diff -u b/core/lib/Drupal/Core/Template/TwigExtension.php b/core/lib/Drupal/Core/Template/TwigExtension.php
--- b/core/lib/Drupal/Core/Template/TwigExtension.php
+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -132,7 +132,7 @@
       // be used in "trans" tags.
       // @see TwigNodeTrans::compileString()
       new \Twig_SimpleFilter('passthrough', 'twig_raw_filter', array('is_safe' => array('html'))),
-      new \Twig_SimpleFilter('placeholder', [$this, 'escapePlaceholder'], array('is_safe' => array('html'), 'needs_environment' => TRUE)),
+      new \Twig_SimpleFilter('placeholder', [$this, 'escapePlaceholder'], ['is_safe' => ['html'], 'needs_environment' => TRUE]),
 
       // Replace twig's escape filter with our own.
       new \Twig_SimpleFilter('drupal_escape', [$this, 'escapeFilter'], array('needs_environment' => true, 'is_safe_callback' => 'twig_escape_filter_is_safe')),
diff -u b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
--- b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
+++ b/core/modules/system/tests/modules/twig_theme_test/twig_theme_test.module
@@ -15,10 +15,10 @@
     'variables' => array(),
     'template' => 'twig_theme_test.trans',
   );
-  $items['twig_theme_test_placeholder_outside_trans'] = array(
-    'variables' => array('var' => ''),
+  $items['twig_theme_test_placeholder_outside_trans'] = [
+    'variables' => ['var' => ''],
     'template' => 'twig_theme_test.placeholder_outside_trans',
-  );
+  ];
   $items['twig_namespace_test'] = array(
     'variables' => array(),
     'template' => 'twig_namespace_test',