[ruff] Ignore stub functions in unused-async (RUF029) (#11026)

## Summary

We should ignore methods that appear to be stubs, e.g.:

```python
async def foo() -> int: ...
```

Closes #11018.

crates/ruff_linter/resources/test/fixtures/ruff/RUF029.py

@@ -22,7 +22,7 @@ async def pass_3():  # OK: uses an async loop
 
 
 class Foo:
-    async def pass_4():  # OK: method of a class
+    async def pass_4(self):  # OK: method of a class
         pass
 
 
@@ -31,6 +31,10 @@ async def pass_5():  # OK: uses an await
         await bla
 
 
+async def pass_6():  # OK: just a stub
+    ...
+
+
 async def fail_1a():  # RUF029
     time.sleep(1)
 
@@ -58,7 +62,7 @@ async def foo():
 
 async def fail_4b():  # RUF029: the /outer/ function does not await
     class Foo:
-        async def foo():
+        async def foo(self):
             await bla
 
 

crates/ruff_linter/src/rules/ruff/rules/unused_async.rs

@@ -3,6 +3,7 @@ use ruff_macros::{derive_message_formats, violation};
 use ruff_python_ast::identifier::Identifier;
 use ruff_python_ast::visitor::preorder;
 use ruff_python_ast::{self as ast, AnyNodeRef, Expr, Stmt};
+use ruff_python_semantic::analyze::function_type::is_stub;
 
 use crate::checkers::ast::Checker;
 
@@ -160,6 +161,11 @@ pub(crate) fn unused_async(
         return;
     }
 
+    // Ignore stubs (e.g., `...`).
+    if is_stub(function_def, checker.semantic()) {
+        return;
+    }
+
     let found_await_or_async = {
         let mut visitor = AsyncExprVisitor::default();
         preorder::walk_body(&mut visitor, body);

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF029_RUF029.py.snap

@@ -1,56 +1,48 @@
 ---
 source: crates/ruff_linter/src/rules/ruff/mod.rs
 ---
-RUF029.py:34:11: RUF029 Function `fail_1a` is declared `async`, but doesn't `await` or use `async` features.
+RUF029.py:38:11: RUF029 Function `fail_1a` is declared `async`, but doesn't `await` or use `async` features.
    |
-34 | async def fail_1a():  # RUF029
+38 | async def fail_1a():  # RUF029
    |           ^^^^^^^ RUF029
-35 |     time.sleep(1)
+39 |     time.sleep(1)
    |
 
-RUF029.py:38:11: RUF029 Function `fail_1b` is declared `async`, but doesn't `await` or use `async` features.
+RUF029.py:42:11: RUF029 Function `fail_1b` is declared `async`, but doesn't `await` or use `async` features.
    |
-38 | async def fail_1b():  # RUF029: yield does not require async
+42 | async def fail_1b():  # RUF029: yield does not require async
    |           ^^^^^^^ RUF029
-39 |     yield "hello"
+43 |     yield "hello"
    |
 
-RUF029.py:42:11: RUF029 Function `fail_2` is declared `async`, but doesn't `await` or use `async` features.
+RUF029.py:46:11: RUF029 Function `fail_2` is declared `async`, but doesn't `await` or use `async` features.
    |
-42 | async def fail_2():  # RUF029
+46 | async def fail_2():  # RUF029
    |           ^^^^^^ RUF029
-43 |     with None as i:
-44 |         pass
+47 |     with None as i:
+48 |         pass
    |
 
-RUF029.py:47:11: RUF029 Function `fail_3` is declared `async`, but doesn't `await` or use `async` features.
+RUF029.py:51:11: RUF029 Function `fail_3` is declared `async`, but doesn't `await` or use `async` features.
    |
-47 | async def fail_3():  # RUF029
+51 | async def fail_3():  # RUF029
    |           ^^^^^^ RUF029
-48 |     for i in []:
-49 |         pass
+52 |     for i in []:
+53 |         pass
    |
 
-RUF029.py:54:11: RUF029 Function `fail_4a` is declared `async`, but doesn't `await` or use `async` features.
+RUF029.py:58:11: RUF029 Function `fail_4a` is declared `async`, but doesn't `await` or use `async` features.
    |
-54 | async def fail_4a():  # RUF029: the /outer/ function does not await
+58 | async def fail_4a():  # RUF029: the /outer/ function does not await
    |           ^^^^^^^ RUF029
-55 |     async def foo():
-56 |         await bla
+59 |     async def foo():
+60 |         await bla
    |
 
-RUF029.py:59:11: RUF029 Function `fail_4b` is declared `async`, but doesn't `await` or use `async` features.
+RUF029.py:63:11: RUF029 Function `fail_4b` is declared `async`, but doesn't `await` or use `async` features.
    |
-59 | async def fail_4b():  # RUF029: the /outer/ function does not await
+63 | async def fail_4b():  # RUF029: the /outer/ function does not await
    |           ^^^^^^^ RUF029
-60 |     class Foo:
-61 |         async def foo():
-   |
-
-RUF029.py:66:15: RUF029 Function `fail_4c` is declared `async`, but doesn't `await` or use `async` features.
-   |
-65 | def foo():
-66 |     async def fail_4c():  # RUF029: the /inner/ function does not await
-   |               ^^^^^^^ RUF029
-67 |         pass
+64 |     class Foo:
+65 |         async def foo(self):
    |