Iterate and store partial results as a new iterator

scan_map.rs

pub struct ScanMap<I, F, T> {
    iter: I,
    f: F,
    acc: T,
}

impl<I, F, T> ScanMap<I, F, T> 
where
    I: Iterator,
    F: FnMut(&T, I::Item) -> T,
    T: Clone,
{
    fn new(iter: I, acc: T, f: F) -> Self {
        Self { iter, f, acc }
    }
}

impl<I, F, T> Iterator for ScanMap<I, F, T>
where
    I: Iterator,
    F: FnMut(&T, I::Item) -> T,
    T: Clone,
{
    type Item = T;

    fn next(&mut self) -> Option<Self::Item> {
        self.acc = (self.f)(&self.acc, self.iter.next()?);
        Some(self.acc.clone())
    }
}

pub trait ScanMapExt: Iterator {
    fn scan_map<T, F>(self, acc: T, f: F) -> ScanMap<Self, F, T>
    where
        Self: Sized,
        F: FnMut(&T, Self::Item) -> T,
        T: Clone,
    {
        ScanMap::new(self, acc, f)
    }
}

impl<I: Iterator> ScanMapExt for I {}

z_main.rs

mod scan_map;

use scan_map::ScanMapExt;

fn main() {
    let v = [1, 2, 3, 4];
    let result: Vec<_> = v.iter().scan_map(0, |acc, x| acc + x).collect();
    println!("{:?}", result); // Outputs: [1, 3, 6, 10]
}