#[may_dangle] types can only be dropped, not moved

[theemathas]

Moving a #[may_dangle] type in Drop::drop asserts the validity of references stored in that type, which is unsound. For example, the following code is UB according to Miri:

#![feature(dropck_eyepatch)]
#![allow(unused)]

struct Thing<T>(Option<T>);

unsafe impl<#[may_dangle] T> Drop for Thing<T> {
    fn drop(&mut self) {
        let _ = self.0.take();
    }
}

fn main() {
    let thing;
    {
        let a = 1;
        thing = Thing(Some(&a));
    }
    // thing is dropped here
}