymbolFlags) -> ScopeSymbolId {
+        let scope = self.current_scope();
+        let symbol_table = self.current_symbol_table();
+
+        symbol_table.add_or_update_symbol(name, scope, flags, None)
+    }
+
+    fn add_or_update_symbol_with_definition(
+        &mut self,
+        name: Name,
+
+        definition: Definition,
+    ) -> ScopeSymbolId {
+        let scope = self.current_scope();
+        let symbol_table = self.current_symbol_table();
+
+        symbol_table.add_or_update_symbol(name, scope, SymbolFlags::IS_DEFINED, Some(definition))
+    }
+
+    fn with_type_params(
+        &mut self,
+        name: &Name,
+        params: &Option<Box<ast::TypeParams>>,
+        definition: Option<Definition>,
+        defining_symbol: FileSymbolId,
+        nested: impl FnOnce(&mut Self) -> FileScopeId,
+    ) -> FileScopeId {
+        if let Some(type_params) = params {
+            self.push_scope(
+                ScopeKind::Annotation,
+                name,
+                Some(defining_symbol),
+                definition,
+            );
+            for type_param in &type_params.type_params {
+                let name = match type_param {
+                    ast::TypeParam::TypeVar(ast::TypeParamTypeVar { name, .. }) => name,
+                    ast::TypeParam::ParamSpec(ast::TypeParamParamSpec { name, .. }) => name,
+                    ast::TypeParam::TypeVarTuple(ast::TypeParamTypeVarTuple { name, .. }) => name,
+                };
+                self.add_or_update_symbol(Name::new(name), SymbolFlags::IS_DEFINED);
+            }
+        }
+        let nested_scope = nested(self);
+
+        if params.is_some() {
+            self.pop_scope();
+        }
+
+        nested_scope
+    }
+
+    pub(super) fn build(mut self) -> SemanticIndex {
+        let module = self.module;
+        self.visit_body(module.suite());
+
+        // Pop the root scope
+        self.pop_scope();
+        assert!(self.scope_stack.is_empty());
+
+        assert!(self.current_definition.is_none());
+
+        let mut symbol_tables: IndexVec<_, _> = self
+            .symbol_tables
+            .into_iter()
+            .map(|builder| Arc::new(builder.finish()))
+            .collect();
+
+        let mut ast_ids: IndexVec<_, _> = self
+            .ast_ids
+            .into_iter()
+            .map(super::ast_ids::AstIdsBuilder::finish)
+            .collect();
+
+        self.scopes.shrink_to_fit();
+        ast_ids.shrink_to_fit();
+        symbol_tables.shrink_to_fit();
+        self.expression_scopes.shrink_to_fit();
+
+        SemanticIndex {
+            symbol_tables,
+            scopes: self.scopes,
+            ast_ids,
+            expression_scopes: self.expression_scopes,
+        }
+    }
+}
+
+impl Visitor<'_> for SemanticIndexBuilder<'_> {
+    fn visit_stmt(&mut self, stmt: &ast::Stmt) {
+        let module = self.module;
+        #[allow(unsafe_code)]
+        let statement_id = unsafe {
+            // SAFETY: The builder only visits nodes that are part of `module`. This guarantees that
+            // the current statement must be a child of `module`.
