not permutations but combinations, cba to change

permutations.rs

  fn perms<T: Clone>(values: &[T], perm_len: usize) -> Vec<Vec<T>> {
    let mut out = Vec::new();
    perms_inner(&mut out, &values, Vec::with_capacity(perm_len), perm_len);
    return out;
}

fn perms_inner<T: Clone>(out: &mut Vec<Vec<T>>, values: &[T], perm: Vec<T>, perm_len: usize) {
    if perm.len() == perm_len {
        out.push(perm);
        return;
    }

    for v in values {
        let mut new_perm = perm.clone();
        new_perm.push(v.clone());
        perms_inner(out, values,  new_perm, perm_len);
    }
}

fn main() {
    for p in perms(&['a', 'b', 'c', 'd'], 2) {
        println!("{:?}", p);
    }
}