> Follow the rules and it won't break.

That's the problem here: you're on the honor system that everyone knows and follows every rule every time. Maybe you have a really top-notch team, great code review, etc. but can you say for a certainty that this will always be true, or that it's true of every bit of code you use?

Being able to prove that in advance, especially in more complicated scenarios, has a significant value from checking on every build, especially when you think of the many bugs which have been caused by maintenance code breaking some of the assumptions which the original authors had.

Virtually no large scale C++ app has successfully consistently followed the safety rules. It is too easy to, for example, store references to the mutex-protected object somewhere and have those references persist past the unlock operation, allowing unsynchronized access. References and raw pointers are created invisibly in C++ (example: "this"), unlike in Rust where "unsafe" is always explicit (and can be forbidden with a compiler switch or source annotation).

It’s an exactly same reasoning behind static & dynamic type system. “Don’t put null value to this function and it will not break. Just put object with these properties into this function and it will not break.” Static type system allow you to ENFORCE those contracts regarding what is the data. Same with this, Rust’s affine type system allows you to ENFORCE the contracts regarding HOW to use the data (i.e. if already be free, it couldn’t be used any more) as opposed to the infamous guideline “Don’t use the data after it’s already been freed “

You can't just wrap a statement the compiler has deemed to not pass the borrow checker and make it compile. Unsafe blocks can bypass the type system, but you have to call extra magic to do so.