[DasIch]

Rust basically offers a Monad-like API there. That's perfectly fine and a well established pattern.

That has nothing to do with primitive control flow nor is that an indication of if_weekday appearing anytime soon.

That being said having primitive control flow implemented as methods also has precedent with languages like Smalltalk or Self. That may be unusual but I don't think that's necessarily bad. I would be interested in reading about why this is bad design though.