Make sure we check the future type is Sized in AsyncFn*

Author: compiler-errors

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

@@ -907,14 +907,21 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     ])
                 });
 
-                // We must additionally check that the return type impls `Future`.
+                // We must additionally check that the return type impls `Future + Sized`.
                 let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
                 nested.push(obligation.with(
                     tcx,
                     sig.output().map_bound(|output_ty| {
                         ty::TraitRef::new(tcx, future_trait_def_id, [output_ty])
                     }),
                 ));
+                let sized_trait_def_id = tcx.require_lang_item(LangItem::Sized, None);
+                nested.push(obligation.with(
+                    tcx,
+                    sig.output().map_bound(|output_ty| {
+                        ty::TraitRef::new(tcx, sized_trait_def_id, [output_ty])
+                    }),
+                ));
 
                 (trait_ref, Ty::from_closure_kind(tcx, ty::ClosureKind::Fn))
             }
@@ -928,14 +935,20 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     ])
                 });
 
-                // We must additionally check that the return type impls `Future`.
-                // See FIXME in last branch for why we instantiate the binder eagerly.
+                // We must additionally check that the return type impls `Future + Sized`.
                 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]),
                 ));
+                let sized_trait_def_id = tcx.require_lang_item(LangItem::Sized, None);
+                nested.push(obligation.with(
+                    tcx,
+                    sig.output().map_bound(|output_ty| {
+                        ty::TraitRef::new(tcx, sized_trait_def_id, [output_ty])
+                    }),
+                ));
 
                 (trait_ref, args.kind_ty())
             }