Don't treat annotations as redefinitions in .pyi files (#10512)

## Summary

In https://github.com/astral-sh/ruff/pull/10341, we fixed some false
positives in `.pyi` files, but introduced others. This PR effectively
reverts the change in #10341 and fixes it in a slightly different way.
Instead of changing the _bindings_ we generate in the semantic model in
`.pyi` files, we instead change how we _resolve_ them.

Closes https://github.com/astral-sh/ruff/issues/10509.

crates/ruff_linter/resources/test/fixtures/pyflakes/F811_29.pyi

@@ -0,0 +1,8 @@
+"""Regression test for: https://github.com/astral-sh/ruff/issues/10509"""
+
+from foo import Bar as Bar
+
+class Eggs:
+    Bar: int  # OK
+
+Bar = 1  # F811

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -1836,7 +1836,7 @@ impl<'a> Checker<'a> {
         if matches!(
             parent,
             Stmt::AnnAssign(ast::StmtAnnAssign { value: None, .. })
-        ) && !(self.semantic.in_annotation() || self.source_type.is_stub())
+        ) && !self.semantic.in_annotation()
         {
             self.add_binding(id, expr.range(), BindingKind::Annotation, flags);
             return;

crates/ruff_linter/src/rules/pyflakes/mod.rs

@@ -125,6 +125,7 @@ mod tests {
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_26.py"))]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_27.py"))]
     #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_28.py"))]
+    #[test_case(Rule::RedefinedWhileUnused, Path::new("F811_29.pyi"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_0.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_1.py"))]
     #[test_case(Rule::UndefinedName, Path::new("F821_2.py"))]

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F811_F811_29.pyi.snap

@@ -0,0 +1,11 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F811_29.pyi:8:1: F811 Redefinition of unused `Bar` from line 3
+  |
+6 |     Bar: int  # OK
+7 | 
+8 | Bar = 1  # F811
+  | ^^^ F811
+  |
+  = help: Remove definition: `Bar`

crates/ruff_python_semantic/src/model.rs

@@ -397,7 +397,10 @@ impl<'a> SemanticModel<'a> {
                     //
                     // The `name` in `print(name)` should be treated as unresolved, but the `name` in
                     // `name: str` should be treated as used.
-                    BindingKind::Annotation => continue,
+                    //
+                    // Stub files are an exception. In a stub file, it _is_ considered valid to
+                    // resolve to a type annotation.
+                    BindingKind::Annotation if !self.in_stub_file() => continue,
 
                     // If it's a deletion, don't treat it as resolved, since the name is now
                     // unbound. For example, given:
@@ -1570,6 +1573,11 @@ impl<'a> SemanticModel<'a> {
             .intersects(SemanticModelFlags::FUTURE_ANNOTATIONS)
     }
 
+    /// Return `true` if the model is in a stub file (i.e., a file with a `.pyi` extension).
+    pub const fn in_stub_file(&self) -> bool {
+        self.flags.intersects(SemanticModelFlags::STUB_FILE)
+    }
+
     /// Return `true` if the model is in a named expression assignment (e.g., `x := 1`).
     pub const fn in_named_expression_assignment(&self) -> bool {
         self.flags
@@ -1875,6 +1883,9 @@ bitflags! {
         /// ```
         const FUTURE_ANNOTATIONS = 1 << 15;
 
+        /// The model is in a Python stub file (i.e., a `.pyi` file).
+        const STUB_FILE = 1 << 16;
+
         /// The model has traversed past the module docstring.
         ///
         /// For example, the model could be visiting `x` in:
@@ -1883,7 +1894,7 @@ bitflags! {
         ///
         /// x: int = 1
         /// ```
-        const MODULE_DOCSTRING_BOUNDARY = 1 << 16;
+        const MODULE_DOCSTRING_BOUNDARY = 1 << 17;
 
         /// The model is in a type parameter definition.
         ///
@@ -1893,7 +1904,7 @@ bitflags! {
         ///
         /// Record = TypeVar("Record")
         ///
-        const TYPE_PARAM_DEFINITION = 1 << 17;
+        const TYPE_PARAM_DEFINITION = 1 << 18;
 
         /// The model is in a named expression assignment.
         ///
@@ -1901,7 +1912,7 @@ bitflags! {
         /// ```python
         /// if (x := 1): ...
         /// ```
-        const NAMED_EXPRESSION_ASSIGNMENT = 1 << 18;
+        const NAMED_EXPRESSION_ASSIGNMENT = 1 << 19;
 
         /// The model is in a comprehension variable assignment.
         ///
@@ -1909,7 +1920,7 @@ bitflags! {
         /// ```python
         /// [_ for x in range(10)]
         /// ```
-        const COMPREHENSION_ASSIGNMENT = 1 << 19;
+        const COMPREHENSION_ASSIGNMENT = 1 << 20;
 
         /// The model is in a module / class / function docstring.
         ///
@@ -1928,7 +1939,7 @@ bitflags! {
         ///     """Function docstring."""
         ///     pass
         /// ```
-        const DOCSTRING = 1 << 20;
+        const DOCSTRING = 1 << 21;
 
         /// The context is in any type annotation.
         const ANNOTATION = Self::TYPING_ONLY_ANNOTATION.bits() | Self::RUNTIME_EVALUATED_ANNOTATION.bits() | Self::RUNTIME_REQUIRED_ANNOTATION.bits();
@@ -1953,6 +1964,7 @@ impl SemanticModelFlags {
     pub fn new(path: &Path) -> Self {
         let mut flags = Self::default();
         if is_python_stub_file(path) {
+            flags |= Self::STUB_FILE;
             flags |= Self::FUTURE_ANNOTATIONS;
         }
         flags