Using mutable mutexed map twice



I’m trying to share a HashMap between two threads, one of which shall update the nested array if the key is already present and initialize a new one if there’s no value associated with it.

Ho wever, because i’m trying to obtain a reference to the value and try inserting within same code block, I get in trouble for trying to use mutable reference twice.

I’m trying to find a language-idiomatic way to solve the problem, although so far without big success. Code follows.

Thank you!

fn put_metric<'a>(metrics: Arc<Mutex<HashMap<&'a str, RingBuffer<Metric>>>>,
                  name: &'a str,
                  metric: Metric) {
        let mut unwrapped = metrics.lock().unwrap();

        match unwrapped.get_mut(name) {
            None      => {
                let mut rb: RingBuffer<Metric> = RingBuffer { size: 5,
                                                              vec: VecDeque::new() };
                unwrapped.insert(name, rb); // Here compiler complains that value was used twice
            Some(rb)   => {


You probably want to use HashMap::entry.


Thank you, that’s a very nice pointer. Didn’t notice it has so much “sugar” on it (such as or_insert).