Rust comments

snippet.rs

 async fn insert_with<F>(&self, key: Key, future: F) -> FetchResult
 where
 F: Future<Output = FetchResult>,
 {
  // Each `OnceCell` represents a fetch task, either complete, in progress, or (if we create it) about to start.
  let once = {
   let mut store = self.store.lock().await;
   store
    .entry(key.clone())
    .or_insert_with(|| Arc::new(OnceCell::new()))
    .clone()
  };

  // We're now out of the lock. Other calls to the downloader may affect the store, but any task involving this specific handle will return the same cell.

  // Case 1: Fetch hasn't started. It will now start. The `await` will only resolve once the fetch is complete.
  // Case 2: Fetch is in progress. The `await` will merge with the ongoing fetch and resolve once the fetch is complete.
  // Case 3: Fetch is complete. The `await` will resolve immediately with the result of the fetch.
  once.get_or_init(|| future).await.clone()
 }