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Auto merge of #128572 - compiler-errors:fix-elaborate-box-derefs-on-debug, r=saethlin

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Fix `ElaborateBoxDerefs` on debug varinfo

Slightly simplifies the `ElaborateBoxDerefs` pass to fix cases where it was applying the wrong projections to debug var infos containing places that deref boxes.

From what I can tell[^1], we don't actually have any tests (or code anywhere, really) that exercise `debug x => *(...: Box<T>)`, and it's very difficult to trigger this in surface Rust, so I wrote a custom MIR test.

What happens is that the pass was turning `*(SOME_PLACE: Box<T>)` into `*(*((((SOME_PLACE).0: Unique<T>).0: NonNull<T>).0: *const T))` in debug var infos. In particular, notice the *double deref*, which was wrong.

This is the root cause of #128554, so this PR fixes #128554 as well. The reason that async closures was affected is because of the way that we compute the [`ByMove` body](https://github.com/rust-lang/rust/blob/master/compiler/rustc_mir_transform/src/coroutine/by_move_body.rs), which resulted in `*(...: Box<T>)` in debug var info. But this really has nothing to do with async closures.

[^1]: Validated by literally replacing the `if elem == PlaceElem::Deref && base_ty.is_box() { ... }` innards with a `panic!()`, which compiled all of stage2 without panicking.
This commit is contained in:
bors 2024-08-10 21:24:25 +00:00
commit 730d5d4095
4 changed files with 77 additions and 8 deletions
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17
compiler/rustc_mir_transform/src/elaborate_box_derefs.rs
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@ -116,29 +116,30 @@ impl<'tcx> MirPass<'tcx> for ElaborateBoxDerefs {
for debug_info in body.var_debug_info.iter_mut() {
if let VarDebugInfoContents::Place(place) = &mut debug_info.value {
let mut new_projections: Option<Vec<_>> = None;
let mut last_deref = 0;
for (i, (base, elem)) in place.iter_projections().enumerate() {
for (base, elem) in place.iter_projections() {
let base_ty = base.ty(&body.local_decls, tcx).ty;
if elem == PlaceElem::Deref && base_ty.is_box() {
let new_projections = new_projections.get_or_insert_default();
// Clone the projections before us, since now we need to mutate them.
let new_projections =
new_projections.get_or_insert_with(|| base.projection.to_vec());
let (unique_ty, nonnull_ty, ptr_ty) =
build_ptr_tys(tcx, base_ty.boxed_ty(), unique_did, nonnull_did);
new_projections.extend_from_slice(&base.projection[last_deref..]);
new_projections.extend_from_slice(&build_projection(
unique_ty, nonnull_ty, ptr_ty,
));
new_projections.push(PlaceElem::Deref);
last_deref = i;
} else if let Some(new_projections) = new_projections.as_mut() {
// Keep building up our projections list once we've started it.
new_projections.push(elem);
}
}
if let Some(mut new_projections) = new_projections {
new_projections.extend_from_slice(&place.projection[last_deref..]);
// Store the mutated projections if we actually changed something.
if let Some(new_projections) = new_projections {
place.projection = tcx.mk_place_elems(&new_projections);
}
}