// Non-copyable types.
struct Empty;
struct Null;

// A trait generic over `T`.
trait DoubleDrop<T> {
    // Define a method on the caller type which takes an
    // additional single parameter `T` and does nothing with it.
    fn double_drop(self, _: T);
}

// Implement `DoubleDrop<T>` for any generic parameter `T` and
// caller `U`.
impl<T, U> DoubleDrop<T> for U {
    // This method takes ownership of both passed arguments,
    // deallocating both.
    fn double_drop(self, _: T) { std::hint::black_box(true); }
}

fn main() {
    let empty = Empty;
    let null  = Null;

    // Deallocate `empty` and `null`.
    empty.double_drop(null);

    Null.double_drop(Empty);
}