Rollup merge of #106200 - compiler-errors:suggest-impl-trait, r=estebank

Suggest `impl Fn*` and `impl Future` in `-> _` return suggestions

Follow-up to #106172, only the last commit is relevant. Can rebase once that PR is landed for easier review.

Suggests `impl Future` and `impl Fn{,Mut,Once}` in `-> _` return suggestions.

r? `@estebank`

Author: matthiaskrgr

core/src/future/future.rs

@@ -37,6 +37,7 @@ use crate::task::{Context, Poll};
 pub trait Future {
     /// The type of value produced on completion.
     #[stable(feature = "futures_api", since = "1.36.0")]
+    #[rustc_diagnostic_item = "FutureOutput"]
     type Output;
 
     /// Attempt to resolve the future to a final value, registering