Rust iterator mystery

main.rs

use std::fs;
use std::path::PathBuf;

fn show_sizes_vec(file_list: &Vec<PathBuf>) -> u64 {
    let sizes = file_list
        .iter()
        .map(|e| fs::metadata(e).map(|m| m.len()).unwrap())
        .collect::<Vec<u64>>();

    let totsize : u64 = sizes.iter().sum();
    println!("Total size = {}", totsize);

    for each in file_list.iter().zip(sizes.iter())
    {
        println!(" {}   {}", each.1, each.0.to_str().unwrap());
    }

    totsize
}

fn show_sizes_iter(file_list: &Vec<PathBuf>) -> u64 {
    let mut sizes = file_list
        .iter()
        .map(|e| fs::metadata(e).map(|m| m.len()).unwrap());

    let totsize : u64 = sizes.by_ref().sum();
    println!("Total size = {}", totsize);

    for each in file_list.iter().zip(sizes.by_ref())
    {
        println!(" {}   {}", each.1, each.0.to_str().unwrap());
    }

    totsize
}


fn main() {
    println!("OK, let's get to work!");

    let file_list = fs::read_dir(".").unwrap().map(|e| e.unwrap().path()).collect::<Vec<PathBuf>>();
    println!("Files: {:?}", file_list);
    
    let count = file_list.len();

    println!("--------------------");
    println!("Sizes (by iteration):");
    let totsize = show_sizes_iter(&file_list);
    println!("{} files, {} bytes", count, totsize);
    
    println!("--------------------");
    println!("Sizes (by vec):");
    let totsize = show_sizes_vec(&file_list);
    println!("{} files, {} bytes", count, totsize);
}