drupal/core/modules/link/link.module

<?php

/**
 * @file
 * Defines simple link field types.
 */

use Drupal\Core\Field\FieldTypeCategoryManagerInterface;
use Drupal\Core\Link;
use Drupal\Core\Url;
use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function link_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    case 'help.page.link':
      $output = '';
      $output .= '<h2>' . t('About') . '</h2>';
      $output .= '<p>' . t('The Link module allows you to create fields that contain internal or external URLs and optional link text. See the <a href=":field">Field module help</a> and the <a href=":field_ui">Field UI help</a> pages for general information on fields and how to create and manage them. For more information, see the <a href=":link_documentation">online documentation for the Link module</a>.', [':field' => Url::fromRoute('help.page', ['name' => 'field'])->toString(), ':field_ui' => (\Drupal::moduleHandler()->moduleExists('field_ui')) ? Url::fromRoute('help.page', ['name' => 'field_ui'])->toString() : '#', ':link_documentation' => 'https://www.drupal.org/documentation/modules/link']) . '</p>';
      $output .= '<h2>' . t('Uses') . '</h2>';
      $output .= '<dl>';
      $output .= '<dt>' . t('Managing and displaying link fields') . '</dt>';
      $output .= '<dd>' . t('The <em>settings</em> and the <em>display</em> of the link field can be configured separately. See the <a href=":field_ui">Field UI help</a> for more information on how to manage fields and their display.', [':field_ui' => (\Drupal::moduleHandler()->moduleExists('field_ui')) ? Url::fromRoute('help.page', ['name' => 'field_ui'])->toString() : '#']) . '</dd>';
      $output .= '<dt>' . t('Setting the allowed link type') . '</dt>';
      $output .= '<dd>' . t('In the field settings you can define the allowed link type to be <em>internal links only</em>, <em>external links only</em>, or <em>both internal and external links</em>. <em>Internal links only</em> and <em>both internal and external links</em> options enable an autocomplete widget for internal links, so a user does not have to copy or remember a URL.') . '</dd>';
      $output .= '<dt>' . t('Adding link text') . '</dt>';
      $output .= '<dd>' . t('In the field settings you can define additional link text to be <em>optional</em> or <em>required</em> in any link field.') . '</dd>';
      $output .= '<dt>' . t('Displaying link text') . '</dt>';
      $output .= '<dd>' . t('If link text has been submitted for a URL, then by default this link text is displayed as a link to the URL. If you want to display both the link text <em>and</em> the URL, choose the appropriate link format from the drop-down menu in the <em>Manage display</em> page. If you only want to display the URL even if link text has been submitted, choose <em>Link</em> as the format, and then change its <em>Format settings</em> to display <em>URL only</em>.') . '</dd>';
      $output .= '<dt>' . t('Adding attributes to links') . '</dt>';
      $output .= '<dd>' . t('You can add attributes to links, by changing the <em>Format settings</em> in the <em>Manage display</em> page. Adding <em>rel="nofollow"</em> notifies search engines that links should not be followed.') . '</dd>';
      $output .= '<dt>' . t('Validating URLs') . '</dt>';
      $output .= '<dd>' . t('All links are validated after a link field is filled in. They can include anchors or query strings.') . '</dd>';
      $output .= '</dl>';
      return $output;
  }
}

/**
 * Implements hook_theme().
 */
function link_theme() {
  return [
    'link_formatter_link_separate' => [
      'variables' => ['title' => NULL, 'url_title' => NULL, 'url' => NULL],
    ],
  ];
}

/**
 * Prepares variables for separated link field templates.
 *
 * This template outputs a separate title and link.
 *
 * Default template: link-formatter-link-separate.html.twig.
 *
 * @param array $variables
 *   An associative array containing:
 *   - title: (optional) A descriptive or alternate title for the link, which
 *     may be different than the actual link text.
 *   - url_title: The anchor text for the link.
 *   - url: A \Drupal\Core\Url object.
 */
function template_preprocess_link_formatter_link_separate(&$variables) {
  $variables['link'] = Link::fromTextAndUrl($variables['url_title'], $variables['url'])->toString();
}

/**
 * Implements hook_field_type_category_info_alter().
 */
function link_field_type_category_info_alter(&$definitions) {
  // The `link` field type belongs in the `general` category, so the libraries
  // need to be attached using an alter hook.
  $definitions[FieldTypeCategoryManagerInterface::FALLBACK_CATEGORY]['libraries'][] = 'link/drupal.link-icon';
}