CODEWITHSUNDEEP
rustborrow-checker
Paperino
Paperino

Reputation: 1019

How to invoke multiple methods on a mutable borrow in Rust

Basically I am having hard time understanding why calling multiple methods on a mutable reference can cause the borrow checker to err. Example snippet:

use std::collections::HashMap;

struct MainState {
    // The HashMap owns the values which is Ok.
    queues: HashMap<String, Vec<String>>,
}

impl MainState {
    fn new() -> Self {
        Self {
            queues: HashMap::new(),
        }
    }

    fn get_queue(&mut self, session_id: &str) -> &mut Vec<String> {
        // borrowing the first time explicitly
        let queues = &mut self.queues;
        let queue = queues.get_mut(session_id);
        if queue.is_none() {
            let queue = Vec::new();
            // Multiple mutable error on the following lines. Why?
            queues.insert(session_id.to_string(), queue);
            queues.get_mut(session_id).unwrap()
        } else {
            queue.unwrap()
        }
    }
}

I can't figure out how to fix this code in an idiomatic acceptable way.

Upvotes: 0

Views: 33

Answers (0)

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