Better issue when `.str` used instead of `.data`

[ArtemkaKun]

fn to_upper(s string) -> string {
    unsafe {
        mut b := mem.alloc(s.len + 1)
        for i in 0 .. s.len {
            if s.str[i] >= `a` && s.str[i] <= `z` {
                b[i] = s.str[i] - 32
            } else {
                b[i] = s.str[i]
            }
        }
        b[s.len] = 0
        return tos(b)
    }
}

C compiler output:
/home/yuart/Projects/spawn/out.c: In function ‘main___to_upper’:
/home/yuart/Projects/spawn/out.c:1349:43: error: subscripted value is neither array nor pointer nor vector
 1349 |                         if (!((string__str[i] < 'a')) && (string__str[i] < 'z' || string__str[i] == 'z')) {
      |                                           ^
/home/yuart/Projects/spawn/out.c:1349:70: error: subscripted value is neither array nor pointer nor vector
 1349 |                         if (!((string__str[i] < 'a')) && (string__str[i] < 'z' || string__str[i] == 'z')) {
      |                                                                      ^
/home/yuart/Projects/spawn/out.c:1349:94: error: subscripted value is neither array nor pointer nor vector
 1349 |                         if (!((string__str[i] < 'a')) && (string__str[i] < 'z' || string__str[i] == 'z')) {
      |                                                                                              ^
/home/yuart/Projects/spawn/out.c:1350:51: error: subscripted value is neither array nor pointer nor vector
 1350 |                                 b[i] = string__str[i] - 32;
      |                                                   ^
/home/yuart/Projects/spawn/out.c:1352:51: error: subscripted value is neither array nor pointer nor vector
 1352 |                                 b[i] = string__str[i];
      |                                                   ^

error: C compiler failed