Fields exposed as immutable references can be mutated when capturing them using a closure

[yorickpeterse]

When a closure captures a field, it exposes the field as a mut T even if the original type was ref T. As a result you can mutate immutable references to fields using a closure:

class Thing {
  let @values: Array[Int]

  fn example {
    fn { @values.pop } # This should error at compile-time, but it doesn't
  }
}