# Error code E0204

The `Copy` trait was implemented on a type which contains a field that doesn't implement the `Copy` trait.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    struct Foo {
        foo: Vec<u32>,
    }

    impl Copy for Foo { } // error!
    }

The `Copy` trait is implemented by default only on primitive types. If your type only contains primitive types, you'll be able to implement `Copy` on it. Otherwise, it won't be possible.

Here's another example that will fail:

    #![allow(unused)]
    fn main() {
    #[derive(Copy)] // error!
    struct Foo<'a> {
        ty: &'a mut bool,
    }
    }

This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this differs from the behavior for `&T`, which is always `Copy`).