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>
    <% topic_url = Discourse.base_url + @topic_view.relative_url %>
    <% lang = SiteSetting.find_by_name('default_locale').try(:value) %>
    <title><%= @topic_view.title %></title>
    <link><%= topic_url %></link>
    <description><%= @topic_view.posts.first.raw %></description>
    <% if lang %>
      <language><%= lang %></language>
    <% end %>
    <lastBuildDate><%= @topic_view.topic.created_at.rfc2822 %></lastBuildDate>
    <category><%= @topic_view.topic.category.name %></category>
    <atom:link href="<%= topic_url %>.rss" rel="self" type="application/rss+xml" />
    <% @topic_view.recent_posts.each do |post| %>
      <% next unless post.user %>
      <item>
        <title><%= @topic_view.title %> at <%= post.created_at %></title>
        <author><%= "@#{post.user.username} (#{post.user.name})" -%></author>
        <description><![CDATA[
          <% post_url = Discourse.base_url + post.url %>
          <p><%= t('author_wrote', author: link_to(post.user.name, post.user)).html_safe %></p>
          <blockquote>
            <%= post.cooked.html_safe %>
          </blockquote>
          <p><%= link_to t('read_full_topic'), post_url %></p>
        ]]></description>
        <link><%= post_url %></link>
        <pubDate><%= post.created_at.rfc2822 %></pubDate>
        <guid><%= post_url %></guid>
        <source url="<%= Discourse.base_url %><%= @topic_view.relative_url %>.rss"><%= @topic_view.title %></source>
      </item>
    <% end %>
  </channel>
</rss>