Bail out NRVO if _0 is mutated after the candidate assignement.

Author: cjgillot

Diff for: compiler/rustc_mir_transform/src/nrvo.rs

@@ -147,15 +147,20 @@ fn find_local_assigned_to_return_place(start: BasicBlock, body: &mut Body<'_>) -
         if vis.seen_indirect {
             discard_borrowed_locals(&mut assigned_locals);
         }
+        if assigned_locals.contains(RETURN_PLACE) {
+            return None;
+        }
 
         for (statement_index, stmt) in bbdata.statements.iter().enumerate().rev() {
             if let StatementKind::Assign(box (lhs, ref rhs)) = stmt.kind
                 && lhs.as_local() == Some(RETURN_PLACE)
-                && let Rvalue::Use(rhs) = rhs
-                && let Some(rhs) = rhs.place()
-                && let Some(rhs) = rhs.as_local()
-                && !assigned_locals.contains(rhs)
             {
+                let Rvalue::Use(rhs) = rhs else { return None };
+                let rhs = rhs.place()?;
+                let rhs = rhs.as_local()?;
+                if assigned_locals.contains(rhs) {
+                    return None;
+                }
                 return Some(rhs);
             }
 
@@ -165,6 +170,9 @@ fn find_local_assigned_to_return_place(start: BasicBlock, body: &mut Body<'_>) -
             if vis.seen_indirect {
                 discard_borrowed_locals(&mut assigned_locals);
             }
+            if assigned_locals.contains(RETURN_PLACE) {
+                return None;
+            }
         }
 
         match body.basic_blocks.predecessors()[block].as_slice() {

Diff for: tests/mir-opt/nrvo_miscompile_111005.constant.RenameReturnPlace.diff

@@ -0,0 +1,16 @@
+- // MIR for `constant` before RenameReturnPlace
++ // MIR for `constant` after RenameReturnPlace
+  
+  fn constant(_1: char) -> char {
+      let mut _0: char;
+      let mut _2: char;
+  
+      bb0: {
+-         _2 = _1;
++         _0 = _1;
+          _0 = const 'b';
+-         _0 = _2;
+          return;
+      }
+  }
+  

Diff for: tests/mir-opt/nrvo_miscompile_111005.multiple.RenameReturnPlace.diff

@@ -8,9 +8,8 @@
 
       bb0: {
           _2 = _1;
--         _3 = _1;
--         _0 = _3;
-+         _0 = _1;
+          _3 = _1;
+          _0 = _3;
           _0 = _2;
           _2 = const 'b';
           return;

Diff for: tests/mir-opt/nrvo_miscompile_111005.rs

@@ -54,6 +54,17 @@ pub fn multiple(arg: char) -> char {
     })
 }
 
+// EMIT_MIR nrvo_miscompile_111005.constant.RenameReturnPlace.diff
+#[custom_mir(dialect = "runtime", phase = "initial")]
+pub fn constant(arg: char) -> char {
+    mir!({
+        let temp = arg;
+        RET = 'b';
+        RET = temp;
+        Return()
+    })
+}
+
 // EMIT_MIR nrvo_miscompile_111005.projection.RenameReturnPlace.diff
 #[custom_mir(dialect = "runtime", phase = "initial")]
 pub fn projection(arg: char) -> (char, u8) {
@@ -143,6 +154,7 @@ fn main() {
     assert_eq!(indirect('a'), 'a');
     assert_eq!(moved('a'), 'a');
     assert_eq!(multiple('a'), 'a');
+    assert_eq!(constant('a'), 'a');
     assert_eq!(call('a'), 'a');
     assert_eq!(call_ok('a'), 'a');
     assert_eq!(multiple_return('a', true), 'z');