Rollup merge of #125259 - compiler-errors:fn-mut-as-a-treat, r=oli-obk

An async closure may implement `FnMut`/`Fn` if it has no self-borrows

There's no reason that async closures may not implement `FnMut` or `Fn` if they don't actually borrow anything with the closure's env lifetime. Specifically, #123660 made it so that we don't always need to borrow captures from the closure's env.

See the doc comment on `should_reborrow_from_env_of_parent_coroutine_closure`:

https://github.com/rust-lang/rust/blob/c00957a3e269219413041a4e3565f33b1f9d0779/compiler/rustc_hir_typeck/src/upvar.rs#L1777-L1823

If there are no such borrows, then we are free to implement `FnMut` and `Fn` as permitted by our closure's inferred `ClosureKind`.

As far as I can tell, this change makes `async || {}` work in precisely the set of places they used to work before #120361.
Fixes #125247.

r? oli-obk

1 files changed, 3 insertions, 12 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index b9e853a0678..fd7c47ad6fb 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -418,20 +418,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     // Ambiguity if upvars haven't been constrained yet
                     && !args.tupled_upvars_ty().is_ty_var()
                 {
-                    let no_borrows = match args.tupled_upvars_ty().kind() {
-                        ty::Tuple(tys) => tys.is_empty(),
-                        ty::Error(_) => false,
-                        _ => bug!("tuple_fields called on non-tuple"),
-                    };
                     // A coroutine-closure implements `FnOnce` *always*, since it may
                     // always be called once. It additionally implements `Fn`/`FnMut`
-                    // only if it has no upvars (therefore no borrows from the closure
-                    // that would need to be represented with a lifetime) and if the
-                    // closure kind permits it.
-                    // FIXME(async_closures): Actually, it could also implement `Fn`/`FnMut`
-                    // if it takes all of its upvars by copy, and none by ref. This would
-                    // require us to record a bit more information during upvar analysis.
-                    if no_borrows && closure_kind.extends(kind) {
+                    // only if it has no upvars referencing the closure-env lifetime,
+                    // and if the closure kind permits it.
+                    if closure_kind.extends(kind) && !args.has_self_borrows() {
                         candidates.vec.push(ClosureCandidate { is_const });
                     } else if kind == ty::ClosureKind::FnOnce {
                         candidates.vec.push(ClosureCandidate { is_const });