Borrowed @mut boxes are not unborrowed during unwinding

[brson]

use std::task::spawn_unlinked;
use std::unstable::finally::Finally;

fn main() {
    let (port, chan) = stream();
    do spawn_unlinked {
        let buf = @mut ~[0];
        do (|| {
            fn take(_b: &mut ~[uint]) {
                fail!();
            }
            take(&mut *buf);
        }).finally {
            // This fails because buf is already borrowed
            let x = &*buf;
            assert!(*x == ~[0]);
            chan.send(());
        }
    }

    // This will fail if the finally block doesn't complete
    port.recv();
}

[brson]

This will also leak one of the two thrown exceptions but that's a different bug.

[nikomatsakis]

This is a pretty trivial fix I suspect. the code in middle/trans/write_guard.rs specifies that the "return_to_mut" cleanup only occurs on the non-fail path.

[emberian]

Triage bump

[thestinger]

There are no more @mut pointers.