Storing session information in a Hashmap


I am trying to store some session information in a HashMap. I wrote a much simplifed example of the problem that I am having. So far this is the best that I was able to do.

use std::collections::HashMap;

struct Session {
    last_number: f32, 
    //Other stuf

fn main() {
    let mut sessions : HashMap<i32, Session> = HashMap::new();
    for n in 0..10 {
        let userid = 32; //This is obtained from the user
        let number = 1.2; //This is obtained from the user
        if !sessions.contains_key(&userid) {
            let session = Session{last_number: 0.};
            sessions.insert(userid, session);
        let mut session = sessions.get_mut(&userid).unwrap();
        session.last_number = number;

It does not look very nice. When the key is not found, I am querying, inserting and then querying again the map. I would like to reuse the session object built for insertion and avoid calling contains_key. I think it should be enough with get_mut as it returns an Option<V>. ButI failed when trying make a change in this direction. The compiler/borrow checker was complaining and I was not able to solve it.

My questions are:

  1. Is it really a bad design or it does not matter for rust in terms of efficiency?
  2. How can I implement the other solution?


You want the “entry API”: