CODEWITHSUNDEEP
rust
Fomalhaut
Fomalhaut

Reputation: 9815

Cannot borrow element as mutable more than once at a time for an array elements in Rust

I have such code that can't be compiled:

fn my_cool_replace<T: Copy>(arr: &mut [T]) {
    swap(&mut arr[2], &mut arr[5]);
}

fn swap<T: Copy>(a: &mut T, b: &mut T) {
    let c: T = *a;
    *a = *b;
    *b = c;
}

The error is:

error[E0499]: cannot borrow `arr[_]` as mutable more than once at a time

I want the function my_cool_replace to replace 2-th and 5-th elements in the given mutable slice of an array or a vector. Why do I have such error and what is the best way to fix it?

Upvotes: 1

Views: 283

Answers (2)

Kaplan
Kaplan

Reputation: 3758

As already correctly stated, you can't do this in one swoop ─ but:

fn my_cool_replace<T: Copy>(arr: &mut [T]) {
    let mut tmp = arr[5];
    tmp = std::mem::replace(&mut arr[2], tmp);
    _ = std::mem::replace(&mut arr[5], tmp);
}

Upvotes: 0

Pandemonium
Pandemonium

Reputation: 8390

From the Rust Book:

Here are the rules for borrowing in Rust:

First, any borrow must last for a scope no greater than that of the owner. Second, you may have one or the other of these two kinds of borrows, but not both at the same time:

  • one or more references (&T) to a resource,
  • exactly one mutable reference (&mut T).

Your swap function smells weird by accepting references as arguments. Instead, it could have been rewritten to accept a mutable reference to an array and the indices the user like to swap:

fn swap<T: Copy>(arr: &mut [T], p1: usize, p2: usize) {
    let tmp = arr[p1];
    arr[p1] = arr[p2];
    arr[p2] = tmp;
}

Upvotes: 3

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