Symfony version(s) affected: 3.2 - master

Description
Using env vars in routing configuration is not supported and actively denied:

#20687

However, if we use indirection with a parameter we can avoid this error, but routing will not work as expected.

How to reproduce
This will be denied correctly, as described above:

# config.yaml
parameters:
    env(route_prefix): 'default'

# routes.yaml
some_route:
  path:       /%env(route_prefix)%/contact
  controller: App\Controller\MainController::contact

However, this will not throw an error:

# config.yaml
parameters:
    env(route_prefix): 'default'
    route_prefix: '%env(route_prefix)%'

# routes.yaml
some_route:
  path:       /%route_prefix%/contact
  controller: App\Controller\MainController::contact

This will allow the container to boot, but defining a different route_prefix env var will never have any effect.

Possible Solution
For good and non confusing DX, detect if parameters are pointing to env vars and deny building routes up front.