Heap while a good idea, is going to take too long, there is a faster approach, that wouldn't require making all the permutations.

I need to get more comfortable making my own iterators, and have utilized the fact that any integer % 10, returns the last digit.

iter().cycle() is also something that, I had no idea about. I absolutely love it.

https://docs.rs/itertools/0.10.1/itertools/fn.join.html
For people that are interested