class ProtectedUserFieldConstraintValidator

class ProtectedUserFieldConstraintValidator extends ConstraintValidator implements ContainerInjectionInterface {
    
    /**
     * User storage handler.
     *
     * @var \Drupal\user\UserStorageInterface
     */
    protected $userStorage;
    
    /**
     * The current user.
     *
     * @var \Drupal\Core\Session\AccountProxyInterface
     */
    protected $currentUser;
    
    /**
     * Constructs the object.
     *
     * @param \Drupal\user\UserStorageInterface $user_storage
     *   The user storage handler.
     * @param \Drupal\Core\Session\AccountProxyInterface $current_user
     *   The current user.
     */
    public function __construct(UserStorageInterface $user_storage, AccountProxyInterface $current_user) {
        $this->userStorage = $user_storage;
        $this->currentUser = $current_user;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('entity_type.manager')
            ->getStorage('user'), $container->get('current_user'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function validate($items, Constraint $constraint) : void {
        if (!isset($items)) {
            return;
        }
        
        /** @var \Drupal\Core\Field\FieldItemListInterface $items */
        $field = $items->getFieldDefinition();
        
        /** @var \Drupal\user\UserInterface $account */
        $account = $items->getEntity();
        if (!isset($account) || !empty($account->_skipProtectedUserFieldConstraint)) {
            // Looks like we are validating a field not being part of a user, or the
            // constraint should be skipped, so do nothing.
            return;
        }
        // Only validate for existing entities and if this is the current user.
        if (!$account->isNew() && $account->id() == $this->currentUser
            ->id()) {
            
            /** @var \Drupal\user\UserInterface $account_unchanged */
            $account_unchanged = $this->userStorage
                ->loadUnchanged($account->id());
            $changed = FALSE;
            // Special case for the password, it being empty means that the existing
            // password should not be changed, ignore empty password fields.
            $value = $items->value;
            if ($field->getName() != 'pass' || !empty($value)) {
                // Compare the values of the field this is being validated on.
                $changed = $items->getValue() != $account_unchanged->get($field->getName())
                    ->getValue();
            }
            if ($changed && !$account->checkExistingPassword($account_unchanged)) {
                $this->context
                    ->addViolation($constraint->message, [
                    '%name' => $field->getLabel(),
                ]);
            }
        }
    }

}