vlib/v/checker/tests/closure_immutable.vv:4:3: error: the closure copy of `a` is immutable, declare it with `mut` to make it mutable
    2 |     a := 1
    3 |     f1 := fn [a] () {
    4 |         a++
      |         ^
    5 |         println(a)
    6 |     }
vlib/v/checker/tests/closure_immutable.vv:7:16: error: original `a` is immutable, declare it with `mut` to make it mutable
    5 |         println(a)
    6 |     }
    7 |     f2 := fn [mut a] () {
      |                   ^
    8 |         a++
    9 |         println(a)
vlib/v/checker/tests/closure_immutable.vv:13:3: error: the closure copy of `b` is immutable, declare it with `mut` to make it mutable
   11 |     mut b := 2
   12 |     f3 := fn [b] () {
   13 |         b++
      |         ^
   14 |         println(b)
   15 |     }
