Slowly.

Objects well-suited to transfer would be shallow mutable structures with immutable data "underneath". Immutable data can be shared, so they'd be ignored by the transfer logic.

Another approach likely to be common would be not transferring objects at all, but sending proxy objects to other guilds which transparently marshal method invocations between those guilds and the object's home guild. For large mutable complexes which are intertwined with everything a guild does, that's probably a more manageable approach.

> Making things worse, you would have to make sure that the ownership transfer is atomic - no other part of the program should see the data structure in a “partially transferred” state

This is easy. You just run the transfer operation with the guild's mutex locked. No other guild can have a reference to a mutable object, and an immutable object doesn't need to be transferred.

> In Ruby, what exactly should happen here? Have these guys really thought about the possibilities?

It seems obvious that every linked mutable object is invalidated, so Qux will be transferred.