fn replace_at_idx_in_each_word(line: &str, delims: &[char], idx: usize, replace: char) -> String {
    let mut res = String::with_capacity(line.len());

    for (word, delim) in WordsBy::new(line, delims) {
        replace_nth_and_append(&mut res, word, idx, replace);
        res.extend(delim);
    }

    res
}

fn replace_nth_in_each_word(line: &str, delims: &[char], pos: usize, replace: char) -> String {
    assert!(pos != 0, "can not replace zeroth character in word");
    replace_at_idx_in_each_word(line, delims, pos - 1, replace)
}

struct WordsBy<'a> {
    line: Option<&'a str>,
    delims: &'a [char],
}

impl<'a> WordsBy<'a> {
    fn new(line: &'a str, delims: &'a [char]) -> Self {
        Self {
            line: Some(line),
            delims,
        }
    }
}

impl<'a> Iterator for WordsBy<'a> {
    type Item = (&'a str, Option<char>);

    fn next(&mut self) -> Option<Self::Item> {
        let line = self.line.take()?;
        if let Some((pos, delim)) = line.char_indices().find(|(_, c)| self.delims.contains(&c)) {
            let (word, rest) = line.split_at(pos);
            let rest = &rest[delim.len_utf8()..];
            if !rest.is_empty() {
                self.line = Some(rest)
            }

            Some((word, Some(delim)))
        } else {
            Some((line, None))
        }
    }
}

fn replace_nth_and_append(acc: &mut String, word: &str, idx: usize, new: char) {
    if let Some((pos, old)) = word.char_indices().nth(idx) {
        let (before, after) = word.split_at(pos);
        acc.push_str(before);
        acc.push(new);
        acc.push_str(&after[old.len_utf8()..])
    } else {
        acc.push_str(word)
    }
}

fn main() {
    let line = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.";
    let delims = [' '];
    let result = replace_nth_in_each_word(line, &delims, 2, '%');
    assert_eq!(
        result,
        "L%rem i%sum d%lor s%t a%et, c%nsectetur a%ipiscing e%it."
    );
}