Potential unsoundness in FuturesUnordered

[robbie01]

I was browsing through the docs for FuturesUnordered when I noticed that its Sync bound is dependent on Fut being Sync rather than Send + Sync. This theoretically allows you to push a value that is !Send + Sync and use poll_next to retrieve it from another thread. Is my understanding of this correct? Apologies for raising an issue if I’m wrong.

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Aside: why do FuturesUnordered::<Fut>::iter and the corresponding impl<'a, Fut> IntoIterator for &'a FuturesUnordered<Fut> require Fut: Unpin? That requirement can be circumvented safely via e.g. Pin::new(&f).iter_pin_ref().map(Pin::get_ref).

Aside aside: why do iter_pin_ref and iter_pin_mut require it to be pinned? FuturesUnordered implements Unpin unconditionally.

[robbie01]

Demonstrative example of unsafe-free UB (MutexGuard is !Send + Sync):

use std::{thread, future, sync::Mutex};
use futures::{executor, stream::{FuturesUnordered, StreamExt as _}};

fn main() {
    let m = Mutex::new(());
    let mut f = FuturesUnordered::new();
    
    thread::scope(|scope| {
        scope.spawn(|| {
            let guard = m.lock();
            f.push(future::ready(guard));
        });
    });

    dbg!(executor::block_on(f.next()));
}

[taiki-e]

Thanks for finding this! I filed #2788 to fix this.