You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: serializer.rs
    path of file: ./repos/Project-47/pr47-core/src/util
    
    the code of the file until where you have to start completion: //! # `serializer.rs`: "User space", runtime unaware coroutine serializer
//!
//! This serializer could make sure only one of the participating `task`s can can run at one time,
//! no matter which runtime the user chose. However, `task`s can still benefit from asynchronous
//! completion of `Future`s.
//!
//! The purpose of making this "coroutine serializer" is that Pr47 heavily relies on
//! *Run-Time Lifetime Checking* (RTLC) and related analysis, which are hard to go multi-threading.
//! Forcing everything to happen in a single-threaded, sequential, serialized behavior would
//! solve this problem easily.

use std::cell::UnsafeCell;
use std::collections::HashMap;
use std::future::Future;
use std::mem::transmute;
use std::sync::Arc;

use futures::future::JoinAll;
use xjbutil::as
    
    fn basic_test_print() {
        async fn test_impl() {
            let serializer: CoroutineContext<()> = CoroutineContext::main_context(()).await;
            eprintln!("line 1");
            serializer.co_spawn_task(|serializer: CoroutineContext<()>, _x: ()| async move {
                eprintln!("line 2");
                serializer.co_yield().await;
                eprintln!("line 3");
            }, ()).await;
            eprintln!("line 4");
            serializer.co_spawn_task(|serializer: CoroutineContext<()>, _x: ()| async move {
                eprintln!("line 5");
                serializer.co_yield().await;
                eprintln!("line 6");
                serializer.co_await(testing_sleep(Duration::from_millis(500))).await;
                eprintln!("line 7");
            }, ()).await;
            eprintln!("line 8");
            serializer.finish().await;
            eprintln!("line 9");
        }

        eprintln!("launching test");
        block_on_future(test_impl());
        eprintln!("mission accomplished");
    }