Do not automatically DerefMut ManuallyDrop through union references

Author: eggyal

compiler/rustc_hir_typeck/src/place_op.rs

@@ -285,9 +285,6 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // Clear previous flag; after a pointer indirection it does not apply any more.
                 inside_union = false;
             }
-            if source.is_union() {
-                inside_union = true;
-            }
             // Fix up the autoderefs. Autorefs can only occur immediately preceding
             // overloaded place ops, and will be fixed by them in order to get
             // the correct region.
@@ -335,6 +332,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
                 self.typeck_results.borrow_mut().adjustments_mut().insert(expr.hir_id, adjustments);
             }
+            // Now that any autoderef of this expression component is complete, if it resolved to a
+            // `union` then remember we're inside a `union` as we iterate over subsequent expression
+            // components. If this test were performed prior to autoderef resolution, it would only
+            // detect expression components of the (owned) `union` type and not references thereto.
+            // See <https://github.com/rust-lang/rust/issues/141621>.
+            if source.is_union() {
+                inside_union = true;
+            }
 
             match expr.kind {
                 hir::ExprKind::Index(base_expr, ..) => {

tests/ui/union/union-deref.rs

@@ -16,11 +16,26 @@ fn main() {
     unsafe { (*u.f).0.push(0) }; // explicit deref, this compiles
     unsafe { u.f.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
 
+    unsafe { (*(&mut u).f).0 = Vec::new() }; // explicit deref, this compiles
+    unsafe { (&mut u).f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { &mut (*(&mut u).f).0 }; // explicit deref, this compiles
+    unsafe { &mut (&mut u).f.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { (*(&mut u).f).0.push(0) }; // explicit deref, this compiles
+    unsafe { (&mut u).f.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+
+
     let mut u : U2<Vec<i32>> = U2 { x: () };
     unsafe { (*u.f.0).0 = Vec::new() }; // explicit deref, this compiles
     unsafe { u.f.0.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
     unsafe { &mut (*u.f.0).0 }; // explicit deref, this compiles
     unsafe { &mut u.f.0.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
     unsafe { (*u.f.0).0.push(0) }; // explicit deref, this compiles
     unsafe { u.f.0.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+
+    unsafe { (*(&mut u).f.0).0 = Vec::new() }; // explicit deref, this compiles
+    unsafe { (&mut u).f.0.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { &mut (*(&mut u).f.0).0 }; // explicit deref, this compiles
+    unsafe { &mut (&mut u).f.0.0 }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
+    unsafe { (*(&mut u).f.0).0.push(0) }; // explicit deref, this compiles
+    unsafe { (&mut u).f.0.0.push(0) }; //~ERROR not automatically applying `DerefMut` on `ManuallyDrop` union field
 }

tests/ui/union/union-deref.stderr

@@ -26,7 +26,34 @@ LL |     unsafe { u.f.0.push(0) };
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
 error: not automatically applying `DerefMut` on `ManuallyDrop` union field
-  --> $DIR/union-deref.rs:21:14
+  --> $DIR/union-deref.rs:20:14
+   |
+LL |     unsafe { (&mut u).f.0 = Vec::new() };
+   |              ^^^^^^^^^^
+   |
+   = help: writing to this reference calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+  --> $DIR/union-deref.rs:22:19
+   |
+LL |     unsafe { &mut (&mut u).f.0 };
+   |                   ^^^^^^^^^^
+   |
+   = help: writing to this reference calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+  --> $DIR/union-deref.rs:24:14
+   |
+LL |     unsafe { (&mut u).f.0.push(0) };
+   |              ^^^^^^^^^^
+   |
+   = help: writing to this reference calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+  --> $DIR/union-deref.rs:29:14
    |
 LL |     unsafe { u.f.0.0 = Vec::new() };
    |              ^^^^^
@@ -35,7 +62,7 @@ LL |     unsafe { u.f.0.0 = Vec::new() };
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
 error: not automatically applying `DerefMut` on `ManuallyDrop` union field
-  --> $DIR/union-deref.rs:23:19
+  --> $DIR/union-deref.rs:31:19
    |
 LL |     unsafe { &mut u.f.0.0 };
    |                   ^^^^^
@@ -44,13 +71,40 @@ LL |     unsafe { &mut u.f.0.0 };
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
 error: not automatically applying `DerefMut` on `ManuallyDrop` union field
-  --> $DIR/union-deref.rs:25:14
+  --> $DIR/union-deref.rs:33:14
    |
 LL |     unsafe { u.f.0.0.push(0) };
    |              ^^^^^
    |
    = help: writing to this reference calls the destructor for the old value
    = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
 
-error: aborting due to 6 previous errors
+error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+  --> $DIR/union-deref.rs:36:14
+   |
+LL |     unsafe { (&mut u).f.0.0 = Vec::new() };
+   |              ^^^^^^^^^^^^
+   |
+   = help: writing to this reference calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+  --> $DIR/union-deref.rs:38:19
+   |
+LL |     unsafe { &mut (&mut u).f.0.0 };
+   |                   ^^^^^^^^^^^^
+   |
+   = help: writing to this reference calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: not automatically applying `DerefMut` on `ManuallyDrop` union field
+  --> $DIR/union-deref.rs:40:14
+   |
+LL |     unsafe { (&mut u).f.0.0.push(0) };
+   |              ^^^^^^^^^^^^
+   |
+   = help: writing to this reference calls the destructor for the old value
+   = help: add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor
+
+error: aborting due to 12 previous errors