class TestController

class TestController extends ControllerBase {
    
    /**
     * Constructs a TestController.
     *
     * @param \Drupal\Core\DrupalKernelInterface $kernel
     *   The Drupal kernel.
     */
    public function __construct(DrupalKernelInterface $kernel) {
    }
    
    /**
     * Displays the path to a module.
     *
     * @param string $module
     *   The module name.
     * @param string $function
     *   The function to check if it exists.
     *
     * @return string[]
     *   A render array.
     */
    public function showModuleInfo(string $module, string $function) {
        $module_handler = \Drupal::moduleHandler();
        $module_message = $module . ': ';
        if ($module_handler->moduleExists($module)) {
            $module_message .= \Drupal::moduleHandler()->getModule($module)
                ->getPath();
        }
        else {
            $module_message .= 'not installed';
        }
        $function_message = $function . ': ' . var_export(function_exists($function), TRUE);
        return [
            '#theme' => 'item_list',
            '#items' => [
                $module_message,
                $function_message,
            ],
        ];
    }
    
    /**
     * Resets the container.
     *
     * @return array
     *   A render array.
     */
    public function containerReset() {
        $this->messenger()
            ->addMessage(t('Before the container was reset.'));
        $this->kernel
            ->resetContainer();
        // The container has been reset, therefore we need to get the new service.
        $this->messenger = NULL;
        $this->messenger()
            ->addMessage(t('After the container was reset.'));
        return [];
    }

}