I've never met an expert who writes c well all of the time, so I have been led to believe it's simply not possible. Folks with 20+ years of experience will still regularly write a use after free but which is not caught until it hits production. I'm not sure I've actually seen C code free of concurrency bugs. If it's possible it takes years of battle hardening with no feature work. Humans are prone to make mistakes and we need to use as many tools as possible to help us avoid making those mistakes. They are recently looking into introducing thread annotations for locks in the Linux kernel and I expect them to find lots of real bugs lurking in otherwise well maintained subsystems. I've rewritten swaths of code into rust and always seen it find a bug in the original code's design that we were lucky to never run into.