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Rollup merge of #123350 - compiler-errors:async-closure-by-move, r=oli-obk

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Actually use the inferred `ClosureKind` from signature inference in coroutine-closures

A follow-up to https://github.com/rust-lang/rust/pull/123349, which fixes another subtle bug: We were not taking into account the async closure kind we infer during closure signature inference.

When I pass a closure directly to an arg like `fn(x: impl async FnOnce())`, that should have the side-effect of artificially restricting the kind of the async closure to `ClosureKind::FnOnce`. We weren't doing this -- that's a quick fix; however, it uncovers a second, more subtle bug with the way that `move`, async closures, and `FnOnce` interact.

Specifically, when we have an async closure like:
```
let x = Struct;
let c = infer_as_fnonce(async move || {
  println!("{x:?}");
}
```

The outer closure captures `x` by move, but the inner coroutine still immutably borrows `x` from the outer closure. Since we've forced the closure to by `async FnOnce()`, we can't actually *do* a self borrow, since the signature of `AsyncFnOnce::call_once` doesn't have a borrowed lifetime. This means that all `async move` closures that are constrained to `FnOnce` will fail borrowck.

We can fix that by detecting this case specifically, and making the *inner* async closure `move` as well. This is always beneficial to closure analysis, since if we have an `async FnOnce()` that's `move`, there's no reason to ever borrow anything, so `move` isn't artificially restrictive.
This commit is contained in:
Guillaume Gomez 2024-04-05 16:38:51 +02:00 • committed by GitHub
commit 02ee8a8cee
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9 changed files with 182 additions and 55 deletions
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20
compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
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@ -91,15 +91,17 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
return;
}
let ty::Coroutine(_, coroutine_args) = *coroutine_ty.kind() else { bug!("{body:#?}") };
// We don't need to generate a by-move coroutine if the kind of the coroutine is
// already `FnOnce` -- that means that any upvars that the closure consumes have
// already been taken by-value.
let coroutine_kind = coroutine_args.as_coroutine().kind_ty().to_opt_closure_kind().unwrap();
if coroutine_kind == ty::ClosureKind::FnOnce {
// We don't need to generate a by-move coroutine if the coroutine body was
// produced by the `CoroutineKindShim`, since it's already by-move.
if matches!(body.source.instance, ty::InstanceDef::CoroutineKindShim { .. }) {
return;
}
let ty::Coroutine(_, args) = *coroutine_ty.kind() else { bug!("{body:#?}") };
let args = args.as_coroutine();
let coroutine_kind = args.kind_ty().to_opt_closure_kind().unwrap();
let parent_def_id = tcx.local_parent(coroutine_def_id);
let ty::CoroutineClosure(_, parent_args) =
*tcx.type_of(parent_def_id).instantiate_identity().kind()
@ -128,6 +130,12 @@ impl<'tcx> MirPass<'tcx> for ByMoveBody {
// the outer closure body -- we need to change the coroutine to take the
// upvar by value.
if coroutine_capture.is_by_ref() && !parent_capture.is_by_ref() {
assert_ne!(
coroutine_kind,
ty::ClosureKind::FnOnce,
"`FnOnce` coroutine-closures return coroutines that capture from \
their body; it will always result in a borrowck error!"
);
by_ref_fields.insert(FieldIdx::from_usize(num_args + idx));
}