Auto merge of #120712 - compiler-errors:async-closures-harmonize, r=oli-obk

Harmonize `AsyncFn` implementations, make async closures conditionally impl `Fn*` traits

This PR implements several changes to the built-in and libcore-provided implementations of `Fn*` and `AsyncFn*` to address two problems:
1. async closures do not implement the `Fn*` family traits, leading to breakage: https://crater-reports.s3.amazonaws.com/pr-120361/index.html
2. *references* to async closures do not implement `AsyncFn*`, as a consequence of the existing blanket impls of the shape `AsyncFn for F where F: Fn, F::Output: Future`.

In order to fix (1.), we implement `Fn` traits appropriately for async closures. It turns out that async closures can:
* always implement `FnOnce`, meaning that they're drop-in compatible with `FnOnce`-bound combinators like `Option::map`.
* conditionally implement `Fn`/`FnMut` if they have no captures, which means that existing usages of async closures should *probably* work without breakage (crater checking this: #120712 (comment)).

In order to fix (2.), we make all of the built-in callables implement `AsyncFn*` via built-in impls, and instead adjust the blanket impls for `AsyncFn*` provided by libcore to match the blanket impls for `Fn*`.

Author: bors

compiler/rustc_hir_typeck/src/callee.rs

@@ -261,23 +261,40 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         adjusted_ty: Ty<'tcx>,
         opt_arg_exprs: Option<&'tcx [hir::Expr<'tcx>]>,
     ) -> Option<(Option<Adjustment<'tcx>>, MethodCallee<'tcx>)> {
+        // HACK(async_closures): For async closures, prefer `AsyncFn*`
+        // over `Fn*`, since all async closures implement `FnOnce`, but
+        // choosing that over `AsyncFn`/`AsyncFnMut` would be more restrictive.
+        // For other callables, just prefer `Fn*` for perf reasons.
+        //
+        // The order of trait choices here is not that big of a deal,
+        // since it just guides inference (and our choice of autoref).
+        // Though in the future, I'd like typeck to choose:
+        // `Fn > AsyncFn > FnMut > AsyncFnMut > FnOnce > AsyncFnOnce`
+        // ...or *ideally*, we just have `LendingFn`/`LendingFnMut`, which
+        // would naturally unify these two trait hierarchies in the most
+        // general way.
+        let call_trait_choices = if self.shallow_resolve(adjusted_ty).is_coroutine_closure() {
+            [
+                (self.tcx.lang_items().async_fn_trait(), sym::async_call, true),
+                (self.tcx.lang_items().async_fn_mut_trait(), sym::async_call_mut, true),
+                (self.tcx.lang_items().async_fn_once_trait(), sym::async_call_once, false),
+                (self.tcx.lang_items().fn_trait(), sym::call, true),
+                (self.tcx.lang_items().fn_mut_trait(), sym::call_mut, true),
+                (self.tcx.lang_items().fn_once_trait(), sym::call_once, false),
+            ]
+        } else {
+            [
+                (self.tcx.lang_items().fn_trait(), sym::call, true),
+                (self.tcx.lang_items().fn_mut_trait(), sym::call_mut, true),
+                (self.tcx.lang_items().fn_once_trait(), sym::call_once, false),
+                (self.tcx.lang_items().async_fn_trait(), sym::async_call, true),
+                (self.tcx.lang_items().async_fn_mut_trait(), sym::async_call_mut, true),
+                (self.tcx.lang_items().async_fn_once_trait(), sym::async_call_once, false),
+            ]
+        };
+
         // Try the options that are least restrictive on the caller first.
-        for (opt_trait_def_id, method_name, borrow) in [
-            (self.tcx.lang_items().fn_trait(), Ident::with_dummy_span(sym::call), true),
-            (self.tcx.lang_items().fn_mut_trait(), Ident::with_dummy_span(sym::call_mut), true),
-            (self.tcx.lang_items().fn_once_trait(), Ident::with_dummy_span(sym::call_once), false),
-            (self.tcx.lang_items().async_fn_trait(), Ident::with_dummy_span(sym::async_call), true),
-            (
-                self.tcx.lang_items().async_fn_mut_trait(),
-                Ident::with_dummy_span(sym::async_call_mut),
-                true,
-            ),
-            (
-                self.tcx.lang_items().async_fn_once_trait(),
-                Ident::with_dummy_span(sym::async_call_once),
-                false,
-            ),
-        ] {
+        for (opt_trait_def_id, method_name, borrow) in call_trait_choices {
             let Some(trait_def_id) = opt_trait_def_id else { continue };
 
             let opt_input_type = opt_arg_exprs.map(|arg_exprs| {
@@ -294,7 +311,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
             if let Some(ok) = self.lookup_method_in_trait(
                 self.misc(call_expr.span),
-                method_name,
+                Ident::with_dummy_span(method_name),
                 trait_def_id,
                 adjusted_ty,
                 opt_input_type.as_ref().map(slice::from_ref),

compiler/rustc_hir_typeck/src/closure.rs

@@ -56,11 +56,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         // It's always helpful for inference if we know the kind of
         // closure sooner rather than later, so first examine the expected
         // type, and see if can glean a closure kind from there.
-        let (expected_sig, expected_kind) = match expected.to_option(self) {
-            Some(ty) => {
-                self.deduce_closure_signature(self.try_structurally_resolve_type(expr_span, ty))
-            }
-            None => (None, None),
+        let (expected_sig, expected_kind) = match closure.kind {
+            hir::ClosureKind::Closure => match expected.to_option(self) {
+                Some(ty) => {
+                    self.deduce_closure_signature(self.try_structurally_resolve_type(expr_span, ty))
+                }
+                None => (None, None),
+            },
+            // We don't want to deduce a signature from `Fn` bounds for coroutines
+            // or coroutine-closures, because the former does not implement `Fn`
+            // ever, and the latter's signature doesn't correspond to the coroutine
+            // type that it returns.
+            hir::ClosureKind::Coroutine(_) | hir::ClosureKind::CoroutineClosure(_) => (None, None),
         };
 
