> Especially you should not ever have to fight to borrow checker because it does the job you should be doing as a C/C++ programmer in your head anyway.
It adds mental overhead.
I don't even like the "const" qualifier for functions in C++. It is so hard to get a design right upfront when thinking about non trivial 'const'-chains.
The borrow checker removes the mental overhead of borrow-checking. You don't have to do it by yourself, you can lean on the borrow-checker to do that for you. In C++ you're all by yourself. You may pretend you don't need to do it, but you will run into troubles pretty fast.
Moving stuff into a closure is a perfect example of that. Try to capture an arcmutex to a bunch of threads in a closure? The compiler will note that arc can't be copied and to try moving it. It will then say you are trying to move the same arc to multiple threads, so you clone the arc. And if you try to just use an Rc it will tell you that can't be sent between threads (not thread safe) use arc instead. Those messages can absolutely guide you a long way before you realize your whole concept won't work, and that is frustrating, but those steps usually make you understand the whole issue.
To me `const` removes mental overhead -- if I pass a const object somewhere, I can be sure it doesn't get changed. I don't need to inspect the code to make sure it doesn't call any methods that modify the object or trust a probably outdated comment or documentation.
When you are using the function, ye. Not when you are writing it. And if you someday want to change a const method to non-const you could have a cascading constness change all over the place.
