combinations.rb

def combos(items, k, s = 0, combo = [])
  return [combo] if k == 0

  (s .. items.size - k).flat_map do |i|
    combos(items, k - 1, i + 1, combo + [items[i]])
  end
end

combos([:a, :b, :c, :d, :e, :f], 3).each do |combo|
  p combo
end

combinations.rs

use std::iter;

fn combos<T>(items: &[T], k: usize) -> impl Iterator<Item = Vec<&T>> {
    combo_helper(items, Vec::new(), k)
}

fn combo_helper<'a, T>(
    items: &'a [T],
    combo: Vec<&'a T>,
    k: usize,
) -> Box<dyn Iterator<Item = Vec<&'a T>> + 'a> {
    if k == 0 {
        return Box::new(iter::once(combo));
    }

    let iter = (0..=items.len() - k).flat_map(move |i| {
        let mut cc = combo.clone();
        cc.push(&items[i]);
        combo_helper(&items[i + 1..], cc, k - 1)
    });

    Box::new(iter)
}

fn main() {
    for combo in combos(&['a', 'b', 'c', 'd', 'e', 'f'], 3) {
        println!("{:?}", combo);
    }
}