Those are probably not for the same use case. Crystal is a whole another language, and you'd have to migrate your whole codebase there. This (in my understanding) is more for compiling certain heavily used functions in your otherwise Ruby codebase to get a quick performance boost.
Doesn't the new ruby JIT already do the same thing?
I haven't looked into the new ruby JIT too deeply, but as far as I remember it had several limitations that made it slow Rails apps down instead. This would make it easier to select and optimise for specific methods instead.
Yeah, this is pretty much how the JIT works.
Current Ruby JIT doesn't take any type hint.
It's certainly related -- not sure what kinds if specialisation the new ruby JIT does.