playground.rs

fn fst<T1, T2>((t1, _t2): (T1, T2)) -> T1 {
    t1
}

fn snd<T1, T2>((_t1, t2): (T1, T2)) -> T2 {
    t2
}

fn main() {
    let xs = vec![42, 43, 44, 45];
    let ys: Vec<usize> = xs.iter().enumerate().map(fst).collect();
    println!("{:?}", ys);

    let xs = vec![42, 43, 44, 45];
    let ys: Vec<&i32> = xs.iter().enumerate().map(snd).collect();
    println!("{:?}", ys);
}