No need to instantiate binder in confirm_async_closure_candidate

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -951,18 +951,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 });
 
                 // We must additionally check that the return type impls `Future`.
-
-                // FIXME(async_closures): Investigate this before stabilization.
-                // We instantiate this binder eagerly because the `confirm_future_candidate`
-                // method doesn't support higher-ranked futures, which the `AsyncFn`
-                // traits expressly allow the user to write. To fix this correctly,
-                // we'd need to instantiate trait bounds before we get to selection,
-                // like the new trait solver does.
                 let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
-                let placeholder_output_ty = self.infcx.enter_forall_and_leak_universe(sig.output());
                 nested.push(obligation.with(
                     tcx,
-                    ty::TraitRef::new(tcx, future_trait_def_id, [placeholder_output_ty]),
+                    sig.output().map_bound(|output_ty| {
+                        ty::TraitRef::new(tcx, future_trait_def_id, [output_ty])
+                    }),
                 ));
 
                 (trait_ref, Ty::from_closure_kind(tcx, ty::ClosureKind::Fn))