It's a good implementation, but by nature converting a number into a string is much more complicated than a simple while loop which runs a dozen or so times.

clean but slow

to_string() is essentially a wrapper for impl Display. I think this is the u64 implementation of it: https://doc.rust-lang.org/src/core/fmt/num.rs.html#199-258 Looks like there is some cost, but I can't speak to how much of it is optimized out in release mode.

Does to_string cost much?