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});
  }

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...

I think you would still need to know the exact type inside of struct_printf(), unless you knew exactly how the compiler would pack a struct onto the stack or registers, and that it was exactly the same as individual parameters.

Instead, you're supposed to #include <inttypes.h> and use:

  "No type promotion here: %f %"PRIu16" %"PRIx8"\n"

Sometimes it's a good way to go.