Lazily boot the rails app during tapioca gem

lib/tapioca/loaders/loader.rb

@@ -29,13 +29,11 @@ def load; end
       def load_bundle(gemfile, initialize_file, require_file, halt_upon_load_error)
         require_helper(initialize_file)
 
-        load_rails_application(halt_upon_load_error: halt_upon_load_error)
-
         gemfile.require_bundle
 
-        require_helper(require_file)
+        load_rails_application(environment_load: true, halt_upon_load_error: halt_upon_load_error)
 
-        load_rails_engines
+        require_helper(require_file)
       end
 
       sig do
@@ -85,116 +83,6 @@ def load_rails_application(environment_load: false, eager_load: false, app_root:
         say("Continuing RBI generation without loading the Rails application.")
       end
 
-      sig { void }
-      def load_rails_engines
-        return if engines.empty?
-
-        with_rails_application do
-          run_initializers
-
-          if zeitwerk_mode?
-            load_engines_in_zeitwerk_mode
-          else
-            load_engines_in_classic_mode
-          end
-        end
-      end
-
-      def run_initializers
-        engines.each do |engine|
-          engine.instance.initializers.tsort_each do |initializer|
-            initializer.run(Rails.application)
-          rescue ScriptError, StandardError
-            nil
-          end
-        end
-      end
-
-      sig { void }
-      def load_engines_in_zeitwerk_mode
-        # Collect all the directories that are already managed by all existing Zeitwerk loaders.
-        managed_dirs = Zeitwerk::Registry.loaders.flat_map(&:dirs).to_set
-        # We use a fresh loader to load the engine directories, so that we don't interfere with
-        # any of the existing loaders.
-        autoloader = Zeitwerk::Loader.new
-
-        engines.each do |engine|
-          eager_load_paths(engine).each do |path|
-            # Zeitwerk only accepts existing directories in `push_dir`.
-            next unless File.directory?(path)
-            # We should not add directories that are already managed by a Zeitwerk loader.
-            next if managed_dirs.member?(path)
-
-            autoloader.push_dir(path)
-          end
-        end
-
-        autoloader.setup
-      end
-
-      sig { void }
-      def load_engines_in_classic_mode
-        # This is code adapted from `Rails::Engine#eager_load!` in
-        # https://github.com/rails/rails/blob/d9e188dbab81b412f73dfb7763318d52f360af49/railties/lib/rails/engine.rb#L489-L495
-        #
-        # We can't use `Rails::Engine#eager_load!` directly because it will raise as soon as it encounters
-        # an error, which is not what we want. We want to try to load as much as we can.
-        engines.each do |engine|
-          eager_load_paths(engine).each do |load_path|
-            Dir.glob("#{load_path}/**/*.rb").sort.each do |file|
-              require_dependency file
-            end
-          rescue ScriptError, StandardError
-            nil
-          end
-        end
-      end
-
-      sig { returns(T::Boolean) }
-      def zeitwerk_mode?
-        Rails.respond_to?(:autoloaders) &&
-          Rails.autoloaders.respond_to?(:zeitwerk_enabled?) &&
-          Rails.autoloaders.zeitwerk_enabled?
-      end
-
-      sig { params(blk: T.proc.void).void }
-      def with_rails_application(&blk)
-        # Store the current Rails.application object so that we can restore it
-        rails_application = T.unsafe(Rails.application)
-
-        # Create a new Rails::Application object, so that we can load the engines.
-        # Some engines and the `Rails.autoloaders` call might expect `Rails.application`
-        # to be set, so we need to create one here.
-        unless rails_application
-          Rails.application = Class.new(Rails::Application)
-        end
-
-        blk.call
-      ensure
-        Rails.app_class = Rails.application = rails_application
-      end
-
-      T::Sig::WithoutRuntime.sig { returns(T::Array[T.class_of(Rails::Engine)]) }
-      def engines
-        return [] unless defined?(Rails::Engine)
-
-        safe_require("active_support/core_ext/class/subclasses")
-
-        project_path = Bundler.default_gemfile.parent.expand_path
-        # We can use `Class#descendants` here, since we know Rails is loaded
-        Rails::Engine
-          .descendants
-          .reject(&:abstract_railtie?)
-          .reject { |engine| gem_in_app_dir?(project_path, engine.config.root.to_path) }
-      end
-
-      sig { params(path: String).void }
-      def safe_require(path)
-        require path
-      rescue LoadError
-        nil
-      end
-
       sig { void }
       def eager_load_rails_app
         application = Rails.application

spec/tapioca/cli/gem_spec.rb

@@ -1272,6 +1272,19 @@ class Post
             RB
           end
 
+          @project.write!("config/application.rb", <<~RB)
+            module Tapioca
+              class Application < Rails::Application
+                config.load_defaults(#{ActiveSupport.gem_version.to_s[0..2]})
+              end
+            end
+          RB
+
+          @project.write!("config/environment.rb", <<~RB)
+            require_relative "application"
+            Rails.application.initialize!
+          RB
+
           @project.require_real_gem("rails", ActiveSupport.gem_version.to_s)
           @project.require_mock_gem(foo)
           @project.bundle_install!
@@ -1329,6 +1342,11 @@ class Application < Rails::Application
             end
           RB
 
+          @project.write!("config/environment.rb", <<~RB)
+            require_relative "application"
+            Rails.application.initialize!
+          RB
+
           response = @project.tapioca("gem turbo-rails")
 
           assert_includes(response.out, "Compiled turbo-rails")
@@ -1360,6 +1378,11 @@ class Application < Rails::Application
             end
           RB
 
+          @project.write!("config/environment.rb", <<~RB)
+            require_relative "application"
+            Rails.application.initialize!
+          RB
+
           response = @project.tapioca("gem turbo-rails")
 
           assert_includes(response.out, "Compiled turbo-rails")