vlib/v/checker/tests/immutable_field.vv:8:4: error: field `i1` of struct `Aaa` is immutable
    6 | fn main() {
    7 |     a := Aaa{1}
    8 |     a.i1 = 2
      |       ~~
    9 | }
vlib/v/checker/tests/immutable_field.vv:8:2: error: `a` is immutable, declare it with `mut` to make it mutable
    6 | fn main() {
    7 |     a := Aaa{1}
    8 |     a.i1 = 2
      |     ^
    9 | }