Re-use struct fields on drop; was: Drop(&mut self) vs. drop(self)

[matklad]

See https://github.com/rust-lang/rust/issues/4330#issuecomment-26852226

The root problem, as I understand it, is that if you have fn drop(self), you somehow need to not Drop the self the second time.

Like

fn foo(x: X) {
     // calling drop here
}

impl DropByValue for X {
    fn drop(self) {
        foo(self); // what does this mean?
    }
}