pub struct Mapscan<I, A, F> {
    accu: Option<A>,
    iter: I,
    f: F,
}

impl<I, A, F> Mapscan<I, A, F> {
    pub fn new(iter: I, accu: Option<A>, f: F) -> Self {
        Self { iter, accu, f }
    }
}

impl<I, A, F> Iterator for Mapscan<I, A, F>
where
    I: Iterator,
    F: FnMut(&A, I::Item) -> Option<A>,
{
    type Item = A;

    fn next(&mut self) -> Option<Self::Item> {
        self.accu.take().map(|accu| {
            self.accu = self.iter.next().and_then(|item| (self.f)(&accu, item));
            accu
        })
    }
}

trait IterPlus: Iterator {
    fn map_scan<A, F>(self, accu: Option<A>, f: F) -> Mapscan<Self, A, F>
    where
        Self: Sized,
        F: FnMut(&A, Self::Item) -> Option<A>,
    {
        Mapscan::new(self, accu, f)
    }
}

impl<T: ?Sized> IterPlus for T where T: Iterator {}

fn main() {
    let xs = [1, 2, 3, 4, 5];

    let vs = xs
        .iter()
        .map_scan(Some(0), |acc, x| Some(acc + x))
        .collect::<Vec<_>>();

    println!("{:?}", vs)
}