That is really cool.

Combine this with struct definitions in function bodies and I see a great teamup with variadic functions.

Take a theoretical struct_printf, which prints out non promoted data with struct packing (not variadic stack packing, which requires type promotion)

  /* Only uses va_start and va_arg to*/
  void struct_printf(const char* format, ...){
  ...
  }

  void print_foo(){
    struct foo_t{
      float f;
      uint16_t u2;
      uint8_t x;
    } input;

    struct_printf("No type promotion here: %f %hu %hhx\n",
     (struct foo_t){ .f = 0.1, .u2 = 1000, .x = 0xF2});
  }

Now we can actually pass what we mean to pass without the compiler changing it.

NOTE: After I wrote this I'm not actually sure what va_args does with a struct or how to get a pointer to the first member...