Running into segmentation fault with new closures

[cburgdorf]

I'm playing with the new closures and ran into a segmentation fault when I tried this:

    #![feature(unboxed_closures)]
    #![feature(overloaded_calls)]

    fn main() {
        let mut test = box 5i;

        let change = ref |:| { *test = 10 };
        change();

        println!("{}", test);
    }

Not sure if this was reported before or a known issue but I thought I better report it.

/cc @pcwalton

[bkoropoff]

I think this was caused by capture-by-ref closures actually being emitted as capture-by-value in trans, resulting in the box being freed twice. This was fixed by #17731. It might be worth adding a test for this specific case, though.