MainContentRenderersPass.php

<?php

namespace Drupal\Core\Render\MainContent;

use Symfony\Component\DependencyInjection\ContainerBuilder;
use Symfony\Component\DependencyInjection\Compiler\CompilerPassInterface;

/**
 * Adds main_content_renderers parameter to the container.
 */
class MainContentRenderersPass implements CompilerPassInterface {
    
    /**
     * {@inheritdoc}
     *
     * Collects the available main content renderer service IDs into the
     * main_content_renderers parameter, keyed by format.
     */
    public function process(ContainerBuilder $container) {
        $main_content_renderers = [];
        foreach ($container->findTaggedServiceIds('render.main_content_renderer') as $id => $attributes_list) {
            foreach ($attributes_list as $attributes) {
                $format = $attributes['format'];
                $main_content_renderers[$format] = $id;
            }
        }
        $container->setParameter('main_content_renderers', $main_content_renderers);
    }

}