iter_fallback

An iterator that falls back to a second iterator when the first one is exusted.

 assert_eq!(
            vec![1, 2, 3, 4, 5],
            vec![1, 2, 3]
                .into_iter()
                .fallback(vec![0, 0, 0, 4, 5].into_iter())
                .collect::<Vec<_>>()
        )