RegistryConstants.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */

namespace Magento\SalesRule\Model;

/**
 * Declarations of core registry keys used by the SalesRule module
 *
 */
class RegistryConstants
{
    /**
     * Key for current sales rule in registry
     */
    const CURRENT_SALES_RULE = 'current_promo_sales_rule';
}