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Unelaborate the &mut deref temps GVN rewrites into copies #351

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Unelaborate the &mut deref temps GVN rewrites into copies #351

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8 changes: 8 additions & 0 deletions src/analyze/local_def.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -415,6 +415,14 @@ impl<'tcx, 'ctx> Analyzer<'tcx, 'ctx> {
return Some((lhs_local, *place));
}

// GVN rewrites the deref temp of a `&mut` from `deref_copy` into a plain `copy`.
if let mir::Rvalue::Use(mir::Operand::Copy(place)) = &rvalue {
let ty = place.ty(&self.body.local_decls, self.tcx).ty;
if !place.projection.is_empty() && ty.ref_mutability() == Some(mir::Mutability::Mut) {
return Some((lhs_local, *place));
Comment on lines +421 to +422

@chatgpt-codex-connector chatgpt-codex-connector Bot Oct 22, 2026

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P1 Badge Restrict copy unelaboration to actual deref temps

Projected copies are not exclusive to GVN-generated deref temps: rustc also emits them for ordinary mutable bindings such as let mut r = &mut *w.a. If r is later reassigned from w.b, ReplacePlacesVisitor retains the permanent r -> w.a replacement, so the reassignment's destination is rewritten into w.a itself. For a borrowed aggregate this makes Thrust analyze the function as changing the caller's reference field even though the real program only reassigns a local, which can corrupt post-state constraints and accept an invalid assertion or reject a valid one. Check that lhs_local is marked as a DerefTemp rather than classifying every projected &mut copy.

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}
}

let unique_did = self.ctx.def_ids.unique()?;
let nonnull_did = self.ctx.def_ids.nonnull()?;

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13 changes: 13 additions & 0 deletions tests/ui/fail/mut_ref_deref_optimized.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
//@error-in-other-file: Unsat
//@compile-flags: -C debug-assertions=off -C opt-level=1

fn bump(w: &mut &mut i64) {
**w += 3;
}

fn main() {
let mut a = 0;
let mut r = &mut a;
bump(&mut r);
assert!(a == 0);
}
13 changes: 13 additions & 0 deletions tests/ui/pass/mut_ref_deref_optimized.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,13 @@
//@check-pass
//@compile-flags: -C debug-assertions=off -C opt-level=1

fn bump(w: &mut &mut i64) {
**w += 3;
}

fn main() {
let mut a = 0;
let mut r = &mut a;
bump(&mut r);
assert!(a == 3);
}
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