Deprecate mime types lookup in auto_discovery_link_tag

Automatically handling mime types for things other than :rss and :atom
is not functionality that justifies dependency on Mime::Type from
actionpack.

actionpack/lib/action_view/helpers/asset_tag_helper.rb

@@ -222,6 +222,14 @@ module AssetTagHelper
       #  auto_discovery_link_tag(:rss, "http://www.example.com/feed.rss", {:title => "Example RSS"})
       #  # => <link rel="alternate" type="application/rss+xml" title="Example RSS" href="http://www.example.com/feed" />
       def auto_discovery_link_tag(type = :rss, url_options = {}, tag_options = {})
+        if !(type == :rss || type == :atom) && tag_options[:type].blank?
+          message = "You have passed type other than :rss or :atom to auto_discovery_link_tag and haven't supplied " +
+                    "the :type option key. This behavior is deprecated and will be remove in Rails 4.1. You should pass " +
+                    ":type option explicitly if you want to use other types, for example: " +
+                    "auto_discovery_link_tag(:xml, '/feed.xml', :type => 'application/xml')"
+          ActiveSupport::Deprecation.warn message
+        end
+
         tag(
           "link",
           "rel"   => tag_options[:rel] || "alternate",

actionpack/test/template/asset_tag_helper_test.rb

@@ -65,7 +65,6 @@ def teardown
     %(auto_discovery_link_tag) => %(<link href="http://www.example.com" rel="alternate" title="RSS" type="application/rss+xml" />),
     %(auto_discovery_link_tag(:rss)) => %(<link href="http://www.example.com" rel="alternate" title="RSS" type="application/rss+xml" />),
     %(auto_discovery_link_tag(:atom)) => %(<link href="http://www.example.com" rel="alternate" title="ATOM" type="application/atom+xml" />),
-    %(auto_discovery_link_tag(:xml)) => %(<link href="http://www.example.com" rel="alternate" title="XML" type="application/xml" />),
     %(auto_discovery_link_tag(:rss, :action => "feed")) => %(<link href="http://www.example.com" rel="alternate" title="RSS" type="application/rss+xml" />),
     %(auto_discovery_link_tag(:rss, "http://localhost/feed")) => %(<link href="http://localhost/feed" rel="alternate" title="RSS" type="application/rss+xml" />),
     %(auto_discovery_link_tag(:rss, "//localhost/feed")) => %(<link href="//localhost/feed" rel="alternate" title="RSS" type="application/rss+xml" />),
@@ -299,6 +298,16 @@ def teardown
     %(audio_tag(["audio.mp3", "audio.ogg"], :autobuffer => true, :controls => true)) => %(<audio autobuffer="autobuffer" controls="controls"><source src="/audios/audio.mp3" /><source src="/audios/audio.ogg" /></audio>)
   }
 
+  def test_autodiscovery_link_tag_deprecated_types
+    result = nil
+    assert_deprecated do
+      result = auto_discovery_link_tag(:xml)
+    end
+
+    expected = %(<link href="http://www.example.com" rel="alternate" title="XML" type="application/xml" />)
+    assert_equal expected, result
+  end
+
   def test_auto_discovery_link_tag
     AutoDiscoveryToTag.each { |method, tag| assert_dom_equal(tag, eval(method)) }
   end