RFC: Replace the internal implementation of `LocalPoolHandle` with `LocalRuntime`

[ADD-SP]

This issue is tracked by #7558, and it can be done after stabilizing the LocalRuntime.

What is the LocalPoolHandle?

It manages several Current Thread Runtimes and uses LocalSet to execute !Send Futures.

Why did we add the LocalPoolHandle?

It was added by #3370, the use case is not only for running !Send Futures, but also for dispatching !Send Futures to other threads.

Why do we need to change the LocalPoolHandle?

There are two reasons.

• The LocalSet is proposed to be deprecated (Proposal: Deprecate Tokio's LocalSet #6741), and we don't want the LocalPoolHandle keeps relying a deprecated feature.
• Almost all downsides of the LocalSet can also be applied to the LocalPoolHandle, see Add a LocalRuntime for replacing Runtime+LocalSet #6739 for the downsides of the LocalSet. Migrating to the LocalRuntime can get rid of these downsides.

What should we do?

I have read the implementation of the LocalPoolHandle and I think it is not hard to replace the internal Current Thread Runtime by the LocalRuntime.

https://github.com/tokio-rs/tokio/blob/925c614c89d0a26777a334612e2ed6ad0e7935c3/tokio-util/src/task/spawn_pinned.rs

Will there be any breaking change?

No, there will not be any breaking change.