error[E0594]: cannot assign to `x`, as it is a captured variable in a `Fn` closure
  --> $DIR/borrow-immutable-upvar-mutation-impl-trait.rs:11:9
   |
LL |   fn bar() -> impl Fn() -> usize {
   |      ---      ------------------ change this to return `FnMut` instead of `Fn`
LL |       let mut x = 0;
LL | /     move || {
LL | |         x += 1;
   | |         ^^^^^^ cannot assign
LL | |         x
LL | |     }
   | |_____- in this closure

error: aborting due to previous error

For more information about this error, try `rustc --explain E0594`.