fn bad(vec1: Vec<u8>, vec2: Vec<u8>) -> Vec<(u8, Option<u8>)> {
    let it1 = vec1.into_iter().rev();
    let mut it2 = vec2.into_iter().rev();
    it1.map(|v1| {
        (v1, it2.next())
    }).rev().collect()
}

fn good(vec1: Vec<u8>, vec2: Vec<u8>) -> Vec<(u8, Option<u8>)> {
    let it1 = vec1.into_iter().rev();
    let mut it2 = vec2.into_iter().rev();
    let vec3: Vec<_> = it1.map(|v1| {
        (v1, it2.next())
    }).collect();
    vec3.into_iter().rev().collect()
}

pub fn main() {
    let count = vec![0, 1, 2, 3, 4, 5];
    println!("Should be  : {:?}'),", good(count.clone(), vec![0, 1, 2, 3]));
    println!("but instead: {:?}", bad(count.clone(), vec![0, 1, 2, 3]));
}