Rust error E0502: cannot borrow as mutable because it is also borrowed as immutable

Q: How do I fix E0502 (cannot borrow as mutable because it is also borrowed as immutable)?

Copy out what you need first: Clone or copy the value so the borrow ends before the mutation. Collect changes, then apply: Read in one pass, write in a second. Use the entry API for maps: One lookup that can insert, with no outstanding borrow. Mutate by index: Indices are plain numbers, so they do not hold a borrow.

What it meansYou are changing something while a shared reference to it is still in use. A shared & borrow promises the data will not change until that borrow ends.

What rustc prints

error[E0502]: cannot borrow `*a` as mutable because it is also borrowed as immutable
 --> main.rs:4:5
  |
3 |     let y = &a; // a is borrowed as immutable.
  |             -- immutable borrow occurs here
4 |     bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
  |     ^^^^^^ mutable borrow occurs here
5 |             //        as immutable
6 |     println!("{}", y);
  |                    - immutable 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

let first = &v[0]; v.push(1); println!("{first}"): push may reallocate, which would leave first dangling.
Iterating with for x in &v (or .iter()) and modifying v inside the loop.
Calling map.get(..), keeping the result, then calling map.insert(..).

How to fix it

1. Copy out what you need first

Clone or copy the value so the borrow ends before the mutation.

let first = v[0];   // i32 is Copy: no borrow kept
v.push(1);
println!("{first}");

2. Collect changes, then apply

Read in one pass, write in a second.

let extra: Vec<_> = v.iter().filter(|x| **x > 2).map(|x| x * 10).collect();
v.extend(extra);

3. Use the entry API for maps

One lookup that can insert, with no outstanding borrow.

*counts.entry(word).or_insert(0) += 1;

4. Mutate by index

Indices are plain numbers, so they do not hold a borrow.

for i in 0..v.len() {
    if v[i] > 2 { v[i] *= 10; }
}

Other wordings of this error

cannot borrow as mutable because it is also borrowed as immutable, immutable borrow occurs here, mutable borrow occurs here, immutable borrow later used here

E0502Ownership and borrowing

Activein rustc 1.99.0

8related errors linked below

The compiler team's explanation

A variable already borrowed with a certain mutability (either mutable or immutable) was borrowed again with a different mutability.

Erroneous code example:

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);
}

To fix this error, ensure that you don't have any other references to the variable before trying to access it with a different mutability:

fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    bar(a);
    let y = &a; // ok!
    println!("{}", y);
}

For more information on Rust's ownership system, take a look at the References & Borrowing section of the Book.

From rustc --explain E0502 (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 →

Also worth your time

Did this answer your question?