also detect DerefMut in nested union fields

src/test/ui/union/union-deref.rs

@@ -4,10 +4,16 @@
 
 use std::mem::ManuallyDrop;
 
-union U<T> { x:(), f: ManuallyDrop<(T,)> }
+union U1<T> { x:(), f: ManuallyDrop<(T,)> }
+
+union U2<T> { x:(), f: (ManuallyDrop<(T,)>,) }
 
 fn main() {
-    let mut u : U<Vec<i32>> = U { x: () };
+    let mut u : U1<Vec<i32>> = U1 { x: () };
     unsafe { (*u.f).0 = Vec::new() }; // explicit deref, this compiles
     unsafe { u.f.0 = Vec::new() }; //~ERROR not automatically applying `DerefMut` on 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 union field
 }

src/test/ui/union/union-deref.stderr

@@ -1,5 +1,5 @@
 error: not automatically applying `DerefMut` on union field
-  --> $DIR/union-deref.rs:12:14
+  --> $DIR/union-deref.rs:14:14
    |
 LL |     unsafe { u.f.0 = Vec::new() };
    |              ^^^
@@ -7,5 +7,14 @@ LL |     unsafe { u.f.0 = Vec::new() };
    = help: writing to this field 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 previous error
+error: not automatically applying `DerefMut` on union field
+  --> $DIR/union-deref.rs:18:14
+   |
+LL |     unsafe { u.f.0.0 = Vec::new() };
+   |              ^^^^^^^
+   |
+   = help: writing to this field 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 2 previous errors