vlib/v/checker/tests/immutable_field_postfix.vv:7:4: error: field `i` of struct `Aaa` is immutable
    5 | fn main() {
    6 |     mut a := Aaa{}
    7 |     a.i++
      |       ^
    8 |     a.i--
    9 | }
vlib/v/checker/tests/immutable_field_postfix.vv:8:4: error: field `i` of struct `Aaa` is immutable
    6 |     mut a := Aaa{}
    7 |     a.i++
    8 |     a.i--
      |       ^
    9 | }
