I am the maintainer of a very large scale multi-threaded C++ application used by large airline/airport/rail companies around the world. I haven’t had a bug in p

I am the maintainer of a very large scale multi-threaded C++ application used by large airline/airport/rail companies around the world. I haven’t had a bug in production for 5+ years. Not one. And I routinely refactor/improve large parts of the code with zero problems. The “secret” obviously is 9000+ system level tests. And I will claim that you need that number of tests. Even if the application was written in (say) Rust. Rust won’t save you from subtle daylight savings errors, country specific governmental requirements, weird special case organisational policy rules etc. Only an executable spec (tests) will do that.