Issue #1987738 by Mile23: Convert menu_test_callback() to a new style controller

core/modules/system/src/Tests/Menu/MenuRouterTest.php

@@ -199,7 +199,7 @@ protected function doTestExoticPath() {
       "%23%25%26%2B%2F%3F" . // Characters that look like a percent-escaped string.
       "éøïвβ中國書۞"; // Characters from various non-ASCII alphabets.
     $this->drupalGet($path);
-    $this->assertRaw('This is menu_test_callback().');
+    $this->assertRaw('This is the menuTestCallback content.');
   }
 
   /**

core/modules/system/tests/modules/menu_test/menu_test.module

@@ -80,20 +80,6 @@ function menu_test_menu_local_tasks_alter(&$data, $route_name) {
   }
 }
 
-/**
- * Page callback: Provides a dummy function which can be used as a placeholder.
- *
- * @return string
- *   A string that can be used for comparison.
- *
- * @see menu_test_menu().
- *
- * @deprecated Use \Drupal\menu_test\Controller\MenuTestController::menuTestCallback()
- */
-function menu_test_callback() {
-  return ['#markup' => 'This is menu_test_callback().'];
-}
-
 /**
  * Page callback: Tests the theme negotiation functionality.
  *

core/modules/system/tests/modules/menu_test/src/Controller/MenuTestController.php

@@ -13,10 +13,13 @@
 class MenuTestController {
 
   /**
-   * @todo Remove menu_test_callback().
+   * Some known placeholder content which can be used for testing.
+   *
+   * @return string
+   *   A string that can be used for comparison.
    */
   public function menuTestCallback() {
-    return menu_test_callback();
+    return ['#markup' => 'This is the menuTestCallback content.'];
   }