deprecates autoloading constants during initialization [closes #35745]

See rationale in the warning message included in the patch.

3 files changed, 96 insertions, 0 deletions

diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index ab400e5982..673cfde3d9 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Autoloading during initialization is deprecated.
+
+    *Xavier Noria*
+
 *   Only force `:async` ActiveJob adapter to `:inline` during seeding.
 
     *BatedUrGonnaDie*
diff --git a/railties/lib/rails/application/finisher.rb b/railties/lib/rails/application/finisher.rb
index 1cf44a480c..109c560c80 100644
--- a/railties/lib/rails/application/finisher.rb
+++ b/railties/lib/rails/application/finisher.rb
@@ -1,5 +1,8 @@
 # frozen_string_literal: true
 
+require "active_support/core_ext/string/inflections"
+require "active_support/core_ext/array/conversions"
+
 module Rails
   class Application
     module Finisher
@@ -21,6 +24,40 @@ module Rails
         end
       end
 
+      # This will become an error if/when we remove classic mode. The plan is
+      # autoloaders won't be configured up to this point in the finisher, so
+      # constants just won't be found, raising regular NameError exceptions.
+      initializer :warn_if_autoloaded, before: :let_zeitwerk_take_over do
+        next if config.cache_classes
+        next if ActiveSupport::Dependencies.autoloaded_constants.empty?
+
+        autoloaded    = ActiveSupport::Dependencies.autoloaded_constants
+        constants     = "constant".pluralize(autoloaded.size)
+        enum          = autoloaded.to_sentence
+        have          = autoloaded.size == 1 ? "has" : "have"
+        these         = autoloaded.size == 1 ? "This" : "These"
+        example       = autoloaded.first
+        example_klass = example.constantize.class
+
+        ActiveSupport::DescendantsTracker.clear
+        ActiveSupport::Dependencies.clear
+
+        ActiveSupport::Deprecation.warn(<<~WARNING)
+          Initialization autoloaded the #{constants} #{enum}.
+
+          Being able to do this is deprecated. Autoloading during initialization is going
+          to be an error condition in future versions of Rails.
+
+          Reloading does not reboot the application, and therefore code executed during
+          initialization does not run again. So, if you reload #{example}, for example,
+          the expected changes won't be reflected in that stale #{example_klass} object.
+
+          #{these} autoloaded #{constants} #{have} been unloaded.
+
+          Please, check the "Autoloading and Reloading Constants" guide for solutions.
+        WARNING
+      end
+
       initializer :let_zeitwerk_take_over do
         if config.autoloader == :zeitwerk
           require "active_support/dependencies/zeitwerk_integration"
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index b8e167b488..a2e3e781c0 100644
--- a/railties/test/application/configuration_test.rb
+++ b/railties/test/application/configuration_test.rb
@@ -1704,6 +1704,61 @@ module ApplicationTests
       end
     end
 
+    test "autoloading during initialization gets deprecation message and clearing if config.cache_classes is false" do
+      app_file "lib/c.rb", <<~EOS
+        class C
+          extend ActiveSupport::DescendantsTracker
+        end
+
+        class X < C
+        end
+      EOS
+
+      app_file "app/models/d.rb", <<~EOS
+        require "c"
+
+        class D < C
+        end
+      EOS
+
+      app_file "config/initializers/autoload.rb", "D"
+
+      app "development"
+
+      # TODO: Test deprecation message, assert_depcrecated { app "development" }
+      # does not collect it.
+
+      assert_equal [X], C.descendants
+      assert_empty ActiveSupport::Dependencies.autoloaded_constants
+    end
+
+    test "autoloading during initialization triggers nothing if config.cache_classes is true" do
+      app_file "lib/c.rb", <<~EOS
+        class C
+          extend ActiveSupport::DescendantsTracker
+        end
+
+        class X < C
+        end
+      EOS
+
+      app_file "app/models/d.rb", <<~EOS
+        require "c"
+
+        class D < C
+        end
+      EOS
+
+      app_file "config/initializers/autoload.rb", "D"
+
+      app "production"
+
+      # TODO: Test no deprecation message is issued.
+
+      assert_equal [X, D], C.descendants
+    end
+
+
     test "raises with proper error message if no database configuration found" do
       FileUtils.rm("#{app_path}/config/database.yml")
       err = assert_raises RuntimeError do