Following program segfaults using rust 0.5 on Fedora 17:

extern mod std;

use core::task::;
use std::;

fn main() {
let value = 0;
let marc = arc::MutexARC(value);

for int::range(0, 5) |_| {
do spawn {
do marc.access |v| { *v += 1; }
}
}
}