task: define task_local in underlying future Drop

[l4l]

Motivation

Currently task_local-s cannot be accessed at the Drop::drop, which seems quite strange behavior.

Solution

Current solution wraps the underlying future with ManuallyDrop and requires some unsafe (not really sure about soundness). Other option is using Option<Fut> that have additional overhead though requires no unsafe blocks.

For some reason, a test within tokio doesn't reproduce the issue but it works for playground/separate crate (thanks @loyd for that one), link.

[Darksonn]

I don't understand the motivation for this. Can you please post some example code that uses this new feature, along with an explanation of what the code does?

[loyd]

We're storing the current trace ID in TLS and accessing it, e.g. for logging. However, in the Drop instances it can fail if it's being called during runtime termination.

[l4l]

I added a test for the case finally. Overall the previous behavior looks quite unexpected. Especially when you see in debugger LocalKey::scope at previous function frames but the task local value is unavailable that's one is really confusing.

[Darksonn]

The motivation for your PR makes a lot of sense, but it isn't quite implemented in the way I would want it to, and I also noticed a bunch of other minor issues in LocalKey. Therefore, I've opened the PR #4795 containing those changes.

Thanks for taking the time to submit a PR.