Trait coherency regression in 1.78 about rust

[lqd]

This is from #120584, I think it was expected because of existing brokenness but I may be wrong and see no FCP, so will cc its author @compiler-errors.

[compiler-errors]

Yeah, this is a legitimate bug in the code sample that was shared. Downstream crates can implement Vendor with a Processor type that implements VendorProcessor<EventGroup = MyEvent>. Coherence doesn't take into account the privacy of the items, so privacy here doesn't really matter.

For the record, this also would have begun to fail when we land the new trait solver for coherence (#121848) which will ideally happen soon.

[w4]

Thanks guys. It is clear now that this code sample only compiled because of the HRTB, but it was nice because the trait solver did actually work correctly here for our particular use case, as it was able to disambiguate between our usages of VendorProcessor<MyEvent> and VendorProcessor<Group> but I understand other 99% of use cases where this is broken. Closing.