Any chance to merge it in 4.1?

It's a challenge to make it without changes in the core so far: ContainerConfigurator requires PhpFileLoader as dependency (probably, FileLoader is enough?). /cc @nicolas-grekas

What about something like

$c->extension('my_ext', ['primitive' => 'config']);

// vs.

/** @var MyExtConfigurator $myExt */
$myExt = $c->extension('my_ext');
$myExt
   ->fluent('config');

Thus support both flavors, as well as multiple extension per-config.

With a corresponding ConfiguratorAwareConfigurationInterface or so?

Or:

$myExt = $c->extension('my_ext')->configure(function (MyExtConfigurator $myExt) {
	return $myExt->foo('bar');
});

Or:

$c->extension(function(MyExtConfigurator $e) {
   $e->fluent('config');
});

Where's my cookie? :)

I'm not convinced this is the approach we need in core.

The idea I proposed in #25630 doesn't require any code at all, thus not any new concept/methods/api:

return function (ContainerConfigurator $c) {
    $myBundle = new MyBundleConfigurator($c);

    $myBundle->setWhatever();
}

To me, configuration is the correct place to do "new".

Note that I'd also really like to be able to autocomplete bundle config using fluent configurator.
Unfortunately, this is currently left as "an exercise" to bundle authors (and the exercise is not trivial).
Would be awesome to be able to somehow autogenerate it from config definition!

@unkind can you check #27065? I think it could fix your issue in a nice way. WDYT? I'm closing this PR as explained above. Thanks for proposing.

@unkind can you check #27065? I think it could fix your issue in a nice way.

Hmm, what's the difference? Closure or anonymous class?

The issue is a bit deeper than it may look.
Maintainers of bundles don't know and don't want to support this type of configuration.
I still don't see it for Monolog bundle, for example (like in my proposal here: #25630).
I'm pretty sure that simple "official" support at extension level could be nice push for them, because it's "official" way.
Just think about it like ... sales manager, not programmer. :)

Also, PhpFileLoader is still cannot be extended in user-land with custom injected file.

YAML/XML are cool when you need to generate them, e.g. you have some kind of "hexagons" (in terms of hexagonal architecture), they have different namespaces, you have to register it for auto wiring, etc., so config generation here is perfect solution.

But when you deal with manual changes, I still don't see better solution than plain PHP.