rvalues don't live long enough when using "ref"

This code does a use-after-free:

```
fn main() {
    let msg;
    match some(~"Hello") {
        some(ref m) => {
            msg = m;
        },  
        none => { fail }
    }   
    io::println(*msg);
}
```

Should it not compile? Or should liveness/borrowck ensure that the string live as long as the escaped reference?
