Modifying a serde_yaml Mapping in a function

serde_yaml_mapping_modify.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: 10
    //   y: 20
}

fn build_mapping(map: &mut Mapping, idx: u32) {
    // made recursive to mimic real code that might generate a Mapping
    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 {
        let mut inner_map = Mapping::new();
        inner_map.insert(Value::String("x".to_string()), Value::Number(Number::from(10u64)));
        inner_map.insert(Value::String("y".to_string()), Value::Number(Number::from(20u64)));
        map.insert(Value::String("key3".to_string()), Value::Mapping(inner_map));
    }
}

The above code can be directly run in the Rust Playground: https://play.rust-lang.org