class EntityLink

class EntityLink extends FieldPluginBase {
  
  /**
   * Stores the result of parent entities build for all rows to reuse it later.
   *
   * @var array
   */
  protected $build;
  
  /**
   * {@inheritdoc}
   */
  protected function defineOptions() {
    $options = parent::defineOptions();
    $options['teaser'] = [
      'default' => FALSE,
    ];
    return $options;
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildOptionsForm(&$form, FormStateInterface $form_state) {
    $form['teaser'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('Show teaser-style link'),
      '#default_value' => $this->options['teaser'],
      '#description' => $this->t('Show the comment link in the form used on standard entity teasers, rather than the full entity form.'),
    ];
    parent::buildOptionsForm($form, $form_state);
  }
  
  /**
   * {@inheritdoc}
   */
  public function query() {
  }
  
  /**
   * {@inheritdoc}
   */
  public function preRender(&$values) {
    // Render all nodes, so you can grep the comment links.
    $entities = [];
    foreach ($values as $row) {
      $entity = $row->_entity;
      $entities[$entity->id()] = $entity;
    }
    if ($entities) {
      $entityTypeId = reset($entities)->getEntityTypeId();
      $viewMode = $this->options['teaser'] ? 'teaser' : 'full';
      $this->build = \Drupal::entityTypeManager()->getViewBuilder($entityTypeId)
        ->viewMultiple($entities, $viewMode);
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function render(ResultRow $values) {
    $entity = $this->getEntity($values);
    // Only render the links, if they are defined.
    return !empty($this->build[$entity->id()]['links']['comment__comment']) ? \Drupal::service('renderer')->render($this->build[$entity->id()]['links']['comment__comment']) : '';
  }

}