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Prefer AsyncFn* over Fn* for coroutine-closures

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This commit is contained in:
Michael Goulet 2024-02-08 15:46:00 +00:00
parent b8c93f1223
commit 3bb384aad6
5 changed files with 79 additions and 29 deletions
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18
compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
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@ -336,11 +336,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
let is_const = self.tcx().is_const_fn_raw(def_id);
match self.infcx.closure_kind(self_ty) {
Some(closure_kind) => {
let no_borrows = self
let no_borrows = match self
.infcx
.shallow_resolve(args.as_coroutine_closure().tupled_upvars_ty())
.tuple_fields()
.is_empty();
.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) {
candidates.vec.push(ClosureCandidate { is_const });
} else if kind == ty::ClosureKind::FnOnce {