[red-knot] Remove match pattern definition visitor (#13209)

## Summary

This PR is based on this discussion:
https://github.com/astral-sh/ruff/pull/13147#discussion_r1739408653.

**Todo**

- [x] Add documentation for `MatchPatternState`

## Test Plan

`cargo insta test` and `cargo clippy`

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -1097,16 +1097,62 @@ match subject:
         );
 
         let use_def = use_def_map(&db, global_scope_id);
-        for name in ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l"] {
+        for (name, expected_index) in [
+            ("a", 0),
+            ("b", 0),
+            ("c", 1),
+            ("d", 2),
+            ("e", 0),
+            ("f", 1),
+            ("g", 0),
+            ("h", 1),
+            ("i", 0),
+            ("j", 1),
+            ("k", 0),
+            ("l", 1),
+        ] {
             let definition = use_def
                 .first_public_definition(
                     global_table.symbol_id_by_name(name).expect("symbol exists"),
                 )
                 .expect("Expected with item definition for {name}");
-            assert!(matches!(
-                definition.node(&db),
-                DefinitionKind::MatchPattern(_)
-            ));
+            if let DefinitionKind::MatchPattern(pattern) = definition.node(&db) {
+                assert_eq!(pattern.index(), expected_index);
+            } else {
+                panic!("Expected match pattern definition for {name}");
+            }
+        }
+    }
+
+    #[test]
+    fn nested_match_case() {
+        let TestCase { db, file } = test_case(
+            "
+match 1:
+    case first:
+        match 2:
+            case second:
+                pass
+",
+        );
+
+        let global_scope_id = global_scope(&db, file);
+        let global_table = symbol_table(&db, global_scope_id);
+
+        assert_eq!(names(&global_table), vec!["first", "second"]);
+
+        let use_def = use_def_map(&db, global_scope_id);
+        for (name, expected_index) in [("first", 0), ("second", 0)] {
+            let definition = use_def
+                .first_public_definition(
+                    global_table.symbol_id_by_name(name).expect("symbol exists"),
+                )
+                .expect("Expected with item definition for {name}");
+            if let DefinitionKind::MatchPattern(pattern) = definition.node(&db) {
+                assert_eq!(pattern.index(), expected_index);
+            } else {
+                panic!("Expected match pattern definition for {name}");
+            }
         }
     }
 

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -36,6 +36,8 @@ pub(super) struct SemanticIndexBuilder<'db> {
     scope_stack: Vec<FileScopeId>,
     /// The assignment we're currently visiting.
     current_assignment: Option<CurrentAssignment<'db>>,
+    /// The match case we're currently visiting.
+    current_match_case: Option<CurrentMatchCase<'db>>,
     /// Flow states at each `break` in the current loop.
     loop_break_states: Vec<FlowSnapshot>,
 
@@ -59,6 +61,7 @@ impl<'db> SemanticIndexBuilder<'db> {
             module: parsed,
             scope_stack: Vec::new(),
             current_assignment: None,
+            current_match_case: None,
             loop_break_states: vec![],
 
             scopes: IndexVec::new(),
@@ -805,7 +808,7 @@ where
         }
     }
 
-    fn visit_parameters(&mut self, parameters: &'ast ruff_python_ast::Parameters) {
+    fn visit_parameters(&mut self, parameters: &'ast ast::Parameters) {
         // Intentionally avoid walking default expressions, as we handle them in the enclosing
         // scope.
         for parameter in parameters.iter().map(ast::AnyParameterRef::as_parameter) {
@@ -813,54 +816,16 @@ where
         }
     }
 
-    fn visit_pattern(&mut self, pattern: &'ast ast::Pattern) {
-        // The definition visitor will recurse into the pattern so avoid walking it here.
-        let mut definition_visitor = MatchPatternDefinitionVisitor::new(self, pattern);
-        definition_visitor.visit_pattern(pattern);
-    }
-}
+    fn visit_match_case(&mut self, match_case: &'ast ast::MatchCase) {
+        debug_assert!(self.current_match_case.is_none());
+        self.current_match_case = Some(CurrentMatchCase::new(&match_case.pattern));
+        self.visit_pattern(&match_case.pattern);
+        self.current_match_case = None;
 
