Problem/Motivation
Symfony 4 has removed the classloader, in favour of using Composer's. This means that all the non-APCu classloaders (xcache etc.) will be gone, with no replacement and we need to find a replacement APCu loader.
Proposed resolution
1. We don't need to provide a replacement for the non-APCu loader in core for Drupal 9, however we should make 8.x work without relying on them being in the code base.
2. For APCu, we'll need to do one of the following issues (or a third option):
- #1818628: Use Composer's optimized ClassLoader for Core/Component classes
- #2704571: Add an APCu classloader with a single entry
The above aren't mutually exclusive, we could do both.
While we need to do this for Drupal 9, we don't actually need to do it to unblock updating other Symfony components, because #3020303: Relax composer.json requirements to allow Drupal 8 to be installed with Symfony 4 can just leave symfony/class-loader at 3.4.0 and that works fine.
For Drupal 9 we need to do it to avoid running unsupported code still though, and it would be good to have the alternatives in Drupal 8 for sites to start using.
Remaining tasks
Discuss.
User interface changes
None.
API changes
TBD.
Data model changes
None.
Release notes snippet
TBD.