> What we’re doing is fine. You should get your serialization and I should get mine. But if either of us declares, process-wide, that one of us has determined the One True Serialization of PairOfInts, I think we are wrong.

Well, fine, but then you need to actually implement a module system or something. Currently trait impls are program-wide, and if you say that you're not allowed to make global impls of a trait then that's the same as saying you're not allowed to implement traits at all.

Rust’s orphan rule has the property that there is no spooky action at all distance in terms of program semantics. If I write a library, my library behaves the same way regardless of whether the main program imports a different library.

In any case, the OP’s proposed “incoherent” scheme actually is a module system of sorts for conflicting trait impls, and it seems about right for something like serialization.