Resolve environment placeholders

[carlos-ea]

When using environment variables as values for config parameters that are read conditionally, symfony can mark that environment variable as unused and throw a fatal error.

For example, the following config:

translation:
    fallback_translation:
        enabled: '%env(bool:FALLBACK_TRANSLATION_ENABLED)%'

Will result in a fatal error if the environment variable is not used elsewhere. Environment variables "bool:FALLBACK_TRANSLATION_ENABLED" are never used. Please, check your container's configuration.

This PR solves this issue, see symfony/symfony#23520 (comment) and the comment below that.

[odolbeau]

Thanks!
That's the first time I heard about that! :D

[Nyholm]

Thank you

[nicolas-grekas]

This change is wrong: it defeats the purpose of env vars.

[odolbeau]

@nicolas-grekas Thanks for let us know.
Sorry if the question sounds silly but what are we supposed to do in a bundle to let end users use environment variables in config?

[nicolas-grekas]

There are places where using env vars doesn't make sense, is not possible, or needs to be supported by runtime factories.

Enabling/disabling features should usually be controlled by per-symfony-env parameters (in the example here e.g.)