.../EntityResourceRestTestCoverageTest.php         | 166 +++++++++++++++++++++
 1 file changed, 166 insertions(+)

diff --git a/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceRestTestCoverageTest.php b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceRestTestCoverageTest.php
new file mode 100644
index 0000000..4d046fb
--- /dev/null
+++ b/core/modules/rest/tests/src/Functional/EntityResource/EntityResourceRestTestCoverageTest.php
@@ -0,0 +1,166 @@
+<?php
+
+namespace Drupal\Tests\rest\Functional\EntityResource;
+
+use Drupal\Tests\BrowserTestBase;
+
+/**
+ * Checks that all core content/config entity types have REST test coverage.
+ *
+ * Every entity type must have test coverage for:
+ * - every format in core (json + hal_json)
+ * - every authentication provider in core (anon, cookie, basic_auth)
+ *
+ * @todo also require xml after https://www.drupal.org/node/2800873 lands
+ *
+ * @group rest
+ */
+class EntityResourceRestTestCoverageTest extends BrowserTestBase {
+
+  /**
+   * Entities definitions array.
+   *
+   * @var array
+   */
+  protected $definitions;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $all_modules = system_rebuild_module_data();
+    $stable_core_modules = array_filter($all_modules, function ($module) {
+      // Filter contrib, hidden, already enabled modules and modules in the
+      // Testing and experimental packages.
+      if ($module->origin !== 'core' || !empty($module->info['hidden']) || $module->status == TRUE || $module->info['package'] == 'Testing' || $module->info['package'] == 'Core (Experimental)') {
+        return FALSE;
+      }
+      return TRUE;
+    });
+
+    $this->container->get('module_installer')->install(array_keys($stable_core_modules));
+    $this->rebuildContainer();
+
+    $this->definitions = $this->container->get('entity_type.manager')->getDefinitions();
+
+    // Remove definitions for which the REST resource plugin definition was
+    // removed via hook_rest_resource_alter(). Entity types which are never
+    // exposed via REST also don't need test coverage.
+    $resource_plugin_ids = array_keys($this->container->get('plugin.manager.rest')->getDefinitions());
+    foreach (array_keys($this->definitions) as $entity_type_id) {
+      if (!in_array("entity:$entity_type_id", $resource_plugin_ids, TRUE)) {
+        unset($this->definitions[$entity_type_id]);
+      }
+    }
+  }
+
+  /**
+   * Tests that all core content/config entity types have REST test coverage.
+   */
+  public function testEntityTypeRestTestCoverage() {
+    $default_test_locations = [
+      // Test coverage for formats provided by the 'serialization' module.
+      'serialization' => [
+        'possible paths' => [
+          '\Drupal\Tests\rest\Functional\EntityResource\CLASS\CLASS',
+        ],
+        'class suffix' => [
+          'JsonAnonTest',
+          'JsonBasicAuthTest',
+          'JsonCookieTest',
+        ],
+      ],
+      // Test coverage for formats provided by the 'hal' module.
+      'hal' => [
+        'possible paths' => [
+          '\Drupal\Tests\hal\Functional\EntityResource\CLASS\CLASS',
+        ],
+        'class suffix' => [
+          'HalJsonAnonTest',
+          'HalJsonBasicAuthTest',
+          'HalJsonCookieTest',
+        ],
+      ],
+    ];
+
+    $problems = [];
+    foreach ($this->definitions as $entity_type_id => $info) {
+      $class_name_full = $info->getClass();
+      $parts = explode('\\', $class_name_full);
+      $class_name = end($parts);
+      $module_name = $parts[1];
+
+      // The test class can live either in the REST/HAL module, or in the module
+      // providing the entity type.
+      $tests = $default_test_locations;
+      $tests['serialization']['possible paths'][] = '\Drupal\Tests\\' . $module_name . '\Functional\Rest\CLASS';
+      $tests['hal']['possible paths'][] = '\Drupal\Tests\\' . $module_name . '\Functional\Hal\CLASS';
+
+      foreach ($tests as $module => $info) {
+        $possible_paths = $info['possible paths'];
+        foreach ($info['class suffix'] as $postfix) {
+          do {
+            $path = array_shift($possible_paths);
+            $class = str_replace('CLASS', $class_name, $path . $postfix);
+            if (class_exists($class)) {
+              continue 3;
+            }
+          } while (!empty($possible_paths));
+          $problems[] = "$entity_type_id: $class_name ($class_name_full)";
+          break 2;
+        }
+      }
+    }
+    $all = count($this->definitions);
+    $good = $all - count($problems);
+    // @todo Remove this in https://www.drupal.org/node/2843139. Having this
+    // work-around in here until then means we can ensure we don't add more
+    // entity types without adding REST test coverage.
+    if ($problems === [0 => 'file: File (Drupal\file\Entity\File)']) {
+      $problems = [];
+    }
+    $this->assertSame([], $problems, $this->getLlamaMessage($good, $all));
+  }
+
+  /**
+   * Message from Llama.
+   *
+   * @param int $good
+   *   A count of entities with test coverage.
+   * @param int $all
+   *   A count of all entities.
+   *
+   * @return string
+   *   An information about progress of REST test coverage.
+   */
+  protected function getLlamaMessage($good, $all) {
+    $a = $all;
+    $g = $good;
+
+    if ($g < 10) {
+      $g = "0$g";
+    }
+
+    $message = "
+☼
+      ________________________
+     /           Hi!          \\
+    |  It's llame to not have  |
+    |   complete REST tests!   |
+    |                          |
+    |     Progress: $g/$a.     |
+    | ________________________/
+    |/
+//  o
+l'>
+ll
+llama
+|| ||
+'' ''
+";
+    return $message;
+  }
+
+}