Or just do it in C.

  #define span(T) struct span_##T { size_t len; T *data; }
  #define span_access(T, x, i) (*({              \
    span(T) *_v = (x);                           \
    auto _i = (i);                               \
    if (((size_t)_i) >= _v->len) abort();        \
    &_v->data[_i];                               \
  }))