+            self.current_ast_ids().record_statement(stmt, module)
+        };
+        match stmt {
+            ast::Stmt::FunctionDef(function_def) => {
+                for decorator in &function_def.decorator_list {
+                    self.visit_decorator(decorator);
+                }
+                let name = Name::new(&function_def.name.id);
+                let definition = Definition::FunctionDef(ScopeFunctionId(statement_id));
+                let scope = self.current_scope();
+                let symbol = FileSymbolId::new(
+                    scope,
+                    self.add_or_update_symbol_with_definition(name.clone(), definition),
+                );
+
+                self.with_type_params(
+                    &name,
+                    &function_def.type_params,
+                    Some(definition),
+                    symbol,
+                    |builder| {
+                        builder.visit_parameters(&function_def.parameters);
+                        for expr in &function_def.returns {
+                            builder.visit_annotation(expr);
+                        }
+
+                        builder.push_scope(
+                            ScopeKind::Function,
+                            &name,
+                            Some(symbol),
+                            Some(definition),
+                        );
+                        builder.visit_body(&function_def.body);
+                        builder.pop_scope()
+                    },
+                );
+            }
+            ast::Stmt::ClassDef(class) => {
+                for decorator in &class.decorator_list {
+                    self.visit_decorator(decorator);
+                }
+
+                let name = Name::new(&class.name.id);
+                let definition = Definition::from(ScopeClassId(statement_id));
+                let id = FileSymbolId::new(
+                    self.current_scope(),
+                    self.add_or_update_symbol_with_definition(name.clone(), definition),
+                );
+                self.with_type_params(&name, &class.type_params, Some(definition), id, |builder| {
+                    if let Some(arguments) = &class.arguments {
+                        builder.visit_arguments(arguments);
+                    }
+
+                    builder.push_scope(ScopeKind::Class, &name, Some(id), Some(definition));
+                    builder.visit_body(&class.body);
+
+                    builder.pop_scope()
+                });
+            }
+            ast::Stmt::Import(ast::StmtImport { names, .. }) => {
+                for (i, alias) in names.iter().enumerate() {
+                    let symbol_name = if let Some(asname) = &alias.asname {
+                        asname.id.as_str()
+                    } else {
+                        alias.name.id.split('.').next().unwrap()
+                    };
+
+                    let def = Definition::Import(ImportDefinition {
+                        import_id: ScopeImportId(statement_id),
+                        alias: u32::try_from(i).unwrap(),
+                    });
+                    self.add_or_update_symbol_with_definition(Name::new(symbol_name), def);
+                }
+            }
+            ast::Stmt::ImportFrom(ast::StmtImportFrom {
+                module: _,
+                names,
+                level: _,
+                ..
+            }) => {
+                for (i, alias) in names.iter().enumerate() {
+                    let symbol_name = if let Some(asname) = &alias.asname {
+                        asname.id.as_str()
+                    } else {
+                        alias.name.id.as_str()
+                    };
+                    let def = Definition::ImportFrom(ImportFromDefinition {
+                        import_id: ScopeImportFromId(statement_id),
+                        name: u32::try_from(i).unwrap(),
+                    });
+                    self.add_or_update_symbol_with_definition(Name::new(symbol_name), def);
+                }
+            }
+            ast::Stmt::Assign(node) => {
+                debug_assert!(self.current_definition.is_none());
+                self.visit_expr(&node.value);
+                self.current_definition =
+                    Some(Definition::Assignment(ScopeAssignmentId(statement_id)));
+                for target in &node.targets {
+                    self.visit_expr(target);
+                }
+                self.current_definition = None;
+            }
+            _ => {
+                walk_stmt(self, stmt);
+            }
+        }
+    }
+
+    fn visit_expr(&mut self, expr: &'_ ast::Expr) {
+        let module = self.module;
+        #[allow(unsafe_code)]
+        let expression_id = unsafe {
+            // SAFETY: The builder only visits nodes that are part of `module`. This guarantees that
+            // the current expression must be a child of `module`.
