In #21837 there was basically a hack used to add some load time code to the container.

Besides that the service configuration defined in the different config files is heavily altered by extensions/configurators/compiler passes so you need to consult a different sources scattered around various places: Resources/config, *Bundle.php (for compiler passes actually used), *Extension.php (dynamic config), *Pass.php (for container building code) and some more depending on how sophisticated your feature's DI integration is.

The container:debug commands helps only to some degree, to at least get a view of the final result, but still gives no clue about where to look for the configuration of a specific service.

To consolidate these various sources, I propose something like "container-scripts". An example how #21837 could be solved with it:

<container ... >
  <services>
    <service id="annotations.reader" class="Doctrine\Common\Annotations\AnnotationReader" public="false">
      ...
    </service>
  <services>
  <factory-script type="php/closure" depends="annotations.reader" on-missing="skip"><![CDATA[
    Doctrine\Common\Annotations\AnnotationRegistry::registerLoader('class_exists');
  ]]></factory-script>
</container>

This would execute the script each time the service "annotations.reader" is initialized/created ("newed").

Script scenarios:

• <config-script> script executed during configuration, maybe to include config resources depending on the env; would be executed right after the config resource is loaded (before following config resources are loaded)
• <build-script> like extensions
• <compile-script> like compiler passes
• <load-script> executed when the container is loaded/booted
• <factory-script> executed when services are "newed"

WDYT