E0506 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0506

An attempt was made to assign to a borrowed value.

Erroneous code example:

    #![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);
    }

Because `fancy_ref` still holds a reference to `fancy_num`, `fancy_num` can't be assigned to a new value as it would invalidate the reference.

Alternatively, we can move out of `fancy_num` into a second `fancy_num`:

    #![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);
    }

If the value has to be borrowed, try limiting the lifetime of the borrow using a scoped block:

    #![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);
    }

Or by moving the reference into a function:

    #![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);
    }