This Google blogpost explains how they displayed which lines of a patch are covered by a test. That's a really cool feature, would be great to have something like that. It's useful both for ensuring that new code comes with tests, and for building confidence for merging trivial changes to well-tested code without manual testing.
Apparently people have worked on this for Gerrit but there is nothing out-of-the-box usable at the moment, so this is more of a tracking task. The upstream issue is #3538.