Function mz_lowertest::deserialize_optional_generic

pub fn deserialize_optional_generic<D, I>(
    stream_iter: &mut I,
    type_name: &'static str
) -> Result<Option<D>, String>where
    D: DeserializeOwned + MzReflect,
    I: Iterator<Item = TokenTree>,

Simpler interface for deserialize_optional when no syntax overrides or extensions are needed.