Well, the C3 developer could add more fine grained control if people need it...

I don't really see what's your problem. It's not so much different than disabling asserts in production. Some people don't do that, because they rather crash than walking into invalid program state - and that's fine too. It largely depends on the project in question.

> It's not so much different than disabling asserts in production.

Disabling asserts would be equivalent to not having them at all, while this feature introduces _new_ UB. In "fast" mode it's equivalent to using C's `__builtin_assume` or Rust's `std::hint::assert_unchecked`, except it's marketed with a name that makes it appear a safety/correctness feature.