Works for r = 3, almost works for other r

combi2.rs

use std::{fmt::Debug, ops::Range};

fn main() {
    let mut v = vec![1, 2, 3, 4, 5, 6];
    let r = 3;

    for x in CombiIterator::new(&mut v, r) {
        println!("{:?}", x);
    }
}

struct CombiIterator<'a, T> {
    slice: &'a mut [T],
    range: Range<usize>,
    r: usize,
    step: usize,
    end: usize,
}

impl<'a, T> CombiIterator<'a, T> {
    fn new(slice: &'a mut [T], r: usize) -> Self {
        let n = slice.len();
        Self {
            slice,
            range: r - 1..n,
            r,
            step: 0,
            end: n,
        }
    }
}

impl<'a, T: Clone + Debug> Iterator for CombiIterator<'a, T> {
    type Item = Vec<T>;

    fn next(&mut self) -> Option<Self::Item> {
        if self.end < self.r {
            return None;
        }

        let output = self.slice[..self.r].to_vec();

        self.slice[self.range.clone()].rotate_left(1);
        self.step += 1;

        if self.step == self.range.len() {
            if self.range.len() == 1 {
                self.slice[1..self.end].rotate_left(self.r - 1);
                self.slice[..].rotate_left(1);

                self.end -= 1;
                self.range.end = self.end;
            } else {
                self.slice[(self.range.start - 1)..self.end].rotate_left(1);
                self.range.end -= 1;
            }
            self.step = 0;
        }

        Some(output)
    }
}