Preparing the iteration ABI for move-only types

[Ben_Cohen]

@Joe_Groff

Collection.first is a property requirement of type Element? . For most collection implementations that store non-optional Element s, this wouldn't be implementable, since you'd somehow have to form a temporary Optional<Element> value without modifying the collection or copying the element out of it.

.first is one of the dubious customization points that probably should be dropped.

Would it make a difference if it were a set as well as a get computed property? That's been considered in the past as separately useful. Then maybe getting it for a move-only collection could also use the set part to consume the collection.

@Joe_Groff

SetAlgebra.insert has the signature mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element) , where the return tuple contains either the new member inserted, or the old member already in the set. This is not implementable for a move-only element since the value would have to be copied out of the set to be returned.

I guess this would need to be converted to return Element?, with nil for nothing displaced.