[atq2119]

Essentially, yes. What I don't see is their claim that it's a "much greater annotation burden". Compared to what? Rust today just doesn't allow this at all, and if you use a wrapper type to simulate it, you definitely end up with more "annotations" (boilerplate).

[couchand]

FWIW It's not at all clear to me how this requirement would be implemented in practice: "This syntax would explicitly be limited to orphan implementations."

[atq2119]

Maybe I'm missing something, but the compiler can tell whether an implementation is an orphan. That's how you get an error message today if you try to write one. So I don't know what difficulty you have in mind.