Add test for dyn Trait in async fn return type

Author: estebank

tests/ui/async-await/async-fn/dyn-in-return-type.rs

@@ -0,0 +1,10 @@
+//@ edition:2024
+
+async fn f() -> dyn core::fmt::Debug {
+//~^ ERROR return type cannot be a trait object without pointer indirection
+//~| HELP consider returning an `impl Trait` instead of a `dyn Trait`
+//~| HELP alternatively, box the return type, and wrap all of the returned values in `Box::new`
+    loop {}
+}
+
+fn main() {}

tests/ui/async-await/async-fn/dyn-in-return-type.stderr

@@ -0,0 +1,25 @@
+error[E0746]: return type cannot be a trait object without pointer indirection
+  --> $DIR/dyn-in-return-type.rs:3:38
+   |
+LL |   async fn f() -> dyn core::fmt::Debug {
+   |  ______________________________________^
+...  |
+LL | | }
+   | |_^ doesn't have a size known at compile-time
+   |
+help: consider returning an `impl Trait` instead of a `dyn Trait`
+   |
+LL | async fn f() -> dyn core::fmt::Debug impl {
+   |                                      ++++
+help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL ~ async fn f() -> dyn core::fmt::Debug Box<dyn {
+LL |
+...
+LL |     loop {}
+LL ~ }>
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0746`.