You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: fib_iter.rs
    path of file: ./repos/fib/optimized
    
    the code of the file until where you have to start completion: fn main() {
    println!("{}", fib(47)); // Runs in ~800 us on my machi
    
fn fib(length: u64) -> u64 {
    let (mut current, mut last) = (0, 1);
    for _ in 0..length {
        std::mem::swap(&mut current, &mut last);
        current += last;
    }
    current
}