_RuneStatePriv
__CTASSERT(alignof(struct _RuneStatePriv) >= alignof(void *));
__CTASSERT(sizeof(_RuneStatePriv) % alignof(void *) == 0);
__CTASSERT(alignof(struct _RuneStatePriv) >= alignof(int));
__CTASSERT(sizeof(_RuneStatePriv) % alignof(int) == 0);
struct _RuneStatePriv __priv;
#define _PRIVSIZE (sizeof(mbstate_t)-offsetof(_RuneStatePriv, __private))