Yes - writing code myself, whenever I need to do something substantial the first question I ask is "is there already a function in this codebase somewhere that

Yes - writing code myself, whenever I need to do something substantial the first question I ask is "is there already a function in this codebase somewhere that does this, or does something close enough to it that I can just tweak it?" and most of the time there is! AI will just write a new function every time. You can also ask it to write tests, but I think that AI-written test coverage of AI-written code is just asking for trouble. When it breaks, you'll probably just ask an AI to fix it.