That makes more sense.

Might be the simplest and fastest solution.

This solution counts the chars for every char. It's redudant work being done. Might be slow.

I honestly do not remember. Presumably the compiler complained when I implemented it with the reference I create in the match pattern, so I explicitly re-borrowed to work around that. Just a guess. There's nothing about the code that stands out to me as actually needing the re-borrow (actually, it perhaps has to do with lifetimes: the initial reference only lives until the if let Some()... expression leaves scope. Although, from my understanding, immutable borrows are Copy, so this shouldn't really be a problem)

Why do you use &*l instead of just l? For me both is working.

You wanna say, there should be more tests and maybe random tests?