new config to opt-out from adding app directories to $LOAD_PATH

guides/source/configuring.md

@@ -64,6 +64,8 @@ These configuration methods are to be called on a `Rails::Railtie` object, such
 
 * `config.autoload_paths` accepts an array of paths from which Rails will autoload constants. Default is all directories under `app`. It is no longer recommended to adjust this. See [Autoloading and Reloading Constants](autoloading_and_reloading_constants.html#autoload-paths-and-eager-load-paths)
 
+* `config.add_autoload_paths_to_load_path` says whether autoload paths have to be added to `$LOAD_PATH`. This flag is `true` by default, but it is recommended to be set to `false` in `:zeitwerk` mode early, in `config/application.rb`. Zeitwerk uses absolute paths internally, and applications running in `:zeitwerk` mode do not need `require_relative`, so models, controllers, jobs, etc. do not need to be in `$LOAD_PATH`. Setting this to `false` saves Ruby from checking these directories when resolving `require` calls with relative paths, and saves Bootsnap work and RAM, since it does not need to build an index for them.
+
 * `config.cache_classes` controls whether or not application classes and modules should be reloaded on each request. Defaults to `false` in development mode, and `true` in test and production modes.
 
 * `config.beginning_of_week` sets the default beginning of week for the

railties/lib/rails/application/configuration.rb

@@ -19,7 +19,7 @@ module Rails
                     :beginning_of_week, :filter_redirect, :x, :enable_dependency_loading,
                     :read_encrypted_secrets, :log_level, :content_security_policy_report_only,
                     :content_security_policy_nonce_generator, :require_master_key, :credentials,
-                    :disable_sandbox
+                    :disable_sandbox, :add_autoload_paths_to_load_path
 
       attr_reader :encoding, :api_only, :loaded_config_version, :autoloader
 
@@ -67,6 +67,7 @@ module Rails
         @credentials.key_path                    = default_credentials_key_path
         @autoloader                              = :classic
         @disable_sandbox                         = false
+        @add_autoload_paths_to_load_path         = true
       end
 
       def load_defaults(target_version)

railties/lib/rails/engine.rb

@@ -559,9 +559,8 @@ module Rails
       end
     end
 
-    # Add configured load paths to Ruby's load path, and remove duplicate entries.
-    initializer :set_load_path, before: :bootstrap_hook do
-      _all_load_paths.reverse_each do |path|
+    initializer :set_load_path, before: :bootstrap_hook do |app|
+      _all_load_paths(app.config.add_autoload_paths_to_load_path).reverse_each do |path|
         $LOAD_PATH.unshift(path) if File.directory?(path)
       end
       $LOAD_PATH.uniq!
@@ -709,8 +708,12 @@ module Rails
         @_all_autoload_paths ||= (config.autoload_paths + config.eager_load_paths + config.autoload_once_paths).uniq
       end
 
-      def _all_load_paths
-        @_all_load_paths ||= (config.paths.load_paths + _all_autoload_paths).uniq
+      def _all_load_paths(add_autoload_paths_to_load_path)
+        @_all_load_paths ||= begin
+          load_paths  = config.paths.load_paths
+          load_paths += _all_autoload_paths if add_autoload_paths_to_load_path
+          load_paths.uniq
+        end
       end
 
       def build_request(env)

railties/test/application/configuration_test.rb

@@ -4,6 +4,7 @@ require "isolation/abstract_unit"
 require "rack/test"
 require "env_helpers"
 require "set"
+require "active_support/core_ext/string/starts_ends_with"
 
 class ::MyMailInterceptor
   def self.delivering_email(email); email; end
@@ -1704,6 +1705,33 @@ module ApplicationTests
       end
     end
 
+    test "autoload paths are added to $LOAD_PATH by default" do
+      app "development"
+
+      # Action Mailer modifies AS::Dependencies.autoload_paths in-place.
+      autoload_paths = ActiveSupport::Dependencies.autoload_paths
+      autoload_paths_from_app_and_engines = autoload_paths.reject do |path|
+        path.ends_with?("mailers/previews")
+      end
+      assert_equal true, Rails.configuration.add_autoload_paths_to_load_path
+      assert_empty autoload_paths_from_app_and_engines - $LOAD_PATH
+
+      # Precondition, ensure we are testing something next.
+      assert_not_empty Rails.configuration.paths.load_paths
+      assert_empty Rails.configuration.paths.load_paths - $LOAD_PATH
+    end
+
+    test "autoload paths are not added to $LOAD_PATH if opted-out" do
+      add_to_config "config.add_autoload_paths_to_load_path = false"
+      app "development"
+
+      assert_empty ActiveSupport::Dependencies.autoload_paths & $LOAD_PATH
+
+      # Precondition, ensure we are testing something next.
+      assert_not_empty Rails.configuration.paths.load_paths
+      assert_empty Rails.configuration.paths.load_paths - $LOAD_PATH
+    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