src/test/run-pass/mutable-huh-variance-vec2.rs

// error-pattern: mismatched types

fn main() {
    let v = ~[~[0]];

    // This is ok because the outer vec is covariant with respect
    // to the inner vec. If the outer vec was mut then we
    // couldn't do this.
    fn f(&&v: ~[const ~[const int]]) {
    }

    f(v);
}