Rust Error E0500: Borrowed Variable Used in Closure

Understanding Rust Error E0500

Rust Error E0500 occurs when a borrowed variable is used by a closure. The borrow checker ensures that a variable cannot be shared between multiple closure scopes.

Example of Erroneous Code

#![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 this example, 'jon_snow' is borrowed by the 'nights_watch' reference. Since it is already borrowed, it cannot be borrowed by the 'starks' closure at the same time.

Fixing the Issue

1. 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;
    };
  }
}

By moving the closure after the borrow, you ensure that the variable is no longer being referenced elsewhere.

2. Clone the variable before using it in the closure.

#![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);
  }
}

If the type implements the Clone trait, you can clone it before passing it to the closure. This will not cause any conflicts, as both closures now work on separate instances of the variable.