Since the compatibility with Twig 2 has been added on LegacyBridge (ref), it would be very useful to have this backported to 5.4.

For the moment, if we have an environment with symfony 2.8.x and twig 2.1.x we get errors like such:

PHP Fatal error: Class 'Twig_Loader_String' not found in /Path_to_ezp54/vendor/ezsystems/ezpublish-kernel/eZ/Publish/Core/MVC/Symfony/Templating/Twig/LoaderString.php on line 18