Understanding Rust Error E0506

Introduction to Rust Error E0506

Rust Error E0506 occurs when there is an attempt to assign a new value to a variable that has been borrowed. In Rust, when a value is borrowed and a reference is active, the original value is immutable. This is to ensure memory safety and prevent any data races.

Official Erroneous Code Example

Consider the following erroneous code example borrowed from Rust official documentation:

#![allow(unused)]
fn main() {
struct FancyNum {
    num: u8,
}

let mut fancy_num = FancyNum { num: 5 };
let fancy_ref = &fancy_num;
fancy_num = FancyNum { num: 6 };
// error: cannot assign to `fancy_num` because it is borrowed

println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
}

Explanation of the Error

In this code example, a struct called FancyNum is defined, and a mutable instance of it, fancy_num, is created with the value 5. A reference fancy_ref is created from fancy_num. After this, there is an attempt to change the value of fancy_num to 6 while fancy_ref is alive. This causes Rust Error E0506, as fancy_num is borrowed and cannot be assigned a new value.

Solutions to Rust Error E0506

Here are three potential solutions to fix Rust Error E0506:

1. Moving the value instead of borrowing

One possible solution is to move the value out of fancy_num into a second variable, eliminating the need for borrowing. In this case, the error will not occur, and the fancy_num variable can be assigned a new value. Here is an example:

#![allow(unused)]
fn main() {
struct FancyNum {
    num: u8,
}

let mut fancy_num = FancyNum { num: 5 };
let moved_num = fancy_num;
fancy_num = FancyNum { num: 6 };

println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num);
}

2. Limiting the lifetime of the borrow using a scoped block

Another solution is to limit the lifetime of the borrow with a scoped block. By doing so, the reference will not be in scope when fancy_num is assigned a new value, thus avoiding the error. Here is an example:

#![allow(unused)]
fn main() {
struct FancyNum {
    num: u8,
}

let mut fancy_num = FancyNum { num: 5 };

{
    let fancy_ref = &fancy_num;
    println!("Ref: {}", fancy_ref.num);
}

// Works because `fancy_ref` is no longer in scope
fancy_num = FancyNum { num: 6 };
println!("Num: {}", fancy_num.num);
}

3. Moving the reference into a function

Another way to handle the borrow is to move the reference into a separate function. This will ensure that the borrow ends after the function call and allows the original variable to be assigned a new value without causing an error. Here's an example:

#![allow(unused)]
fn main() {
struct FancyNum {
    num: u8,
}

fn print_fancy_ref(fancy_ref: &FancyNum){
    println!("Ref: {}", fancy_ref.num);
}

let mut fancy_num = FancyNum { num: 5 };

print_fancy_ref(&fancy_num);

// Works because function borrow has ended
fancy_num = FancyNum { num: 6 };
println!("Num: {}", fancy_num.num);
}