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: https://github.com/rust-lang/rust/pull/120712#issuecomment-1930587805).

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*`.

1 files changed, 31 insertions, 0 deletions

diff --git a/compiler/rustc_ty_utils/src/instance.rs b/compiler/rustc_ty_utils/src/instance.rs
index 9faad10dd14..eae80199ce5 100644
--- a/compiler/rustc_ty_utils/src/instance.rs
+++ b/compiler/rustc_ty_utils/src/instance.rs
@@ -278,6 +278,24 @@ fn resolve_associated_item<'tcx>(
                         def: ty::InstanceDef::FnPtrShim(trait_item_id, rcvr_args.type_at(0)),
                         args: rcvr_args,
                     }),
+                    ty::CoroutineClosure(coroutine_closure_def_id, args) => {
+                        // When a coroutine-closure implements the `Fn` traits, then it
+                        // always dispatches to the `FnOnce` implementation. This is to
+                        // ensure that the `closure_kind` of the resulting closure is in
+                        // sync with the built-in trait implementations (since all of the
+                        // implementations return `FnOnce::Output`).
+                        if ty::ClosureKind::FnOnce == args.as_coroutine_closure().kind() {
+                            Some(Instance::new(coroutine_closure_def_id, args))
+                        } else {
+                            Some(Instance {
+                                def: ty::InstanceDef::ConstructCoroutineInClosureShim {
+                                    coroutine_closure_def_id,
+                                    target_kind: ty::ClosureKind::FnOnce,
+                                },
+                                args,
+                            })
+                        }
+                    }
                     _ => bug!(
                         "no built-in definition for `{trait_ref}::{}` for non-fn type",
                         tcx.item_name(trait_item_id)
@@ -306,6 +324,19 @@ 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)