When services are autowired, parameter values still needs to be declared in definition like:

    app.my_service:
        class: AppBundle\Service\MyService
        arguments: 
            - $clientHostParameter: "%client_host%"

What about guessing them based on variable name? For example:

class MyService
{
    public function __construct(TwigEngine $twigEngine, $clientHostParameter)
    {
        //...
    }

So if variable ends with word parameter, S3 would tableize clientHost, look for parameter client_host and inject it.

An example taken from Symfony Flex demo would then become:

class RecipeLoader
{
    public function __construct(LoggerInterface $logger, $kernelDebugParameter)
    {
        //...
    }

What do you think?

Sorry if someone posted this idea before.