Rails router automatically calculated for you the controller and named routes in the following scenarios:

match "home/about"                 #=> maps to home#about with named route home_about_path
  match "about"                      #=> does not work because it cannot guess the controller
  match "about" => "home#about"      #=> maps to home#about with named route home_about_path
  match "home/about", :as => "about" #=> maps to home#about with named route about_path

actionpack/lib/action_dispatch/routing/mapper.rb

@@ -55,10 +55,8 @@ module ActionDispatch
             path = normalize_path(path)
 
             if using_match_shorthand?(path, options)
-              options = {
-                :to => path[1..-1].sub(%r{/([^/]*)$}, '#\1'),
-                :as => path[1..-1].gsub("/", "_")
-              }.merge!(options)
+              options[:to] ||= path[1..-1].sub(%r{/([^/]*)$}, '#\1')
+              options[:as] ||= path[1..-1].gsub("/", "_")
             end
 
             [ path, options ]
@@ -71,7 +69,7 @@ module ActionDispatch
 
           # match "account/overview"
           def using_match_shorthand?(path, options)
-            path && options.except(:via, :anchor).empty? && !path.include?(':')
+            path && options.except(:via, :anchor, :to, :as).empty? && path =~ %r{^/[\w\/]+$}
           end
 
           def normalize_path(path)

actionpack/test/dispatch/routing_test.rb

@@ -18,10 +18,9 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
       default_url_options :host => "rubyonrails.org"
 
       controller :sessions do
-        get  'login' => :new, :as => :login
+        get  'login' => :new
         post 'login' => :create
-
-        delete 'logout' => :destroy, :as => :logout
+        delete 'logout' => :destroy
       end
 
       resource :session do
@@ -35,6 +34,7 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
 
       match 'account/overview'
       match '/account/nested/overview'
+      match 'sign_in' => "sessions#new"
 
       match 'account/modulo/:name', :to => redirect("/%{name}s")
       match 'account/proc/:name', :to => redirect {|params| "/#{params[:name].pluralize}" }
@@ -673,6 +673,14 @@ class TestRoutingMapper < ActionDispatch::IntegrationTest
     end
   end
 
+  def test_convention_with_explicit_end
+    with_test_routes do
+      get '/sign_in'
+      assert_equal 'sessions#new', @response.body
+      assert_equal '/sign_in', sign_in_path
+    end
+  end
+
   def test_redirect_with_complete_url
     with_test_routes do
       get '/account/google'