+            self.current_ast_ids().record_expression(expr, module)
+        };
+
+        self.expression_scopes
+            .insert(NodeKey::from_node(expr), self.current_scope());
+
+        match expr {
+            ast::Expr::Name(ast::ExprName { id, ctx, .. }) => {
+                let flags = match ctx {
+                    ast::ExprContext::Load => SymbolFlags::IS_USED,
+                    ast::ExprContext::Store => SymbolFlags::IS_DEFINED,
+                    ast::ExprContext::Del => SymbolFlags::IS_DEFINED,
+                    ast::ExprContext::Invalid => SymbolFlags::empty(),
+                };
+                match self.current_definition {
+                    Some(definition) if flags.contains(SymbolFlags::IS_DEFINED) => {
+                        self.add_or_update_symbol_with_definition(Name::new(id), definition);
+                    }
+                    _ => {
+                        self.add_or_update_symbol(Name::new(id), flags);
+                    }
+                }
+
+                walk_expr(self, expr);
+            }
+            ast::Expr::Named(node) => {
+                debug_assert!(self.current_definition.is_none());
+                self.current_definition =
+                    Some(Definition::NamedExpr(ScopeNamedExprId(expression_id)));
+                // TODO walrus in comprehensions is implicitly nonlocal
+                self.visit_expr(&node.target);
+                self.current_definition = None;
+                self.visit_expr(&node.value);
+            }
+            ast::Expr::If(ast::ExprIf {
+                body, test, orelse, ..
+            }) => {
+                // TODO detect statically known truthy or falsy test (via type inference, not naive
+                // AST inspection, so we can't simplify here, need to record test expression in CFG
+                // for later checking)
+
+                self.visit_expr(test);
+
+                // let if_branch = self.flow_graph_builder.add_branch(self.current_flow_node());
+
+                // self.set_current_flow_node(if_branch);
+                // self.insert_constraint(test);
+                self.visit_expr(body);
+
+                // let post_body = self.current_flow_node();
+
+                // self.set_current_flow_node(if_branch);
+                self.visit_expr(orelse);
+
+                // let post_else = self
+                //     .flow_graph_builder
+                //     .add_phi(self.current_flow_node(), post_body);
+
+                // self.set_current_flow_node(post_else);
+            }
+            _ => {
+                walk_expr(self, expr);
+            }
+        }
+    }
+}

crates/ruff_python_semantic/src/red_knot/semantic_index/definition.rs

@@ -0,0 +1,76 @@
+use crate::red_knot::semantic_index::ast_ids::{
+    ScopeAnnotatedAssignmentId, ScopeAssignmentId, ScopeClassId, ScopeFunctionId,
+    ScopeImportFromId, ScopeImportId, ScopeNamedExprId,
+};
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum Definition {
+    Import(ImportDefinition),
+    ImportFrom(ImportFromDefinition),
+    ClassDef(ScopeClassId),
+    FunctionDef(ScopeFunctionId),
+    Assignment(ScopeAssignmentId),
+    AnnotatedAssignment(ScopeAnnotatedAssignmentId),
+    NamedExpr(ScopeNamedExprId),
+    /// represents the implicit initial definition of every name as "unbound"
+    Unbound,
+    // TODO with statements, except handlers, function args...
+}
+
+impl From<ImportDefinition> for Definition {
+    fn from(value: ImportDefinition) -> Self {
+        Self::Import(value)
+    }
+}
+
+impl From<ImportFromDefinition> for Definition {
+    fn from(value: ImportFromDefinition) -> Self {
+        Self::ImportFrom(value)
+    }
+}
+
+impl From<ScopeClassId> for Definition {
+    fn from(value: ScopeClassId) -> Self {
+        Self::ClassDef(value)
+    }
+}
+
+impl From<ScopeFunctionId> for Definition {
+    fn from(value: ScopeFunctionId) -> Self {
+        Self::FunctionDef(value)
+    }
+}
+
+impl From<ScopeAssignmentId> for Definition {
+    fn from(value: ScopeAssignmentId) -> Self {
+        Self::Assignment(value)
+    }
+}
+
+impl From<ScopeAnnotatedAssignmentId> for Definition {
+    fn from(value: ScopeAnnotatedAssignmentId) -> Self {
+        Self::AnnotatedAssignment(value)
+    }
+}
+
+impl From<ScopeNamedExprId> for Definition {
+    fn from(value: ScopeNamedExprId) -> Self {
+        Self::NamedExpr(value)
+    }
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct ImportDefinition {
+    pub(super) import_id: ScopeImportId,
+
+    /// Index into [`ruff_python_ast::StmtImport::names`].
