This is presumably Rust's u8::to_ascii_lowercase rather than C's tolower since tolower is locale sensitive (which you don't care about) and has Undefined Behavi

This is presumably Rust's u8::to_ascii_lowercase rather than C's tolower since tolower is locale sensitive (which you don't care about) and has Undefined Behaviour (because of course it does it's a C function and who cares about correctness) Or possibly u8::make_ascii_lowercase which is the same function but with in-place mutation.