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Understanding Rust Error E0502: Immutable and Mutable Borrow Conflict

Rust Error E0502: Immutable and Mutable Borrow Conflict

This error occurs when a mutable borrow of a variable is attempted while there's an existing immutable borrow on the same variable. Rust enforces this rule to preserve safety and prevent data races.

Erroneous code example:

#![allow(unused)]
fn main() {
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    let y = &a; // a is borrowed as immutable.
    bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
            //        as immutable
    println!("{}", y);
}
}

Solution

To fix Rust Error E0502, either adjust the borrowing order or remove one of the borrows. Ensure that there are no other references to the variable before borrowing it mutably:

Corrected code example:

#![allow(unused)]
fn main() {
fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    bar(a);
    let y = &a; // ok!
    println!("{}", y);
}
}

Common Scenarios

It's essential to understand common scenarios where Rust Error E0502 can occur. These situations include loops, pattern matching, and nested structures handling.

Scenario 1: Error caused by an index operation inside a loop:

fn update_value(v: &mut Vec<&mut i32>, index: usize, value: i32) {
    let r = &v[index];
    *r = value;
}

Use an iterator to handle the vector elements to resolve the borrowing conflict:

fn update_value(v: &mut Vec<&mut i32>, index: usize, value: i32) {
    let r = v.get_mut(index).unwrap();
    *r = value;
}

Scenario 2: Error caused by incorrect pattern matching:

fn main() {
    let mut x = Some(42);
    match x {
        Some(ref mut v) => {
            let y = x;
            *v = 13;
        },
        None => (),
    }
}

The solution is to change the match arm's pattern:

fn main() {
    let mut x = Some(42);
    match x {
        Some(ref mut v) => {
            let y = &x;
            *v = 13;
        },
        None => (),
    }
}

Remember to check both the borrowing order and removing unnecessary borrows to fix Rust Error E0502. It's essential to comprehend the borrowing rules to navigate the Rust programming language successfully.

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