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

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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`:

c00957a3e2/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
This commit is contained in:
León Orell Valerian Liehr 2024-05-22 19:04:45 +02:00 • committed by GitHub
commit 44c7a2dbff
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4 changed files with 70 additions and 20 deletions
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13
compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs
View file

@ -300,14 +300,11 @@ pub(in crate::solve) fn extract_tupled_inputs_and_output_from_callable<'tcx>(
return Err(NoSolution);
}
// If `Fn`/`FnMut`, we only implement this goal if we
// have no captures.
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"),
};
if closure_kind != ty::ClosureKind::FnOnce && !no_borrows {
// A coroutine-closure implements `FnOnce` *always*, since it may
// always be called once. It additionally implements `Fn`/`FnMut`
// only if it has no upvars referencing the closure-env lifetime,
// and if the closure kind permits it.
if closure_kind != ty::ClosureKind::FnOnce && args.has_self_borrows() {
return Err(NoSolution);
}

15
compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
View file

@ -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 });