Drops support for classic mode

This starts a series of patches in which we drop classic mode. The final
result no longer has a const_missing callback, there is no hook/unhook,
and so on.

So, in this patch we remove the ability of configuring classic, but some
of the code that remains will be further refactored.

activesupport/lib/active_support/dependencies/zeitwerk_integration.rb

@@ -2,6 +2,7 @@
 
 require "set"
 require "active_support/core_ext/string/inflections"
+require "zeitwerk"
 
 module ActiveSupport
   module Dependencies

guides/source/autoloading_and_reloading_constants.md

@@ -438,25 +438,3 @@ While in common names these operations match, if acronyms or custom inflection r
 ### More Differences
 
 There are some other subtle differences. Please check the [autoloading section](upgrading_ruby_on_rails.html#autoloading) of the _Upgrading Ruby on Rails_] guide for details.
-
-Classic Mode is Deprecated
---------------------------
-
-Currently, it is still possible to use `classic` mode. However, `classic` is deprecated and will be eventually removed.
-
-New applications should use `zeitwerk` mode (which is the default), and applications being upgraded are strongly encouraged to migrate to `zeitwerk` mode. Please check the [_Upgrading Ruby on Rails_](upgrading_ruby_on_rails.html#autoloading) guide for details.
-
-Opting Out
-----------
-
-Applications can load Rails 6 defaults and still use the `classic` autoloader this way:
-
-```ruby
-# config/application.rb
-config.load_defaults 6.0
-config.autoloader = :classic
-```
-
-That may be handy if upgrading to Rails 6 in different phases, but `classic` mode is discouraged for new applications.
-
-`zeitwerk` mode is not available in versions of Rails prior to 6.0.

guides/source/configuring.md

@@ -159,13 +159,6 @@ numbers. It also filters out sensitive values of database columns when call `#in
 
 * `config.time_zone` sets the default time zone for the application and enables time zone awareness for Active Record.
 
-* `config.autoloader` sets the autoloading mode. This option defaults to `:zeitwerk` when `config.load_defaults` is called with `6.0` or greater. Applications can still use the classic autoloader by setting this value to `:classic` after loading the framework defaults:
-
-    ```ruby
-    config.load_defaults 6.0
-    config.autoloader = :classic
-    ```
-
 ### Configuring Assets
 
 * `config.assets.enabled` a flag that controls whether the asset

railties/lib/rails/application/configuration.rb

@@ -23,7 +23,7 @@ class Configuration < ::Rails::Engine::Configuration
                     :require_master_key, :credentials, :disable_sandbox, :add_autoload_paths_to_load_path,
                     :rake_eager_load
 
-      attr_reader :encoding, :api_only, :loaded_config_version, :autoloader
+      attr_reader :encoding, :api_only, :loaded_config_version
 
       def initialize(*)
         super
@@ -69,7 +69,6 @@ def initialize(*)
         @credentials                             = ActiveSupport::OrderedOptions.new
         @credentials.content_path                = default_credentials_content_path
         @credentials.key_path                    = default_credentials_key_path
-        @autoloader                              = :classic
         @disable_sandbox                         = false
         @add_autoload_paths_to_load_path         = true
         @permissions_policy                      = nil
@@ -128,8 +127,6 @@ def load_defaults(target_version)
         when "6.0"
           load_defaults "5.2"
 
-          self.autoloader = :zeitwerk if RUBY_ENGINE == "ruby"
-
           if respond_to?(:action_view)
             action_view.default_enforce_utf8 = false
           end
@@ -155,8 +152,6 @@ def load_defaults(target_version)
         when "6.1"
           load_defaults "6.0"
 
-          self.autoloader = :zeitwerk if %w[ruby truffleruby].include?(RUBY_ENGINE)
-
           if respond_to?(:active_record)
             active_record.has_many_inversing = true
             active_record.legacy_connection_handling = false
@@ -365,18 +360,6 @@ def permissions_policy(&block)
         end
       end
 
-      def autoloader=(autoloader)
-        case autoloader
-        when :classic
-          @autoloader = autoloader
-        when :zeitwerk
-          require "zeitwerk"
-          @autoloader = autoloader
-        else
-          raise ArgumentError, "config.autoloader may be :classic or :zeitwerk, got #{autoloader.inspect} instead"
-        end
-      end
-
       def default_log_file
         path = paths["log"].first
         unless File.exist? File.dirname path

railties/lib/rails/application/finisher.rb

@@ -2,6 +2,7 @@
 
 require "active_support/core_ext/string/inflections"
 require "active_support/core_ext/array/conversions"
+require "zeitwerk"
 
 module Rails
   class Application
@@ -39,14 +40,10 @@ module Finisher
         example       = autoloaded.first
         example_klass = example.constantize.class
 
-        if config.autoloader == :zeitwerk
-          ActiveSupport::DescendantsTracker.clear
-          ActiveSupport::Dependencies.clear
+        ActiveSupport::DescendantsTracker.clear
+        ActiveSupport::Dependencies.clear
 
-          unload_message = "#{these} autoloaded #{constants} #{have} been unloaded."
-        else
-          unload_message = "`config.autoloader` is set to `#{config.autoloader}`. #{these} autoloaded #{constants} would have been unloaded if `config.autoloader` had been set to `:zeitwerk`."
-        end
+        unload_message = "#{these} autoloaded #{constants} #{have} been unloaded."
 
