title: Finding out rails' test coverage
body: <p>I've just seen this issue on rails' issue tracker:</p>
  
  <p><a href="https://github.com/rails/rails/issues/2667">https://github.com/rails/rails/issues/2667</a></p>
  
  <p>And I'd like to find out which parts of the code aren't covered. I couldn't find a coverage tool into the Rakefiles, and searching for it is a bit frustrating, since it returns far more results about test coverage on your rails app than test coverage of the framework itself.</p>
  
  <p>Has anybody set up a code coverage tool? Is there any documentation on how to do it?</p>