From 7be8e8add8ba15c197e8181c74ed6ecb18faa158 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Thu, 22 Oct 2026 21:34:40 +0000
Subject: [PATCH] Unelaborate the `&mut` deref temps GVN rewrites into copies

From `-C opt-level=1`, GVN rewrites a deref temp `_t = deref_copy P` of a
`&mut`-typed place into `_t = copy P`. `extract_elaborated_deref` only
recognized `CopyForDeref`, so `_t` survived as an independent local and
each such copy was analyzed as its own borrow of `P`. With two of them
alive at once (`**w += 3` reads through one and writes through the other)
the prophecies contradict, the body is vacuous, and callers verify any
assertion, `assert!(false)` included.

Treat a copy of a projected `&mut`-typed place as such a deref temp.
Whole-local copies are left alone: rustc emits those for ordinary moves
of a `&mut` local.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KKfGn15ekWG5r4cKqzoP9a
---
 src/analyze/local_def.rs                 |  8 ++++++++
 tests/ui/fail/mut_ref_deref_optimized.rs | 13 +++++++++++++
 tests/ui/pass/mut_ref_deref_optimized.rs | 13 +++++++++++++
 3 files changed, 34 insertions(+)
 create mode 100644 tests/ui/fail/mut_ref_deref_optimized.rs
 create mode 100644 tests/ui/pass/mut_ref_deref_optimized.rs

diff --git a/src/analyze/local_def.rs b/src/analyze/local_def.rs
index 6a9cdf1b5..6e9acc387 100644
--- a/src/analyze/local_def.rs
+++ b/src/analyze/local_def.rs
@@ -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));
+            }
+        }
+
         let unique_did = self.ctx.def_ids.unique()?;
         let nonnull_did = self.ctx.def_ids.nonnull()?;
 
diff --git a/tests/ui/fail/mut_ref_deref_optimized.rs b/tests/ui/fail/mut_ref_deref_optimized.rs
new file mode 100644
index 000000000..8b092d39c
--- /dev/null
+++ b/tests/ui/fail/mut_ref_deref_optimized.rs
@@ -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);
+}
diff --git a/tests/ui/pass/mut_ref_deref_optimized.rs b/tests/ui/pass/mut_ref_deref_optimized.rs
new file mode 100644
index 000000000..1fe487590
--- /dev/null
+++ b/tests/ui/pass/mut_ref_deref_optimized.rs
@@ -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);
+}