rust - How to idiomatically copy a slice?

Question

In Go, copying slices is standard-fare and looks like this:

# It will figure out the details to match slice sizes
dst = copy(dst[n:], src[:m])

In Rust, I couldn't find a similar method as replacement. Something I came up with looks like this:

fn copy_slice(dst: &mut [u8], src: &[u8]) -> usize {
    let mut c = 0;
    for (&mut d, &s) in dst.iter_mut().zip(src.iter()) {
        d = s;
        c += 1;
    }
    c
}

Unfortunately, I get this compile-error that I am unable to solve:

error[E0384]: re-assignment of immutable variable `d`
 --> src/main.rs:4:9
  |
3 |     for (&mut d, &s) in dst.iter_mut().zip(src.iter()) {
  |               - first assignment to `d`
4 |         d = s;
  |         ^^^^^ re-assignment of immutable variable

How can I set d? Is there a better way to copy a slice?

Answer 1

Yes, use the method clone_from_slice(), it is generic over any element type that implements Clone.

fn main() {
    let mut x = vec![0; 8];
    let y = [1, 2, 3];
    x[..3].clone_from_slice(&y);
    println!("{:?}", x);
    // Output:
    // [1, 2, 3, 0, 0, 0, 0, 0]
}

The destination x is either a &mut [T] slice, or anything that derefs to that, like a mutable Vec<T> vector. You need to slice the destination and source so that their lengths match.

---

As of Rust 1.9, you can also use copy_from_slice(). This works the same way but uses the Copy trait instead of Clone, and is a direct wrapper of memcpy. The compiler can optimize clone_from_slice to be equivalent to copy_from_slice when applicable, but it can still be useful.