Tweak feature error, add test

src/test/ui/async-await/async-trait-fn.stderr

@@ -62,7 +62,7 @@ LL |     async fn bar(&self) {}
 LL | pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
    |                                           ------------------------------- the found opaque type
    |
-   = note: expected associated type `<Self as T>::bar::{opaque#0}`
+   = note: expected associated type `<Self as T>::bar::{opaque#0}<'_>`
                   found opaque type `impl Future<Output = ()>`
 
 error[E0308]: mismatched types

src/test/ui/feature-gates/feature-gate-return_position_impl_trait_in_trait.stderr

@@ -3,6 +3,9 @@ error[E0562]: `impl Trait` only allowed in function and inherent method return t
    |
 LL |     fn bar() -> impl Sized;
    |                 ^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/in-trait/reveal.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+#![feature(return_position_impl_trait_in_trait)]
+#![allow(incomplete_features)]
+
+trait Foo {
+    fn f() -> Box<impl Sized>;
+}
+
+impl Foo for () {
+    fn f() -> Box<String> {
+        Box::new(String::new())
+    }
+}
+
+fn main() {
+    let x: Box<String> = <() as Foo>::f();
+}

src/test/ui/impl-trait/where-allowed.stderr

@@ -162,12 +162,18 @@ error[E0562]: `impl Trait` only allowed in function and inherent method return t
    |
 LL |     fn in_return() -> impl Debug;
    |                       ^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `impl` method return
   --> $DIR/where-allowed.rs:125:34
    |
 LL |     fn in_trait_impl_return() -> impl Debug { () }
    |                                  ^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = help: add `#![feature(return_position_impl_trait_in_trait)]` to the crate attributes to enable
 
 error[E0562]: `impl Trait` only allowed in function and inherent method return types, not in `extern fn` param
   --> $DIR/where-allowed.rs:138:33