This proposal adds a new container parameter .extension.processed_configs that holds all processed extension configs from all registered extensions in order to improve the DX during the DI compilation phase (load extension, compiler pass).

Before

Currently, it's not possible to access processed configs, so this new parameter will avoid unsafely and hacking workarounds just to query a config value that belongs to another (already processed) extension and derives some DI definitions from it.

Some workarounds involve:

• Collect extension configs during the "prepend extension" phase (even if other extensions can prepend more config after)
• Duplicated configs with similar meanings in two or more extensions
• Call/override the processConfiguration() method from another extension is forbidden as it's protected and final, meaning you'll have to process twice this config outside the extension owner.
• Useless container parameters have to be defined to pass config information from "prepend" to "load" or/and from "load" to "compiler passes".

After

With this proposal, you'll access these configs through the .extension.processed_configs parameter. It's an array of extensions configs, where the array key matches the extension alias:

class AppExtension extends AbstractExtension
{
    public function loadExtension(array $config, ContainerConfigurator $container, ContainerBuilder $builder): void
    {
        $processedConfigs = $builder->getParameter('.extension.processed_configs');

        // $processedConfigs['framework']
    }
}

Note that in load or loadExtension you have access only to already loaded extensions depending on the extension/bundle definition order. If you need access to other extension configs that are not loaded yet then move your logic to a compiler pass or change the bundle/extension order in the bundle.php file of your project.

Also available in any compiler pass with the capability to resolve env placeholders:

class AppCompilerPass implements CompilerPassInterface
{
    public function process(ContainerBuilder $container): void
    {
        $processedConfigs = $container->getParameter('.extension.processed_configs');
        $processedConfigs = $container->resolveEnvPlaceholders($processedConfigs, true);

        // $processedConfigs['framework']
    }
}

After merging #47680 this .extension.processed_configs parameter is considered a "build parameter" finally, so it will be used during compiler time only and will be removed at the end of the process.