[red-knot] Add symbols for for loop variables (#13075)

## Summary

This PR adds symbols introduced by `for` loops to red-knot:
- `x` in `for x in range(10): pass`
- `x` and `y` in `for x, y in d.items(): pass`
- `a`, `b`, `c` and `d` in `for [((a,), b), (c, d)] in foo: pass`

## Test Plan

Several tests added, and the assertion in the benchmarks has been
updated.

---------

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

crates/red_knot_python_semantic/src/semantic_index.rs

@@ -1095,4 +1095,56 @@ match subject:
             vec!["subject", "a", "b", "c", "d", "f", "e", "h", "g", "Foo", "i", "j", "k", "l"]
         );
     }
+
+    #[test]
+    fn for_loops_single_assignment() {
+        let TestCase { db, file } = test_case("for x in a: pass");
+        let scope = global_scope(&db, file);
+        let global_table = symbol_table(&db, scope);
+
+        assert_eq!(&names(&global_table), &["a", "x"]);
+
+        let use_def = use_def_map(&db, scope);
+        let definition = use_def
+            .first_public_definition(global_table.symbol_id_by_name("x").unwrap())
+            .unwrap();
+
+        assert!(matches!(definition.node(&db), DefinitionKind::For(_)));
+    }
+
+    #[test]
+    fn for_loops_simple_unpacking() {
+        let TestCase { db, file } = test_case("for (x, y) in a: pass");
+        let scope = global_scope(&db, file);
+        let global_table = symbol_table(&db, scope);
+
+        assert_eq!(&names(&global_table), &["a", "x", "y"]);
+
+        let use_def = use_def_map(&db, scope);
+        let x_definition = use_def
+            .first_public_definition(global_table.symbol_id_by_name("x").unwrap())
+            .unwrap();
+        let y_definition = use_def
+            .first_public_definition(global_table.symbol_id_by_name("y").unwrap())
+            .unwrap();
+
+        assert!(matches!(x_definition.node(&db), DefinitionKind::For(_)));
+        assert!(matches!(y_definition.node(&db), DefinitionKind::For(_)));
+    }
+
+    #[test]
+    fn for_loops_complex_unpacking() {
+        let TestCase { db, file } = test_case("for [((a,) b), (c, d)] in e: pass");
+        let scope = global_scope(&db, file);
+        let global_table = symbol_table(&db, scope);
+
+        assert_eq!(&names(&global_table), &["e", "a", "b", "c", "d"]);
+
+        let use_def = use_def_map(&db, scope);
+        let definition = use_def
+            .first_public_definition(global_table.symbol_id_by_name("a").unwrap())
+            .unwrap();
+
+        assert!(matches!(definition.node(&db), DefinitionKind::For(_)));
+    }
 }

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -15,7 +15,7 @@ use crate::semantic_index::ast_ids::node_key::ExpressionNodeKey;
 use crate::semantic_index::ast_ids::AstIdsBuilder;
 use crate::semantic_index::definition::{
     AssignmentDefinitionNodeRef, ComprehensionDefinitionNodeRef, Definition, DefinitionNodeKey,
-    DefinitionNodeRef, ImportFromDefinitionNodeRef,
+    DefinitionNodeRef, ForStmtDefinitionNodeRef, ImportFromDefinitionNodeRef,
 };
 use crate::semantic_index::expression::Expression;
 use crate::semantic_index::symbol::{
@@ -578,6 +578,27 @@ where
             ast::Stmt::Break(_) => {
                 self.loop_break_states.push(self.flow_snapshot());
             }
+
+            ast::Stmt::For(
+                for_stmt @ ast::StmtFor {
+                    range: _,
+                    is_async: _,
+                    target,
+                    iter,
+                    body,
+                    orelse,
+                },
+            ) => {
+                // TODO add control flow similar to `ast::Stmt::While` above
+                self.add_standalone_expression(iter);
+                self.visit_expr(iter);
+                debug_assert!(self.current_assignment.is_none());
+                self.current_assignment = Some(for_stmt.into());
+                self.visit_expr(target);
+                self.current_assignment = None;
+                self.visit_body(body);
+                self.visit_body(orelse);
+            }
             _ => {
                 walk_stmt(self, stmt);
             }
@@ -624,6 +645,15 @@ where
                         Some(CurrentAssignment::AugAssign(aug_assign)) => {
                             self.add_definition(symbol, aug_assign);
                         }
+                        Some(CurrentAssignment::For(node)) => {
+                            self.add_definition(
+                                symbol,
+                                ForStmtDefinitionNodeRef {
+                                    iterable: &node.iter,
+                                    target: name_node,
+                                },
+                            );
+                        }
                         Some(CurrentAssignment::Named(named)) => {
                             // TODO(dhruvmanila): If the current scope is a comprehension, then the
                             // named expression is implicitly nonlocal. This is yet to be
@@ -796,6 +826,7 @@ enum CurrentAssignment<'a> {
     Assign(&'a ast::StmtAssign),
     AnnAssign(&'a ast::StmtAnnAssign),
     AugAssign(&'a ast::StmtAugAssign),
+    For(&'a ast::StmtFor),
     Named(&'a ast::ExprNamed),
     Comprehension {
         node: &'a ast::Comprehension,
@@ -822,6 +853,12 @@ impl<'a> From<&'a ast::StmtAugAssign> for CurrentAssignment<'a> {
     }
 }
 
