add test for correct await span

src/test/ui/async-await/issue-71137.rs

@@ -0,0 +1,21 @@
+// edition:2018
+
+use std::future::Future;
+use std::sync::Mutex;
+
+fn fake_spawn<F: Future + Send + 'static>(f: F) { }
+
+async fn wrong_mutex() {
+  let m = Mutex::new(1);
+  {
+    let mut guard = m.lock().unwrap();
+    (async { "right"; }).await;
+    *guard += 1;
+  }
+
+  (async { "wrong"; }).await;
+}
+
+fn main() {
+  fake_spawn(wrong_mutex()); //~ Error future cannot be sent between threads safely
+}

src/test/ui/async-await/issue-71137.stderr

@@ -0,0 +1,23 @@
+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 std::future::Future`, the trait `std::marker::Send` is not implemented for `std::sync::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 `std::sync::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
+