Well, for whatever reason, it doesn't meet their requirements... See the String section of https://github.com/fish-shell/fish-shell/blob/master/doc_int...

edit: here is a more specific rationale: https://github.com/fish-shell/fish-shell/pull/9512#discussio...

The Cygwin issue isn't strings (well, that could be another issue) but that Rust doesn't support Cygwin in the first place, at least according to the comments in the linked thread.