Решение задачи с Хабра от @Milein: найти максимальное количество идущих подряд символов для каждого сивола строки

main.rs

fn cut_repeating_prefix(source: &str) -> Option<(char, usize, &str)> {
    let mut chars = source.char_indices();
    let first = chars.next()?.1;
    let len = chars
        .find_map(|(i, ch)| if ch != first { Some(i) } else { None })
        .unwrap_or(source.len());
    let count = len / first.len_utf8();
    let (_pre, post) = source.split_at(len);
    Some((first, count, post))
}

use std::collections::HashMap;

fn max_repeating(mut s: &str) -> HashMap<char, usize> {
    let mut lens = HashMap::new();
    let counts = std::iter::from_fn(|| {
        let (ch, count, rest) = cut_repeating_prefix(s)?;
        s = rest;
        Some((ch, count))
    });

    for (ch, count) in counts {
        let max = lens.entry(ch).or_insert(0);
        *max = (*max).max(count);
    }

    lens
}

fn main() {
    println!("{:?}", max_repeating("abbb"));
    println!("{:?}", max_repeating("аяяя"));
}