fallback for construct_generic_bound_failure

Author: bvanjoi

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2444,18 +2444,22 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 let suggestion =
                     if has_lifetimes { format!(" + {lt_name}") } else { format!(": {lt_name}") };
                 suggs.push((sp, suggestion))
-            } else {
-                let generics = self.tcx.hir().get_generics(suggestion_scope).unwrap();
+            } else if let Some(generics) = self.tcx.hir().get_generics(suggestion_scope) {
                 let pred = format!("{bound_kind}: {lt_name}");
-                let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred,);
+                let suggestion = format!("{} {}", generics.add_where_or_trailing_comma(), pred);
                 suggs.push((generics.tail_span_for_predicate_suggestion(), suggestion))
+            } else {
+                let consider = format!("{msg} `{bound_kind}: {sub}`...");
+                err.help(consider);
             }
 
-            err.multipart_suggestion_verbose(
-                format!("{msg}"),
-                suggs,
-                Applicability::MaybeIncorrect, // Issue #41966
-            );
+            if !suggs.is_empty() {
+                err.multipart_suggestion_verbose(
+                    format!("{msg}"),
+                    suggs,
+                    Applicability::MaybeIncorrect, // Issue #41966
+                );
+            }
         }
 
         err

tests/ui/impl-trait/in-trait/async-and-ret-ref.rs

@@ -0,0 +1,11 @@
+// edition:2021
+// https://github.com/rust-lang/rust/issues/117547
+
+trait T {}
+
+trait MyTrait {
+    async fn foo() -> &'static impl T;
+    //~^ ERROR the associated type `<Self as MyTrait>::{opaque#0}` may not live long enough
+}
+
+fn main() {}

tests/ui/impl-trait/in-trait/async-and-ret-ref.stderr

@@ -0,0 +1,14 @@
+error[E0310]: the associated type `<Self as MyTrait>::{opaque#0}` may not live long enough
+  --> $DIR/async-and-ret-ref.rs:7:5
+   |
+LL |     async fn foo() -> &'static impl T;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |     |
+   |     the associated type `<Self as MyTrait>::{opaque#0}` must be valid for the static lifetime...
+   |     ...so that the reference type `&'static impl T` does not outlive the data it points at
+   |
+   = help: consider adding an explicit lifetime bound `<Self as MyTrait>::{opaque#0}: 'static`...
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0310`.