ICE related to pointer comparisons

[whitequark]

Repro:

pub trait Widget {}

pub trait Container {
    fn remove(&mut self, widget: &Widget) -> Box<Widget>;
}

pub struct BoxLayout {
    children: Vec<Box<Widget>>,
}

impl Container for BoxLayout {
    fn remove(&mut self, widget: &Widget) -> Box<Widget> {
        let index = self.children.iter().position(|elem| {
            (&**elem as *const _) == (widget as *const _)
        });
        panic!();
    }
}

fn main(){}

Backtrace: unable to acquire; rustc dies with SIGILL inside the unwinder.

Assertion:

thread 'rustc' panicked at 'assertion failed: self.appropriate_rvalue_mode(bcx.ccx()) == ByValue', src/librustc_trans/trans/datum.rs:634

[mitaa]

dup of #25768 (see rust-lang/rfcs#1135)