Add a lint for forgetting futures

[tdittr]

This adds a new lint to check when a Future is passed to mem::forget, thus not running any cancellation logic.

changelog: Add [mem_forget_future] lint

Open question: Should it be called forget_future instead? Similar to forget_non_drop.

[tdittr]

r? @llogiq

[tdittr]

CC: @datdenkikniet we talked about this at the embedded unconf

[samueltardieu]

Aren't you also linting Future types even if they don't implement Drop? What would be the problem there?

View changes since the review

[tdittr]

Oh yes that is a very good point. I'll change that

[tdittr]

Weirdly the future created by an async block or async fn always implements Drop? No matter if it captures anything that implements Drop.

[samueltardieu]

Weirdly the future created by an async block or async fn always implements Drop? No matter if it captures anything that implements Drop.

I'd expect that, at least if any object inside the async block implements Drop and can be alive around any .await. You should try and check if this is easy to determine.

[tdittr]

It looks like the drop glue is always generated, but no explicit Drop impl is around. So for example this will fail to compile:

async fn foo() {
    let x = Box::new(());
    core::future::ready(()).await;
    let _ = x;
}

fn bar() -> impl Future<Output = ()> + Drop {
    foo()
}

Are you by any chance also at RustWeek at the moment?

[samueltardieu]

Are you by any chance also at RustWeek at the moment?

Yes, but leaving right after lunch.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[tdittr]

I finally found some time again to work on this after RustWeek and addressed the suggestions by @datdenkikniet (thanks those texts were much better than what I came up with).

The Drop glue that is generated still makes this lint complain about more cases than it strictly needs to.