error: cannot borrow immutable field `z.x` as mutable
  --> $DIR/issue-39544.rs:21:18
   |
20 |     let z = Z { x: X::Y };
   |         - consider changing this to `mut z`
21 |     let _ = &mut z.x;
   |                  ^^^ cannot mutably borrow immutable field

error: cannot borrow immutable field `z.x` as mutable
  --> $DIR/issue-39544.rs:25:18
   |
24 | pub fn with_arg(z: Z, w: &Z) {
   |                 - consider changing this to `mut z`
25 |     let _ = &mut z.x;
   |                  ^^^ cannot mutably borrow immutable field

error: cannot borrow immutable field `w.x` as mutable
  --> $DIR/issue-39544.rs:26:18
   |
24 | pub fn with_arg(z: Z, w: &Z) {
   |                          -- use `&mut Z` here to make mutable
25 |     let _ = &mut z.x;
26 |     let _ = &mut w.x;
   |                  ^^^ cannot mutably borrow immutable field

error: aborting due to 3 previous errors