drop in `Box<Box<T>>` as `Box<Trait>` broken

[oli-obk]

rust-lang/rust#39230 changed some things around with boxed, especially that there is a "Drop impl" now: https://github.com/rust-lang/rust/blame/master/src/liballoc/boxed.rs#L297 (it's empty, doesn't drop its contents)

We should just wait for rustc to fix it.

repro:

#![feature(box_syntax)]

struct DroppableStruct;

static mut DROPPED: bool = false;

impl Drop for DroppableStruct {
    fn drop(&mut self) {
        unsafe { DROPPED = true; }
    }
}

trait MyTrait { fn dummy(&self) { } }
impl MyTrait for Box<DroppableStruct> {}

struct Whatever { w: Box<MyTrait+'static> }
impl  Whatever {
    fn new(w: Box<MyTrait+'static>) -> Whatever {
        Whatever { w: w }
    }
}

fn main() {
    {
        let f: Box<_> = box DroppableStruct;
        let _a = Whatever::new(box f as Box<MyTrait>);
    }
    assert!(unsafe { DROPPED });
}

[eddyb]

We don't plan to fix this in rustc, you have to check if it's a Box specifically.