Match alternative: squashed, uses fewer lines, not necessarily easier to read.

client.rs

    pub fn start_event_loop_cb<F, G>(&mut self,
                                     notify_handler: Option<F>,
                                     request_handler: Option<G>)
        where F: FnMut(&str, Vec<Value>) + Send + 'static,
              G: FnMut(&str, Vec<Value>) -> Result<Value, Value> + Send + 'static
    {
        let queue = self.queue.clone();
        let reader = self.reader.take().unwrap();
        let writer = self.writer.clone();

        self.dispatch_guard = match (notify_handler, request_handler) {
            (None, None) => {
                let (f, g) = (default_notify_handler, default_request_handler);
                Some(Self::dispatch_thread(queue, reader, writer, f, g))
            }
            (Some(f), None) => {
                let g = default_request_handler;
                Some(Self::dispatch_thread(queue, reader, writer, f, g))
            }
            (None, Some(g)) => {
                let f = default_notify_handler;
                Some(Self::dispatch_thread(queue, reader, writer, f, g))
            }
            (Some(f), Some(g)) => Some(Self::dispatch_thread(queue, reader, writer, f, g)),
        };
        self.event_loop_started = true;
    }