Async recursive function that takes a mutex

Question

How do you create an async recursive function that takes a mutex? Rust claims that this code holds a mutex across an await point. However, the value is dropped before the .await.

#[async_recursion]
async fn f(mutex: &Arc<Mutex<u128>>) {
    let mut unwrapped = mutex.lock().unwrap();
    *unwrapped += 1;
    let value = *unwrapped;
    drop(unwrapped);
    tokio::time::sleep(tokio::time::Duration::from_millis(1000)).await;
    if value < 100 {
        f(mutex);
    }
}

Error

future cannot be sent between threads safely
within `impl futures::Future<Output = ()>`, the trait `std::marker::Send` is not implemented for `std::sync::MutexGuard<'_, u128>`
required for the cast to the object type `dyn futures::Future<Output = ()> + std::marker::Send`rustc
lib.rs(251, 65): future is not `Send` as this value is used across an await

Answer 1

In this case, you can restructure the code to make it so unwrapped can't be used across an await:

let value = {
    let mut unwrapped = mutex.lock().unwrap();
    *unwrapped += 1;
    *unwrapped
};
tokio::time::sleep(tokio::time::Duration::from_millis(1000)).await;
if value < 100 {
    f(mutex);
}

If you weren't able to do this, then you'd need to make it so you don't return a Future that implements Send. The async_recursion docs specify an option you can pass to the macro to disable the Send bound it adds:

#[async_recursion(?Send)]
async fn f(mutex: &Arc<Mutex<u128>>) {
    ...

(playground)

You wouldn't be able to send such a Future across threads though.