Rollup merge of #134933 - compiler-errors:async-fn-future-sized, r=lqd

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

Fixes #134817

1 files changed, 20 insertions, 3 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 962b6b94fa6..3619d16cde2 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -896,6 +896,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         sig.tupled_inputs_ty,
                     ])
                 });
+
+                // Note that unlike below, we don't need to check `Future + Sized` for
+                // the output coroutine because they are `Future + Sized` by construction.
+
                 (trait_ref, args.kind_ty())
             }
             ty::FnDef(..) | ty::FnPtr(..) => {
@@ -907,7 +911,7 @@ 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,
@@ -915,6 +919,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         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 +939,20 @@ 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())
             }