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`.