Understanding and Fixing Rust Error E0595

Introduction to Rust Error E0595

Rust Error E0595 occurs when a closure attempts to mutate an immutable captured variable. This error message is designed to prevent closure-related issues, as Rust enforces borrowing and ownership rules to ensure safe and predictable program behavior.

E0595 Erroneous Code Example

#![allow(unused)]
fn main() {
let x = 3; // error: closure cannot assign to immutable local variable `x`
let mut c = || { x += 1 };
}

In the erroneous code example above, the closure attempts to increment the value of the immutable variable `x`. Rust compiler produces error E0595 because the captured variable `x` is immutable and cannot be mutated.

Fixing Rust Error E0595

To fix error E0595, the captured variable should be declared as mutable to allow the closure to change its value. Here's a fixed version of the previous erroneous code example:

#![allow(unused)]
fn main() {
let mut x = 3; // ok!
let mut c = || { x += 1 };
}

After declaring the variable `x` as mutable by using the keyword `mut`, the closure can now successfully increment the value of `x`. The Rust compiler will no longer generate error E0595.