Don't use `join_all` on `JoinHandle`

What it does

The join_all method is used to run multiple futures concurrently by polling all of them simultaneously. However, if used together with tokio::spawn, it provides no advantage over a simple loop, and has a performance impact since the join_all does extra work to poll all of them simultaneously.

Lint Name

join-all-joinhandles

Category

perf

Advantage

• Using join_all on a vector of JoinHandle is inefficient.
• Using join_all makes it easy to ignore errors because Vec<Result<...>> is not #[must_use].

Drawbacks

Calling join_all is shorter.

Example

let join_handles: Vec<JoinHandle<T>> = ...;
join_all(join_handles);

Could be written as:

let join_handles: Vec<JoinHandle<T>> = ...;
for handle in join_handles {
    handle.await;
}

Note that the handle.await will trigger a #[must_use] warning in the above, which is not triggered in the original example.

Start by locating the implementation entry point for the join-all-joinhandles lint in rust-clippy and review how existing performance lints inspect join_all and JoinHandle. Done means the lint detects the described pattern, recommends awaiting handles in a loop, and preserves the #[must_use] warning behavior shown in the issue.