Rust deep clone and recursive merge for nested maps

use std::collections::BTreeMap;

#[derive(Clone, Debug, PartialEq)]
enum Value {
    Scalar(String),
    Obj(BTreeMap<String, Value>),
}

fn merge(base: &Value, patch: &Value) -> Value {
    match (base, patch) {
        (Value::Obj(a), Value::Obj(b)) => {
            let mut out = a.clone();
            for (k, v) in b {
                let next = a.get(k).map_or_else(|| v.clone(), |old| merge(old, v));
                out.insert(k.clone(), next);
            }
            Value::Obj(out)
        }
        (_, other) => other.clone(),
    }
}