Attempt to subtract with overflow in FuturesUnordered's IntoIter after calling .take(n)

[wfraser]

Construct a FuturesUnordered, put n futures in it, call .into_iter().take(n-1) on it, and iterate the result.

Minimal example:

let _ = FuturesUnordered::from_iter(vec![ready(1), ready(2), ready(3), ready(4)])
    .into_iter()
    .take(3) // any number less than the number of futures in the collection, except zero
    .count(); // or collect into another FuturesUnordered, or whatever.

playground

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Separately, while testing this, I found that if you only partially iterate that IntoIter type before dropping it, you get a different panic: future still here when dropping:

let _ = FuturesUnordered::from_iter(vec![ready(1), ready(2), ready(3), ready(4)])
    .into_iter()
    .take(3)
    .next();

playground

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I also debugged this a bit and think I see what's going on. It looks like FuturesUnordered's IntoIter implementation does some brain surgery on the linked list while iterating which leaves it in a partially invalid state unless the iteration is fully completed. I'm preparing a possible fix.