Pointers that store array size data, x86_64 only; not recommended for use.

fatptr.c

#include <inttypes.h>

#define IS_ARRAY(x) _Generic(&x, default: 1, typeof(*x)**: 0)
#define COUNT_OF(x) _Generic(&x, default: (sizeof(x) / sizeof(0[x])), typeof(*x)**: 0)

#define array_to_fatptr(x) ((fatptr_t){.addr = x, .n_elems = (int16_t)COUNT_OF(x)})
#define fatptr_to_ptr(x, type) ((type*)(intptr_t)x.addr)

typedef union fatptr_t{
	void *ptr;
	struct{
		intptr_t addr:48;
		int16_t n_elems;
	};
} fatptr_t;