Mutability of variables captured by procs is ignored

[huonw]

fn main() {
    let mut x = 1;
    proc() { x = 2 };
}

proc-mut.rs:3:14: 3:15 error: cannot assign to immutable captured outer variable in a heap closure
proc-mut.rs:3     proc() { x = 2 };
                           ^

[emberian]

cc @nikomatsakis @pnkfelix

[alexcrichton]

Closing as a dupe of #11958, moving the example to that bug.

[huonw]

Closing in sloooowww motion.