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Rollup merge of #138514 - compiler-errors:fake-borrow-ref-to-value, r=oli-obk

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Remove fake borrows of refs that are converted into non-refs in `MakeByMoveBody`

Remove fake borrows of closure captures if that capture has been replaced with a by-move version of that capture.

For example, given an async closure that looks like:

```
let f: Foo;
let c = async move || {
    match f { ... }
};
```

... in this pair of coroutine-closure + coroutine, we capture `Foo` in the parent and `&Foo` in the child. We will emit two fake borrows like:

```
_2 = &fake shallow (*(_1.0: &Foo));
_3 = &fake shallow (_1.0: &Foo);
```

However, since the by-move-body transform is responsible for replacing `_1.0: &Foo` with `_1.0: Foo` (since the `AsyncFnOnce` coroutine will own `Foo` by value), that makes the second fake borrow obsolete since we never have an upvar of type `&Foo`, and we should replace it with a `nop`.

As a side-note, we don't actually even care about fake borrows here at all since they're fully a MIR borrowck artifact, and we don't need to borrowck by-move MIR bodies. But it's best to preserve as much as we can between these two bodies :)

Fixes #138501

r? oli-obk
This commit is contained in:
Matthias Krüger 2025-03-15 11:29:27 +01:00 • committed by GitHub
commit 81ba55746d
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4 changed files with 202 additions and 2 deletions
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41
compiler/rustc_mir_transform/src/coroutine/by_move_body.rs
View file

@ -178,7 +178,7 @@ pub(crate) fn coroutine_by_move_body_def_id<'tcx>(
),
};
(
Some((
FieldIdx::from_usize(child_field_idx + num_args),
(
FieldIdx::from_usize(parent_field_idx + num_args),
@ -186,9 +186,10 @@ pub(crate) fn coroutine_by_move_body_def_id<'tcx>(
peel_deref,
child_precise_captures,
),
)
))
},
)
.flatten()
.collect();
if coroutine_kind == ty::ClosureKind::FnOnce {
@ -312,10 +313,46 @@ impl<'tcx> MutVisitor<'tcx> for MakeByMoveBody<'tcx> {
self.super_place(place, context, location);
}
fn visit_statement(&mut self, statement: &mut mir::Statement<'tcx>, location: mir::Location) {
// Remove fake borrows of closure captures if that capture has been
// replaced with a by-move version of that capture.
//
// For example, imagine we capture `Foo` in the parent and `&Foo`
// in the child. We will emit two fake borrows like:
//
// ```
// _2 = &fake shallow (*(_1.0: &Foo));
// _3 = &fake shallow (_1.0: &Foo);
// ```
//
// However, since this transform is responsible for replacing
// `_1.0: &Foo` with `_1.0: Foo`, that makes the second fake borrow
// obsolete, and we should replace it with a nop.
//
// As a side-note, we don't actually even care about fake borrows
// here at all since they're fully a MIR borrowck artifact, and we
// don't need to borrowck by-move MIR bodies. But it's best to preserve
// as much as we can between these two bodies :)
if let mir::StatementKind::Assign(box (_, rvalue)) = &statement.kind
&& let mir::Rvalue::Ref(_, mir::BorrowKind::Fake(mir::FakeBorrowKind::Shallow), place) =
rvalue
&& let mir::PlaceRef {
local: ty::CAPTURE_STRUCT_LOCAL,
projection: [mir::ProjectionElem::Field(idx, _)],
} = place.as_ref()
&& let Some(&(_, _, true, _)) = self.field_remapping.get(&idx)
{
statement.kind = mir::StatementKind::Nop;
}
self.super_statement(statement, location);
}
fn visit_local_decl(&mut self, local: mir::Local, local_decl: &mut mir::LocalDecl<'tcx>) {
// Replace the type of the self arg.
if local == ty::CAPTURE_STRUCT_LOCAL {
local_decl.ty = self.by_move_coroutine_ty;
}
self.super_local_decl(local, local_decl);
}
}