Extracting arbitrary field sets from JSON

main.rs

use serde::Deserialize;

struct HNil;

impl<'de> Deserialize<'de> for HNil {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: serde::Deserializer<'de>,
    {
        deserializer.deserialize_any(serde::de::IgnoredAny)?;
        Ok(Self)
    }
}

#[derive(Deserialize)]
struct HCons<H, T> {
    #[serde(flatten)]
    head: H,
    #[serde(flatten)]
    tail: T,
}

macro_rules! HList {
    () => { HNil };
    ($head:ty $(, $rest:ty)* $(,)?) => { HCons<$head, HList![$($rest),*]> };
}

macro_rules! hlist_pat {
    () => { HNil };
    ($head:pat $(, $rest:pat)* $(,)?) => { HCons { head: $head, tail: hlist_pat!($($rest),*) } };
}

#[derive(Deserialize)]
struct Items {
    items: Vec<String>,
}

#[derive(Deserialize)]
struct Version {
    version: u32,
}

#[derive(Deserialize)]
struct User {
    user_id: String,
}

fn main() {
    let json = r#"{
        "user_id": "john_doe",
        "items": ["salad", "juice", "beer", "fork"],
        "version": 0,
        "bogus": [null, 0.3, 4, "nope", {}]
    }"#;

    use serde_json::from_str as from_json;
    let hlist_pat!(user, items, version) = from_json::<HList![User, Items, Version]>(json).unwrap();
    assert_eq!(user.user_id, "john_doe");
    assert_eq!(items.items, ["salad", "juice", "beer", "fork"]);
    assert_eq!(version.version, 0);

    let json = r#"{
        "user_id": "john_doe",
        "items": ["salad", "juice", "beer", "fork"],
        "bogus": [null, 0.3, 4, "nope", {}]
    }"#;

    assert!(from_json::<HList![User, Items, Version]>(json).is_err());
    assert!(from_json::<HList![User, Items]>(json).is_ok());

    let json = r#"{
        "version": 0,
        "bogus": [null, 0.3, 4, "nope", {}]
    }"#;

    assert!(from_json::<HList![User, Items, Version]>(json).is_err());
    assert!(from_json::<HList![Option<User>, Option<Items>, Version]>(json).is_ok());

    type Rest = serde_json::Map<String, serde_json::Value>;
    let json = r#"{
        "user_id": "john_doe",
        "items": ["salad", "juice", "beer", "fork"],
        "version": 0,
        "bogus": [null, 0.3, 4, "nope", {}]
    }"#;

    let hlist_pat!(_, _, _, rest) = from_json::<HList![User, Items, Version, Rest]>(json).unwrap();
    assert_eq!(
        serde_json::Value::Object(rest),
        serde_json::json!({
            "bogus": [null, 0.3, 4, "nope", {}],
        })
    );
}