Enter tokio context outside of a future

[jplatte]

Sometimes, non-poll methods expect to be able to spawn tasks. One common case is Drop implementations of types that want to do some async cleanup. Would you accept PRs for any of the following?

• enter_tokio_context + TokioEnterGuard wrapping TOKIO1.enter()
• impl Drop<T> for Compat<T> that enters the tokio context while dropping the inner value (requires some unsafe for ManuallyDrop, I think)

[notgull]

I'm more of a fan of the second option. You shouldn't need ManuallyDrop, just wrap the type in Option and take it out to drop it. I would accept this PR.

[jplatte]

Yes, Option works too but has unnecessary overhead.

[jplatte]

Just realized that Compat uses pin_project! for its definition, which prohibits custom Drop impls. So the Drop impl is only an option if that safe abstraction is replaced with hand-checked unsafe.

[taiki-e]

pin_project! for its definition, which prohibits custom Drop impls.

pin-project-lite supports custom Drop impl: taiki-e/pin-project-lite#25

[notgull]

Yes, Option works too but has unnecessary overhead.

I doubt the overhead is noticeable, especially since most futures and I/O types can use the non-zero optimization.

[jplatte]

Right, but given PinnedDrop (good thing I was wrong about pin-project-lite not having it!) gives us Pin<&mut Self>, Option::take does not work and we'll have to resort to unsafe either way (I think). It also introduces pointless branches for every use of the inner field.

I'm working on a PR right now and it's quite gnarly (which might be due to ManuallyDrop to some extent, but I think it's mostly just due to pinning). I think it will be easier to discuss the solution in the context of an actual diff.

[notgull]

The value of a pinned type can be overwritten with set. So if you set the type to None it will work.

[jplatte]

Oohh, clever! I was aware of that method, but hadn't connected the dots. That should indeed make an unsafe-free solution possible. (together with Option::as_pin_mut which I found is also a thing)