vlib/v/checker/tests/assign_immutable_reference_var_err.vv:8:11: error: `x` is immutable, cannot have a mutable reference to an immutable object
    6 |
    7 | fn y(x &Foo) {
    8 |     mut m := x
      |              ^
    9 |     m.value = 42
   10 | }
