1 files changed, 164 insertions, 0 deletions
diff --git a/railties/test/application/zeitwerk_integration_test.rb b/railties/test/application/zeitwerk_integration_test.rb
new file mode 100644
index 0000000000..d44b7f7e4d
--- /dev/null
+++ b/railties/test/application/zeitwerk_integration_test.rb
@@ -0,0 +1,164 @@
+# frozen_string_literal: true
+
+require "isolation/abstract_unit"
+require "active_support/dependencies/zeitwerk_integration"
+
+class ZeitwerkIntegrationTest < ActiveSupport::TestCase
+  include ActiveSupport::Testing::Isolation
+
+  def setup
+    build_app
+  end
+
+  def boot(env = "development")
+    app(env)
+  end
+
+  def teardown
+    teardown_app
+  end
+
+  def deps
+    ActiveSupport::Dependencies
+  end
+
+  def decorated?
+    deps.singleton_class < deps::ZeitwerkIntegration::Decorations
+  end
+
+  test "ActiveSupport::Dependencies is decorated by default" do
+    boot
+
+    assert decorated?
+    assert_instance_of Zeitwerk::Loader, Rails.autoloader
+    assert_instance_of Zeitwerk::Loader, Rails.once_autoloader
+    assert_equal [Rails.autoloader, Rails.once_autoloader], Rails.autoloaders
+  end
+
+  test "ActiveSupport::Dependencies is not decorated in classic mode" do
+    add_to_config 'config.autoloader = :classic'
+    boot
+
+    assert_not decorated?
+    assert_nil Rails.autoloader
+    assert_nil Rails.once_autoloader
+    assert_empty Rails.autoloaders
+  end
+
+  test "constantize returns the value stored in the constant" do
+    app_file "app/models/admin/user.rb", "class Admin::User; end"
+    boot
+
+    assert_same Admin::User, deps.constantize("Admin::User")
+  end
+
+  test "constantize raises if the constant is unknown" do
+    boot
+
+    assert_raises(NameError) { deps.constantize("Admin") }
+  end
+
+  test "safe_constantize returns the value stored in the constant" do
+    app_file "app/models/admin/user.rb", "class Admin::User; end"
+    boot
+
+    assert_same Admin::User, deps.safe_constantize("Admin::User")
+  end
+
+  test "safe_constantize returns nil for unknown constants" do
+    boot
+
+    assert_nil deps.safe_constantize("Admin")
+  end
+
+  test "autoloaded_constants returns autoloaded constant paths" do
+    app_file "app/models/admin/user.rb", "class Admin::User; end"
+    app_file "app/models/post.rb", "class Post; end"
+    boot
+
+    assert Admin::User
+    assert_equal ["Admin", "Admin::User"], deps.autoloaded_constants
+  end
+
+  test "autoloaded? says if a constant has been autoloaded" do
+    app_file "app/models/user.rb", "class User; end"
+    app_file "app/models/post.rb", "class Post; end"
+    boot
+
+    assert Post
+    assert deps.autoloaded?("Post")
+    assert deps.autoloaded?(Post)
+    assert_not deps.autoloaded?("User")
+  end
+
+  test "eager loading loads the application code" do
+    $zeitwerk_integration_test_user = false
+    $zeitwerk_integration_test_post = false
+
+    app_file "app/models/user.rb", "class User; end; $zeitwerk_integration_test_user = true"
+    app_file "app/models/post.rb", "class Post; end; $zeitwerk_integration_test_post = true"
+    boot("production")
+
+    assert $zeitwerk_integration_test_user
+    assert $zeitwerk_integration_test_post
+  end
+
+  test "eager loading loads anything managed by Zeitwerk" do
+    $zeitwerk_integration_test_user  = false
+    app_file "app/models/user.rb", "class User; end; $zeitwerk_integration_test_user = true"
+
+    $zeitwerk_integration_test_extras = false
+    app_dir "extras"
+    app_file "extras/webhook_hacks.rb", "WebhookHacks = 1; $zeitwerk_integration_test_extras = true"
+
+    require "zeitwerk"
+    autoloader = Zeitwerk::Loader.new
+    autoloader.push_dir("#{app_path}/extras")
+    autoloader.setup
+
+    boot("production")
+
+    assert $zeitwerk_integration_test_user
+    assert $zeitwerk_integration_test_extras
+  end
+
+  test "autoload paths that are below Gem.path go to the once autoloader" do
+    app_dir "extras"
+    add_to_config 'config.autoload_paths << "#{Rails.root}/extras"'
+
+    # Mocks Gem.path to include the extras directory.
+    Gem.singleton_class.prepend(
+      Module.new do
+        def path
+          super + ["#{Rails.root}/extras"]
+        end
+      end
+    )
+    boot
+
+    refute_includes Rails.autoloader.dirs, "#{app_path}/extras"
+    assert_includes Rails.once_autoloader.dirs, "#{app_path}/extras"
+  end
+
+  test "clear reloads the main autoloader, and does not reload the once one" do
+    boot
+
+    $zeitwerk_integration_reload_test = []
+
+    autoloader = Rails.autoloader
+    def autoloader.reload
+      $zeitwerk_integration_reload_test << :autoloader
+      super
+    end
+
+    once_autoloader = Rails.once_autoloader
+    def once_autoloader.reload
+      $zeitwerk_integration_reload_test << :once_autoloader
+      super
+    end
+
+    ActiveSupport::Dependencies.clear
+
+    assert_equal %i(autoloader), $zeitwerk_integration_reload_test
+  end
+end