so48244892-1.rs

use std::collections::HashMap;

struct ListOfUsers(Vec<(String, String)>);  // (User, Email)
struct UserToEmail<'a>(HashMap<&'a str, &'a str>);
struct EmailToUser<'a>(HashMap<&'a str, &'a str>);

impl<'a> From<&'a ListOfUsers> for UserToEmail<'a> {
    fn from(value: &'a ListOfUsers) -> Self {
        Self(value.0.iter()
            .map(|(u, e)| (u.as_ref(), e.as_ref()))
            .collect::<HashMap<&'_ str, &'_ str>>()
        )
    }
}

impl<'a> From<&'a ListOfUsers> for EmailToUser<'a> {
    fn from(value: &'a ListOfUsers) -> Self {
        Self(value.0.iter()
            .map(|(u, e)| (e.as_ref(), u.as_ref()))
            .collect::<HashMap<&'_ str, &'_ str>>()
        )
    }
}

fn main() {
    let users = ListOfUsers(vec![
        ("John".into(), "john@example.com".into()),
        ("Mary".into(), "mary@example.com".into()),
        ("Takao".into(), "takao@example.com".into()),
        ("Darcy".into(), "darcy@example.com".into()),
    ]);
    let user2email = UserToEmail::from(&users);
    println!("{}", *user2email.0.get("Darcy").unwrap());  // darcy@example.com
    let email2user = EmailToUser::from(&users);
    println!("{}", *email2user.0.get("mary@example.com").unwrap());  // Mary
}