Create iterator that returns lenghts of runs of identical elements of slice

main.rs

use std::iter;

fn run_lengths<'a, T>(a: &'a[T]) -> impl Iterator<Item=usize> + 'a
where
    T: std::cmp::PartialEq 
{
    a
    .windows(2)
    .enumerate()
    .filter_map(|(i, ab)| if ab[0]==ab[1] {None} else {Some(i as isize)})
    .chain(iter::once(a.len() as isize - 1)) //append edge at end of slice
    .scan(-1, |last_pos, pos| { //first edge is before slice
        let res = pos - *last_pos;
        *last_pos=pos; 
        Some(res as usize)
    })
}

fn main() {
    let a = [1,2,3,3,1,1,1];

    let r: Vec<_> = run_lengths(&a).collect();
    dbg!(&r);
}

Playground at https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=713a989f351f9474c59c1cf12df4e7d1