diff --git a/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
index 01e37a4..69e0bb3 100644
--- a/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -52,7 +52,8 @@ public function getFilters() {
       new \Twig_SimpleFilter('drupal_escape', 'twig_drupal_escape_filter', array('needs_environment' => true, 'is_safe_callback' => 'twig_escape_filter_is_safe')),
 
       // Implements safe joining.
-      // @todo Make that the default for |join?
+      // @todo Make that the default for |join? Upstream issue:
+      //   https://github.com/fabpot/Twig/issues/1420
       new \Twig_SimpleFilter('safe_join', 'twig_drupal_join_filter', array('is_safe' => array('html'))),
 
       // Array filters.
diff --git a/core/themes/engines/twig/twig.engine b/core/themes/engines/twig/twig.engine
index a951a8b..67bfdcf 100644
--- a/core/themes/engines/twig/twig.engine
+++ b/core/themes/engines/twig/twig.engine
@@ -130,7 +130,7 @@ function twig_render_var($arg) {
     return NULL;
   }
 
-  // Optimize for strings as its likely they come from escape filter.
+  // Optimize for strings as it is likely they come from the escape filter.
   if (is_string($arg)) {
     return $arg;
   }
@@ -189,14 +189,29 @@ function twig_without($element) {
  * Overrides twig_escape_filter().
  *
  * Replacement function for twig's escape filter.
+ *
+ * @param Twig_Environment $env
+ *   A Twig_Environment instance.
+ * @param string $string
+ *   The value to be escaped.
+ * @param string $strategy
+ *   The escaping strategy. Defaults to 'html'.
+ * @param string $charset
+ *  The charset.
+ * @param Boolean $autoescape
+ *   Whether the function is called by the auto-escaping feature (TRUE) or by
+ *   the developer (FALSE).
+ *
+ * @return string|null
+ *   The escaped, renderd output, or NULL if there is no valid output.
  */
-function twig_drupal_escape_filter(\Twig_Environment $env, $arg, $strategy = 'html', $charset = null, $autoescape = false) {
-  // Check for numeric zero.
+function twig_drupal_escape_filter(\Twig_Environment $env, $arg, $strategy = 'html', $charset = NULL, $autoescape = FALSE) {
+  // Check for a numeric zero.
   if ($arg === 0) {
     return 0;
   }
 
-  // Return early for NULL and also true for empty arrays.
+  // Return early for NULL or an empty array.
   if ($arg == NULL) {
     return NULL;
   }
@@ -210,21 +225,23 @@ function twig_drupal_escape_filter(\Twig_Environment $env, $arg, $strategy = 'ht
 
   if (is_scalar($arg)) {
     $return = (string) $arg;
-  } else if (is_object($arg)) {
+  }
+  elseif (is_object($arg)) {
     if (method_exists($arg, '__toString')) {
       $return = (string) $arg;
     }
     else {
-      throw new Exception(t('Object of type "@class" cannot be printed.', array('@class' => get_class($arg))));
+      throw new \Exception(t('Object of type "@class" cannot be printed.', array('@class' => get_class($arg))));
     }
   }
 
   // We have a string or an object converted to a string: Autoescape it!
   if (isset($return)) {
     if ($strategy != 'html') {
-      // Drupal only supports HTML strategy, fallback for other strategies.
-      // @todo Add optional strategy parameter to SafeMarkup function calls,
-      //   to avoid this when its already safe for this strategy.
+      // Drupal only supports the HTML escaping strategy, so provide a
+      // fallback for other strategies.
+      // @todo Add an optional strategy parameter to SafeMarkup function calls,
+      //   to avoid this when it is already safe for this strategy.
       return twig_escape_filter($env, $return, $strategy, $charset, $autoescape);
     }
     if (!SafeMarkup::isSafe($return)) {
@@ -242,11 +259,17 @@ function twig_drupal_escape_filter(\Twig_Environment $env, $arg, $strategy = 'ht
  *
  * Safely joins several strings together.
  *
- * Note: Glue is considered safe here and _not_ escaped.
+ * @param array|Traversable $value
+ *   The pieces to join.
+ * @param string $glue
+ *   The delimiter with which to join the string. Defaults to an empty string.
+ *
+ * @return \Drupal\Component\Utility\SafeMarkup|string
+ *   The imploded string, which is now also marked as safe.
  */
 function twig_drupal_join_filter($value, $glue = '') {
-  if (is_object($value) && $value instanceof Traversable) {
-      $value = iterator_to_array($value, false);
+  if (is_object($value) && $value instanceof \Traversable) {
+    $value = iterator_to_array($value, false);
   }
 
   return SafeMarkup::implode($glue, (array) $value);