This would work if `Integer` would implement the `Copy` trait. But I guess that type is heavy enough for it to be too expensive, therefore forcing you to explicitly call `clone()`.

That `Integer` type can't implement `Copy` because it manages a resource, meaning you can't just do a simple memcpy of the type.

  let b = &a + &a;

will work, but I agree it's unfortunate that this is necessary.