example : where is #render ?

def index
  p method(:render).source_location
  render params[:id]
end

then curl http://localhost:3000/users/xxxx (or wherever your index is) and check your server output :

Processing by UsersController#show as */*
  Parameters: {"id"=>"xxxx"}
["/Users/aaron/git/rails/actionpack/lib/action_controller/metal/instrumentation.rb", 40]
Completed 500 Internal Server Error in 35ms (ActiveRecord: 0.0ms)

from : https://tenderlovemaking.com/2016/02/05/i-am-a-puts-debuggerer.html