Have you thought of two state machines communicating?

[RustMania]

Hi,
have you thought of the use case of a key refresh in TSS lib? Two independent machines have to send messages sometimes to each other during key refresh. Abstract sink in messages_tx forwards all msgs to another layer. In case two machines need to talk directly occasionally, that layer should accommodate a logic to bridge machines' channels, and this logic, in my opinion, depends on the protocol and it does not look like a part of that layer's architecture. Your new design allows simple and transparent implementation of the feature.
It could also be a subject for the article in Medium, part 2.

Best
Oleg