-/// A visitor that adds symbols and definitions for the identifiers in a match pattern.
-struct MatchPatternDefinitionVisitor<'a, 'db> {
-    /// The semantic index builder in which to add the symbols and definitions.
-    builder: &'a mut SemanticIndexBuilder<'db>,
-    /// The index of the current node in the pattern.
-    index: u32,
-    /// The pattern being visited. This pattern is the outermost pattern that is being visited
-    /// and is required to add the definitions.
-    pattern: &'a ast::Pattern,
+        if let Some(expr) = &match_case.guard {
+            self.visit_expr(expr);
         }
-
-impl<'a, 'db> MatchPatternDefinitionVisitor<'a, 'db> {
-    fn new(builder: &'a mut SemanticIndexBuilder<'db>, pattern: &'a ast::Pattern) -> Self {
-        Self {
-            index: 0,
-            builder,
-            pattern,
-        }
-    }
-
-    fn add_symbol_and_definition(&mut self, identifier: &ast::Identifier) {
-        let symbol = self
-            .builder
-            .add_or_update_symbol(identifier.id().clone(), SymbolFlags::IS_DEFINED);
-        self.builder.add_definition(
-            symbol,
-            MatchPatternDefinitionNodeRef {
-                pattern: self.pattern,
-                identifier,
-                index: self.index,
-            },
-        );
-    }
-}
-
-impl<'ast, 'db> Visitor<'ast> for MatchPatternDefinitionVisitor<'_, 'db>
-where
-    'ast: 'db,
-{
-    fn visit_expr(&mut self, expr: &'ast ast::Expr) {
-        self.builder.visit_expr(expr);
+        self.visit_body(&match_case.body);
     }
 
     fn visit_pattern(&mut self, pattern: &'ast ast::Pattern) {
@@ -869,7 +834,16 @@ where
             range: _,
         }) = pattern
         {
-            self.add_symbol_and_definition(name);
+            let symbol = self.add_or_update_symbol(name.id().clone(), SymbolFlags::IS_DEFINED);
+            let state = self.current_match_case.as_ref().unwrap();
+            self.add_definition(
+                symbol,
+                MatchPatternDefinitionNodeRef {
+                    pattern: state.pattern,
+                    identifier: name,
+                    index: state.index,
+                },
+            );
         }
 
         walk_pattern(self, pattern);
@@ -881,10 +855,19 @@ where
             rest: Some(name), ..
         }) = pattern
         {
-            self.add_symbol_and_definition(name);
+            let symbol = self.add_or_update_symbol(name.id().clone(), SymbolFlags::IS_DEFINED);
+            let state = self.current_match_case.as_ref().unwrap();
+            self.add_definition(
+                symbol,
+                MatchPatternDefinitionNodeRef {
+                    pattern: state.pattern,
+                    identifier: name,
+                    index: state.index,
+                },
+            );
         }
 
-        self.index += 1;
+        self.current_match_case.as_mut().unwrap().index += 1;
     }
 }
 
@@ -937,3 +920,27 @@ impl<'a> From<&'a ast::WithItem> for CurrentAssignment<'a> {
         Self::WithItem(value)
     }
 }
+
+struct CurrentMatchCase<'a> {
+    /// The pattern that's part of the current match case.
+    pattern: &'a ast::Pattern,
+
+    /// The index of the sub-pattern that's being currently visited within the pattern.
+    ///
+    /// For example:
+    /// ```py
+    /// match subject:
+    ///     case a as b: ...
+    ///     case [a, b]: ...
+    ///     case a | b: ...
+    /// ```
+    ///
+    /// In all of the above cases, the index would be 0 for `a` and 1 for `b`.
+    index: u32,
+}
+
+impl<'a> CurrentMatchCase<'a> {
+    fn new(pattern: &'a ast::Pattern) -> Self {
+        Self { pattern, index: 0 }
+    }
+}