Comment by JoshTriplett

Yes, that's the downside of relaxing the orphan rule. That doesn't mean there has to be a way for the top-level crate to work around that, other than avoiding having both of those in its dependency tree.

Ideally, people would tend to put trait implementations for a given pair of crates in a unique library crate, implement them in the obvious way, and provide that as a library for people to use.