internal compiler error: Got a fat pointer where a scalar was expected

[Amanieu]

The following code causes an internal error in miri:

use std::rc::Rc;
use std::fmt::Debug;

fn main() {
    let p = Rc::new(1) as Rc<Debug>;
    let a: *const Debug = &*p;
    let r = Rc::into_raw(p);
    a == r;
}

error: internal compiler error: src/librustc_mir/interpret/operand.rs:73: Got a fat pointer where a scalar was expected

[RalfJung]

This will be fixed by #623