Encapsulated Unsafe Rust

pub fn copy(dest: &mut [u8], source: &[u8]) {
    let len = dest.len().min(source.len());
    let mut i = 0;
    while i < len {
        // SAFETY: `i` must be in-bounds as it was produced by source.len()
        let new = unsafe { source.get_unchecked(i) };

        // SAFETY: `i` must be in-bounds as it was produced by dest.len()
        let old = unsafe { dest.get_unchecked_mut(i) };

        *old = *new;
        i += 1;
    }

    for (dest, src) in dest.iter_mut().zip(source) {
        *dest = *src;
    }
}

fn main() {
    let a = &[114, 117, 115, 116];
    let b = &mut [82, 85, 83, 84];

    println!("{}", String::from_utf8_lossy(b));
    copy(b, a);
    println!("{}", String::from_utf8_lossy(b));
}