Fatal error ServiceNotFoundException: ... "Drupal\Core\Theme\ThemePreprocess" on Drupal 10 in facets 3.0.4

Problem/Motivation

Since Facets 3.0.4, facets_preprocess_facets_item_list() — and the equivalent preprocess functions in the facets_summary and facets_searchbox_widget submodules — call:

\Drupal::service(ThemePreprocess::class)->preprocessItemList($variables);

Drupal\Core\Theme\ThemePreprocess is a core service/class that only exists in Drupal 11 (added as part of the OOP conversion of template_preprocess_* functions, see core issue #3504381). It does not exist in Drupal 10.x.

facets.composer.json still only requires drupal/core: <10.1.0 as a floor, so the module installs cleanly on Drupal 10 but fatals at render time on any page that renders a facets item list.

Note that DeprecationHelper::backwardsCompatibleCall() is already used elsewhere in facets.module for exactly this kind of core-version compatibility — it just wasn't applied to these 4 new ThemePreprocess call sites, which looks like an oversight in the 3.0.4 release.

Steps to reproduce

1. Install Facets 3.0.4 on a site running Drupal core 10.x (e.g. 10.6.15).
2. Visit any page that renders a facets item list (e.g. a Search API facets block, or a view page with facets enabled).
3. Observe a fatal error:

Symfony\Component\DependencyInjection\Exception\ServiceNotFoundException: You have requested a non-existent service "Drupal\Core\Theme\ThemePreprocess". in Drupal\Component\DependencyInjection\Container->get() (line 159 of core/lib/Drupal/Component/DependencyInjection/Container.php).

Drupal::service() (Line: 233)
facets_preprocess_facets_item_list()

Proposed resolution

Guard each ThemePreprocess::class call with class_exists(), falling back to core's procedural template_preprocess_item_list() (still present in Drupal 10) when the class isn't available:

if (class_exists(ThemePreprocess::class)) {
  \Drupal::service(ThemePreprocess::class)->preprocessItemList($variables);
}
else {
  template_preprocess_item_list($variables);
}

Patch attached, applied at the following 4 call sites:

• facets.module: facets_preprocess_facets_item_list()
• modules/facets_summary/facets_summary.module: facets_summary_preprocess_facets_summary_item_list()
• modules/facets_searchbox_widget/facets_searchbox_widget.module: facets_searchbox_widget_preprocess_facets_item_list__searchbox_checkbox() and ..._searchbox_links()

Remaining tasks

• Review/apply patch
• Consider whether facets.info.yml / composer.json should instead gate on core_version_requirement if the intent was to drop Drupal 10 support in 3.0.4 (unclear from release notes)
• Add test coverage rendering a facets item list on Drupal 10 to catch this regression

User interface changes

None.

API changes

None.

Data model changes

None.