[red-knot] treat annotated assignments without RHS in stubs as bindings (#16409)

crates/red_knot_project/tests/check.rs

@@ -283,4 +283,9 @@ const KNOWN_FAILURES: &[(&str, bool, bool)] = &[
     // related to circular references in f-string annotations (invalid syntax)
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_15.py", true, true),
     ("crates/ruff_linter/resources/test/fixtures/pyflakes/F821_14.py", false, true),
+    // related to circular references in stub type annotations (salsa cycle panic):
+    ("crates/ruff_linter/resources/test/fixtures/pycodestyle/E501_4.py", false, true),
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F401_0.py", false, true),
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F401_12.py", false, true),
+    ("crates/ruff_linter/resources/test/fixtures/pyflakes/F401_14.py", false, true),
 ];

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

@@ -15,3 +15,30 @@ class Bar(Foo[Bar]): ...
 reveal_type(Bar)  # revealed: Literal[Bar]
 reveal_type(Bar.__mro__)  # revealed: tuple[Literal[Bar], Unknown, Literal[object]]
 ```
+
+## Access to attributes declarated in stubs
+
+Unlike regular Python modules, stub files often omit the right-hand side in declarations, including
+in class scope. However, from the perspective of the type checker, we have to treat them as bindings
+too. That is, `symbol: type` is the same as `symbol: type = ...`.
+
+One implication of this is that we'll always treat symbols in class scope as safe to be accessed
+from the class object itself. We'll never infer a "pure instance attribute" from a stub.
+
+`b.pyi`:
+
+```pyi
+from typing import ClassVar
+
+class C:
+    class_or_instance_var: int
+```
+
+```py
+from typing import ClassVar, Literal
+
+from b import C
+
+# No error here, since we treat `class_or_instance_var` as bound on the class.
+reveal_type(C.class_or_instance_var)  # revealed: int
+```

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

@@ -0,0 +1,17 @@
+# Declarations in stubs
+
+Unlike regular Python modules, stub files often declare module-global variables without initializing
+them. If these symbols are then used in the same stub, applying regular logic would lead to an
+undefined variable access error.
+
+However, from the perspective of the type checker, we should treat something like `symbol: type` the
+same as `symbol: type = ...`. In other words, assume these are bindings too.
+
+```pyi
+from typing import Literal
+
+CONSTANT: Literal[42]
+
+# No error here, even though the variable is not initialized.
+uses_constant: int = CONSTANT
+```

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -353,7 +353,7 @@ impl<'db> SemanticIndexBuilder<'db> {
         #[allow(unsafe_code)]
         // SAFETY: `definition_node` is guaranteed to be a child of `self.module`
         let kind = unsafe { definition_node.into_owned(self.module.clone()) };
-        let category = kind.category();
+        let category = kind.category(self.file.is_stub(self.db.upcast()));
         let is_reexported = kind.is_reexported();
         let definition = Definition::new(
             self.db,

crates/red_knot_python_semantic/src/semantic_index/definition.rs

@@ -515,7 +515,7 @@ impl DefinitionKind<'_> {
         }
     }
 
-    pub(crate) fn category(&self) -> DefinitionCategory {
+    pub(crate) fn category(&self, in_stub: bool) -> DefinitionCategory {
         match self {
             // functions, classes, and imports always bind, and we consider them declarations
             DefinitionKind::Function(_)
@@ -543,9 +543,10 @@ impl DefinitionKind<'_> {
                     DefinitionCategory::Binding
                 }
             }
-            // annotated assignment is always a declaration, only a binding if there is a RHS
+            // Annotated assignment is always a declaration. It is also a binding if there is a RHS
+            // or if we are in a stub file. Unfortunately, it is common for stubs to omit even an `...` value placeholder.
