error[E0204]: the trait `Copy` may not be implemented for this type
  --> $DIR/union-copy.rs:24:6
   |
LL |     a: String
   |     --------- this field does not implement `Copy`
...
LL | impl Copy for W {} //~ ERROR the trait `Copy` may not be implemented for this type
   |      ^^^^

error: aborting due to previous error

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