drupal/core/modules/user/user.views_execution.inc

<?php

/**
 * @file
 * Provide views runtime hooks for user.module.
 */

use Drupal\views\ViewExecutable;

/**
 * Implements hook_views_query_substitutions().
 *
 * Allow replacement of current userid so we can cache these queries.
 */
function user_views_query_substitutions(ViewExecutable $view) {
  global $user;
  return array('***CURRENT_USER***' => $user->id());
}