Module depends on a search api service and it shouldn't

See also #2878792: You have requested a non-existent service "search_api.query_helper". in Drupal\Component\DependencyInjection\Container->get()

I think this is also why the tests are currently all failing on HEAD. Let's try to get to this with priority!

I'm afraid that the easiest answer will be to create a new submodule that depends on search api.

Either that or something like this in the ::create method of \Drupal\facets\Plugin\facets\facet_source\SearchApiDisplay

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {

    if (!$container->get('module_handler')->moduleExists('search_api')) {
      return new \stdClass();
    }

    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('plugin.manager.facets.query_type'),
      $container->get('search_api.query_helper'),
      $container->get('plugin.manager.search_api.display'),
      $container->get('request_stack')->getMasterRequest()
    );
  }

Let's get opinions in before we do this, tagging as beta blocker - because if we need to create the submodule, we really have to do that before beta.

I prefer a moduleExists for this case
I think most users will use facets with search_api enabled, so I don't mind having this in the core facets module.

Added this as a patch; with more docs.

Updated the title.

Patch got applied pretty well on the latest dev version of the module but somehow after enabling it I'm getting this message:

Fatal error: Call to undefined method stdClass::isRenderedInCurrentRequest() in /var/www/drupal/modules/contrib/facets/facets.module on line 249

This is the code in facets.module on line 249:

if (!$facet_source_plugin->isRenderedInCurrentRequest()) {

Ah, great catch. I figured this should work, but looks like we'll have to do more work to actually get this to work. I'll try to get to this issue over the weekend.

I think this resolves the problem. Would you mind testing this?

Fixes the tests, not related to this issue.

Patch provided in [#13] got easily applied to the latest dev version of the module.

Checking patch src/FacetSource/FacetSourcePluginManager.php...
Checking patch src/Plugin/facets/facet_source/SearchApiDisplay.php...
Applied patch src/FacetSource/FacetSourcePluginManager.php cleanly.
Applied patch src/Plugin/facets/facet_source/SearchApiDisplay.php cleanly.

I was able to enable this module without any dependency of search_api, hence marking this as RTBC.

Committed, thanks!