+impl<'a> From<&'a ast::StmtFor> for CurrentAssignment<'a> {
+    fn from(value: &'a ast::StmtFor) -> Self {
+        Self::For(value)
+    }
+}
+
 impl<'a> From<&'a ast::ExprNamed> for CurrentAssignment<'a> {
     fn from(value: &'a ast::ExprNamed) -> Self {
         Self::Named(value)

crates/red_knot_python_semantic/src/semantic_index/definition.rs

@@ -39,6 +39,7 @@ impl<'db> Definition<'db> {
 pub(crate) enum DefinitionNodeRef<'a> {
     Import(&'a ast::Alias),
     ImportFrom(ImportFromDefinitionNodeRef<'a>),
+    For(ForStmtDefinitionNodeRef<'a>),
     Function(&'a ast::StmtFunctionDef),
     Class(&'a ast::StmtClassDef),
     NamedExpression(&'a ast::ExprNamed),
@@ -92,6 +93,12 @@ impl<'a> From<ImportFromDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
     }
 }
 
+impl<'a> From<ForStmtDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
+    fn from(value: ForStmtDefinitionNodeRef<'a>) -> Self {
+        Self::For(value)
+    }
+}
+
 impl<'a> From<AssignmentDefinitionNodeRef<'a>> for DefinitionNodeRef<'a> {
     fn from(node_ref: AssignmentDefinitionNodeRef<'a>) -> Self {
         Self::Assignment(node_ref)
@@ -134,6 +141,12 @@ pub(crate) struct WithItemDefinitionNodeRef<'a> {
     pub(crate) target: &'a ast::ExprName,
 }
 
+#[derive(Copy, Clone, Debug)]
+pub(crate) struct ForStmtDefinitionNodeRef<'a> {
+    pub(crate) iterable: &'a ast::Expr,
+    pub(crate) target: &'a ast::ExprName,
+}
+
 #[derive(Copy, Clone, Debug)]
 pub(crate) struct ComprehensionDefinitionNodeRef<'a> {
     pub(crate) node: &'a ast::Comprehension,
@@ -174,6 +187,12 @@ impl DefinitionNodeRef<'_> {
             DefinitionNodeRef::AugmentedAssignment(augmented_assignment) => {
                 DefinitionKind::AugmentedAssignment(AstNodeRef::new(parsed, augmented_assignment))
             }
+            DefinitionNodeRef::For(ForStmtDefinitionNodeRef { iterable, target }) => {
+                DefinitionKind::For(ForStmtDefinitionKind {
+                    iterable: AstNodeRef::new(parsed.clone(), iterable),
+                    target: AstNodeRef::new(parsed, target),
+                })
+            }
             DefinitionNodeRef::Comprehension(ComprehensionDefinitionNodeRef { node, first }) => {
                 DefinitionKind::Comprehension(ComprehensionDefinitionKind {
                     node: AstNodeRef::new(parsed, node),
@@ -212,6 +231,10 @@ impl DefinitionNodeRef<'_> {
             }) => target.into(),
             Self::AnnotatedAssignment(node) => node.into(),
             Self::AugmentedAssignment(node) => node.into(),
+            Self::For(ForStmtDefinitionNodeRef {
+                iterable: _,
+                target,
+            }) => target.into(),
             Self::Comprehension(ComprehensionDefinitionNodeRef { node, first: _ }) => node.into(),
             Self::Parameter(node) => match node {
                 ast::AnyParameterRef::Variadic(parameter) => parameter.into(),
@@ -232,6 +255,7 @@ pub enum DefinitionKind {
     Assignment(AssignmentDefinitionKind),
     AnnotatedAssignment(AstNodeRef<ast::StmtAnnAssign>),
     AugmentedAssignment(AstNodeRef<ast::StmtAugAssign>),
+    For(ForStmtDefinitionKind),
     Comprehension(ComprehensionDefinitionKind),
     Parameter(AstNodeRef<ast::Parameter>),
     ParameterWithDefault(AstNodeRef<ast::ParameterWithDefault>),
@@ -302,6 +326,22 @@ impl WithItemDefinitionKind {
     }
 }
 
