error: future cannot be sent between threads safely
  --> $DIR/issue-71137.rs:20:3
   |
LL | fn fake_spawn<F: Future + Send + 'static>(f: F) { }
   |                           ---- required by this bound in `fake_spawn`
...
LL |   fake_spawn(wrong_mutex());
   |   ^^^^^^^^^^ future returned by `wrong_mutex` is not `Send`
   |
   = help: within `impl Future`, the trait `Send` is not implemented for `MutexGuard<'_, i32>`
note: future is not `Send` as this value is used across an await
  --> $DIR/issue-71137.rs:12:5
   |
LL |     let mut guard = m.lock().unwrap();
   |         --------- has type `MutexGuard<'_, i32>` which is not `Send`
LL |     (async { "right"; }).await;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ await occurs here, with `mut guard` maybe used later
LL |     *guard += 1;
LL |   }
   |   - `mut guard` is later dropped here

error: aborting due to previous error