Do we still have a compiler pass validating that services are either instantiable classes or have a factory configured, so that we get a clean error at build time for services that miss this configuration ? And if yes, do we need to update its error message to mention the possibility to use#[Autoconfigure(constructor: ...)] for autoconfigured definitions ?

We do have a compiler pass but it didn't tell about the attribute. PR updated:

Invalid service "App\Foo": its constructor must be public. Did you miss configuring a factory or a static constructor? Try using the "#[Autoconfigure(constructor: ...)]" attribute for the latter.