case BSON_TYPE_UTF8: {
   uint32_t utf8_len;
   const char *utf8;

   utf8 = bson_iter_utf8 (iter, &utf8_len);

   if (!bson_utf8_validate (utf8, utf8_len, true)) {
      iter->err_off = iter->off;
      return true;
   }

   if (VISIT_UTF8 (iter, key, utf8_len, utf8, data)) {
      return true;
   }
} break;

#include <bson/bson.h>

int
main (int argc, char *argv[])
{
   static const char invalid[] = {0xed, 0xa0, 0xa5, 0};

   printf ("validate utf8: %d\n",
           bson_utf8_validate (invalid, strlen (invalid), false));
   // output: validate utf8: 0

   bson_t b;
   bson_init (&b);
   printf ("append utf8: %d\n", bson_append_utf8 (&b, "key", -1, invalid, -1));
   // output: append utf8: 1

   bson_iter_t child;
   if (!bson_iter_init (&child, &b)) {
      printf ("bson_iter_init failed\n");
      return 1;
   }

   static bson_visitor_t bson_validate_funcs = {};
   bson_iter_visit_all (&child, &bson_validate_funcs, NULL);
   printf ("bson iter err: %d\n", child.err_off > 0);
   // output: bson iter err: 1

   printf ("bson validate: %d\n", bson_validate (&b, BSON_VALIDATE_UTF8, NULL));
   // output: bson validate: 1

   return 0;
}