macros: run new_current_thread() inside LocalSet

[bnoordhuis]

Allows using tokio::task::spawn_local() without further setup.

Reapplies 33f0a1f, reverted for making tokio::task::block_in_place()
panic with the multi-threaded variant of #[tokio::test].

This commit creates the LocalSet only for the single-threaded runtime.
Still a significant Quality-of-Life improvement, IMO.

edit: refs #4142

[bnoordhuis]

Feedback, por favor?

[Darksonn]

I haven't forgotten you, but I only look at non-simple things once a week, and I didn't get around to this PR this week.

[bnoordhuis]

Quick recap: 33f0a1f was reverted because the newly introduced LocalSet interacts badly with block_in_place().

tokio/tokio/src/runtime/thread_pool/worker.rs

Lines 284 to 286 in d047584

	// This probably means we are on the basic_scheduler or in a
	// LocalSet, where it is _not_ okay to block.
	panic!("can call blocking only when running on the multi-threaded runtime");

[Noah-Kennedy]

I've tested it with #[tokio::main] locally, and that works as well. We should get a couple more reviewers on this though; I am not familiar enough with some of the implementation details for localset to know if this leads to any changes in performance, and others may have opinions on this.

[Darksonn]

One of the concerns we had with the original PR is that it gives #[tokio::main] and #[tokio::test] an important place where their behavior differs, namely that only one of them allows use of spawn_local.

[carllerche]

One of the concerns we had with the original PR is that it gives #[tokio::main] and #[tokio::test] an important place where their behavior differs, namely that only one of them allows use of spawn_local.

We could disable spawn_local in #[tokio::test] unless explicitly asked for?

[Darksonn]

Right, having a local_set option on the macro is one of the options we considered in the previous discussion.

[bnoordhuis]

I'm retracting this. It turned out to be (a lot) quicker to just make appropriate changes to our code base than trying to upstream this change.