> I don't understand the concern regarding the orphan rule or "downstream usage by dependents" - that's not relevant here and no language semantics change.
I don't understand how you don't think it's relevant or a semantic change. If I don't put Debug on my type, and I export it in a library, you don't have Debug today. Under what you're proposing, you'd be able to implement Debug on it by using the Debug implementation on it. If you don't allow that in your system, then it becomes impossible to ever Debug types from dependencies (unless the library authors just manually invoke the Debug implementation on their types I guess, but at that point you're back to the same thing we already have, just with a more annoying way of doing it)
I don't understand how you don't think it's relevant or a semantic change. If I don't put Debug on my type, and I export it in a library, you don't have Debug today. Under what you're proposing, you'd be able to implement Debug on it by using the Debug implementation on it. If you don't allow that in your system, then it becomes impossible to ever Debug types from dependencies (unless the library authors just manually invoke the Debug implementation on their types I guess, but at that point you're back to the same thing we already have, just with a more annoying way of doing it)