Moves from `self` during the drop hook

internals ideas (deprecated)
3

I didn’t have time to write this up anywhere, but I came up with a simpler system a few days ago:

#[lang="interior"]
struct Interior<T>(T);

impl<T> Interior<T> {
    unsafe fn of(x: T) -> Interior<T> {
        Interior(x)
    }
}

#[lang="drop"]
trait Drop {
    fn drop(self: Interior<Self>);
}

Interior<T> would provide field access and pattern-matching as if it were T, but otherwise act like a different type entirely, e.g. it has none of the methods implemented on T.

It wouldn’t have Drop implemented on it, so dropping Interior<T> will only destroy the fields, but won’t call T's Drop implementation, solving the infinite drop recursion issue.

We can also remove the restriction on calling Drop::drop directly, since it needs Interior<T> and that’s unsafe to get from T.

Personally, I think DropPtr is an unnecessary complication and I won’t support it.

Linear types don’t need the ability, but I am sympathetic to finally fixing Drop, if we don’t make it worse.

cc @nikomatsakis

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