static <A> O mapMethodLike vmBus_onMessage_weakRef(S msg, IVF1<A> onMessage) {
  Map<S, Set> map = vm_busListenersByMessage_live();
  synchronized(map) {
    Set listeners = map.get(msg);
    if (listeners == null) map.put(msg, listeners = syncIdentityHashSet());
    WeakRefVMBusListener listener = new(listeners, (_msg, arg) -> onMessage.get((A) arg));
    // We're technically violating the one-synchronized-object-per-thread rule,
    // but it should be OK here.
    // TODO: remove empty sets from map
    listeners.add(listener);
    ret listener;
  }
}

Began life as a copy of #1018419