farm-2.x-dev/modules/core/settings/farm_settings.module

<?php

/**
 * @file
 * Contains farm_settings.module.
 */

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Url;

/**
 * Implements hook_help().
 */
function farm_settings_help($route_name, RouteMatchInterface $route_match) {
  $output = '';

  // Modules form.
  if ($route_name == 'farm_settings.modules_form') {
    $output .= '<p>' . t('Select the core and community farmOS modules that you would like to be installed.') . '</p>';
  }

  return $output;
}

/**
 * Implements hook_toolbar().
 */
function farm_settings_toolbar() {
  $user = \Drupal::currentUser();

  $items = [];
  $items['farm_settings'] = [
    '#cache' => [
      'contexts' => [
        'user.permissions',
      ],
    ],
  ];

  // Add settings to the toolbar if the user has permission.
  if ($user->hasPermission('administer farm settings')) {
    $items['farm_settings'] += [
      '#type' => 'toolbar_item',
      'tab' => [
        '#type' => 'link',
        '#title' => t('Settings'),
        '#url' => Url::fromRoute('farm_settings.settings_page'),
        '#attributes' => [
          'title' => t('Settings'),
          'class' => ['toolbar-icon', 'toolbar-icon-farm-settings'],
        ],
        '#attached' => [
          'library' => [
            'farm_settings/toolbar',
          ],
        ],
      ],
      '#weight' => -5,
    ];
  }

  return $items;
}