async/await: improve obligation errors

This commit improves obligation errors for async/await:

```
note: future does not implement `std::marker::Send` because this value is used across an
      await
  --> $DIR/issue-64130-non-send-future-diags.rs:15:5
   |
LL |     let g = x.lock().unwrap();
   |         - has type `std::sync::MutexGuard<'_, u32>`
LL |     baz().await;
   |     ^^^^^^^^^^^ await occurs here, with `g` maybe used later
LL | }
   | - `g` is later dropped here
```

Signed-off-by: David Wood <david@davidtw.co>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/libstd/future.rs b/src/libstd/future.rs
index 0406549ff07..c65f71fb1a4 100644
--- a/src/libstd/future.rs
+++ b/src/libstd/future.rs
@@ -26,6 +26,7 @@ pub fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T:
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "gen_future")]
 struct GenFuture<T: Generator<Yield = ()>>(T);
 
 // We rely on the fact that async/await futures are immovable in order to create