The blocks might end up calling each other

That doesn't matter if they're not called.

I like it in principle, but give it a more formal construct that includes the promise that no more than 1 of them will be asked for and they are not interdependant.

IMHO that's a (premature) optimization that can be added later, when a compiler actually wants to do such an optimization, via a trait or so.

That said, if somebody comes up with a nice syntax for them, why not?