fn take_vec<T>(_v: Vec<T>) {} // requires a Vec

fn main() {
    // collect returns any kind of collection
    let x = [1, 2, 3].into_iter().collect();
    // the type of x is not determined yet

    take_vec(x); // now x has to be a Vec
    // if you remove this line, this fails to compile because it does not know what type x should be
}