Pre-pre-RFC: `NoDrop` marker trait

[earthengine]

I have an alternative solution

Christmas proposal: "Don't call" annotation

Marry Christmas to you all. There are two different motivations lead to this proposal. One is in this existing proposal: https://internals.rust-lang.org/t/pre-pre-rfc-nodrop-marker-trait/15682/20 Me, and the author of the post above wanted to have a way to say "this object cannot be dropped unless you have done the check that it is a good time to drop it". My precise challenge is that the object may have active reference leaked so it may not in a safe status to be dropped. To write unsafe co…

With this we don't use a marker trait, instead we have a special annotation, so we can write our own marker object (line PhantomPinned).

struct PhantomNoDrop;
impl Drop for PhantomNoDrop {
    #[do_not_call]
    fn drop(&mut self) { unreachable!() }
}

With this any type that in its generated drop clue cannot call this drop implementation otherwise the compiler will complain.