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Understanding Rust Error E0007

Overview of Error E0007

Rust error E0007 occurs when a match arm attempts to move a value into more than one location, violating unique ownership rules. Although this error code is no longer emitted by the compiler, understanding the cause and solution can help you avoid similar issues in the future.

Understanding Unique Ownership

Rust enforces unique ownership throughout the code, ensuring a value can only be owned by one variable at a time. When using match expressions, this can lead to the E0007 error if the code tries to move a value into multiple locations.

Example of Error E0007

This erroneous code demonstrates where the error E0007 happens:

#![allow(unused)]
#![feature(bindings_after_at)]

fn main() {
    let x = Some("s".to_string());

    match x {
        op_string @ Some(s) => {}, // error: use of moved value
        None => {},
    }
}

In this example, the entire `Option<String>` value is moved into the `op_string` variable, whereas the inner `String` value is moved into variable `s`. This violates unique ownership.

Solution for Error E0007

To resolve this issue, you can use a match guard with a reference or an if-let expression. Here's an example of using a match guard:

fn main() {
    let x = Some("s".to_string());

    match &x {
        Some(s) if { let op_string = &x; true } => {},
        None => {},
    }
}

This code introduces a match guard that first creates a reference to `x` with `&x`, ensuring we don't move the entire value. This way, we maintain unique ownership within the match arm.

Another solution is using an if-let expression:

fn main() {
    let x = Some("s".to_string());

    if let Some(s) = &x {
        let op_string = &x;
        // Perform operations
    } else {
        // Handle None case
    }
}

This approach avoids the issue by using an if-let expression and handling the Option value with a reference instead of moving the value.

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