borrowck unsoundness

``` rust
fn read_lines_borrowed() -> ~[&str] {
    let rawLines: ~[~str] = ~[~"foo  ", ~"  bar"];
    rawLines.iter().map(|l| l.trim()).collect()
}

fn main() {
    println!("{}", read_lines_borrowed());
}
```

read_lines_borrowed is not rejected. but the value it returns cannot possibly be valid, since rawLines is going to be free'd after it returns, leaving the collected references dangling.
