It's currently not possible to write a Rust function that's generic over a tuple with arbitrarily many entries, so they had to implement a trait by hand for tuple sizes up to 21 (presumably that limit was chosen arbitrarily).

Does Rust have a template or macro system that would facilitate generating up to e.g. 221 instead?

If I'm understanding correctly, some Regex features may require implementing more than 21 rules for a thorough and complete implementation.

Yes, rust declarative macros shine at exactly this kind of problem. And nom does use this to generate the implementation.

But for some reason it's customary in rust to stick to a lower number of autogenerated implementations. My first guess would be it might be a compile time issue?

https://docs.rs/nom/latest/src/nom/branch/mod.rs.html#245

The workaround is just to nest multiple `alt`s inside each other.