Rust error E0506: cannot assign to because it is borrowed

Q: How do I fix E0506 (cannot assign to because it is borrowed)?

End the borrow before assigning:

What it meansYou assigned a new value while a reference to the old one was still in use.

What rustc prints

error[E0506]: cannot assign to `fancy_num` because it is borrowed
  --> main.rs:9:1
   |
 8 | let fancy_ref = &fancy_num;
   |                 ---------- `fancy_num` is borrowed here
 9 | fancy_num = FancyNum { num: 6 };
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `fancy_num` is assigned to here but it was already borrowed
...
12 | println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num);
   |                                             ------------- borrow later used here

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

Why it happens

Updating a variable inside a loop while holding a reference to it from earlier.
Assigning to a struct field while a borrow of it lives on.

How to fix it

1. End the borrow before assigning

let len = s.len();  // copy what you need
s = String::from("new");
println!("{len}");

Other wordings of this error

cannot assign to because it is borrowed, assignment to borrowed occurs here

E0506Ownership and borrowing

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

An attempt was made to assign to a borrowed value.

Erroneous code example:

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:

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:

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:

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);

From rustc --explain E0506 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

Up next

Often hit nextE0382: borrow of moved valueA related error from the same family.Read it next →

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