[jstrong]

I've had good luck in the past using From<T> and TryFrom<T> to go from State<A> -> State<B>. It's similar to what you propose insofar as there are only some transitions defined, just relying on the built-in traits instead of using your own. Since State::from(prev) is so much more ergonomic than the other ways you could manually construct an illegal transition, it's not much of a problem to prevent that in practice. But I would never use this approach without being able to generate code with macros, otherwise it'd be a pain.

you mention "never have to match against an enum" - I think that's the biggest thing I learned from my experiences, is that generics or traits + associated types are much nicer to use than enums for the same purposes. With an enum, you have to match to pull any data out. It becomes annoying quickly.