+#[derive(Clone, Debug)]
+pub struct ForStmtDefinitionKind {
+    iterable: AstNodeRef<ast::Expr>,
+    target: AstNodeRef<ast::ExprName>,
+}
+
+impl ForStmtDefinitionKind {
+    pub(crate) fn iterable(&self) -> &ast::Expr {
+        self.iterable.node()
+    }
+
+    pub(crate) fn target(&self) -> &ast::ExprName {
+        self.target.node()
+    }
+}
+
 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
 pub(crate) struct DefinitionNodeKey(NodeKey);
 
@@ -347,6 +387,12 @@ impl From<&ast::StmtAugAssign> for DefinitionNodeKey {
     }
 }
 
+impl From<&ast::StmtFor> for DefinitionNodeKey {
+    fn from(value: &ast::StmtFor) -> Self {
+        Self(NodeKey::from_node(value))
+    }
+}
+
 impl From<&ast::Comprehension> for DefinitionNodeKey {
     fn from(node: &ast::Comprehension) -> Self {
         Self(NodeKey::from_node(node))

crates/red_knot_python_semantic/src/types/infer.rs

@@ -138,7 +138,6 @@ pub(crate) struct TypeInference<'db> {
 }
 
 impl<'db> TypeInference<'db> {
-    #[allow(unused)]
     pub(crate) fn expression_ty(&self, expression: ScopedExpressionId) -> Type<'db> {
         self.expressions[&expression]
     }
@@ -317,6 +316,13 @@ impl<'db> TypeInferenceBuilder<'db> {
             DefinitionKind::AugmentedAssignment(augmented_assignment) => {
                 self.infer_augment_assignment_definition(augmented_assignment.node(), definition);
             }
+            DefinitionKind::For(for_statement_definition) => {
+                self.infer_for_statement_definition(
+                    for_statement_definition.target(),
+                    for_statement_definition.iterable(),
+                    definition,
+                );
+            }
             DefinitionKind::NamedExpression(named_expression) => {
                 self.infer_named_expression_definition(named_expression.node(), definition);
             }
@@ -865,11 +871,48 @@ impl<'db> TypeInferenceBuilder<'db> {
         } = for_statement;
 
         self.infer_expression(iter);
+        // TODO more complex assignment targets
+        if let ast::Expr::Name(name) = &**target {
+            self.infer_definition(name);
+        } else {
             self.infer_expression(target);
+        }
         self.infer_body(body);
         self.infer_body(orelse);
     }
 
+    fn infer_for_statement_definition(
+        &mut self,
+        target: &ast::ExprName,
+        iterable: &ast::Expr,
+        definition: Definition<'db>,
+    ) {
+        let expression = self.index.expression(iterable);
+        let result = infer_expression_types(self.db, expression);
+        self.extend(result);
+        let iterable_ty = self
+            .types
+            .expression_ty(iterable.scoped_ast_id(self.db, self.scope));
+
+        // TODO(Alex): only a valid iterable if the *type* of `iterable_ty` has an `__iter__`
+        // member (dunders are never looked up on an instance)
+        let _dunder_iter_ty = iterable_ty.member(self.db, &ast::name::Name::from("__iter__"));
+
+        // TODO(Alex):
+        // - infer the return type of the `__iter__` method, which gives us the iterator
+        // - lookup the `__next__` method on the iterator
+        // - infer the return type of the iterator's `__next__` method,
+        //   which gives us the type of the variable being bound here
+        //   (...or the type of the object being unpacked into multiple definitions, if it's something like
+        //   `for k, v in d.items(): ...`)
+        let loop_var_value_ty = Type::Unknown;
+
+        self.types
+            .expressions
+            .insert(target.scoped_ast_id(self.db, self.scope), loop_var_value_ty);
+        self.types.definitions.insert(definition, loop_var_value_ty);
+    }
+
     fn infer_while_statement(&mut self, while_statement: &ast::StmtWhile) {
         let ast::StmtWhile {
             range: _,

crates/ruff_benchmark/benches/red_knot.rs

@@ -40,23 +40,8 @@ static EXPECTED_DIAGNOSTICS: &[&str] = &[
     "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.",
 ];