State-machine with variadic generics and trait specialization?

I tend to represent a lot of programs as state-machines as most of what I write are implementation of protocols. The transaction layer of protocols can typically be implemented as a state-machine of message sequences. Right now, I represent this state-machine using the type state pattern

impl Transaction for StateA {}
impl Transaction for StateB {}

But I was thinking how nice it would be to represent the state transitions as purely sequences of the messages themselves, implemented with tuples + variadic generics + trait specialization and checked at compile time.

trait Transaction { fn next(self: Box<Self>) -> Box<dyn Transaction>; }
impl Transaction for (MessageA, ...MessageB, MessageC)

Implementation where first state is MessageA, then zero or more MessageB, terminated by MessageC.

Then if I want a catch-all for the above implementation I can combine specialization with the variadic

impl<T> Transaction for (...T, MessageC) // default implementation

This will probably never be possible in Rust. But man do I want this.

I think Coq is a better language to do this.

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