I was building a service with a service locater and it occurred to me that you're forced to define both a key and id (service id) to add a service to the service locator.

Enter service subscribers, in which it's possible to omit the key if it's the same internally and externally. Example:

public static function getSubscribedServices()
{
    return [
        LoggerInterface::class,
    ];
}

This is enough* to add the logger service to your service with a service subscriber because the key is that same as its service id and it links the two.

<service id="my_locator" class="Symfony\Component\DependencyInjection\ServiceLocator">
    <argument type="collection">
        <argument type="service" key="Psr\Logger\LoggerInterface" id="Psr\Logger\LoggerInterface" />
    </argument>
</service>

This is necessary to add the logger service as an argument to a service locator. With the PSR Logger this isn't a big deal, but with longer classes it's just a hassle.

I would propose to add a new way to add a service to a service locator by only passing a string:

<service id="my_locator" class="Symfony\Component\DependencyInjection\ServiceLocator">
    <argument type="collection">
        <argument>Psr\Logger\LoggerInterface</argument>
    </argument>
</service>

This is similar to how it's handled in the service subscribers pass. I've made an attempt to create a simple PR to achieve the functionality, but was unable to do so due to memory errors when building the container and have yet to make another attempt.