error[E0204]: the trait `Copy` may not be implemented for this type
  --> $DIR/union-copy.rs:12:15
   |
LL |     a: std::mem::ManuallyDrop<String>
   |     --------------------------------- this field does not implement `Copy`
...
LL | impl Copy for W {}
   |               ^
   |
note: the `Copy` impl for `ManuallyDrop<String>` requires that `String: Copy`
  --> $DIR/union-copy.rs:8:8
   |
LL |     a: std::mem::ManuallyDrop<String>
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0204`.