> "buffer overflow within a struct" is often considered a feature.

That's lying to the compiler, because there's no way to say what you meant.

That's why I proposed the syntax

    typedef struct msgitem {
        const size_t len;
        char itemvalue[len];
    };
    struct msgitem firstmsg = { 100, 0 }; // empty msgitem, size 100

The last item of a struct could be a variable sized array. That's sound, and enough for things such as network packets. It's important to avoid bikeshedding.