Issue #3115159 by longwave, daffie, alexpott: Properly deprecate config.storage.staging service

core/core.services.yml

@@ -330,14 +330,12 @@ services:
     class: Drupal\Core\Config\ExportStorageManager
     arguments: ['@config.storage', '@database', '@event_dispatcher', '@lock']
     public: false
-  # @deprecated in Drupal 8.0.x and will be removed before 10.0.0. Use
-  #   config.storage.sync instead.
-  # @see https://www.drupal.org/node/2574957
-  config.storage.staging:
+  config.storage.sync:
     class: Drupal\Core\Config\FileStorage
     factory: Drupal\Core\Config\FileStorageFactory::getSync
-  config.storage.sync:
-    alias: config.storage.staging
+  config.storage.staging:
+    alias: config.storage.sync
+    deprecated: The "%alias_id%" service is deprecated in drupal:8.0.0 and is removed from drupal:10.0.0. Use the "config.storage.sync" service instead. See https://www.drupal.org/node/2574957
   config.storage.snapshot:
     class: Drupal\Core\Config\DatabaseStorage
     arguments: ['@database', config_snapshot]

core/lib/Drupal/Core/DependencyInjection/YamlFileLoader.php

@@ -149,7 +149,11 @@ private function parseDefinition($id, $service, $file)
 
         if (isset($service['alias'])) {
             $public = !array_key_exists('public', $service) || (bool) $service['public'];
-            $this->container->setAlias($id, new Alias($service['alias'], $public));
+            $alias = $this->container->setAlias($id, new Alias($service['alias'], $public));
+
+            if (array_key_exists('deprecated', $service)) {
+                $alias->setDeprecated(true, $service['deprecated']);
+            }
 
             return;
         }

core/tests/Drupal/KernelTests/Core/Config/ConfigStorageStagingTest.php

+<?php
+
+namespace Drupal\KernelTests\Core\Config;
+
+use Drupal\Core\Config\FileStorage;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests that the service "config.storage.staging" has been deprecated.
+ *
+ * @group Test
+ * @group legacy
+ */
+class ConfigStorageStagingTest extends KernelTestBase {
+
+  /**
+   * @expectedDeprecation The "config.storage.staging" service is deprecated in drupal:8.0.0 and is removed from drupal:10.0.0. Use the "config.storage.sync" service instead. See https://www.drupal.org/node/2574957
+   */
+  public function testConfigStorageStagingDeprecation() {
+    $storage_staging = \Drupal::service('config.storage.staging');
+    // Ensure at least one assertion.
+    $this->assertInstanceOf(FileStorage::class, $storage_staging);
+  }
+
+}