[flake8-return] Ignore assignments to annotated variables in `unnec…

…essary-assign` (#10741)

## Summary

Closes #10732.

crates/ruff_linter/resources/test/fixtures/flake8_return/RET504.py

@@ -406,3 +406,18 @@ def foo():
         with contextlib.suppress(Exception):
             y = 2
         return y
+
+
+# See: https://github.com/astral-sh/ruff/issues/10732
+def func(a: dict[str, int]) -> list[dict[str, int]]:
+    services: list[dict[str, int]]
+    if "services" in a:
+        services = a["services"]
+        return services
+
+
+# See: https://github.com/astral-sh/ruff/issues/10732
+def func(a: dict[str, int]) -> list[dict[str, int]]:
+    if "services" in a:
+        services = a["services"]
+        return services

crates/ruff_linter/src/rules/flake8_return/rules/function.rs

@@ -567,6 +567,12 @@ fn unnecessary_assign(checker: &mut Checker, stack: &Stack) {
             continue;
         }
 
+        // Ignore variables that have an annotation defined elsewhere.
+        if stack.annotations.contains(assigned_id.as_str()) {
+            continue;
+        }
+
+        // Ignore `nonlocal` and `global` variables.
         if stack.non_locals.contains(assigned_id.as_str()) {
             continue;
         }

crates/ruff_linter/src/rules/flake8_return/snapshots/ruff_linter__rules__flake8_return__tests__RET504_RET504.py.snap

@@ -241,4 +241,19 @@ RET504.py:400:12: RET504 [*] Unnecessary assignment to `y` before `return` state
 402 401 | 
 403 402 | def foo():
 
+RET504.py:423:16: RET504 [*] Unnecessary assignment to `services` before `return` statement
+    |
+421 |     if "services" in a:
+422 |         services = a["services"]
+423 |         return services
+    |                ^^^^^^^^ RET504
+    |
+    = help: Remove unnecessary assignment
 
+ℹ Unsafe fix
+419 419 | # See: https://github.com/astral-sh/ruff/issues/10732
+420 420 | def func(a: dict[str, int]) -> list[dict[str, int]]:
+421 421 |     if "services" in a:
+422     |-        services = a["services"]
+423     |-        return services
+    422 |+        return a["services"]

crates/ruff_linter/src/rules/flake8_return/visitor.rs

@@ -13,6 +13,21 @@ pub(super) struct Stack<'data> {
     pub(super) elifs_elses: Vec<(&'data [Stmt], &'data ElifElseClause)>,
     /// The non-local variables in the current function.
     pub(super) non_locals: FxHashSet<&'data str>,
+    /// The annotated variables in the current function.
+    ///
+    /// For example, consider:
+    /// ```python
+    /// x: int
+    ///
+    /// if True:
+    ///    x = foo()
+    ///    return x
+    /// ```
+    ///
+    /// In this case, the annotation on `x` is used to cast the return value
+    /// of `foo()` to an `int`. Removing the `x = foo()` statement would
+    /// change the return type of the function.
+    pub(super) annotations: FxHashSet<&'data str>,
     /// Whether the current function is a generator.
     pub(super) is_generator: bool,
     /// The `assignment`-to-`return` statement pairs in the current function.
@@ -86,6 +101,14 @@ impl<'semantic, 'a> Visitor<'a> for ReturnVisitor<'semantic, 'a> {
                     .non_locals
                     .extend(names.iter().map(Identifier::as_str));
             }
+            Stmt::AnnAssign(ast::StmtAnnAssign { target, value, .. }) => {
+                // Ex) `x: int`
+                if value.is_none() {
+                    if let Expr::Name(name) = target.as_ref() {
+                        self.stack.annotations.insert(name.id.as_str());
+                    }
+                }
+            }
             Stmt::Return(stmt_return) => {
                 // If the `return` statement is preceded by an `assignment` statement, then the
                 // `assignment` statement may be redundant.
@@ -163,19 +186,6 @@ impl<'semantic, 'a> Visitor<'a> for ReturnVisitor<'semantic, 'a> {
     }
 }
 
-/// RET504
-/// If the last statement is a `return` statement, and the second-to-last statement is a
-/// `with` statement that suppresses an exception, then we should not analyze the `return`
-/// statement for unnecessary assignments. Otherwise we will suggest removing the assignment
-/// and the `with` statement, which would change the behavior of the code.
-///
-/// Example:
-/// ```python
-/// def foo(data):
-///    with suppress(JSONDecoderError):
-///       data = data.decode()
-///   return data
-
 /// Returns `true` if the [`With`] statement is known to have a conditional body. In other words:
 /// if the [`With`] statement's body may or may not run.
 ///