Understanding Rust Error E0005: Irrefutable Patterns

What is Rust Error E0005?

Rust Error E0005 occurs when a pattern used to bind names is refutable, which means it does not guarantee a name will be extracted in all cases. In Rust, patterns must be irrefutable to ensure that the program does not encounter any undefined behavior.

Example of Erroneous Code

#![allow(unused)]
fn main() {
    let x = Some(1);
    let Some(y) = x;
    // error: refutable pattern in local binding: `None` not covered
}

In the example above, the code tries to directly destructure the Option enum with the pattern 'Some(y)', which is refutable because the pattern does not cover the 'None' variant of the 'Option' enum. This leads to Rust error E0005.

How to Fix E0005

To fix Rust Error E0005, you can either use a match expression or the if let structure to handle the possibility of failure correctly. Below are examples of both approaches.

Using match Expression

#![allow(unused)]
fn main() {
    let x = Some(1);

    match x {
        Some(y) => {
            // do something
        },
        None => {}
    }
}

The match expression above handles both the 'Some' and 'None' variants of the 'Option' enum, ensuring that the pattern is irrefutable and fixing Error E0005.

Using if let Structure

#![allow(unused)]
fn main() {
    let x = Some(1);

    if let Some(y) = x {
        // do something
    }
}