Returning a serde_yaml Mapping for building a nested Mapping

serde_yaml_return_mapping.rs

// adapted from serde_json example: https://stackoverflow.com/a/39147207

extern crate serde_yaml;
use serde_yaml::{Value, Mapping, Number};

fn main() {
    let mut map = Mapping::new();
    build_mapping(&mut map, 1);
    println!("{}", serde_yaml::to_string(&map).unwrap());
    // ---
    // key1: test
    // key2:
    //   - a
    //   - b
    // key3:
    //   x: 12
    //   y: 34
}

fn build_mapping(map: &mut Mapping, idx: u32) {
    if idx == 1 {
        map.insert(Value::String("key1".to_string()), Value::String("test".to_string()));
        build_mapping(map, idx+1);
    } else if idx == 2 {
        map.insert(
            Value::String("key2".to_string()),
            Value::Sequence(vec![
                Value::String("a".to_string()),
                Value::String("b".to_string()),
            ]),
        ); 
        build_mapping(map, idx+1);
    } else {
        map.insert(
            Value::String("key3".to_string()),
            Value::Mapping(build_inner_map(12, 34))
        );
    }
}

fn build_inner_map(val1: u32, val2: u32) -> Mapping {
    let mut inner_map = Mapping::new();
    inner_map.insert(Value::String("x".to_string()), Value::Number(Number::from(val1)));
    inner_map.insert(Value::String("y".to_string()), Value::Number(Number::from(val2)));
    return inner_map;
}