Elementary tests, like unit tests, should be mecanically generated by walking the AST, differences ack`d and snapshoted when commiting. Every language should come with this built-in.
Very important. Very valuable.
Imagine if `if err != nil { return err }` just stopped working tomorrow. Your tests would detect it! Outage prevented!
Only past few days though, I did find two bugs that would've been prevented if the original code was covered by a decent unit test.
So you've never made a change caused a unit test to fail? If not, how large is your codebase, and is ownership shared across multiple teams?
I caught dozens of latent or unreported bugs by writing unit tests for a 6kloc JS app which had 0% coverage before.