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Retagging: do not retag 'raw reborrows' #78597

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Nov 3, 2020
Merged

Retagging: do not retag 'raw reborrows' #78597

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26 changes: 20 additions & 6 deletions compiler/rustc_mir/src/transform/add_retag.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -73,6 +73,19 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
// a temporary and retag on that.
is_stable(place.as_ref()) && may_be_reference(place.ty(&*local_decls, tcx).ty)
};
let place_base_raw = |place: &Place<'tcx>| {
// If this is a `Deref`, get the type of what we are deref'ing.
let deref_base =
place.projection.iter().rposition(|p| matches!(p, ProjectionElem::Deref));
if let Some(deref_base) = deref_base {
let base_proj = &place.projection[..deref_base];
let ty = Place::ty_from(place.local, base_proj, &*local_decls, tcx).ty;
ty.is_unsafe_ptr()
} else {
// Not a deref, and thus not raw.
false
}
};

// PART 1
// Retag arguments at the beginning of the start block.
Expand Down Expand Up @@ -136,13 +149,14 @@ impl<'tcx> MirPass<'tcx> for AddRetag {
// iterate backwards using indices.
for i in (0..block_data.statements.len()).rev() {
let (retag_kind, place) = match block_data.statements[i].kind {
// Retag-as-raw after escaping to a raw pointer.
StatementKind::Assign(box (place, Rvalue::AddressOf(..))) => {
(RetagKind::Raw, place)
// Retag-as-raw after escaping to a raw pointer, if the referent
// is not already a raw pointer.
StatementKind::Assign(box (lplace, Rvalue::AddressOf(_, ref rplace)))
if !place_base_raw(rplace) =>
{
(RetagKind::Raw, lplace)
}
// Assignments of reference or ptr type are the ones where we may have
// to update tags. This includes `x = &[mut] ...` and hence
// we also retag after taking a reference!
// Retag after assignments of reference type.
StatementKind::Assign(box (ref place, ref rvalue)) if needs_retag(place) => {
let kind = match rvalue {
Rvalue::Ref(_, borrow_kind, _)
Expand Down