Add initializers that run before Zeitwerk

We need this to setup inflections that Zeitwerk needs

config/application.rb

@@ -292,6 +292,13 @@ module Gitlab
     initializer :move_initializers, before: :load_config_initializers, after: :let_zeitwerk_take_over do
     end
 
+    # We need this for initializers that need to be run before Zeitwerk is loaded
+    initializer :before_zeitwerk, before: :let_zeitwerk_take_over, after: :prepend_helpers_path do
+      Dir[Rails.root.join('config/initializers_before_autoloader/*.rb')].sort.each do |initializer|
+        load_config_initializer(initializer)
+      end
+    end
+
     config.after_initialize do
       Rails.application.reload_routes!
 

doc/development/README.md

@@ -97,6 +97,7 @@ Complementary reads:
 - [Issue types vs first-class types](issue_types.md)
 - [Application limits](application_limits.md)
 - [Redis guidelines](redis.md)
+- [Rails initializers](rails_initializers.md)
 
 ## Performance guides
 

doc/development/rails_initializers.md

+# Rails initializers
+
+By default, Rails loads Zeitwerk after the initializers in `config/initializers` are loaded.
+Autoloading before Zeitwerk is loaded is now deprecated but because we use a lot of autoloaded
+constants in our initializers, we had to move the loading of Zeitwerk earlier than these
+initializers.
+
+A side-effect of this is that in the initializers, `config.autoload_paths` is already frozen.
+
+To run an initializer before Zeitwerk is loaded, you need put them in `config/initializers_before_autoloader`.
+Ruby files in this folder are loaded in alphabetical order just like the default Rails initializers.
+
+Some examples where you would need to do this are:
+
+1. Modifying Rails' `config.autoload_paths`
+1. Changing configuration that Zeitwerk uses, e.g. inflections