Move Zeitwerk to railties

Active Support should not know about Rails. Once classic has been removed,
AS should get anything it needs from the application as usual.

Author: fxn

activesupport/activesupport.gemspec

@@ -36,6 +36,5 @@ Gem::Specification.new do |s|
   s.add_dependency "i18n",            ">= 1.6", "< 2"
   s.add_dependency "tzinfo",          "~> 2.0"
   s.add_dependency "concurrent-ruby", "~> 1.0", ">= 1.0.2"
-  s.add_dependency "zeitwerk",        "~> 2.5.0.beta3"
   s.add_dependency "minitest",        ">= 5.1"
 end

activesupport/lib/active_support/dependencies.rb

@@ -58,6 +58,12 @@ def self.unload_interlock
     # main autoloader. Used to clear state.
     mattr_accessor :_autoloaded_tracked_classes, default: Set.new
 
+    # If reloading is enabled, this private attribute stores the main autoloader
+    # of a Rails application. It is `nil` otherwise.
+    #
+    # The public interface for this autoloader is `Rails.autoloaders.main`.
+    mattr_accessor :autoloader
+
     # Private method that reloads constants autoloaded by the main autoloader.
     #
     # Rails.application.reloader.reload! is the public interface for application
@@ -66,9 +72,7 @@ def self.unload_interlock
     def self.clear
       unload_interlock do
         _autoloaded_tracked_classes.clear
-        Rails.autoloaders.main.reload
-      rescue Zeitwerk::ReloadingDisabledError
-        raise "reloading is disabled because config.cache_classes is true"
+        autoloader.reload
       end
     end
 

railties/lib/rails/application/finisher.rb

@@ -29,6 +29,7 @@ module Finisher
 
         unless config.cache_classes
           autoloader.enable_reloading
+          ActiveSupport::Dependencies.autoloader = autoloader
 
           autoloader.on_load do |_cpath, value, _abspath|
             if value.is_a?(Class) && value.singleton_class < ActiveSupport::DescendantsTracker

railties/railties.gemspec

@@ -42,6 +42,7 @@ Gem::Specification.new do |s|
   s.add_dependency "rake", ">= 0.13"
   s.add_dependency "thor", "~> 1.0"
   s.add_dependency "method_source"
+  s.add_dependency "zeitwerk", "~> 2.5.0.beta3"
 
   s.add_development_dependency "actionview", version
 end

railties/test/application/zeitwerk_integration_test.rb

@@ -152,15 +152,6 @@ class RESTfulController < ApplicationController
     assert_not Rails.autoloaders.once.reloading_enabled?
   end
 
-  test "reloading raises if config.cache_classes is true" do
-    boot("production")
-
-    e = assert_raises(StandardError) do
-      deps.clear
-    end
-    assert_equal "reloading is disabled because config.cache_classes is true", e.message
-  end
-
   test "eager loading loads code in engines" do
     $test_blog_engine_eager_loaded = false