Issue #3310271 by catch, longwave, benjifisher, samuel.mortenson: Container...

    $mapping = [];

    foreach ($this->getServiceIds() as $service_id) {

      if ($this->initialized($service_id) && $service_id !== 'service_container') {

        $mapping[$this->generateServiceIdHash($this->get($service_id))] = $service_id;

        $service = $this->get($service_id);

        if (is_object($service)) {

          $mapping[$this->generateServiceIdHash($service)] = $service_id;

        }

      }

    }

    return $mapping;

<?php

namespace Drupal\KernelTests\Core\DependencyInjection;

use Drupal\KernelTests\KernelTestBase;

/**

 * Tests serialization of string services.

 *

 * @group DependencyInjection

 * @group legacy

 */

class StringSerializationTest extends KernelTestBase {

  /**

   * Tests that strings are not put into the container class mapping.

   */

  public function testSerializeString() {

    $this->assertIsString($this->container->get('app.root'));

    $this->container->getServiceIdMappings();

    $this->assertIsString($this->container->get('app.root'));

  }

}