Group By a Key (fold into HashMap)

use std::collections::HashMap;

#[derive(Debug)]
struct User<'a> { name: &'a str, team: &'a str }

fn main() {
    let users = vec![
        User { name: "Alice", team: "A" },
        User { name: "Bob",   team: "B" },
        User { name: "Cara",  team: "A" },
        User { name: "Dave",  team: "B" },
    ];

    let by_team: HashMap<&str, Vec<&User>> = users.iter().fold(HashMap::new(), |mut acc, u| {
        acc.entry(u.team).or_default().push(u);
        acc
    });

    for (team, members) in &by_team {
        let names: Vec<&str> = members.iter().map(|u| u.name).collect();
        println!("{team}: {:?}", names);
    }
}