discourse/app/views/topics/show.rss.erb

<?xml version="1.0" encoding="UTF-8" ?>
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
  <channel>
    <title><%= @topic_view.title %></title>
    <link><%= Discourse.base_url %><%= @topic_view.relative_url %></link>
    <description><%= @topic_view.posts.first.raw %></description>
    <atom:link href="<%= Discourse.base_url %><%= @topic_view.relative_url %>.rss" rel="self" type="application/rss+xml" />
    <% @topic_view.recent_posts.each do |post| %>
      <% if post.user %>
        <item>
          <title><%= @topic_view.title %> at <%= post.created_at %></title>
          <description><![CDATA[
            <p><%= post.author_readable %> wrote:</p>
            <%= post.cooked.html_safe %>
          ]]></description>
          <link><%= Discourse.base_url %><%= post.url %></link>
          <pubDate><%= post.created_at.rfc2822 %></pubDate>
          <guid><%= Discourse.base_url %><%= post.url %></guid>
          <source url="<%= Discourse.base_url %><%= @topic_view.relative_url %>.rss"><%= @topic_view.title %></source>
        </item>
      <% end %>
    <% end %>
  </channel>
</rss>