Code shared from the Rust Playground

playground.rs

use std::ops::{Add, RangeInclusive};

struct Collate<Iter, Item> {
    iter: Iter,
    current: Option<RangeInclusive<Item>>,
}

impl<Iter, Item> Collate<Iter, Item> {
    fn new(iter: Iter) -> Self {
        Self {
            iter,
            current: None,
        }
    }
}

impl<Iter, Item> Iterator for Collate<Iter, Item>
where
    Iter: Iterator<Item = Item>,
    Item: PartialOrd + Add<u64, Output = Item> + Copy,
{
    type Item = RangeInclusive<Item>;

    fn next(&mut self) -> Option<Self::Item> {
        while let Some(item) = self.iter.next() {
            if let Some(current) = self.current.take() {
                if item == *current.end() {
                    self.current = Some(current);
                } else if item == *current.end() + 1 {
                    self.current = Some(*current.start()..=item);
                } else {
                    self.current = Some(item..=item);
                    return Some(current);
                }
            } else {
                self.current = Some(item..=item);
            }
        }

        self.current.take()
    }
}

fn collate<Iter, Item>(iter: Iter) -> Collate<Iter, Item>
where
    Iter: Iterator<Item = Item>,
    Item: PartialOrd,
{
    Collate::new(iter)
}

fn main() {
    let items = vec![
        1, 2, 3, 10, 10, 10, 11, 11, 20, 30, 31, 31, 32, 33, 35, 40, 40, 40, 40,
    ];
    dbg!(&items);
    let collated = collate(items.into_iter()).collect::<Vec<_>>();
    dbg!(&collated);
}