RoleForm::form

public RoleForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $entity = $this->entity;
  $form['label'] = array(
    '#type' => 'textfield',
    '#title' => $this->t('Role name'),
    '#default_value' => $entity->label(),
    '#size' => 30,
    '#required' => TRUE,
    '#maxlength' => 64,
    '#description' => $this->t('The name for this role. Example: "Moderator", "Editorial board", "Site architect".'),
  );
  $form['id'] = array(
    '#type' => 'machine_name',
    '#default_value' => $entity->id(),
    '#required' => TRUE,
    '#disabled' => !$entity->isNew(),
    '#size' => 30,
    '#maxlength' => 64,
    '#machine_name' => array(
      'exists' => ['\Drupal\user\Entity\Role', 'load'],
    ),
  );
  $form['weight'] = array(
    '#type' => 'value',
    '#value' => $entity->getWeight(),
  );

  return parent::form($form, $form_state, $entity);
}