Don't pass `FuturesUnordered` to `join_all`

What it does

The FuturesUnordered collection is a way to run code concurrently, however it is also possible to use FuturesUnordered as a linked-list stack and ignoring its concurrency features. The linked list can be created via collect on an iterator or with its push method. Then, its IntoIterator impl can be used to get the items back out in FILO order like a stack, destroying the FuturesUnordered in the progress.

This is what happens when you pass a FuturesUnordered to the join_all method. The futures are added to the collection, then immediately removed again when join_all collects the iterator its given into a Vec.

Lint Name

futures-unordered-with-join-all

Category

complexity

Advantage

Any code that passes a FuturesUnordered to join_all is semantically incorrect and should use a Vec instead.

Drawbacks

None.

Example

let tasks = tasks.
    .into_iter()
    .map(|task| my_async_fn(task))
    .collect::<FuturesUnordered<_>>();

let outputs = join_all(tasks).await;

Could be written as:

let tasks = tasks.
    .into_iter()
    .map(|task| my_async_fn(task))
    .collect::<Vec<_>>();

let outputs = join_all(tasks).await;

No implementation files or tests are named. Start by searching the repository for existing lint implementations and tests, then use the lint name futures-unordered-with-join-all and the supplied example to trace the relevant patterns. Done means the lint detects passing FuturesUnordered to join_all and recommends collecting into a Vec instead.