+    pub(super) alias: u32,
+}
+
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct ImportFromDefinition {
+    pub(super) import_id: ScopeImportFromId,
+
+    /// Index into [`ruff_python_ast::StmtImportFrom::names`].
+    pub(super) name: u32,
+}

crates/ruff_python_semantic/src/red_knot/semantic_index/symbol.rs

@@ -0,0 +1,362 @@
+// Allow unused underscore violations generated by the salsa macro
+// TODO(micha): Contribute fix upstream
+#![allow(clippy::used_underscore_binding)]
+
+use std::hash::{Hash, Hasher};
+use std::ops::Range;
+
+use bitflags::bitflags;
+use hashbrown::hash_map::RawEntryMut;
+use rustc_hash::FxHasher;
+use smallvec::SmallVec;
+
+use ruff_db::vfs::VfsFile;
+use ruff_index::{newtype_index, IndexVec};
+
+use crate::name::Name;
+use crate::red_knot::semantic_index::definition::Definition;
+use crate::red_knot::semantic_index::{scopes_map, symbol_table, SymbolMap};
+use crate::Db;
+
+#[derive(Eq, PartialEq, Debug)]
+pub struct Symbol {
+    name: Name,
+    flags: SymbolFlags,
+    scope: FileScopeId,
+
+    /// The nodes that define this symbol, in source order.
+    definitions: SmallVec<[Definition; 4]>,
+}
+
+impl Symbol {
+    fn new(name: Name, scope: FileScopeId, definition: Option<Definition>) -> Self {
+        Self {
+            name,
+            scope,
+            flags: SymbolFlags::empty(),
+            definitions: definition.into_iter().collect(),
+        }
+    }
+
+    fn push_definition(&mut self, definition: Definition) {
+        self.definitions.push(definition);
+    }
+
+    fn insert_flags(&mut self, flags: SymbolFlags) {
+        self.flags.insert(flags);
+    }
+
+    /// The symbol's name.
+    pub fn name(&self) -> &Name {
+        &self.name
+    }
+
+    /// The scope in which this symbol is defined.
+    pub fn scope(&self) -> FileScopeId {
+        self.scope
+    }
+
+    /// Is the symbol used in its containing scope?
+    pub fn is_used(&self) -> bool {
+        self.flags.contains(SymbolFlags::IS_USED)
+    }
+
+    /// Is the symbol defined in its containing scope?
+    pub fn is_defined(&self) -> bool {
+        self.flags.contains(SymbolFlags::IS_DEFINED)
+    }
+
+    pub fn definitions(&self) -> &[Definition] {
+        &self.definitions
+    }
+}
+
+bitflags! {
+    #[derive(Copy, Clone, Debug, Eq, PartialEq)]
+    pub(super) struct SymbolFlags: u8 {
+        const IS_USED         = 1 << 0;
+        const IS_DEFINED      = 1 << 1;
+        /// TODO: This flag is not yet set by anything
+        const MARKED_GLOBAL   = 1 << 2;
+        /// TODO: This flag is not yet set by anything
+        const MARKED_NONLOCAL = 1 << 3;
+    }
+}
+
+/// ID that uniquely identifies a public symbol defined in a module's root scope.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
+pub struct PublicSymbolId {
+    scope: ScopeId,
+    symbol: ScopeSymbolId,
+}
+
+impl PublicSymbolId {
+    pub(crate) fn new(scope: ScopeId, symbol: ScopeSymbolId) -> Self {
+        Self { scope, symbol }
+    }
+
+    pub fn scope(self) -> ScopeId {
+        self.scope
+    }
+
+    pub(crate) fn scope_symbol(self) -> ScopeSymbolId {
+        self.symbol
+    }
+}
+
+impl From<PublicSymbolId> for ScopeSymbolId {
+    fn from(val: PublicSymbolId) -> Self {
+        val.scope_symbol()
+    }
+}
+
+/// ID that uniquely identifies a symbol in a file.
