It's a little bit surprising for me we can re-assign to self in a yielding borrow. In the original pitch [Pitch] Modify and read accessors, it was spelled as mutating read, so maybe this should be mutating yielding borrow?
PS: I think the gap is, maybe we should clarify the semantics of self in a yielding borrow of non-copyable types in the first place: is it a borrowing self or a consuming self? And, is consuming yielding borrow a valid accessor?