Extracting translation keys from source is nice, and saves us a few steps and makes thing less error prone in general.

Currently, in PHP, only TranslatorInterface::trans() in controllers calls are extracted as such. However, often we have just a translatable string, which is passed to the translator later on. E.g. with knp menus:

$menu->addChild('translation.key', ['extras' => ['translation_domain' => 'nav']]);

To extract translation.key i propose to add $string = Translatable::_('translation.key'). Which is an identity function defined in contracts. It enables extractors to act upon. More short, maybe t('string') even ...

Next, we should scrape entire src IMHO, or any source file from the DI container.

Related, can we consider https://github.com/php-translation/extractor for core? Or why is there something competing doing a better job?