Update to Drupal 8.0.0-beta15. For more information, see: https://www.drupal.org/node/2563023

core/lib/Drupal/Component/Utility/SafeStringInterface.php

@@ -10,20 +10,29 @@ namespace Drupal\Component\Utility;
 /**
  * Marks an object's __toString() method as returning safe markup.
  *
+ * All objects that implement this interface should be marked @internal.
+ *
  * This interface should only be used on objects that emit known safe strings
  * from their __toString() method. If there is any risk of the method returning
  * user-entered data that has not been filtered first, it must not be used.
  *
+ * If the object is going to be used directly in Twig templates it should
+ * implement \Countable so it can be used in if statements.
+ *
  * @internal
  *   This interface is marked as internal because it should only be used by
- *   objects used during rendering. Currently, there is no use case for this
- *   interface in contrib or custom code.
+ *   objects used during rendering. This interface should be used by modules if
+ *   they interrupt the render pipeline and explicitly deal with SafeString
+ *   objects created by the render system. Additionally, if a module reuses the
+ *   regular render pipeline internally and passes processed data into it. For
+ *   example, Views implements a custom render pipeline in order to render JSON
+ *   and to fast render fields.
  *
- * @see \Drupal\Component\Utility\SafeMarkup::set()
+ * @see \Drupal\Component\Utility\SafeStringTrait
  * @see \Drupal\Component\Utility\SafeMarkup::isSafe()
  * @see \Drupal\Core\Template\TwigExtension::escapeFilter()
  */
-interface SafeStringInterface {
+interface SafeStringInterface extends \JsonSerializable {
 
   /**
    * Returns a safe string.