Regular closures now built-in impls for AsyncFn*

compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs

@@ -394,7 +394,78 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_async_callable<'tc
             ))
         }
 
-        ty::FnDef(..) | ty::FnPtr(..) | ty::Closure(..) => Err(NoSolution),
+        ty::FnDef(..) | ty::FnPtr(..) => {
+            let bound_sig = self_ty.fn_sig(tcx);
+            let sig = bound_sig.skip_binder();
+            let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
+            // `FnDef` and `FnPtr` only implement `AsyncFn*` when their
+            // return type implements `Future`.
+            let nested = vec![
+                bound_sig
+                    .rebind(ty::TraitRef::new(tcx, future_trait_def_id, [sig.output()]))
+                    .to_predicate(tcx),
+            ];
+            let future_output_def_id = tcx
+                .associated_items(future_trait_def_id)
+                .filter_by_name_unhygienic(sym::Output)
+                .next()
+                .unwrap()
+                .def_id;
+            let future_output_ty = Ty::new_projection(tcx, future_output_def_id, [sig.output()]);
+            Ok((
+                bound_sig.rebind((Ty::new_tup(tcx, sig.inputs()), sig.output(), future_output_ty)),
+                nested,
+            ))
+        }
+        ty::Closure(_, args) => {
+            let args = args.as_closure();
+            let bound_sig = args.sig();
+            let sig = bound_sig.skip_binder();
+            let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
+            // `Closure`s only implement `AsyncFn*` when their return type
+            // implements `Future`.
+            let mut nested = vec![
+                bound_sig
+                    .rebind(ty::TraitRef::new(tcx, future_trait_def_id, [sig.output()]))
+                    .to_predicate(tcx),
+            ];
+
+            // Additionally, we need to check that the closure kind
+            // is still compatible.
+            let kind_ty = args.kind_ty();
+            if let Some(closure_kind) = kind_ty.to_opt_closure_kind() {
+                if !closure_kind.extends(goal_kind) {
+                    return Err(NoSolution);
+                }
+            } else {
+                let async_fn_kind_trait_def_id =
+                    tcx.require_lang_item(LangItem::AsyncFnKindHelper, None);
+                // When we don't know the closure kind (and therefore also the closure's upvars,
+                // which are computed at the same time), we must delay the computation of the
+                // generator's upvars. We do this using the `AsyncFnKindHelper`, which as a trait
+                // goal functions similarly to the old `ClosureKind` predicate, and ensures that
+                // the goal kind <= the closure kind. As a projection `AsyncFnKindHelper::Upvars`
+                // will project to the right upvars for the generator, appending the inputs and
+                // coroutine upvars respecting the closure kind.
+                nested.push(
+                    ty::TraitRef::new(
+                        tcx,
+                        async_fn_kind_trait_def_id,
+                        [kind_ty, Ty::from_closure_kind(tcx, goal_kind)],
+                    )
+                    .to_predicate(tcx),
+                );
+            }
+
+            let future_output_def_id = tcx
+                .associated_items(future_trait_def_id)
+                .filter_by_name_unhygienic(sym::Output)
+                .next()
+                .unwrap()
+                .def_id;
+            let future_output_ty = Ty::new_projection(tcx, future_output_def_id, [sig.output()]);
+            Ok((bound_sig.rebind((sig.inputs()[0], sig.output(), future_output_ty)), nested))
+        }
 
         ty::Bool
         | ty::Char

compiler/rustc_trait_selection/src/traits/project.rs

@@ -2450,14 +2450,6 @@ fn confirm_async_closure_candidate<'cx, 'tcx>(
 ) -> Progress<'tcx> {
     let tcx = selcx.tcx();
     let self_ty = selcx.infcx.shallow_resolve(obligation.predicate.self_ty());
-    let ty::CoroutineClosure(def_id, args) = *self_ty.kind() else {
-        unreachable!(
-            "expected coroutine-closure self type for coroutine-closure candidate, found {self_ty}"
-        )
-    };
-    let args = args.as_coroutine_closure();
-    let kind_ty = args.kind_ty();
-    let sig = args.coroutine_closure_sig().skip_binder();
 
