rust serde skip_null_values helper

skip_null_values.rs

fn skip_null_values<S, K, M>(map: &M, serializer: S) -> Result<S::Ok, S::Error>
where
    S: serde::Serializer,
    K: serde::Serialize,
    M: ?Sized,
    for<'a> &'a M: IntoIterator<Item = (&'a K, &'a Value)>,
{
    use serde::ser::SerializeMap;
    let mut ser_map = serializer.serialize_map(None)?;
    for (k, v) in map {
        if !v.is_null() {
            ser_map.serialize_entry(k, v)?;
        }
    }
    ser_map.end()
}

Usage:

#[derive(serde::Serialize, serde::Deserialize)]
struct User {
    id: String,
    username: String,

    #[serde(flatten, serialize_with = "skip_null_values")]
    extra: std::collections::HashMap<String, serde_json::Value>,
}