Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

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

@@ -0,0 +1,36 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Component\Utility\SafeStringInterface.
+ */
+
+namespace Drupal\Component\Utility;
+
+/**
+ * Marks an object's __toString() method as returning safe markup.
+ *
+ * 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.
+ *
+ * @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.
+ *
+ * @see \Drupal\Component\Utility\SafeMarkup::set()
+ * @see \Drupal\Component\Utility\SafeMarkup::isSafe()
+ * @see \Drupal\Core\Template\TwigExtension::escapeFilter()
+ */
+interface SafeStringInterface {
+
+  /**
+   * Returns a safe string.
+   *
+   * @return string
+   *   The safe string.
+   */
+  public function __toString();
+
+}