const JSON: &str = r#"
    {
        "name": "string",
        "optional": null
    }
    "#;

    let file = config::File::from_str(JSON, config::FileFormat::Json5);
    let config = Config::builder().add_source(file).build().unwrap();
    // called `Result::unwrap()` on an `Err` value: invalid type: unit value, expected a boolean, integer, float, string, null, array or map

// json5.rs, line 18:
impl<'de> serde::de::Deserialize<'de> for Val {
    fn deserialize<D>(d: D) -> Result<Self, D::Error>
    where
        D: serde::de::Deserializer<'de>,
    {
        serde_untagged::UntaggedEnumVisitor::new()
            .bool(|value| Ok(Self::Boolean(value)))
            .i64(|value| Ok(Self::Integer(value)))
            .f64(|value| Ok(Self::Float(value)))
            .string(|value| Ok(Val::String(value.to_owned())))
            .none(|| Ok(Self::Null))
            .unit(|| Ok(Self::Null))  // Adding this allows parsing of null values !
            .seq(|value| value.deserialize().map(Val::Array))
            .map(|value| value.deserialize().map(Val::Object))
            .deserialize(d)
    }
}