fn foo<T: Clone>(slices: &[&[T]]) -> Vec<T> {
    slices
        .iter()
        .map(|s| s.iter().cloned())
        .flatten()
        .collect::<Vec<T>>()
}

fn bar<T: Clone>(slices: &[&[T]]) -> Vec<T> {
    let n: usize = slices.iter().fold(0, |acc, s| acc + s.len());
    let mut vec = Vec::with_capacity(n);
    slices.iter().for_each(|s| {
        vec.extend_from_slice(s);
    });
    vec
}

fn main() {
    let a: [u8; 4] = 42u32.to_le_bytes();
    let v: Vec<u8> = vec![1, 2, 3, 4, 5, 6];

    let j: Vec<u8> = foo(&[&a, &v, &a, &a, &a, &v]);
    println!("{:?}", j);

    let j2: Vec<u8> = bar(&[&a, &v, &a, &a, &a, &v]);
    println!("{:?}", j2);
}