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

3 files changed, 38 insertions, 6 deletions

diff --git a/railties/lib/rails/application/configuration.rb b/railties/lib/rails/application/configuration.rb
index ac86cc89be..0b758dd3dd 100644
--- a/railties/lib/rails/application/configuration.rb
+++ b/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)
diff --git a/railties/lib/rails/engine.rb b/railties/lib/rails/engine.rb
index 06b4019fc7..d1b8c7803f 100644
--- a/railties/lib/rails/engine.rb
+++ b/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)
diff --git a/railties/test/application/configuration_test.rb b/railties/test/application/configuration_test.rb
index 62d9b1c813..7c613585e0 100644
--- a/railties/test/application/configuration_test.rb
+++ b/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