     let goal_kind =
         tcx.async_fn_trait_kind_from_def_id(obligation.predicate.trait_def_id(tcx)).unwrap();
@@ -2465,8 +2457,14 @@ fn confirm_async_closure_candidate<'cx, 'tcx>(
         ty::ClosureKind::Fn | ty::ClosureKind::FnMut => obligation.predicate.args.region_at(2),
         ty::ClosureKind::FnOnce => tcx.lifetimes.re_static,
     };
-
     let item_name = tcx.item_name(obligation.predicate.def_id);
+
+    let poly_cache_entry = match *self_ty.kind() {
+        ty::CoroutineClosure(def_id, args) => {
+            let args = args.as_coroutine_closure();
+            let kind_ty = args.kind_ty();
+            let sig = args.coroutine_closure_sig().skip_binder();
+
             let term = match item_name {
                 sym::CallOnceFuture | sym::CallMutFuture | sym::CallFuture => {
                     if let Some(closure_kind) = kind_ty.to_opt_closure_kind() {
@@ -2524,9 +2522,11 @@ fn confirm_async_closure_candidate<'cx, 'tcx>(
                 name => bug!("no such associated type: {name}"),
             };
             let projection_ty = match item_name {
-        sym::CallOnceFuture | sym::Output => {
+                sym::CallOnceFuture | sym::Output => ty::AliasTy::new(
-            ty::AliasTy::new(tcx, obligation.predicate.def_id, [self_ty, sig.tupled_inputs_ty])
+                    tcx,
-        }
+                    obligation.predicate.def_id,
+                    [self_ty, sig.tupled_inputs_ty],
+                ),
                 sym::CallMutFuture | sym::CallFuture => ty::AliasTy::new(
                     tcx,
                     obligation.predicate.def_id,
@@ -2535,13 +2535,84 @@ fn confirm_async_closure_candidate<'cx, 'tcx>(
                 name => bug!("no such associated type: {name}"),
             };
 
-    confirm_param_env_candidate(
-        selcx,
-        obligation,
             args.coroutine_closure_sig()
-            .rebind(ty::ProjectionPredicate { projection_ty, term: term.into() }),
+                .rebind(ty::ProjectionPredicate { projection_ty, term: term.into() })
-        true,
+        }
-    )
+        ty::FnDef(..) | ty::FnPtr(..) => {
+            let bound_sig = self_ty.fn_sig(tcx);
+            let sig = bound_sig.skip_binder();
+
+            let term = match item_name {
+                sym::CallOnceFuture | sym::CallMutFuture | sym::CallFuture => sig.output(),
+                sym::Output => {
+                    let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
+                    let future_output_def_id = tcx
+                        .associated_items(future_trait_def_id)
+                        .filter_by_name_unhygienic(sym::Output)
+                        .next()
+                        .unwrap()
+                        .def_id;
+                    Ty::new_projection(tcx, future_output_def_id, [sig.output()])
+                }
+                name => bug!("no such associated type: {name}"),
+            };
+            let projection_ty = match item_name {
+                sym::CallOnceFuture | sym::Output => ty::AliasTy::new(
+                    tcx,
+                    obligation.predicate.def_id,
+                    [self_ty, Ty::new_tup(tcx, sig.inputs())],
+                ),
+                sym::CallMutFuture | sym::CallFuture => ty::AliasTy::new(
+                    tcx,
+                    obligation.predicate.def_id,
+                    [
+                        ty::GenericArg::from(self_ty),
+                        Ty::new_tup(tcx, sig.inputs()).into(),
+                        env_region.into(),
+                    ],
+                ),
+                name => bug!("no such associated type: {name}"),
+            };
+
+            bound_sig.rebind(ty::ProjectionPredicate { projection_ty, term: term.into() })
+        }
+        ty::Closure(_, args) => {
+            let args = args.as_closure();
+            let bound_sig = args.sig();
+            let sig = bound_sig.skip_binder();
+
+            let term = match item_name {
+                sym::CallOnceFuture | sym::CallMutFuture | sym::CallFuture => sig.output(),
+                sym::Output => {
+                    let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
+                    let future_output_def_id = tcx
+                        .associated_items(future_trait_def_id)
+                        .filter_by_name_unhygienic(sym::Output)
+                        .next()
+                        .unwrap()
+                        .def_id;
+                    Ty::new_projection(tcx, future_output_def_id, [sig.output()])
+                }
+                name => bug!("no such associated type: {name}"),
+            };
+            let projection_ty = match item_name {
+                sym::CallOnceFuture | sym::Output => {
+                    ty::AliasTy::new(tcx, obligation.predicate.def_id, [self_ty, sig.inputs()[0]])
+                }
+                sym::CallMutFuture | sym::CallFuture => ty::AliasTy::new(
+                    tcx,
+                    obligation.predicate.def_id,
+                    [ty::GenericArg::from(self_ty), sig.inputs()[0].into(), env_region.into()],
+                ),
+                name => bug!("no such associated type: {name}"),
+            };
+
+            bound_sig.rebind(ty::ProjectionPredicate { projection_ty, term: term.into() })
+        }
+        _ => bug!("expected callable type for AsyncFn candidate"),
+    };
+
+    confirm_param_env_candidate(selcx, obligation, poly_cache_entry, true)
         .with_addl_obligations(nested)
 }
 

