Unsoundness: free variable in proc can be consumed twice

[kmcallister]

use std::io;

fn make_proc() -> proc() {
    let s = ~"foo";
    proc() {
        drop(s);
        io::stderr().write_str(s).unwrap();
    }
}

fn main() {
    let p = make_proc();
    p();
}

$ rustc -v
rustc 0.10-pre (68a4f7d 2014-02-24 12:42:02 -0800)
host: x86_64-unknown-linux-gnu

$ rustc foo.rs
$ ./foo
Segmentation fault

(Apologies if this has already been fixed; I'm using Servo's rustc.)

[flaper87]

Sounds like a duplicate of #12223