pub trait Single {
    type Item;

    fn single(self) -> Option<Self::Item>;
}

impl<T, I> Single for I
where
    I: IntoIterator<Item = T>,
{
    type Item = T;

    fn single(self) -> Option<Self::Item> {
        let mut iter = self.into_iter();

        // There are three cases of interest:
        //
        // 1. `self` has zero items -> return None.
        // 2. `self` has two or more items -> return None.
        // 3. `self` has exactly one item -> return `iter.next()` unchanged, as
        //    it holds that item.
        iter.next().filter(|_| iter.next().is_none())
    }
}