How can I write asynchronous code? (Coroutine not copyable)

[samufi]

How can I implement asynchroneous code in mojo (using async etc.)?

In the Changelog, I find the following example (slightly adjusted):

async fn add_three(a: Int, b: Int, c: Int) -> Int:
    return a + b + c

async fn call_it():
    var task = add_three(1, 2, 3)
    print(await task)

This gives me the following compiler error:

test_async.mojo:39:17: error: 'Coroutine[Int, {}]' is not copyable because it has no '__copyinit__'
    print(await task)
                ^~~~

I do not understand this error, as I thought that Coroutine is register passable (as visible in the source code). What works is

async fn add_three(a: Int, b: Int, c: Int) -> Int:
    return a + b + c

async fn call_it():
    var task = await add_three(1, 2, 3)
    print(task)

However, if I always put await on the right hand side, I will never be able to execute code asynchroneously, as I am always waiting until the line has finished. What am I doing wrong / where is my misconception? (Or is this simply a bug?)

In the changelog it furthermore says that tasks can be "resumed" after completion (see here). What is meant by "resume" and how do I do that?

I am working on the current nightly built: mojo 2024.7.2005 (96a1562c).