Test coverage gem for ruby that checks lines are actually covered

I used a tool that would help me improve test coverage by checking that the lines of the file that I am testing actually mattered to the spec.

It was done by disabling each line in the tested file and seeing if any tests failed. If no tests failed after a line was disabled, it was considered not covered.

I can't seem to find it today, maybe it was in a different language. Any ideas?