Understanding Rust Error E0301

Introduction to Rust Error E0301

Rust error E0301 was used to indicate that mutable borrows were not allowed in pattern guards. However, this error code is no longer emitted by the compiler; the explanation provided here serves as historical context or for those using older Rust versions.

Why Mutable Borrows were not allowed in Pattern Guards

Pattern guards were designed to allow additional conditions during pattern matching, however, Rust did not allow mutable borrows in pattern guards to preserve safety and exhaustiveness of the match expression. Mutable borrows could introduce side effects that could alter the matched object or its environment in such a way that the match would no longer be exhaustive.

Example of Potential Problems with Mutable Borrows in Pattern Guards

Consider the following example:

fn main() {
match Some(()) {
    None => { },
    option if option.take().is_none() => {
        /* impossible, option is `Some` */
    },
    Some(_) => { } // When the previous match failed, the option became `None`.
}
}

In this example, allowing mutable borrows could lead to a situation where no arm of the match expression is executed. This is because the .take() method would consume the `Some` value and replace it with `None`. Since this example is not allowed to compile, Rust prevents this unintended behavior by disallowing mutable borrows in pattern guards.

How to Correctly Use Pattern Guards

To avoid Rust error E0301, avoid mutable borrows in pattern guards and instead perform mutable operations within the match arm itself. Consider the following corrected example:

fn main() {
match Some(()) {
    None => { },
    Some(option) => {
        if option.is_none() {
            /* process when option is `None` */
        } else {
            /* process when option is `Some` */
        }
    }
}
}

This version of the code refrains from using mutable borrows within the pattern guard, and instead handles the different cases within the match arm itself.