Automatically link to Ruby files referenced in notes [ci skip]

To ease reading the "Active Support Core Extensions" guide, let's
automatically link references to Ruby files.

It's also possible to reference other components' files in the even
though it's not used in the guides at the moment.

[Petr Skocik & Robin Dupret]

guides/rails_guides/markdown/renderer.rb

@@ -33,7 +33,9 @@ def header(text, header_level)
       end
 
       def paragraph(text)
-        if text =~ /^(TIP|IMPORTANT|CAUTION|WARNING|NOTE|INFO|TODO)[.:]/
+        if text =~ %r{^NOTE:\s+Defined\s+in\s+<code>(.*?)</code>\.?$}
+          %(<div class="note"><p>Defined in <code><a href="#{github_file_url($1)}">#{$1}</a></code>.</p></div>)
+        elsif text =~ /^(TIP|IMPORTANT|CAUTION|WARNING|NOTE|INFO|TODO)[.:]/
           convert_notes(text)
         elsif text.include?("DO NOT READ THIS FILE ON GITHUB")
         elsif text =~ /^\[<sup>(\d+)\]:<\/sup> (.+)$/
@@ -90,6 +92,22 @@ def convert_notes(body)
           end
         end
 
+        def github_file_url(file_path)
+          root, rest = file_path.split('/', 2)
+
+          case root
+          when 'abstract_controller', 'action_controller', 'action_dispatch'
+            path = ['actionpack', 'lib', root, rest].join('/')
+          when 'active_support', 'active_record', 'active_model', 'action_view',
+               'action_cable', 'action_mailer', 'action_pack', 'active_job'
+            path = [root.sub('_', ''), 'lib', root, rest].join('/')
+          else
+            path = file_path
+          end
+
+          ["https://github.com/rails/rails/tree", version || 'master', path].join('/')
+        end
+
         def version
           ENV['RAILS_VERSION']
         end