             DefinitionKind::AnnotatedAssignment(ann_assign) => {
-                if ann_assign.value.is_some() {
+                if in_stub || ann_assign.value.is_some() {
                     DefinitionCategory::DeclarationAndBinding
                 } else {
                     DefinitionCategory::Declaration

crates/red_knot_python_semantic/src/types/infer.rs

@@ -119,7 +119,7 @@ fn infer_definition_types_cycle_recovery<'db>(
 ) -> TypeInference<'db> {
     tracing::trace!("infer_definition_types_cycle_recovery");
     let mut inference = TypeInference::empty(input.scope(db));
-    let category = input.kind(db).category();
+    let category = input.kind(db).category(input.file(db).is_stub(db.upcast()));
     if category.is_declaration() {
         inference
             .declarations
@@ -903,7 +903,10 @@ impl<'db> TypeInferenceBuilder<'db> {
     }
 
     fn add_binding(&mut self, node: AnyNodeRef, binding: Definition<'db>, ty: Type<'db>) {
-        debug_assert!(binding.kind(self.db()).category().is_binding());
+        debug_assert!(binding
+            .kind(self.db())
+            .category(self.context.in_stub())
+            .is_binding());
         let use_def = self.index.use_def_map(binding.file_scope(self.db()));
         let declarations = use_def.declarations_at_binding(binding);
         let mut bound_ty = ty;
@@ -938,7 +941,10 @@ impl<'db> TypeInferenceBuilder<'db> {
         declaration: Definition<'db>,
         ty: TypeAndQualifiers<'db>,
     ) {
-        debug_assert!(declaration.kind(self.db()).category().is_declaration());
+        debug_assert!(declaration
+            .kind(self.db())
+            .category(self.context.in_stub())
+            .is_declaration());
         let use_def = self.index.use_def_map(declaration.file_scope(self.db()));
         let prior_bindings = use_def.bindings_at_declaration(declaration);
         // unbound_ty is Never because for this check we don't care about unbound
@@ -968,8 +974,14 @@ impl<'db> TypeInferenceBuilder<'db> {
         definition: Definition<'db>,
         declared_and_inferred_ty: &DeclaredAndInferredType<'db>,
     ) {
-        debug_assert!(definition.kind(self.db()).category().is_binding());
-        debug_assert!(definition.kind(self.db()).category().is_declaration());
+        debug_assert!(definition
+            .kind(self.db())
+            .category(self.context.in_stub())
+            .is_binding());
+        debug_assert!(definition
+            .kind(self.db())
+            .category(self.context.in_stub())
+            .is_declaration());
 
         let (declared_ty, inferred_ty) = match *declared_and_inferred_ty {
             DeclaredAndInferredType::AreTheSame(ty) => (ty.into(), ty),
@@ -2226,7 +2238,15 @@ impl<'db> TypeInferenceBuilder<'db> {
                 },
             );
         } else {
-            self.add_declaration(assignment.into(), definition, declared_ty);
+            if self.in_stub() {
+                self.add_declaration_with_binding(
+                    assignment.into(),
+                    definition,
+                    &DeclaredAndInferredType::AreTheSame(declared_ty.inner_type()),
+                );
+            } else {
+                self.add_declaration(assignment.into(), definition, declared_ty);
+            }
         }
 
         self.infer_expression(target);

crates/ruff_benchmark/benches/red_knot.rs

@@ -96,13 +96,6 @@ static EXPECTED_DIAGNOSTICS: &[KeyDiagnosticFields] = &[
         Cow::Borrowed("Unused blanket `type: ignore` directive"),
         Severity::Warning,
     ),
-    (
-        DiagnosticId::lint("invalid-attribute-access"),
-        Some("/src/tomllib/__init__.py"),
-        Some(270..296),
-        Cow::Borrowed("Cannot assign to instance attribute `__module__` from the class object `Literal[TOMLDecodeError]`"),
-        Severity::Error,
-    ),
 ];
 
 fn tomllib_path(file: &TestFile) -> SystemPathBuf {