final State<?> state;

    FinalizableReference(ClosingFuture<?> referent) {
      super(referent, referenceQueue);
      this.state = referent.state;
    }

    @Override
    public void finalizeReferent() {
      references.remove(this);
      state.closeIfLeaked();
    }
  }

  /** Ensures that {@code o} is not garbage collected until this point in the code. */
  @VisibleForTesting