Differential Coverage for Debugging. Take a failing test. You have no idea where the underlying issue might be. The system you’re working with is possibly deep, wide, and eldritch.

Your move:

  • run the tests and capture code coverage without the failing test
  • run the tests and capture code coverage with the failing test
  • diff the line-by-line code coverage(s) to see which lines are executed in the failing case only
  • eliminate further red herrings by mental elimination

Very clever! Via Thorsten Ball.