the semantics of C make that virtually impossible. the compiler would have to make some semantics of the language invalid, invalidating patterns that are more than likely highly utilized in existing code, thus we have Rust, which built its semantics around those safety concepts from the beginning. there’s just no way for the compiler to know the lifetime of some variables without some semantic indication

At the cost of sounding naive and stupid

It may be a naive question, but it's a very important naive question. Naive doesn't mean bad.

The answer is that that is not possible, because the compiler is supposed to translate the very specific language of C into mostly very specific machine instructions. The programmers who wrote the code, did so because they usually expect a very specific behavior. So, that would be broken.

But also, the "unsafety" is in the behavior of the system and built into the language and the compiler.

It's a bit of a flawed comparison, but you can't build a house on a foundation of wooden poles, because of the advantages that wood offers, and then complain that they are flammable. You can build it in steel, but you have to replace all of the poles. Just the poles on the left side won't do.

And you can't automatically detect the unsafe parts and just patch those either. If we could, we could just fix them directly or we could automatically transpile them. Darpa is trying that at the moment.

The problem is that C is a prehistoric language and don't have any of the complex types for example. So, in a modern language you create a String. That string will have a length, and some well defined properties (like encoding and such). With C you have a char * , which is just a pointer to the memory that contains bytes, and hopefully is null terminated. The null termination is defined, but not enforced. Any encoding is whatever the developer had in mind. So the compiler just don't have the information to make any decisions. In rust you know exactly how long something lives, if something try to use it after that, the compiler can tell you. With C, all lifetimes lives in the developers head, and the compiler have no way of knowing. So, all these typing and properties of modern languages, are basically the implementation of your suggestion.

Modern C compilers have a lot of features you can use to check for example for memory errors. Rusts borrow-checker is much stricter as it's designed to be part of the language, but for low-level code like the Linux kernel you'll end up having to use Rust's unsafe feature on a lot of code to do things from talking to actual hardware to just implementing certain data structures and then Rust is about as good as C.

If you write unsafe code then how should it compile?