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

@@ -361,8 +361,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
                 candidates.vec.push(AsyncClosureCandidate);
             }
-            ty::Infer(ty::TyVar(_)) => {
+            // Closures and fn pointers implement `AsyncFn*` if their return types
-                candidates.ambiguous = true;
+            // implement `Future`, which is checked later.
+            ty::Closure(_, args) => {
+                if let Some(closure_kind) = args.as_closure().kind_ty().to_opt_closure_kind()
+                    && !closure_kind.extends(goal_kind)
+                {
+                    return;
+                }
+                candidates.vec.push(AsyncClosureCandidate);
+            }
+            ty::FnDef(..) | ty::FnPtr(..) => {
+                candidates.vec.push(AsyncClosureCandidate);
             }
             _ => {}
         }

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

@@ -883,40 +883,86 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         obligation: &PolyTraitObligation<'tcx>,
     ) -> Result<Vec<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
-        // Okay to skip binder because the args on closure types never
+        let tcx = self.tcx();
-        // touch bound regions, they just capture the in-scope
-        // type/region parameters.
         let self_ty = self.infcx.shallow_resolve(obligation.self_ty().skip_binder());
-        let ty::CoroutineClosure(closure_def_id, args) = *self_ty.kind() else {
-            bug!("async closure candidate for non-coroutine-closure {:?}", obligation);
-        };
 
-        let trait_ref = args.as_coroutine_closure().coroutine_closure_sig().map_bound(|sig| {
+        let mut nested = vec![];
+        let (trait_ref, kind_ty) = match *self_ty.kind() {
+            ty::CoroutineClosure(_, args) => {
+                let args = args.as_coroutine_closure();
+                let trait_ref = args.coroutine_closure_sig().map_bound(|sig| {
                     ty::TraitRef::new(
                         self.tcx(),
                         obligation.predicate.def_id(),
                         [self_ty, sig.tupled_inputs_ty],
                     )
                 });
+                (trait_ref, args.kind_ty())
+            }
+            ty::FnDef(..) | ty::FnPtr(..) => {
+                let sig = self_ty.fn_sig(tcx);
+                let trait_ref = sig.map_bound(|sig| {
+                    ty::TraitRef::new(
+                        self.tcx(),
+                        obligation.predicate.def_id(),
+                        [self_ty, Ty::new_tup(tcx, sig.inputs())],
+                    )
+                });
+                // We must additionally check that the return type impls `Future`.
+                let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
+                nested.push(obligation.with(
+                    tcx,
+                    sig.map_bound(|sig| {
+                        ty::TraitRef::new(tcx, future_trait_def_id, [sig.output()])
+                    }),
+                ));
+                (trait_ref, Ty::from_closure_kind(tcx, ty::ClosureKind::Fn))
+            }
+            ty::Closure(_, args) => {
+                let sig = args.as_closure().sig();
+                let trait_ref = sig.map_bound(|sig| {
+                    ty::TraitRef::new(
+                        self.tcx(),
+                        obligation.predicate.def_id(),
+                        [self_ty, sig.inputs()[0]],
+                    )
+                });
+                // We must additionally check that the return type impls `Future`.
+                let future_trait_def_id = tcx.require_lang_item(LangItem::Future, None);
+                nested.push(obligation.with(
+                    tcx,
+                    sig.map_bound(|sig| {
+                        ty::TraitRef::new(tcx, future_trait_def_id, [sig.output()])
+                    }),
+                ));
+                (trait_ref, Ty::from_closure_kind(tcx, ty::ClosureKind::Fn))
+            }
+            _ => bug!("expected callable type for AsyncFn candidate"),
+        };
 
