serde_json 1.0.150: Preserving JSON object key insertion order in serde_json Value and Map requires the cargo feature preserve_order backed by IndexMap instead of default BTreeMap sorting

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# CodeSampleX Notes: serde_json Key Ordering and `preserve_order` Feature Flag

## Search Result
`search_known_solution` returned existing samples for other patterns:
- `sha256:f3054adccb9b7e7b714ef0724134ea047bd70836a71c6f313d1b11bff9b1aae9`: "Parsing scientific notation numbers with serde_json arbitrary_precision normalizes positive and uppercase exponents to explicit lowercase e+ formatting matching zmij float serialization starting in 1.0.149"
- `sha256:e583be8e3c3e9cb793e93e1be28e4fe841fe73406f39ed227bf494cc911856f0`: "Dependency crates declaring serde without the derive feature compile and serialize via derive macros when unified with a dependency graph that activates serde/derive, masking missing feature flags until built in isolation"
- `sha256:861889cfdd0a5dd5ceeb51b6ec6359baf342d7c5c30f7d1fbc598a53e175fac2`: "Disabling chrono default features strips Utc::now and Local which require the clock feature rather than std, while serde support requires an explicit opt-in serde feature and serializes DateTime as an RFC 3339 string"

No existing sample addressed key ordering or the `preserve_order` Cargo build feature in `serde_json`.

## What a Model Would Have Written Instead
A model writes JSON object key iteration or serialization code assuming `serde_json::Value` and `serde_json::Map` preserve key insertion order by default (as in JavaScript `JSON.parse` or Python 3.7+ `json.loads`) using `serde_json = "1.0"` without enabling the `preserve_order` cargo feature.

## How the Wrong Version Fails
Silently with a green build — without `features = ["preserve_order"]`, `serde_json::Map` is backed by `std::collections::BTreeMap`, which compiles cleanly but silently sorts all object keys in lexicographical order (e.g. `["z", "a", "m"]` becomes `["a", "m", "z"]`), corrupting canonical serialization, cryptographic HMAC signatures, and order-dependent pipeline operations.

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use serde_json::{Map, Value};

/// Parses a JSON object string and returns the keys in the order they are stored.
pub fn parse_object_keys(input: &str) -> Result<Vec<String>, serde_json::Error> {
    let value: Value = serde_json::from_str(input)?;
    if let Value::Object(map) = value {
        Ok(map.keys().cloned().collect())
    } else {
        Ok(Vec::new())
    }
}

/// Serializes a JSON object map to string.
pub fn serialize_map(map: &Map<String, Value>) -> Result<String, serde_json::Error> {
    serde_json::to_string(map)
}

#[cfg(test)]
mod tests {
    use super::*;
    use serde_json::json;

    #[test]
    fn test_deserialization_preserves_object_key_insertion_order() {
        // Without features = ["preserve_order"], Map is backed by BTreeMap which sorts keys to ["a", "m", "z"].
        // With features = ["preserve_order"], Map is backed by IndexMap which preserves source order ["z", "a", "m"].
        let input = r#"{"z": 100, "a": 200, "m": 300}"#;
        let keys = parse_object_keys(input).expect("valid JSON object");
        assert_eq!(keys, vec!["z", "a", "m"]);
    }

    #[test]
    fn test_value_serialization_preserves_source_key_order() {
        // Without features = ["preserve_order"], serializing outputs sorted keys `{"a":200,"m":300,"z":100}`.
        // With features = ["preserve_order"], serializing preserves source order `{"z":100,"a":200,"m":300}`.
        let input = r#"{"z": 100, "a": 200, "m": 300}"#;
        let value: Value = serde_json::from_str(input).expect("valid JSON object");
        let serialized = serde_json::to_string(&value).expect("serialization succeeds");
        assert_eq!(serialized, r#"{"z":100,"a":200,"m":300}"#);
    }

    #[test]
    fn test_map_insert_appends_to_order() {
        // With IndexMap backing, inserting a new key appends to the iteration order rather than sorting lexicographically.
        let mut map = Map::new();
        map.insert("z".to_string(), json!(1));
        map.insert("a".to_string(), json!(2));
        map.insert("m".to_string(), json!(3));
        map.insert("b".to_string(), json!(4));

        let keys: Vec<&String> = map.keys().collect();
        assert_eq!(keys, vec!["z", "a", "m", "b"]);

        let serialized = serialize_map(&map).expect("serialization succeeds");
        assert_eq!(serialized, r#"{"z":1,"a":2,"m":3,"b":4}"#);
    }

    #[test]
    fn test_json_macro_preserves_declaration_order() {
        // Constructing a Value via the json! macro preserves key order only when preserve_order is enabled.
        let val = json!({
            "delta": 1,
            "alpha": 2,
            "gamma": 3,
            "beta": 4
        });

        let keys: Vec<&str> = val
            .as_object()
            .expect("is object")
            .keys()
            .map(|k| k.as_str())
            .collect();

        assert_eq!(keys, vec!["delta", "alpha", "gamma", "beta"]);
    }
}