code: @GuardedBy("evictionLock")
    void drainKeyReferences() {
      if (!collectKeys()) {
        return;
      }
      Reference<? extends K> keyRef;
      while ((keyRef = keyReferenceQueue().poll()) != null) {
        Node<K, V> node = data.get(keyRef);
        if (node != null) {
          evictEntry(node, RemovalCause.COLLECTED, 0L);
        }
      }
    }
docs: Drains the weak key references queue.