remove side effects on :controller option in #url_for

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -412,7 +412,7 @@ def initialize(options, recall, set, extras = false)
           normalize_options!
           normalize_controller_action_id!
           use_relative_controller!
-          controller.sub!(%r{^/}, '') if controller
+          normalize_controller_leading_slash
           handle_nil_action!
         end
 
@@ -478,6 +478,11 @@ def use_relative_controller!
           end
         end
 
+        # remove leading slashes from the controller: '/foo/bar' becomes 'foo/bar'
+        def normalize_controller_leading_slash
+          @controller = @options[:controller] = controller.sub(%r{^/}, '') if controller
+        end
+
         # This handles the case of :action => nil being explicitly passed.
         # It is identical to :action => "index"
         def handle_nil_action!

actionpack/test/controller/url_for_test.rb

@@ -333,6 +333,14 @@ def test_false_url_params_are_included_in_query
         assert_equal("/c/a?show=false", W.new.url_for(:only_path => true, :controller => 'c', :action => 'a', :show => false))
       end
 
+      def test_url_for_has_no_side_effects
+        add_host!
+
+        controller_name = '/C'
+        assert_equal('http://www.basecamphq.com/C', W.new.url_for(:controller => controller_name))
+        assert_equal('/C', controller_name)
+      end
+
       private
         def extract_params(url)
           url.split('?', 2).last.split('&').sort