Error code E0500

A borrowed variable was used by a closure.

Erroneous code example:

#![allow(unused)] fn main() { 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:

#![allow(unused)] fn main() { 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:

#![allow(unused)] fn main() { fn you_know_nothing(jon_snow: &mut i32) { let mut jon_copy = jon_snow.clone(); let starks = || { *jon_snow = 3; }; println!("{}", jon_copy); } }