Wouldn't it be better to use this declaration though:

    struct RString {

      struct RBasic basic;

      union {
        struct {
          long len;
          char *ptr;
          union {
            long capa;
            VALUE shared;
          } aux;
        } heap;
    
        char ary[];
      } as;
    };

    /* apologies if I messed up the syntax here */
    #define RSTRING_EMBED_LEN_MAX (sizeof(((RString*)(0))->as) - 1)

Then you can even use the padding the compiler added, if any, plus you can add more things to heap and the embed length will grow automatically.