Serde double encoding JSON

as_json_string.rs

mod as_json_string {
    use serde::de::{Deserialize, DeserializeOwned, Deserializer};
    use serde::ser::{Serialize, Serializer};
    use serde_json;

    pub fn serialize<T: Serialize, S: Serializer>(
        value: &T,
        serializer: S,
    ) -> Result<S::Ok, S::Error> {
        let json = serde_json::to_string(value).map_err(serde::ser::Error::custom)?;
        json.serialize(serializer)
    }

    pub fn deserialize<'de, T: DeserializeOwned, D: Deserializer<'de>>(
        deserializer: D,
    ) -> Result<T, D::Error> {
        let json = String::deserialize(deserializer)?;
        serde_json::from_str(&json).map_err(serde::de::Error::custom)
    }
}