gistfile1.php

/**
 * Implements hook_field_default_field_instance_alter().
 *
 * Add digests if message_digest is enabled.
 */
function oa_messages_field_default_field_instance_alter(&$items) {
  if (module_exists('message_digest')) {
    // Replace '...' with actual values.
    $items[...] = array(
      // ...
    );
  }
}