I think unit tests can be divided into two camps as well: there are the trivial ones that are very close-coupled to the implementation, like, check if my ad hoc

I think unit tests can be divided into two camps as well: there are the trivial ones that are very close-coupled to the implementation, like, check if my ad hoc stack implementation will actually add the element and return sensible outputs. Something like this can easily be a private method as well, and I think they make sense to include right next to the function itself. Some languages allow these reality checks right in the documentation itself - and I don’t see who would it hurt. They are closely coupled to the implementation so sure, they will have to be rewritten with it, but that’s completely fine. The usually called unit tests indeed should not directly test implementation details, like private methods, but then we have to find a different name for the former because I do think they are useful sometimes