Code shared from the Rust Playground

playground.rs

#[derive(Debug)]
pub struct CharsStep<'a> {
    string: &'a str,
    match_len: usize,
}
impl<'a> CharsStep<'a> {
    fn step(&self) -> Option<CharsStep<'a>> {
        let string = &self.string[self.match_len..];
        if self.match_len > 5 {
            Some(CharsStep {string, match_len: self.match_len + 1})
        } else {
            None        
        }
    }
}
// VVVVVVVVVVV  Comment out this code and it compiles/runs
#[derive(Debug,Copy)]
pub struct Matched<'a> {
    full_string: &'a str,
    start: usize,
    end: usize,
}
impl<'a> Clone for Matched<'a> {
    fn clone(&self) -> Self { Matched {full_string: &self.full_string[..], start: self.start, end: self.end} }
}

impl<'a> Matched<'a> {
    fn len(&self) -> usize { self.end - self.start }
    fn len_chars(&self) -> usize { self.string().chars().count() }
    fn string(&self) -> &str { &self.full_string[self.start..self.end] }
    fn unterminated(&self) -> &str { &self.full_string[self.start..] }
    fn remainder(&self) -> &str { &self.full_string[self.end..] }
    fn next(&self, len: usize) -> Matched { Matched {full_string: &self.full_string[..], start: self.end, end: self.end + len }}
}

#[derive(Debug)]
pub struct CharsStepBad<'a> {
    matched: Matched<'a>,
}

impl<'a> CharsStepBad<'a> {
    fn step(&self) -> Option<CharsStepBad<'a>> {
        if self.matched.end == self.matched.full_string.len() { return None; }
        if let Some(size) = Some(self.matched.unterminated().len()) {
            Some(CharsStepBad {matched: self.matched.next(size)})
        } else { None }
    }
}
// ^^^^^^^^^^^ Comment out this code and it compiles/runs
fn main() {
    let step = CharsStep {string: &"fourscrore and seven years ago", match_len: 6};
    let step2 = step.step();
    print!("{:?}", step2 );
    
// VVVVVVVVVVV  Comment out this code and it compiles/runs
    let matched = Matched {full_string: "our forefathers brought forth", start: 0, end: 10};
    let step_bad = CharsStepBad { matched };
    let step_bad_2 = step_bad.step();
    print!("{:?}", step_bad_2 );
// ^^^^^^^^^^^ Comment out this code and it compiles/runs
}