Let’s break down the design of CachedParkThread:

1. Purpose: CachedParkThread is designed to provide thread parking functionality with caching, optimized for use within a single thread.

2. Structure:

   pub(crate) struct CachedParkThread {
       _anchor: PhantomData<Rc<()>>,
   }

   It uses a PhantomData<Rc<()>> to ensure the struct is not Send or Sync, tying it to a single thread.

3. Key methods:

   • new(): Creates a new instance.
   • waker(): Returns a Waker for the current thread.
   • park(): Parks the current thread.
   • park_timeout(): Parks the thread with a timeout.
   • block_on<F: Future>(): Blocks the thread while polling a future to completion.

4. Caching mechanism: It uses a thread-local storage (TLS) to cache the ParkThread:

   tokio_thread_local! {
       static CURRENT_PARKER: ParkThread = ParkThread::new();
   }

   This allows efficient reuse of the parking mechanism within a thread.

5. Thread-safety: The use of PhantomData<Rc<()>> and TLS ensures that CachedParkThread is bound to a single thread and cannot be shared or sent between threads.

6. Waker creation: It can create a Waker from the underlying UnparkThread, allowing integration with Rust’s async/await system.

7. Future execution: The block_on method implements a basic executor, running a future to completion by repeatedly polling and parking the thread when necessary.

8. Error handling: Methods return Result<_, AccessError> to handle cases where TLS access fails.

This design allows for efficient thread parking and waking operations within Tokio’s runtime, particularly useful for blocking operations and integrating with the async ecosystem.