Ruby iteration primitive isn’t block based, either.

Of course it is. `for a in b` desugars to `b#each`, not the other way around.

How do “while” and “until” desugar? Kernel#loop?

Those are two rare exceptions and you're right they don't desugar.

Those are not “exceptions” to Ruby’s primitives, those are Ruby’s primitives. Things that desugar to methods aren’t primitives.

The thing is, they’re rare in practice for cultural reasons.