\Drupal\Core\StringTranslation\Translator\CustomStrings should be serializable, but contain \Drupal\Core\Site\Settings which is not

Problem/Motivation

Getting

LogicException: Settings can not be serialized. This probably means you are serializing an object that has an indirect reference to the Settings object. Adjust your code so that is not necessary. in Drupal\Core\Site\Settings->__sleep() (line 67 of core/lib/Drupal/Core/Site/Settings.php).

When try to render a form with datetime element.

Steps to reproduce

1. Create a form with an element below:

  $form['sample'] = array(
        '#title' => 'Sample field,
        '#type' => 'datelist',
        '#default_value' => new DrupalDateTime('2000-01-01 00:00:00'),
        '#date_part_order' => array('month', 'day', 'year', 'hour', 'minute', 'ampm'),
        '#date_text_parts' => array('year'),
        '#date_year_range' => '2010:2020',
        '#date_increment' => 15,
      );

2. Try to visit the form UL.

Full exception trace if it helps:

LogicException: Settings can not be serialized. This probably means you are serializing an object that has an indirect reference to the Settings object. Adjust your code so that is not necessary. in Drupal\Core\Site\Settings->__sleep() (line 67 of core/lib/Drupal/Core/Site/Settings.php).
serialize(Array)
Drupal\Component\Serialization\PhpSerialize::encode(Array)
Drupal\Core\KeyValueStore\DatabaseStorageExpirable->setWithExpire('form-zRDOGhal0mExQf8tb4Wt4jN283dcztARg82WW3doFpc', Array, 21600)
Drupal\Core\Form\FormCache->setCache('form-zRDOGhal0mExQf8tb4Wt4jN283dcztARg82WW3doFpc', Array, Object)
Drupal\Core\Form\FormBuilder->setCache('form-zRDOGhal0mExQf8tb4Wt4jN283dcztARg82WW3doFpc', Array, Object)
Drupal\Core\Form\FormBuilder->processForm('error_style_form', Array, Object)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\DependencyInjection\Container\prod\Symfony_Component_HttpKernel_HttpKernel_Proxy->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\DependencyInjection\Container\prod\Drupal_Core_StackMiddleware_Session_Proxy->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\Core\DependencyInjection\Container\prod\Drupal_Core_StackMiddleware_KernelPreHandle_Proxy->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

Proposed resolution

Looks like #2199795: Make the Settings class prevent serialization of actual settings has introduced the exception and might need some test coverage to prove it.

Looks like the flow is DrupalDateTime => TranslationManager => translator => CustomStrings => Settings. So if we might need to remove the Settings as a dependency for CustomStrings.

Remaining tasks

User interface changes

N/A

API changes

N/A