+#[derive(Copy, Clone, Debug, Eq, PartialEq, Hash)]
+pub struct FileSymbolId {
+    scope: FileScopeId,
+    symbol: ScopeSymbolId,
+}
+
+impl FileSymbolId {
+    pub(super) fn new(scope: FileScopeId, symbol: ScopeSymbolId) -> Self {
+        Self { scope, symbol }
+    }
+
+    pub fn scope(self) -> FileScopeId {
+        self.scope
+    }
+
+    pub(crate) fn symbol(self) -> ScopeSymbolId {
+        self.symbol
+    }
+}
+
+impl From<FileSymbolId> for ScopeSymbolId {
+    fn from(val: FileSymbolId) -> Self {
+        val.symbol()
+    }
+}
+
+/// Symbol ID that uniquely identifies a symbol inside a [`Scope`].
+#[newtype_index]
+pub(crate) struct ScopeSymbolId;
+
+/// Maps from the file specific [`FileScopeId`] to the global [`ScopeId`] that can be used as a Salsa query parameter.
+///
+/// The [`SemanticIndex`] uses [`FileScopeId`] on a per-file level to identify scopes
+/// because they allow for more efficient storage of associated data
+/// (use of an [`IndexVec`] keyed by [`FileScopeId`] over an [`FxHashMap`] keyed by [`ScopeId`]).
+#[derive(Eq, PartialEq, Debug)]
+pub(crate) struct ScopesMap {
+    scopes: IndexVec<FileScopeId, ScopeId>,
+}
+
+impl ScopesMap {
+    pub(super) fn new(scopes: IndexVec<FileScopeId, ScopeId>) -> Self {
+        Self { scopes }
+    }
+
+    /// Gets the program-wide unique scope id for the given file specific `scope_id`.
+    fn get(&self, scope_id: FileScopeId) -> ScopeId {
+        self.scopes[scope_id]
+    }
+}
+
+/// A cross-module identifier of a scope that can be used as a salsa query parameter.
+#[salsa::tracked]
+pub struct ScopeId {
+    #[allow(clippy::used_underscore_binding)]
+    pub file: VfsFile,
+    pub scope_id: FileScopeId,
+}
+
+impl ScopeId {
+    /// Resolves the symbol named `name` in this scope.
+    pub fn symbol(self, db: &dyn Db, name: &str) -> Option<PublicSymbolId> {
+        let symbol_table = symbol_table(db, self);
+        let in_scope_id = symbol_table.symbol_id_by_name(name)?;
+
+        Some(PublicSymbolId::new(self, in_scope_id))
+    }
+}
+
+/// ID that uniquely identifies a scope inside of a module.
+#[newtype_index]
+pub struct FileScopeId;
+
+impl FileScopeId {
+    /// Returns the scope id of the Root scope.
+    pub fn root() -> Self {
+        FileScopeId::from_u32(0)
+    }
+
+    pub fn to_scope_id(self, db: &dyn Db, file: VfsFile) -> ScopeId {
+        scopes_map(db, file).get(self)
+    }
+}
+
+#[derive(Debug, Eq, PartialEq)]
+pub struct Scope {
+    pub(super) name: Name,
+    pub(super) parent: Option<FileScopeId>,
+    pub(super) definition: Option<Definition>,
+    pub(super) defining_symbol: Option<FileSymbolId>,
+    pub(super) kind: ScopeKind,
+    pub(super) descendents: Range<FileScopeId>,
+}
+
+impl Scope {
+    pub fn name(&self) -> &Name {
+        &self.name
+    }
+
+    pub fn definition(&self) -> Option<Definition> {
+        self.definition
+    }
+
+    pub fn defining_symbol(&self) -> Option<FileSymbolId> {
+        self.defining_symbol
+    }
+
+    pub fn parent(self) -> Option<FileScopeId> {
+        self.parent
+    }
+
+    pub fn kind(&self) -> ScopeKind {
+        self.kind
+    }
+}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub enum ScopeKind {
+    Module,
+    Annotation,
+    Class,
+    Function,
+}
+
+/// Symbol table for a specific [`Scope`].
