CachedCiviContainer generated with Symfony 4 removes private services

Overview

When using Symfony 4.4, private services & aliases are removed from the generated CachedCiviContainer.*.php file making them missing/unavailable.

Reproduction steps

1. Set up a Drupal 9 site using composer (requires symfony/dependency-injection and other Symfony packages with version ^4.4). (I know Drupal 9 isn't officially supported yet, but civicrm-core does claim it can use Symfony ~3.0 || ~4.4 and I think Symfony is the problem here)
2. Follow instructions from "Install CiviCRM on Drupal 8" documentation to add/install CiviCRM.
3. If you look at the CachedCiviContainer.*.php file generated in the templates_c folder, you can see $this->privates and $this->aliases are empty arrays, whereas with Symfony 3 there was a 'cache.short' alias and a number of private services that are now missing.
4. Visit .../civicrm/admin/extensions?reset=1 page and it will break.

Current behaviour

When visiting .../civicrm/admin/extensions?reset=1 page:

The "cache.short" service or alias has been removed or inlined when the container was compiled. You should either make it public, or stop using the container directly and use dependency injection instead.

There are probably other pages/actions that try & fail to use one of the services/alias that were removed, but this is the one I found.

Expected behaviour

Page should load without error!

Environment information

• CiviCRM: 5.29.0
• PHP: 7.3
• CMS: Drupal 9.0.5
• Database: MariaDB 10.4
• Web Server: Nginx

Comments

I believe Symfony 4 may remove these private services/alias with RemoveUnusedDefinitionsPass because they look "unused"? The error message offers a solution, make these services/alias public or "stop using the container directly and use dependency injection instead". I'm not sure what the implications of making them public are, or how much work would be involved in using dependency injection.