use serde::{de, Deserialize, Deserializer};

#[derive(Debug, Deserialize)]
struct Foo {
    foo: u32,
    #[serde(deserialize_with = "deserialize_from_str")]
    bar: Bar,
}

#[derive(Debug, Deserialize)]
struct Bar {
    foo: String,
}

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

fn main() {
    let data = r#"
        {
            "foo": 42,
            "bar": "{\"foo\":\"bar\"}"
        }
    "#;
    println!("{:?}", serde_json::from_str::<Foo>(data));
}