rename Owned to Send

[thestinger]

Types with the Owned kind are sendable, and those without it are not sendable. It does not represent ownership because a shared ownership type like those in std::arc have to be Owned if they're intended to be sendable.

I think this is going to confuse people and lead to bugs. It's tempting to treat Owned as meaning the types are owned, but that ends up treating ARCs incorrectly.

[graydon]

Note: it used to be called Send

[thestinger]

@graydon: yup, I think the old name was better :)

I wasn't around when it was changed, so I don't know if naming it Owned pre-dated shared-ownership sendable types.

[glaebhoerl]

Do the various clients even want the same semantics? It's being used in some places (e.g. for ruling out cycles) that have nothing to do with tasks. Maybe some of them actually do want "single ownership of whole tree" and not just "safe to send between tasks"?

[thestinger]

@glehel: the places using it to rule out cycles don't really care what it means, only that they can mark themselves as #[non_send] and then require Send as a trait bound for the constructor. It's fine for them to contain @ as long as they don't cycle back to themselves (Const allows that for Rc).

[thestinger]

This landed as part of #7451.