`fast_xs` support has been removed. Use 'String#encode(xml: :attr)`.

activesupport/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   `fast_xs` support has been removed.  Use 'String#encode(xml: :attr)`.
+
 *   `ActiveSupport::Notifications::Instrumenter#instrument` should yield
     its payload.
 

activesupport/lib/active_support/core_ext/string.rb

@@ -4,7 +4,6 @@
 require 'active_support/core_ext/string/starts_ends_with'
 require 'active_support/core_ext/string/inflections'
 require 'active_support/core_ext/string/access'
-require 'active_support/core_ext/string/xchar'
 require 'active_support/core_ext/string/behavior'
 require 'active_support/core_ext/string/output_safety'
 require 'active_support/core_ext/string/exclude'

activesupport/lib/active_support/core_ext/string/xchar.rb

-begin
-  # See http://fast-xs.rubyforge.org/ by Eric Wong.
-  # Also included with hpricot.
-  require 'fast_xs'
-rescue LoadError
-  # fast_xs extension unavailable
-else
-  begin
-    require 'builder'
-  rescue LoadError
-    # builder demands the first shot at defining String#to_xs
-  end
-
-  class String
-    alias_method :original_xs, :to_xs if method_defined?(:to_xs)
-    alias_method :to_xs, :fast_xs
-  end
-end