練習問題の解答例? https://gist.github.com/KOBA789/605939bf3458c7a2a1597532625948f6

main.rs

use std::collections::HashMap;

fn greet_map(id: usize, name: String) -> HashMap<usize, String> {
    let mut map = HashMap::new();
    let message = format!("Hello, {}!", name);
    map.insert(id, message);
    map
}

fn main() {
    let map = greet_map(0, String::from("world"));
    for (&i, s) in map.iter() {
        println!("{} {}", i, s);
    }
}