Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/lib/Drupal/Core/GeneratedLink.php

@@ -8,6 +8,7 @@
 namespace Drupal\Core;
 
 use Drupal\Component\Render\MarkupInterface;
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Render\BubbleableMetadata;
 
 /**
@@ -16,7 +17,7 @@ use Drupal\Core\Render\BubbleableMetadata;
  * Note: not to be confused with \Drupal\Core\Link, which is for passing around
  *   ungenerated links (typically link text + route name + route parameters).
  */
-class GeneratedLink extends BubbleableMetadata implements MarkupInterface {
+class GeneratedLink extends BubbleableMetadata implements MarkupInterface, \Countable {
 
   /**
    * The HTML string value containing a link.
@@ -61,4 +62,11 @@ class GeneratedLink extends BubbleableMetadata implements MarkupInterface {
     return $this->__toString();
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function count() {
+    return Unicode::strlen($this->__toString());
+  }
+
 }