let environments configure load-related paths

Co-authored-by: Allen Hsu <allen.hsusp@gmail.com>

guides/source/autoloading_and_reloading_constants_classic_mode.md

@@ -482,8 +482,6 @@ What is described above are the defaults with a newly generated Rails app. There
 
 See also [Autoloading in the Test Environment](#autoloading-in-the-test-environment).
 
-`config.autoload_paths` is not changeable from environment-specific configuration files.
-
 The value of `autoload_paths` can be inspected. In a just-generated application
 it is (edited):
 

railties/lib/rails/engine.rb

@@ -560,6 +560,12 @@ module Rails
       end
     end
 
+    initializer :load_environment_config, before: :load_environment_hook, group: :all do
+      paths["config/environments"].existent.each do |environment|
+        require environment
+      end
+    end
+
     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)
@@ -608,12 +614,6 @@ module Rails
       end
     end
 
-    initializer :load_environment_config, before: :load_environment_hook, group: :all do
-      paths["config/environments"].existent.each do |environment|
-        require environment
-      end
-    end
-
     initializer :prepend_helpers_path do |app|
       if !isolated? || (app == self)
         app.config.helpers_paths.unshift(*paths["app/helpers"].existent)

railties/test/application/configuration_test.rb

@@ -1751,6 +1751,31 @@ module ApplicationTests
       assert_empty Rails.configuration.paths.load_paths - $LOAD_PATH
     end
 
+    test "autoload paths can be set in the config file of the environment" do
+      app_dir "custom_autoload_path"
+      app_dir "custom_autoload_once_path"
+      app_dir "custom_eager_load_path"
+
+      restore_default_config
+      add_to_env_config "development", <<-RUBY
+        config.autoload_paths      << "#{app_path}/custom_autoload_path"
+        config.autoload_once_paths << "#{app_path}/custom_autoload_once_path"
+        config.eager_load_paths    << "#{app_path}/custom_eager_load_path"
+      RUBY
+
+      app "development"
+
+      Rails.application.config.tap do |config|
+        assert_includes config.autoload_paths, "#{app_path}/custom_autoload_path"
+        assert_includes config.autoload_once_paths, "#{app_path}/custom_autoload_once_path"
+        assert_includes config.eager_load_paths, "#{app_path}/custom_eager_load_path"
+      end
+
+      assert_includes $LOAD_PATH, "#{app_path}/custom_autoload_path"
+      assert_includes $LOAD_PATH, "#{app_path}/custom_autoload_once_path"
+      assert_includes $LOAD_PATH, "#{app_path}/custom_eager_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