         ActiveSupport::Deprecation.warn(<<~WARNING)
           Initialization autoloaded the #{constants} #{enum}.
@@ -76,10 +73,8 @@ module Finisher
       end
 
       initializer :let_zeitwerk_take_over do
-        if config.autoloader == :zeitwerk
-          require "active_support/dependencies/zeitwerk_integration"
-          ActiveSupport::Dependencies::ZeitwerkIntegration.take_over(enable_reloading: !config.cache_classes)
-        end
+        require "active_support/dependencies/zeitwerk_integration"
+        ActiveSupport::Dependencies::ZeitwerkIntegration.take_over(enable_reloading: !config.cache_classes)
       end
 
       # Setup default session store if not already set in config/application.rb
@@ -113,10 +108,7 @@ module Finisher
       initializer :eager_load! do
         if config.eager_load
           ActiveSupport.run_load_hooks(:before_eager_load, self)
-          # Checks defined?(Zeitwerk) instead of zeitwerk_enabled? because we
-          # want to eager load any dependency managed by Zeitwerk regardless of
-          # the autoloading mode of the application.
-          Zeitwerk::Loader.eager_load_all if defined?(Zeitwerk)
+          Zeitwerk::Loader.eager_load_all
           config.eager_load_namespaces.each(&:eager_load!)
         end
       end

railties/lib/rails/autoloaders.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/dependencies/zeitwerk_integration"
+require "zeitwerk"
 
 module Rails
   module Autoloaders # :nodoc:
@@ -41,7 +42,7 @@ def log!
       end
 
       def zeitwerk_enabled?
-        Rails.configuration.autoloader == :zeitwerk
+        true
       end
     end
   end

railties/test/application/configuration_test.rb

@@ -1219,7 +1219,6 @@ def index
     test "autoloaders" do
       app "development"
 
-      config = Rails.application.config
       assert Rails.autoloaders.zeitwerk_enabled?
       assert_instance_of Zeitwerk::Loader, Rails.autoloaders.main
       assert_equal "rails.main", Rails.autoloaders.main.tag
@@ -1228,24 +1227,6 @@ def index
       assert_equal [Rails.autoloaders.main, Rails.autoloaders.once], Rails.autoloaders.to_a
       assert_equal ActiveSupport::Dependencies::ZeitwerkIntegration::Inflector, Rails.autoloaders.main.inflector
       assert_equal ActiveSupport::Dependencies::ZeitwerkIntegration::Inflector, Rails.autoloaders.once.inflector
-
-      config.autoloader = :classic
-      assert_not Rails.autoloaders.zeitwerk_enabled?
-      assert_nil Rails.autoloaders.main
-      assert_nil Rails.autoloaders.once
-      assert_equal 0, Rails.autoloaders.count
-
-      config.autoloader = :zeitwerk
-      assert Rails.autoloaders.zeitwerk_enabled?
-      assert_instance_of Zeitwerk::Loader, Rails.autoloaders.main
-      assert_equal "rails.main", Rails.autoloaders.main.tag
-      assert_instance_of Zeitwerk::Loader, Rails.autoloaders.once
-      assert_equal "rails.once", Rails.autoloaders.once.tag
-      assert_equal [Rails.autoloaders.main, Rails.autoloaders.once], Rails.autoloaders.to_a
-      assert_equal ActiveSupport::Dependencies::ZeitwerkIntegration::Inflector, Rails.autoloaders.main.inflector
-      assert_equal ActiveSupport::Dependencies::ZeitwerkIntegration::Inflector, Rails.autoloaders.once.inflector
-
-      assert_raises(ArgumentError) { config.autoloader = :unknown }
     end
 
     test "config.action_view.cache_template_loading with cache_classes default" do

railties/test/application/zeitwerk_integration_test.rb

@@ -26,7 +26,7 @@ def decorated?
     deps.singleton_class < deps::ZeitwerkIntegration::Decorations
   end
 
-  test "ActiveSupport::Dependencies is decorated by default" do
+  test "ActiveSupport::Dependencies is decorated" do
     boot
 
     assert decorated?
@@ -36,17 +36,6 @@ def decorated?
     assert_equal [Rails.autoloaders.main, Rails.autoloaders.once], Rails.autoloaders.to_a
   end
 
-  test "ActiveSupport::Dependencies is not decorated in classic mode" do
-    add_to_config "config.autoloader = :classic"
-    boot
-
-    assert_not decorated?
-    assert_not Rails.autoloaders.zeitwerk_enabled?
-    assert_nil Rails.autoloaders.main
-    assert_nil Rails.autoloaders.once
-    assert_equal 0, Rails.autoloaders.count
-  end
-
   test "autoloaders inflect with Active Support" do
     app_file "config/initializers/inflections.rb", <<-RUBY
       ActiveSupport::Inflector.inflections(:en) do |inflect|