Error code E0503

A value was used after it was mutably borrowed.

Erroneous code example:

fn main() {
    let mut value = 3;
    // Create a mutable borrow of `value`.
    let borrow = &mut value;
    let _sum = value + 1; // error: cannot use `value` because
                          //        it was mutably borrowed
    println!("{}", borrow);
}

In this example, value is mutably borrowed by borrow and cannot be used to calculate sum. This is not possible because this would violate Rust's mutability rules.

You can fix this error by finishing using the borrow before the next use of the value:

fn main() {
    let mut value = 3;
    let borrow = &mut value;
    println!("{}", borrow);
    // The block has ended and with it the borrow.
    // You can now use `value` again.
    let _sum = value + 1;
}

Or by cloning value before borrowing it:

fn main() {
    let mut value = 3;
    // We clone `value`, creating a copy.
    let value_cloned = value.clone();
    // The mutable borrow is a reference to `value` and
    // not to `value_cloned`...
    let borrow = &mut value;
    // ... which means we can still use `value_cloned`,
    let _sum = value_cloned + 1;
    // even though the borrow only ends here.
    println!("{}", borrow);
}

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