[JonChesterfield]

A 'C' -> C compiler which preserves most source code unchanged (i.e. would be the identity transform on some input) and which implements something like constexpr on functions (by running the interpreter during the transform) could be argued to be a forward looking C implementation. Specifically C23 has constexpr, but in an extremely limited form, and aspires to extend that to be more useful later.

Equally one which replaces 'auto' with the name of the type (and similar desugaring games) is still a C to C compiler, just running as a C23 to C99 or whatever. Resolve the branch in _Generic before emitting code as part of downgrading C11.

The lifetime annotations are an interesting one because they're a different language which, if it typechecks, can be losslessly converted into C (by dropping the annotations on the way out).

I'm not sure where in that design space the current implementation lies. In particular folding preprocessed code back into code that has the #defines and #includes in is a massive pain and only really valuable if you want to lean into the round trip capability.

[thradams]

auto, typeof, _Generic are implemented in cake. Sometimes when they are used inside macros the macros needs to be expanded. Then cake has #pragma expand MACRO. for this task.

Sample macro NEW using c23 typeof.

    #include <stdlib.h>
    #include <string.h>

    static inline void* allocate_and_copy(void* s, size_t n) {
        void* p = malloc(n);
        if (p) {
            memcpy(p, s, n);
        }
        return p;
    }

    #define NEW(...) (typeof(__VA_ARGS__)*) allocate_and_copy(&(__VA_ARGS__), sizeof(__VA_ARGS__))
    #pragma expand NEW

    struct X {
        const int i;
    };

    int main() { 
        auto p = NEW((struct X) {});     
    }

The generated code is

    #include <stdlib.h>
    #include <string.h>

    static inline void* allocate_and_copy(void* s, size_t n) {
        void* p = malloc(n);
        if (p) {
            memcpy(p, s, n);
        }
        return p;
    }

    #define NEW(...) (typeof(__VA_ARGS__)*) allocate_and_copy(&(__VA_ARGS__), sizeof(__VA_ARGS__))
    #pragma expand NEW

    struct X {
        const int i;
    };

    int main() { 
        struct X  * p =  (struct X*) allocate_and_copy(&((struct X) {0}), sizeof((struct X) {0}));     
    }