let array = [40];
let mut var = 60;

for element in array.iter().filter(|&x| {*x < var}) {
    var += 1; // Error
}

var += 1; // Fine again

error[E0506]: cannot assign to `var` because it is borrowed
 --> src/main.rs:6:9
  |
5 |     for element in array.iter().filter(|&x| {*x < var}) {
  |                                        ---- borrow of `var` occurs here
6 |         var += 1; // Error
  |         ^^^^^^^^ assignment to borrowed `var` occurs here