Replace reopening the class with App.configure as an alias to class_eval

railties/lib/rails/application.rb

@@ -66,6 +66,10 @@ def config
       self.class.config
     end
 
+    class << self
+      alias configure class_eval
+    end
+
     def root
       config.root
     end

railties/lib/rails/generators/rails/app/templates/config/environments/development.rb.tt

@@ -1,4 +1,4 @@
-class <%= app_const %>
+<%= app_const %>.configure do
   # Settings specified here will take precedence over those in config/environment.rb
 
   # In the development environment your application's code is reloaded on

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -1,4 +1,4 @@
-class <%= app_const %>
+<%= app_const %>.configure do
   # Settings specified here will take precedence over those in config/environment.rb
 
   # The production environment is meant for finished, "live" apps.

railties/lib/rails/generators/rails/app/templates/config/environments/test.rb.tt

@@ -1,4 +1,4 @@
-class <%= app_const %>
+<%= app_const %>.configure do
   # Settings specified here will take precedence over those in config/environment.rb
 
   # The test environment is used exclusively to run your application's