add suggestion to use the async_recursion crate

compiler/rustc_typeck/src/check/check.rs

@@ -1446,6 +1446,9 @@ fn async_opaque_type_cycle_error(tcx: TyCtxt<'tcx>, span: Span) {
     struct_span_err!(tcx.sess, span, E0733, "recursion in an `async fn` requires boxing")
         .span_label(span, "recursive `async fn`")
         .note("a recursive `async fn` must be rewritten to return a boxed `dyn Future`")
+        .note(
+            "consider using the `async_recursion` crate: https://crates.io/crates/async_recursion",
+        )
         .emit();
 }
 

src/test/ui/async-await/mutually-recursive-async-impl-trait-type.stderr

@@ -5,6 +5,7 @@ LL | async fn rec_1() {
    |                  ^ recursive `async fn`
    |
    = note: a recursive `async fn` must be rewritten to return a boxed `dyn Future`
+   = note: consider using the `async_recursion` crate: https://crates.io/crates/async_recursion
 
 error[E0733]: recursion in an `async fn` requires boxing
   --> $DIR/mutually-recursive-async-impl-trait-type.rs:9:18
@@ -13,6 +14,7 @@ LL | async fn rec_2() {
    |                  ^ recursive `async fn`
    |
    = note: a recursive `async fn` must be rewritten to return a boxed `dyn Future`
+   = note: consider using the `async_recursion` crate: https://crates.io/crates/async_recursion
 
 error: aborting due to 2 previous errors
 

src/test/ui/async-await/recursive-async-impl-trait-type.stderr

@@ -5,6 +5,7 @@ LL | async fn recursive_async_function() -> () {
    |                                        ^^ recursive `async fn`
    |
    = note: a recursive `async fn` must be rewritten to return a boxed `dyn Future`
+   = note: consider using the `async_recursion` crate: https://crates.io/crates/async_recursion
 
 error: aborting due to previous error