The absolute simplest semantic I can come up with here (too simple?) would be for a struct containing a non-null owner-pointer to be bound by its owner’s owners

The absolute simplest semantic I can come up with here (too simple?) would be for a struct containing a non-null owner-pointer to be bound by its owner’s ownership/lifetime-scope. For most commonly used graph-types this should lead to pretty simple one-directional ownership chains which should be doable (although probably not trivial) for a compiler to enforce.