[pyflakes] Detect assignments that shadow definitions (F811) (#11961)

## Summary
This PR updates `F811` rule to include assignment as possible shadowed
binding. This will fix issue: #11828 .

## Test Plan

Add a test file, F811_30.py, which includes a redefinition after an
assignment and a verified snapshot file.

crates/ruff_linter/resources/test/fixtures/pyflakes/F811_30.py

@@ -0,0 +1,37 @@
+"""Regression test for: https://github.com/astral-sh/ruff/issues/11828"""
+
+
+class A:
+    """A."""
+
+    def foo(self) -> None:
+        """Foo."""
+
+    bar = foo
+
+    def bar(self) -> None:
+        """Bar."""
+
+
+class B:
+    """B."""
+    def baz(self) -> None:
+        """Baz."""
+
+    baz = 1
+
+
+class C:
+    """C."""
+    def foo(self) -> None:
+        """Foo."""
+
+    bar = (foo := 1)
+
+
+class D:
+    """D."""
+    foo = 1
+    foo = 2
+    bar = (foo := 3)
+    bar = (foo := 4)

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

@@ -125,6 +125,7 @@ mod tests {
     #[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::RedefinedWhileUnused, Path::new("F811_30.py"))]
     #[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_30.py.snap

@@ -0,0 +1,30 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F811_30.py:12:9: F811 Redefinition of unused `bar` from line 10
+   |
+10 |     bar = foo
+11 | 
+12 |     def bar(self) -> None:
+   |         ^^^ F811
+13 |         """Bar."""
+   |
+   = help: Remove definition: `bar`
+
+F811_30.py:21:5: F811 Redefinition of unused `baz` from line 18
+   |
+19 |         """Baz."""
+20 | 
+21 |     baz = 1
+   |     ^^^ F811
+   |
+   = help: Remove definition: `baz`
+
+F811_30.py:29:12: F811 Redefinition of unused `foo` from line 26
+   |
+27 |         """Foo."""
+28 | 
+29 |     bar = (foo := 1)
+   |            ^^^ F811
+   |
+   = help: Remove definition: `foo`

crates/ruff_python_semantic/src/binding.rs

@@ -177,16 +177,31 @@ impl<'a> Binding<'a> {
             | BindingKind::Builtin => {
                 return false;
             }
+            // Assignment-assignment bindings are not considered redefinitions, as in:
+            // ```python
+            // x = 1
+            // x = 2
+            // ```
+            BindingKind::Assignment | BindingKind::NamedExprAssignment => {
+                if matches!(
+                    existing.kind,
+                    BindingKind::Assignment | BindingKind::NamedExprAssignment
+                ) {
+                    return false;
+                }
+            }
             _ => {}
         }
-        // Otherwise, the shadowed binding must be a class definition, function definition, or
+        // Otherwise, the shadowed binding must be a class definition, function definition,
-        // import to be considered a redefinition.
+        // import, or assignment to be considered a redefinition.
         matches!(
             existing.kind,
             BindingKind::ClassDefinition(_)
                 | BindingKind::FunctionDefinition(_)
                 | BindingKind::Import(_)
                 | BindingKind::FromImport(_)
+                | BindingKind::Assignment
+                | BindingKind::NamedExprAssignment
         )
     }