Add test of non-Send async fn default with return type

Author: dtolnay

tests/ui/send-not-implemented.rs

@@ -10,6 +10,13 @@ trait Test {
  let _guard = mutex.lock().unwrap();
  f().await;
  }
+
+ async fn test_ret(&self) -> bool {
+ let mutex = Mutex::new(());
+ let _guard = mutex.lock().unwrap();
+ f().await;
+ true
+ }
 }
 
 fn main() {}

tests/ui/send-not-implemented.stderr

@@ -20,3 +20,22 @@ note: future is not `Send` as this value is used across an await
 12 | }
  | - `_guard` is later dropped here
  = note: required for the cast to the object type `dyn Future<Output = ()> + Send`
+
+error: future cannot be sent between threads safely
+ --> $DIR/send-not-implemented.rs:14:33
+ |
+14 | async fn test_ret(&self) -> bool {
+ | ^^^^ future created by async block is not `Send`
+ |
+ = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, ()>`
+note: future is not `Send` as this value is used across an await
+ --> $DIR/send-not-implemented.rs:17:9
+ |
+14 | async fn test_ret(&self) -> bool {
+ | - `_guard` is later dropped here
+15 | let mutex = Mutex::new(());
+16 | let _guard = mutex.lock().unwrap();
+ | ------ has type `MutexGuard<'_, ()>` which is not `Send`
+17 | f().await;
+ | ^^^^^^^^^ await occurs here, with `_guard` maybe used later
+ = note: required for the cast to the object type `dyn Future<Output = bool> + Send`