Add lib to load paths when application is inherited to be able to load lib code during configuration.

railties/lib/rails/application.rb

@@ -67,6 +67,7 @@ module Rails
         raise "You cannot have more than one Rails::Application" if Rails.application
         super
         Rails.application = base.instance
+        Rails.application.add_lib_to_load_paths!
         ActiveSupport.run_load_hooks(:before_configuration, base.instance)
       end
 
@@ -83,11 +84,21 @@ module Rails
 
     delegate :middleware, :to => :config
 
+    def add_lib_to_load_paths!
+      path = config.root.join('lib').to_s
+      $LOAD_PATH.unshift(path) if File.exists?(path)
+    end
+
     def require_environment!
       environment = paths.config.environment.to_a.first
       require environment if environment
     end
 
+    def eager_load!
+      railties.all(&:eager_load!)
+      super
+    end
+
     def routes
       @routes ||= ActionDispatch::Routing::RouteSet.new
     end

railties/lib/rails/application/finisher.rb

@@ -38,7 +38,7 @@ module Rails
       initializer :eager_load! do
         if config.cache_classes && !$rails_rake_task
           ActiveSupport.run_load_hooks(:before_eager_load, self)
-          railties.all(&:eager_load!)
+          eager_load!
         end
       end
 

railties/test/application/initializers/load_path_test.rb

@@ -19,6 +19,23 @@ module ApplicationTests
       assert $:.include?("#{app_path}/app/models")
     end
 
+    test "initializing an application adds lib path on inheritance hook" do
+      app_file "lib/foo.rb", <<-RUBY
+        module Foo; end
+      RUBY
+
+      add_to_config <<-RUBY
+        require "foo"
+        raise "Expected Foo to be defined" unless defined?(Foo)
+      RUBY
+
+      assert_nothing_raised do
+        require "#{app_path}/config/environment"
+      end
+
+      assert $:.include?("#{app_path}/lib")
+    end
+
     test "initializing an application eager load any path under app" do
       app_file "app/anything/foo.rb", <<-RUBY
         module Foo; end