Copy from slice. Suppose we want to copy data into an array from a slice. The slice could be from another array or a vector. Copy_from_slice can help here.

With this function, we use range syntax to copy data. The implementation is accelerated and will often perform faster than a for-loop.

First example. Here we demonstrate the usage of copy_from_slice in a simple Rust program. We copy a part of the data vector into an array.

fn main() {
    // Part 1: The data we wish to copy.
    let data = vec![10, 20, 30, 40];

    // Part 2: The memory we want to copy the data to.
    let mut empty = [0, 0, 0, 0];

    // Part 3: Use copy from slice with slices.
    empty[0..2].copy_from_slice(&data[0..2]);
    println!("{:?}", empty);
}
[10, 20, 0, 0]

HashMap keys. Sometimes in Rust we want to quickly look up keys in a HashMap. We can use copy_from_slice() to perform part of this task.

use std::collections::HashMap;

fn main() {
    // Step 1: Create HashMap with array keys.
    let mut h = HashMap::new();
    h.insert([5, 15, 25], "OK");

    // Step 2: Copy data into key with copy from slice.
    let data = [5, 15, 25, 35, 45];
    let mut key = [0, 0, 0];
    key.copy_from_slice(&data[0..3]);

    // Step 3: Access HashMap value with key.
    if let Some(value) = h.get(&key) {
        println!("{value}");
    }
}
OK

Final notes. Much like extend_from_slice(), copy_from_slice has optimizations that help it perform faster than a loop. Thus it should be preferred when copying from an array or vector.