         let ClosureSignatures { bound_sig, mut liberated_sig } =

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

@@ -177,12 +177,13 @@ pub fn report_object_safety_error<'tcx>(
             )));
         }
         impls => {
-            let types = impls
+            let mut types = impls
                 .iter()
                 .map(|t| {
                     with_no_trimmed_paths!(format!("  {}", tcx.type_of(*t).instantiate_identity(),))
                 })
                 .collect::<Vec<_>>();
+            types.sort();
             err.help(format!(
                 "the following types implement the trait, consider defining an enum where each \
                  variant holds one of these types, implementing `{}` for this new enum and using \

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

@@ -323,34 +323,27 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_async_callable<'tc
     self_ty: Ty<'tcx>,
     goal_kind: ty::ClosureKind,
     env_region: ty::Region<'tcx>,
-) -> Result<
-    (ty::Binder<'tcx, (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>)>, Option<ty::Predicate<'tcx>>),
-    NoSolution,
-> {
+) -> Result<(ty::Binder<'tcx, (Ty<'tcx>, Ty<'tcx>, Ty<'tcx>)>, Vec<ty::Predicate<'tcx>>), NoSolution>
+{
     match *self_ty.kind() {
         ty::CoroutineClosure(def_id, args) => {
             let args = args.as_coroutine_closure();
             let kind_ty = args.kind_ty();
-
-            if let Some(closure_kind) = kind_ty.to_opt_closure_kind() {
+            let sig = args.coroutine_closure_sig().skip_binder();
+            let mut nested = vec![];
+            let coroutine_ty = if let Some(closure_kind) = kind_ty.to_opt_closure_kind() {
                 if !closure_kind.extends(goal_kind) {
                     return Err(NoSolution);
                 }
-                Ok((
-                    args.coroutine_closure_sig().map_bound(|sig| {
-                        let coroutine_ty = sig.to_coroutine_given_kind_and_upvars(
-                            tcx,
-                            args.parent_args(),
-                            tcx.coroutine_for_closure(def_id),
-                            goal_kind,
-                            env_region,
-                            args.tupled_upvars_ty(),
-                            args.coroutine_captures_by_ref_ty(),
-                        );
-                        (sig.tupled_inputs_ty, sig.return_ty, coroutine_ty)
-                    }),
-                    None,
-                ))
+                sig.to_coroutine_given_kind_and_upvars(
+                    tcx,
+                    args.parent_args(),
+                    tcx.coroutine_for_closure(def_id),
+                    goal_kind,
+                    env_region,
+                    args.tupled_upvars_ty(),
+                    args.coroutine_captures_by_ref_ty(),
+                )
             } else {
                 let async_fn_kind_trait_def_id =
                     tcx.require_lang_item(LangItem::AsyncFnKindHelper, None);
@@ -367,42 +360,117 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_async_callable<'tc
                 // 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.
-                Ok((
-                    args.coroutine_closure_sig().map_bound(|sig| {
-                        let tupled_upvars_ty = Ty::new_projection(
-                            tcx,
-                            upvars_projection_def_id,
-                            [
-                                ty::GenericArg::from(kind_ty),
-                                Ty::from_closure_kind(tcx, goal_kind).into(),
-                                env_region.into(),
-                                sig.tupled_inputs_ty.into(),
-                                args.tupled_upvars_ty().into(),
-                                args.coroutine_captures_by_ref_ty().into(),
-                            ],
-                        );
-                        let coroutine_ty = sig.to_coroutine(
-                            tcx,
-                            args.parent_args(),
-                            Ty::from_closure_kind(tcx, goal_kind),
-                            tcx.coroutine_for_closure(def_id),
-                            tupled_upvars_ty,
-                        );
-                        (sig.tupled_inputs_ty, sig.return_ty, coroutine_ty)
-                    }),
-                    Some(
-                        ty::TraitRef::new(
-                            tcx,
-                            async_fn_kind_trait_def_id,
-                            [kind_ty, Ty::from_closure_kind(tcx, goal_kind)],
-                        )
-                        .to_predicate(tcx),
-                    ),
-                ))
-            }
+                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 tupled_upvars_ty = Ty::new_projection(
+                    tcx,
+                    upvars_projection_def_id,
+                    [
+                        ty::GenericArg::from(kind_ty),
+                        Ty::from_closure_kind(tcx, goal_kind).into(),
+                        env_region.into(),
+                        sig.tupled_inputs_ty.into(),
+                        args.tupled_upvars_ty().into(),
+                        args.coroutine_captures_by_ref_ty().into(),
+                    ],
+                );
+                sig.to_coroutine(
+                    tcx,
+                    args.parent_args(),
+                    Ty::from_closure_kind(tcx, goal_kind),
+                    tcx.coroutine_for_closure(def_id),
+                    tupled_upvars_ty,
+                )
+            };
+
+            Ok((
+                args.coroutine_closure_sig().rebind((
+                    sig.tupled_inputs_ty,
+                    sig.return_ty,
+                    coroutine_ty,
+                )),
+                nested,
+            ))
         }
 
-        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