So, please don't make (or invite) comparisons to Rust if you know you aren't there yet. Not everyone will agree that you are just making such a comparison "tongue-in-cheek". "Safer than Rust" is an extremely high bar to clear, and you haven't done so, so why title your blog post that way?

I am curious to know how you would handle tagged unions, say, something like this:

    struct object {
        int type; /* OBJ_INT, OBJ_STR, etc. */
        union {
            int i;
            char *s; /* heap allocated */
            /* etc */
        } val;
    };

Does Xr0 prevent type confusion, and can it model the data-dependent behaviour of any functions that operate on struct object?

How would you model things like reference counting? In CPython, for example, one of the most pernicious programming problems is invalid reference counting because the C API functions are rather inconsistent in their handling of reference counts. One has to carefully peruse the documentation to understand if the function will steal a reference (from a parameter), borrow a reference (for the return value), or if it makes a new reference. Such setups are perfect candidates for annotation.