Using futures::select macro with async functions

1

I can't seem to figure out how to a do a thing...

I have an async function that takes a mutable reference:

async fn do_thing(foo: &mut Foo)

I want to call it using futures::select!, but I also want to mutate its argument in a separate branch.

futures::select! {
    x = do_thing(&mut foo) => { ... },
    y = something_else() => {
        foo.mutate();
    },
}

This should be safe since select allows you to use the same variable mutably in separate branches. The problem is that I can't call an async fn directly as an argument to select because select requires it's arguments to be Unpin. So I can try pinning the async call separately like this:

let do_thing_future = do_thing(&mut foo);
pin_mut!(do_thing_future);
futures::select! {
    x = do_thing_future => { ... },
    y = something_else() => {
        foo.mutate();
    },
}

But now do_thing_future borrows foo for the duration of the select, so I can no longer mutate it in the other branch. Another option might be to use Box::pin on the async call:

futures::select! {
    x = Box::pin(do_thing(&mut foo)) => { ... },
    y = something_else() => {
        foo.mutate();
    },
}

But this also borrows foo for the duration of the select. From what I understand (though I could be wrong) I can't wrap &mut foo in anything that requires a destructor if I want the borrow checker to be flexible.

So are there any other options here? Is there some way to make this work?

2

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