Optional iterators: `Vec<Nested>: LinkedDataSubject<WithGenerator<rdf_types::generator::Blank>>` not implemented

[Luke-ebbis]

This is a nice macro system! For my usecase, I sometimes have vectors that are missing.

I have adapted the example:

    use linked_data::iref::IriBuf;
    use linked_data::rdf_types::RdfDisplay;
    use rdf_types::iref::Iri;
    use rdf_types::static_iref::iri;


    fn test_ld() -> () {
        #[derive(linked_data::Serialize, linked_data::Deserialize)]
        #[ld(prefix("ex" = "http://example.org/"))]
        struct Foo {
            #[ld(id)]
            id: IriBuf,

            #[ld("ex:name")]
            name: String,

            #[ld("ex:email")]
            email: String,

            #[ld("ex:numbers")]
            numbers: Vec<i64>,
            #[ld("ex:maybe")]
            maybe: Option<String>,
            #[ld("ex:alot")]
            alot: Vec<Nested>,
            #[ld("ex:maybealot")] // does not work
            //  #[ld(ignore)] // does  work
            maybe_alot: Option<Vec<Nested>>,
        }

        #[derive(linked_data::Serialize, linked_data::Deserialize)]
        #[ld(prefix("ex" = "http://example.org/"))]
        #[ld(type = "ex:object")]
        struct Nested {
            #[ld("ex:num")]
            m: i64,
        }

        let value = Foo {
            id: iri!("http://example.org/JohnSmith").to_owned(),
            name: "John Smith".to_owned(),
            email: "john.smith@example.org".to_owned(),
            numbers: vec![1, 133],
            maybe: Some("S".into()),
            alot: vec![Nested { m: 10 }],
            maybe_alot: Some(vec![Nested { m: 10 }]),
        };

        let quads = linked_data::to_quads(rdf_types::generator::Blank::new(), &value)
            .expect("RDF serialization failed");

        dbg!(&quads);

        for quad in quads {
            use rdf_types::RdfDisplay;
            println!("{} .", quad.rdf_display())
        }
    }

This does not work,

error[E0277]: the trait bound `Vec<Nested>: LinkedDataSubject<WithGenerator<rdf_types::generator::Blank>>` is not satisfied
   --> src/biordf/Omicsdi.rs:228:79
    |
228 |         let quads = linked_data::to_quads(rdf_types::generator::Blank::new(), &value)
    |                     ---------------------                                     ^^^^^^ the trait `LinkedDataSubject<WithGenerator<rdf_types::generator::Blank>>` is not implemented for `Vec<Nested>`
    |                     |
    |                     required by a bound introduced by this call
    |
    = help: the following other types implement trait `LinkedDataSubject<I, V>`:
              `&'a T` implements `LinkedDataSubject<I, V>`
              `()` implements `LinkedDataSubject<I, V>`
              `AnonymousBinding<'a, T>` implements `LinkedDataSubject<I, V>`
              `AnonymousGraph<T>` implements `LinkedDataSubject<I, V>`
              `BlankIdBuf` implements `LinkedDataSubject<I, V>`
              `Box<T>` implements `LinkedDataSubject<I, V>`
              `DatasetGraphView<'a, D>` implements `LinkedDataSubject<I, V>`
              `Foo` implements `LinkedDataSubject<I_, V_>`
            and 18 others
    = note: required for `std::option::Option<Vec<Nested>>` to implement `LinkedDataPredicateObjects<WithGenerator<rdf_types::generator::Blank>>`
note: required for `Foo` to implement `LinkedData<WithGenerator<rdf_types::generator::Blank>>`
   --> src/biordf/Omicsdi.rs:188:18
    |
188 |         #[derive(linked_data::Serialize, linked_data::Deserialize)]
    |                  ^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
189 |         #[ld(prefix("ex" = "http://example.org/"))]
190 |         struct Foo {
    |                ^^^
note: required by a bound in `to_quads`
   --> /home/sibbe/.cargo/registry/src/index.crates.io-6f17d22bba15001f/linked-data-0.1.2/src/quads.rs:198:15
    |
196 | pub fn to_quads<G: Generator>(
    |        -------- required by a bound in this function
197 |     generator: G,
198 |     value: &impl LinkedData<interpretation::WithGenerator<G>>,
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `to_quads`
    = note: this error originates in the derive macro `linked_data::Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0277]: the trait bound `Vec<Nested>: LinkedDataResource<WithGenerator<rdf_types::generator::Blank>>` is not satisfied
   --> src/biordf/Omicsdi.rs:228:79
    |
228 |         let quads = linked_data::to_quads(rdf_types::generator::Blank::new(), &value)
    |                     ---------------------                                     ^^^^^^ the trait `LinkedDataResource<WithGenerator<rdf_types::generator::Blank>>` is not implemented for `Vec<Nested>`
    |                     |
    |                     required by a bound introduced by this call
    |
    = help: the following other types implement trait `LinkedDataResource<I, V>`:
              `&'a T` implements `LinkedDataResource<I, V>`
              `()` implements `LinkedDataResource<I, V>`
              `AnonymousBinding<'a, T>` implements `LinkedDataResource<I, V>`
              `AnonymousGraph<T>` implements `LinkedDataResource<I, V>`
              `BlankIdBuf` implements `LinkedDataResource<I, V>`
              `Box<T>` implements `LinkedDataResource<I, V>`
              `Foo` implements `LinkedDataResource<I_, V_>`
              `IriBuf` implements `LinkedDataResource<I, V>`
            and 21 others
    = note: required for `std::option::Option<Vec<Nested>>` to implement `LinkedDataPredicateObjects<WithGenerator<rdf_types::generator::Blank>>`
note: required for `Foo` to implement `LinkedData<WithGenerator<rdf_types::generator::Blank>>`
   --> src/biordf/Omicsdi.rs:188:18
    |
188 |         #[derive(linked_data::Serialize, linked_data::Deserialize)]
    |                  ^^^^^^^^^^^^^^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
189 |         #[ld(prefix("ex" = "http://example.org/"))]
190 |         struct Foo {
    |                ^^^
note: required by a bound in `to_quads`
   --> /home/sibbe/.cargo/registry/src/index.crates.io-6f17d22bba15001f/linked-data-0.1.2/src/quads.rs:198:15
    |
196 | pub fn to_quads<G: Generator>(
    |        -------- required by a bound in this function
197 |     generator: G,
198 |     value: &impl LinkedData<interpretation::WithGenerator<G>>,
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `to_quads`
    = note: this error originates in the derive macro `linked_data::Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)

but if I change #[ld(ex:maybealot")] to #[ld(ignore)], it all works fine.

How may I implement LinkedDataSubject<WithGenerator<rdf_types::generator::Blank>>? I am interested to learn more rust, but I don't yet understand macro programming.

If I get this working, I would love to issue a pull request for the documentation and the derived trait.