toRenderArray support

gistfile1.txt

diff --git a/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
index bb29cd1..5461283 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -420,7 +420,11 @@ public function renderVar($arg) {
     }
 
     if (is_object($arg)) {
-      if (method_exists($arg, '__toString')) {
+      // @todo Replace with a proper interface.
+      if (method_exists($arg, 'toRenderArray')) {
+        return $this->renderer->render($arg->toRenderArray());
+      }
+      elseif (method_exists($arg, '__toString')) {
         return (string) $arg;
       }
       // You can't throw exceptions in the magic PHP __toString methods, see