Avoid calling report_forbidden_specialization for RPITITs

compiler/rustc_hir_analysis/src/check/check.rs

@@ -722,7 +722,14 @@ pub(super) fn check_specialization_validity<'tcx>(
     let result = opt_result.unwrap_or(Ok(()));
 
     if let Err(parent_impl) = result {
+        if !tcx.is_impl_trait_in_trait(impl_item) {
             report_forbidden_specialization(tcx, impl_item, parent_impl);
+        } else {
+            tcx.sess.delay_span_bug(
+                DUMMY_SP,
+                format!("parent item: {:?} not marked as default", parent_impl),
+            );
+        }
     }
 }
 
@@ -1485,7 +1492,9 @@ fn opaque_type_cycle_error(
                 }
 
                 for closure_def_id in visitor.closures {
-                    let Some(closure_local_did) = closure_def_id.as_local() else { continue; };
+                    let Some(closure_local_did) = closure_def_id.as_local() else {
+                        continue;
+                    };
                     let typeck_results = tcx.typeck(closure_local_did);
 
                     let mut label_match = |ty: Ty<'_>, span| {

tests/ui/async-await/in-trait/missing-feature-flag.current.stderr

@@ -0,0 +1,30 @@
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/missing-feature-flag.rs:14:1
+   |
+LL |     async fn foo(_: T) -> &'static str;
+   |     ----------------------------------- `foo` from trait
+...
+LL | impl<T> MyTrait<T> for MyStruct {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/missing-feature-flag.rs:18:5
+   |
+LL | impl<T> MyTrait<T> for MyStruct {}
+   | ------------------------------- parent `impl` is here
+...
+LL |     async fn foo(_: i32) -> &'static str {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot specialize default item `foo`
+   |
+   = note: to specialize, `foo` in the parent `impl` must be marked `default`
+
+error[E0308]: mismatched types
+  --> $DIR/missing-feature-flag.rs:18:42
+   |
+LL |     async fn foo(_: i32) -> &'static str {}
+   |                                          ^^ expected `&str`, found `()`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0046, E0308, E0520.
+For more information about an error, try `rustc --explain E0046`.

tests/ui/async-await/in-trait/missing-feature-flag.next.stderr

@@ -0,0 +1,30 @@
+error[E0046]: not all trait items implemented, missing: `foo`
+  --> $DIR/missing-feature-flag.rs:14:1
+   |
+LL |     async fn foo(_: T) -> &'static str;
+   |     ----------------------------------- `foo` from trait
+...
+LL | impl<T> MyTrait<T> for MyStruct {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `foo` in implementation
+
+error[E0520]: `foo` specializes an item from a parent `impl`, but that item is not marked `default`
+  --> $DIR/missing-feature-flag.rs:18:5
+   |
+LL | impl<T> MyTrait<T> for MyStruct {}
+   | ------------------------------- parent `impl` is here
+...
+LL |     async fn foo(_: i32) -> &'static str {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot specialize default item `foo`
+   |
+   = note: to specialize, `foo` in the parent `impl` must be marked `default`
+
+error[E0308]: mismatched types
+  --> $DIR/missing-feature-flag.rs:18:42
+   |
+LL |     async fn foo(_: i32) -> &'static str {}
+   |                                          ^^ expected `&str`, found `()`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0046, E0308, E0520.
+For more information about an error, try `rustc --explain E0046`.

tests/ui/async-await/in-trait/missing-feature-flag.rs

@@ -0,0 +1,23 @@
+// edition:2018
+// [next] compile-flags: -Zlower-impl-trait-in-trait-to-assoc-ty
+// revisions: current next
+
+#![feature(async_fn_in_trait)]
+#![feature(min_specialization)]
+
+struct MyStruct;
+
+trait MyTrait<T> {
+    async fn foo(_: T) -> &'static str;
+}
+
+impl<T> MyTrait<T> for MyStruct {}
+//~^ ERROR: not all trait items implemented, missing: `foo` [E0046]
+
+impl MyTrait<i32> for MyStruct {
+    async fn foo(_: i32) -> &'static str {}
+    //~^ ERROR: `foo` specializes an item from a parent `impl`, but that item is not marked `default` [E0520]
+    //~| ERROR: mismatched types [E0308]
+}
+
+fn main() {}