Agreed, although to do this at acceptable cost you'll often also want a decent build system (a la blaze, that can rebuild and re-test just what needs rebuilding

Agreed, although to do this at acceptable cost you'll often also want a decent build system (a la blaze, that can rebuild and re-test just what needs rebuilding and re-testing) and a lot of CI slaves. That's not a typical setup. Right now build systems don't even do the "build and test the final commit" bit right; it should of course test against a pre-merge (not on the branch) but again that tends to be a pain to set up.