Use #[derive(RlpEncodableWrapper, RlpDecodableWrapper)]

struct StructWithSingleField<T> {
    field: T
}

impl<T: Encodable> Encodable for StructWithSingleField<T> {
    fn rlp_append(&self, s: &mut RlpStream) {
        self.field.rlp_append(s);
    }
}

can be simplified to

#[derive(RlpEncodableWrapper)]
struct StructWithSingleField<T> {
    field: T
}

It can be applied to struct StructWithSingleTuple<T>(T), and RlpDecodable too.