A borrowed variable was used by a closure.
Erroneous code example:
```compile_fail,E0500
fn you_know_nothing(jon_snow: &mut i32) {
let nights_watch = &jon_snow;
let starks = || {
*jon_snow = 3; // error: closure requires unique access to `jon_snow`
// but it is already borrowed
};
println!("{}", nights_watch);
}
```
In here, `jon_snow` is already borrowed by the `nights_watch` reference, so it
cannot be borrowed by the `starks` closure at the same time. To fix this issue,
you can create the closure after the borrow has ended:
```
fn you_know_nothing(jon_snow: &mut i32) {
let nights_watch = &jon_snow;
println!("{}", nights_watch);
let starks = || {
*jon_snow = 3;
};
}
```
Or, if the type implements the `Clone` trait, you can clone it between
closures:
```
fn you_know_nothing(jon_snow: &mut i32) {
let mut jon_copy = jon_snow.clone();
let starks = || {
*jon_snow = 3;
};
println!("{}", jon_copy);
}
```