error[E0774]: `derive` may only be applied to `struct`s, `enum`s and `union`s
  --> $DIR/macros-in-extern-derive.rs:2:5
   |
LL |     #[derive(Copy)]
   |     ^^^^^^^^^^^^^^^ not applicable here
LL |     fn f();
   |     ------- not a `struct`, `enum` or `union`

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0774`.