Make get_impl_future_output_ty work with AFIT

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

@@ -341,7 +341,15 @@ pub fn unexpected_hidden_region_diagnostic<'tcx>(
 
 impl<'tcx> InferCtxt<'tcx> {
     pub fn get_impl_future_output_ty(&self, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
-        let ty::Opaque(def_id, substs) = *ty.kind() else { return None; };
+        let (def_id, substs) = match *ty.kind() {
+            ty::Opaque(def_id, substs) => (def_id, substs),
+            ty::Projection(data)
+                if self.tcx.def_kind(data.item_def_id) == DefKind::ImplTraitPlaceholder =>
+            {
+                (data.item_def_id, data.substs)
+            }
+            _ => return None,
+        };
 
         let future_trait = self.tcx.require_lang_item(LangItem::Future, None);
         let item_def_id = self.tcx.associated_item_def_ids(future_trait)[0];

src/test/ui/async-await/in-trait/return-type-suggestion.rs

@@ -0,0 +1,14 @@
+// edition: 2021
+
+#![feature(async_fn_in_trait)]
+//~^ WARN the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
+
+trait A {
+    async fn e() {
+        Ok(())
+        //~^ ERROR mismatched types
+        //~| HELP consider using a semicolon here
+    }
+}
+
+fn main() {}

src/test/ui/async-await/in-trait/return-type-suggestion.stderr

@@ -0,0 +1,23 @@
+warning: the feature `async_fn_in_trait` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/return-type-suggestion.rs:3:12
+   |
+LL | #![feature(async_fn_in_trait)]
+   |            ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #91611 <https://github.com/rust-lang/rust/issues/91611> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error[E0308]: mismatched types
+  --> $DIR/return-type-suggestion.rs:8:9
+   |
+LL |         Ok(())
+   |         ^^^^^^- help: consider using a semicolon here: `;`
+   |         |
+   |         expected `()`, found enum `Result`
+   |
+   = note: expected unit type `()`
+                   found enum `Result<(), _>`
+
+error: aborting due to previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0308`.