Services can now define service closures as arguments in services.yml files

You can delay instantiation of performance heavy or conditionally used services using service closures. https://symfony.com/doc/current/service_container/service_closures.html

Previously, you could only use service closures by declaring services to be autowired, then using the #[AutowireServiceClosure('foo.service')] attribute.

Now you can use the !service_closure custom tag in services.yml files.

For example:

my_service:
    class: Drupal\Example\ExampleService
    arguments: [!service_closure '@another_service']

Then in you class constructor, your service param is defined as \Closure.

public function __construct(
        protected \Closure $anotherServiceClosure,
    ) {
}

When you need to instantiate your service you call the closure like so:

protected function getAnotherService(): AnotherService {
    return ($this->anotherServiceClosure)();
}