Added drupal_set_message function.

src/TwigExtension.php

@@ -95,6 +95,9 @@ class TwigExtension extends \Twig_Extension {
       new \Twig_SimpleFunction('drupal_field', [$this, 'drupalField']),
       new \Twig_SimpleFunction('drupal_menu', [$this, 'drupalMenu']),
       new \Twig_SimpleFunction('drupal_config', [$this, 'drupalConfig']),
+      // Wrap drupal_set_message() because it returns some value which is not
+      // suitable for Twig template.
+      new \Twig_SimpleFunction('drupal_set_message', [$this, 'drupalSetMessage']),
     ];
   }
 
@@ -333,4 +336,21 @@ class TwigExtension extends \Twig_Extension {
     return file_url_transform_relative($url);
   }
 
+  /**
+   * Sets a message to display to the user.
+   *
+   * @param string|\Drupal\Component\Render\MarkupInterface $message
+   *   (optional) The translated message to be displayed to the user.
+   * @param string $type
+   *   (optional) The message's type. Defaults to 'status'.
+   * @param bool $repeat
+   *   (optional) If this is FALSE and the message is already set, then the
+   *   message won't be repeated. Defaults to FALSE.
+   *
+   * @see drupal_set_message()
+   */
+  public function drupalSetMessage($message = NULL, $type = 'status', $repeat = FALSE) {
+    drupal_set_message($message, $type, $repeat);
+  }
+
 }

tests/src/Functional/TwigTweakTest.php

@@ -117,6 +117,10 @@ class TwigTweakTest extends BrowserTestBase {
     $xpath = '//div[@class = "tt-config" and . = "Anonymous"]';
     $this->assertByXpath($xpath);
 
+    // Test status message.
+    $xpath = '//div[@class = "messages messages--warning" and contains(., "Hi!")]';
+    $this->assertByXpath($xpath);
+
     // Test token replacement.
     $xpath = '//div[@class = "tt-token-replace" and . = "Site name: Drupal"]';
     $this->assertByXpath($xpath);

tests/twig_tweak_test/templates/twig-tweak-test.html.twig

@@ -13,6 +13,7 @@
   <div class="tt-token">{{ drupal_token('site:name') }}</div>
   <div class="tt-token-data">{{ drupal_token('node:title', {'node': node}) }}</div>
   <div class="tt-config">{{ drupal_config('user.settings', 'anonymous') }}</div>
+  <div class="tt-status-message">{{ drupal_set_message('Hi!', 'warning') }}</div>
   <div class="tt-token-replace">{{ 'Site name: [site:name]' | token_replace }}</div>
   <div class="tt-preg-replace">{{ 'foo' | preg_replace('(foo)', '$1-bar') }}</div>
   <div class="tt-image-style">{{ 'public://images/ocean.jpg' | image_style('thumbnail') }}</div>