2 files changed, 34 insertions, 20 deletions
diff --git a/compiler/rustc_mir_transform/src/copy_prop.rs b/compiler/rustc_mir_transform/src/copy_prop.rs
index c565d6f13b1..319f3a79705 100644
--- a/compiler/rustc_mir_transform/src/copy_prop.rs
+++ b/compiler/rustc_mir_transform/src/copy_prop.rs
@@ -162,20 +162,22 @@ impl<'tcx> MutVisitor<'tcx> for Replacer<'_, 'tcx> {
     }
 
     fn visit_statement(&mut self, stmt: &mut Statement<'tcx>, loc: Location) {
-        match stmt.kind {
-            // When removing storage statements, we need to remove both (#107511).
-            StatementKind::StorageLive(l) | StatementKind::StorageDead(l)
-                if self.storage_to_remove.contains(l) =>
-            {
-                stmt.make_nop()
-            }
-            StatementKind::Assign(box (ref place, ref mut rvalue))
-                if place.as_local().is_some() =>
-            {
-                // Do not replace assignments.
-                self.visit_rvalue(rvalue, loc)
-            }
-            _ => self.super_statement(stmt, loc),
+        // When removing storage statements, we need to remove both (#107511).
+        if let StatementKind::StorageLive(l) | StatementKind::StorageDead(l) = stmt.kind
+            && self.storage_to_remove.contains(l)
+        {
+            stmt.make_nop();
+            return
+        }
+
+        self.super_statement(stmt, loc);
+
+        // Do not leave tautological assignments around.
+        if let StatementKind::Assign(box (lhs, ref rhs)) = stmt.kind
+            && let Rvalue::Use(Operand::Copy(rhs) | Operand::Move(rhs)) | Rvalue::CopyForDeref(rhs) = *rhs
+            && lhs == rhs
+        {
+            stmt.make_nop();
         }
     }
 }
diff --git a/compiler/rustc_mir_transform/src/ssa.rs b/compiler/rustc_mir_transform/src/ssa.rs
index a7b45366662..e5877feb8b2 100644
--- a/compiler/rustc_mir_transform/src/ssa.rs
+++ b/compiler/rustc_mir_transform/src/ssa.rs
@@ -267,11 +267,6 @@ fn compute_copy_classes(ssa: &mut SsaVisitor, body: &Body<'_>) -> IndexVec<Local
     for &local in &ssa.assignment_order {
         debug!(?local);
 
-        if local == RETURN_PLACE {
-            // `_0` is special, we cannot rename it.
-            continue;
-        }
-
         // This is not SSA: mark that we don't know the value.
         debug!(assignments = ?ssa.assignments[local]);
         let Set1::One(LocationExtended::Plain(loc)) = ssa.assignments[local] else { continue };
@@ -290,7 +285,23 @@ fn compute_copy_classes(ssa: &mut SsaVisitor, body: &Body<'_>) -> IndexVec<Local
 
         // We visit in `assignment_order`, ie. reverse post-order, so `rhs` has been
         // visited before `local`, and we just have to copy the representing local.
-        copies[local] = copies[rhs];
+        let head = copies[rhs];
+
+        if local == RETURN_PLACE {
+            // `_0` is special, we cannot rename it. Instead, rename the class of `rhs` to
+            // `RETURN_PLACE`. This is only possible if the class head is a temporary, not an
+            // argument.
+            if body.local_kind(head) != LocalKind::Temp {
+                continue;
+            }
+            for h in copies.iter_mut() {
+                if *h == head {
+                    *h = RETURN_PLACE;
+                }
+            }
+        } else {
+            copies[local] = head;
+        }
         ssa.direct_uses[rhs] -= 1;
     }
 
@@ -302,6 +313,7 @@ fn compute_copy_classes(ssa: &mut SsaVisitor, body: &Body<'_>) -> IndexVec<Local
     for &head in copies.iter() {
         assert_eq!(copies[head], head);
     }
+    debug_assert_eq!(copies[RETURN_PLACE], RETURN_PLACE);
 
     copies
 }