> a waste of build times There's an assumption here that tests need to be run for each build. This rules out more laborious testing that could be exposing bug

> a waste of build times There's an assumption here that tests need to be run for each build. This rules out more laborious testing that could be exposing bugs not found by time-limited tests. For compiler testing, one can set up property-based tests that run for unlimited lengths of time. These can be set up to be always running in the background. Literally billions of tests can be constructed and run.