/usr/lib/ruby/vendor_ruby/action_controller/templates/rescues/diagnostics.erb is in ruby-actionpack-2.3 2.3.14-5.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 | <h1>
<%=h @exception.class.to_s %>
<% if request.parameters['controller'] %>
in <%=h request.parameters['controller'].humanize %>Controller<% if request.parameters['action'] %>#<%=h request.parameters['action'] %><% end %>
<% end %>
</h1>
<pre><%=h @exception.clean_message %></pre>
<%= render :file => @rescues_path["rescues/_trace.erb"] %>
<%= render :file => @rescues_path["rescues/_request_and_response.erb"] %>
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