Understanding and Solving Rust Error E0510

Introduction to Rust Error E0510

Rust Error E0510 occurs when the matched value is assigned within a match guard. Mutating the matched value within a match guard is disallowed, and doing so may result in the match becoming non-exhaustive.

Erroneous Code Example

#![allow(unused)]
fn main() {
    let mut x = Some(0);
    match x {
        None => {},
        Some(_) if { x = None; false } => {}, // error!
        Some(_) => {}
    }
}

In this code example, Error E0510 occurs because the match value 'x' is being mutated within the match guard, which is prohibited.

Solution to Rust Error E0510

To fix Rust Error E0510, move the assignment or mutation of the matched variable outside the match guard and into the match arm body.

Corrected Code Example

#![allow(unused)]
fn main() {
    let mut x = Some(0);
    match x {
        None => {},
        Some(_) => {
            x = None; // ok!
        }
    }
}

In this corrected code example, the assignment of 'x' to None has been moved out of the match guard and into the match arm body, which is allowed.

Summary