+#[derive(Debug)]
+pub struct SymbolTable {
+    /// The symbols in this scope.
+    symbols: IndexVec<ScopeSymbolId, Symbol>,
+
+    /// The symbols indexed by name.
+    symbols_by_name: SymbolMap,
+}
+
+impl SymbolTable {
+    fn new() -> Self {
+        Self {
+            symbols: IndexVec::new(),
+            symbols_by_name: SymbolMap::default(),
+        }
+    }
+
+    fn shrink_to_fit(&mut self) {
+        self.symbols.shrink_to_fit();
+    }
+
+    pub(crate) fn symbol(&self, symbol_id: impl Into<ScopeSymbolId>) -> &Symbol {
+        &self.symbols[symbol_id.into()]
+    }
+
+    #[allow(unused)]
+    pub(crate) fn symbol_ids(&self) -> impl Iterator<Item = ScopeSymbolId> {
+        self.symbols.indices()
+    }
+
+    pub fn symbols(&self) -> impl Iterator<Item = &Symbol> {
+        self.symbols.iter()
+    }
+
+    /// Returns the symbol named `name`.
+    #[allow(unused)]
+    pub(crate) fn symbol_by_name(&self, name: &str) -> Option<&Symbol> {
+        let id = self.symbol_id_by_name(name)?;
+        Some(self.symbol(id))
+    }
+
+    /// Returns the [`ScopeSymbolId`] of the symbol named `name`.
+    pub(crate) fn symbol_id_by_name(&self, name: &str) -> Option<ScopeSymbolId> {
+        let (id, ()) = self
+            .symbols_by_name
+            .raw_entry()
+            .from_hash(Self::hash_name(name), |id| {
+                self.symbol(*id).name().as_str() == name
+            })?;
+
+        Some(*id)
+    }
+
+    fn hash_name(name: &str) -> u64 {
+        let mut hasher = FxHasher::default();
+        name.hash(&mut hasher);
+        hasher.finish()
+    }
+}
+
+impl PartialEq for SymbolTable {
+    fn eq(&self, other: &Self) -> bool {
+        // We don't need to compare the symbols_by_name because the name is already captured in `Symbol`.
+        self.symbols == other.symbols
+    }
+}
+
+impl Eq for SymbolTable {}
+
+#[derive(Debug)]
+pub(super) struct SymbolTableBuilder {
+    table: SymbolTable,
+}
+
+impl SymbolTableBuilder {
+    pub(super) fn new() -> Self {
+        Self {
+            table: SymbolTable::new(),
+        }
+    }
+
+    pub(super) fn add_or_update_symbol(
+        &mut self,
+        name: Name,
+        scope: FileScopeId,
+        flags: SymbolFlags,
+        definition: Option<Definition>,
+    ) -> ScopeSymbolId {
+        let hash = SymbolTable::hash_name(&name);
+        let entry = self
+            .table
+            .symbols_by_name
+            .raw_entry_mut()
+            .from_hash(hash, |id| self.table.symbols[*id].name() == &name);
+
+        match entry {
+            RawEntryMut::Occupied(entry) => {
+                let symbol = &mut self.table.symbols[*entry.key()];
+                symbol.insert_flags(flags);
+
+                if let Some(definition) = definition {
+                    symbol.push_definition(definition);
+                }
+
+                *entry.key()
+            }
+            RawEntryMut::Vacant(entry) => {
+                let mut symbol = Symbol::new(name, scope, definition);
+                symbol.insert_flags(flags);
+
+                let id = self.table.symbols.push(symbol);
+                entry.insert_with_hasher(hash, id, (), |id| {
+                    SymbolTable::hash_name(self.table.symbols[*id].name().as_str())
+                });
+                id
+            }
+        }
+    }
+
+    pub(super) fn finish(mut self) -> SymbolTable {
+        self.table.shrink_to_fit();
+        self.table
+    }
+}