pretty parse tree output (ala pest.rs)

pretty_parse_tree.rs

pub fn pretty_parse_tree(pairs: Pairs<Rule>) -> String {
    #[derive(Clone)]
    struct State {
        depth: usize,
        prefix: String,
    };

    fn go(pairs: Pairs<Rule>, lines: &mut Vec<String>, mut state: State) {
        if state.prefix.is_empty() {
            let indent: String = iter::repeat("  ").take(state.depth).collect();
            state.prefix = format!("{}- ", indent);
        }

        for pair in pairs.into_iter() {
            let rule = format!("{:?}", pair.as_rule());

            let repr = String::from(pair.as_str());
            let pairs = pair.into_inner();

            if pairs.peek().is_some() {
                mem::drop(repr);

                let children: Vec<_> = pairs.clone().into_iter().collect();
                let child_count = children.len();

                if child_count == 1 {
                    let mut state = state.clone();
                    state.prefix = format!("{}{} > ", state.prefix, rule);
                    go(pairs, lines, state);
                } else {
                    lines.push(format!("{}{}", state.prefix, rule));

                    let mut state = state.clone();
                    state.depth += 1;
                    state.prefix = String::new();

                    go(pairs, lines, state);
                }
            } else {
                lines.push(format!("{}{}: {:?}", state.prefix, rule, repr));
            }
        }
    }

    let mut lines = vec![];
    go(pairs, &mut lines, State {
        depth: 0,
        prefix: String::new(),
    });
    lines.join("\n")
}