-        let mut nested = self.confirm_poly_trait_refs(obligation, trait_ref)?;
+        nested.extend(self.confirm_poly_trait_refs(obligation, trait_ref)?);
 
         let goal_kind =
             self.tcx().async_fn_trait_kind_from_def_id(obligation.predicate.def_id()).unwrap();
+
+        // If we have not yet determiend the `ClosureKind` of the closure or coroutine-closure,
+        // then additionally register an `AsyncFnKindHelper` goal which will fail if the kind
+        // is constrained to an insufficient type later on.
+        if let Some(closure_kind) = self.infcx.shallow_resolve(kind_ty).to_opt_closure_kind() {
+            if !closure_kind.extends(goal_kind) {
+                return Err(SelectionError::Unimplemented);
+            }
+        } else {
             nested.push(obligation.with(
                 self.tcx(),
                 ty::TraitRef::from_lang_item(
                     self.tcx(),
                     LangItem::AsyncFnKindHelper,
                     obligation.cause.span,
-                [
+                    [kind_ty, Ty::from_closure_kind(self.tcx(), goal_kind)],
-                    args.as_coroutine_closure().kind_ty(),
-                    Ty::from_closure_kind(self.tcx(), goal_kind),
-                ],
                 ),
             ));
-
+        }
-        debug!(?closure_def_id, ?trait_ref, ?nested, "confirm closure candidate obligations");
 
         Ok(nested)
     }

compiler/rustc_ty_utils/src/instance.rs

@@ -306,6 +306,19 @@ fn resolve_associated_item<'tcx>(
                             Some(Instance::new(coroutine_closure_def_id, args))
                         }
                     }
+                    ty::Closure(closure_def_id, args) => {
+                        let trait_closure_kind = tcx.fn_trait_kind_from_def_id(trait_id).unwrap();
+                        Some(Instance::resolve_closure(
+                            tcx,
+                            closure_def_id,
+                            args,
+                            trait_closure_kind,
+                        ))
+                    }
+                    ty::FnDef(..) | ty::FnPtr(..) => Some(Instance {
+                        def: ty::InstanceDef::FnPtrShim(trait_item_id, rcvr_args.type_at(0)),
+                        args: rcvr_args,
+                    }),
                     _ => bug!(
                         "no built-in definition for `{trait_ref}::{}` for non-lending-closure type",
                         tcx.item_name(trait_item_id)

tests/ui/async-await/async-fn/simple.rs

@@ -1,5 +1,5 @@
 // edition: 2021
-// check-pass
+// build-pass
 
 #![feature(async_fn_traits)]
 

tests/ui/did_you_mean/bad-assoc-ty.stderr

@@ -191,14 +191,7 @@ error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:33:10
    |
 LL | type H = Fn(u8) -> (u8)::Output;
-   |          ^^^^^^^^^^^^^^^^^^^^^^
+   |          ^^^^^^^^^^^^^^^^^^^^^^ help: use fully-qualified syntax: `<(dyn Fn(u8) -> u8 + 'static) as IntoFuture>::Output`
-   |
-help: use fully-qualified syntax
-   |
-LL | type H = <(dyn Fn(u8) -> u8 + 'static) as AsyncFnOnce>::Output;
-   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-LL | type H = <(dyn Fn(u8) -> u8 + 'static) as IntoFuture>::Output;
-   |          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error[E0223]: ambiguous associated type
   --> $DIR/bad-assoc-ty.rs:39:19