RegisterForm::save

public RegisterForm::save(array $form, FormStateInterface $form_state)

public function save(array $form, FormStateInterface $form_state) {
  $account = $this->entity;
  $pass = $account->getPassword();
  $admin = $form_state->getValue('administer_users');
  $notify = !$form_state->isValueEmpty('notify');

  // Save has no return value so this cannot be tested.
  // Assume save has gone through correctly.
  $account->save();

  $form_state->set('user', $account);
  $form_state->setValue('uid', $account->id());

  $this->logger('user')->notice('New user: %name %email.', array('%name' => $form_state->getValue('name'), '%email' => '<' . $form_state->getValue('mail') . '>', 'type' => $account->link($this->t('Edit'), 'edit-form')));

  // Add plain text password into user account to generate mail tokens.
  $account->password = $pass;

  // New administrative account without notification.
  if ($admin && !$notify) {
    drupal_set_message($this->t('Created a new user account for <a href=":url">%name</a>. No email has been sent.', array(':url' => $account->url(), '%name' => $account->getUsername())));
  }
  // No email verification required; log in user immediately.
  elseif (!$admin && !\Drupal::config('user.settings')->get('verify_mail') && $account->isActive()) {
    _user_mail_notify('register_no_approval_required', $account);
    user_login_finalize($account);
    drupal_set_message($this->t('Registration successful. You are now logged in.'));
    $form_state->setRedirect('<front>');
  }
  // No administrator approval required.
  elseif ($account->isActive() || $notify) {
    if (!$account->getEmail() && $notify) {
      drupal_set_message($this->t('The new user <a href=":url">%name</a> was created without an email address, so no welcome message was sent.', array(':url' => $account->url(), '%name' => $account->getUsername())));
    }
    else {
      $op = $notify ? 'register_admin_created' : 'register_no_approval_required';
      if (_user_mail_notify($op, $account)) {
        if ($notify) {
          drupal_set_message($this->t('A welcome message with further instructions has been emailed to the new user <a href=":url">%name</a>.', array(':url' => $account->url(), '%name' => $account->getUsername())));
        }
        else {
          drupal_set_message($this->t('A welcome message with further instructions has been sent to your email address.'));
          $form_state->setRedirect('<front>');
        }
      }
    }
  }
  // Administrator approval required.
  else {
    _user_mail_notify('register_pending_approval', $account);
    drupal_set_message($this->t('Thank you for applying for an account. Your account is currently pending approval by the site administrator.<br />In the meantime, a welcome message with further instructions has been sent to your email address.'));
    $form_state->setRedirect('<front>');
  }
}