How to properly update struct values in a Vec field

help
1 
use std::time::Instant;
use std::collections::HashMap;
use core::cell::RefCell;

struct Focus(/*start*/ Instant, /*end*/ Instant);

#[derive(Default)]
struct Session {
    pid: u32,
    had_focus: Vec<Focus>,
    run_time: u64,
}

#[derive(Default)]
struct Game {
    name: String,
    focus_time_total: u64,
    focus_time_7_days: u64,
    run_time_total: u64,
    run_time_7_days: u64,
    sessions: RefCell<Vec<Session>>,
}

fn main() {
    let mut games: HashMap<String, Game> = HashMap::new();
    
    // ...
    
    let name = "some game";
    let pid = 123;
    
    if !games.contains_key(name) {
        let g = Game {
            name: name.to_string(),
            focus_time_total: 0,
            focus_time_7_days: 0,
            run_time_total: 0,
            run_time_7_days: 0,
            sessions: Default::default(),
        };

        games.insert(name.to_string(), g);
    }
    
    // if there's no session for this pid, create one, then update the session info
    let mut sessions = games.get(name).unwrap().sessions.borrow_mut();
    let last_session = sessions.last();

    if last_session.is_none() || last_session.unwrap().pid != pid {
        let session = Session {
            pid: pid,
            had_focus: vec![Focus(Instant::now(), Instant::now())],
            run_time: 0,
        };

        sessions.push(session);
    }
    
    let mut current_session = &mut sessions.last().unwrap();
    current_session.run_time = 123; // breaks
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0594]: cannot assign to `current_session.run_time`, which is behind a `&` reference
  --> src/main.rs:60:5
   |
60 |     current_session.run_time = 123;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot assign

For more information about this error, try `rustc --explain E0594`.
error: could not compile `playground` due to previous error

I'm new to Rust and have been struggling to get struct vector fields to do what I want them to do. I have a struct, Game with a vector of Sessions. I want to push a Session if one doesn't exist and update one if it does exist.

The use of RefCell on Game::sessions also feels wrong to me for some reason. I feel like I'm trying to force Rust to operate under the paradigms I'm used to instead of doing things the "Rust way".

Any help?

Many thanks in advance.

2

The error is because of a common mistake that it will be useful to learn to spot: in order to mutate something (when not using interior mutability), the entire “path” of operations from the original owned value must not involve any & references, only &mut references. The common way this shows up obviously is borrowing a field with & somewhere, but in your case, you called sessions.last() (which returns an & reference) when you needed sessions.last_mut().

With this change, your program compiles:

let current_session = sessions.last_mut().unwrap();

Note that no muts are needed to make this successfully mutable, except for last_mut() itself. The &mut you had before doesn't change anything about the mutability; you can only ever remove mutability by omitting a mut, not add it by adding &mut.

Similarly, in order to fix the situation where you used RefCell, you need to write get_mut() and last_mut() instead of get() and last():

struct Game {
    ...
    sessions: Vec<Session>,
}

...

let sessions = &mut games.get_mut(name).unwrap().sessions;
let last_session = sessions.last_mut();

As to “get or insert if absent” patterns, for a HashMap you can use Entry to do that; for a Vec the easiest thing will probably be to just check is_empty().

3 Likes
3

@kpreid My code just got so much cleaner--more importantly, it works. Thank you!