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

language design
22

I do not have time to implement it right now (I say as I struggle to prevent myself from going and implementing it), but a #[derive(AfterDrop)] could be set up for this. Given that a safe abstraction can be built without language support, getting it into the stdlib is going to be an uphill battle if you want to fight for it; the Rust stdlib prefers to stick to fundamental abstractions.

Playground example

#[derive(AfterDrop)]
struct Guard {
    #[after_drop]
    resource: ManuallyDrop<Resource>,
    trivial: Trivial,
}

// derived
impl Drop for Guard {
    fn drop(&mut self) {
        let mut resource;
        unsafe {
            resource = mem::uninitialized();
            ptr::copy_nonoverlapping(&*self.resource, &mut resource, 1);
        }
        Self::after_drop(resource);
    }
}

You could probably use frunk's (Labelled)Generic to make AfterDrop<HList![..]> a trait, but it seems unnecessary. You could make the trait AfterDrop<(..,)> if you really want it to be a trait; the derive macro can construct the flat tuple (wheras HList is a cons-list tuple).