Add a test of serde(bound = "") attribute

Without serde(bound = ""), serde_derive infers a bound of `T: Serialize`
for the generated Serialize impl and `T: Deserialize<'de> + Default` for
the Deserialize impl. `X` implements none of these so the generated code
would fail to compile.

    error[E0277]: the trait bound `X: Serialize` is not satisfied
       --> test_suite/tests/test_gen.rs:268:14
        |
    268 |     assert::<PhantomDataWrapper<X>>();
        |              ^^^^^^^^^^^^^^^^^^^^^ the trait `Serialize` is not implemented for `X`
        |
        = help: the following other types implement trait `Serialize`:
                  &'a T
                  &'a mut T
                  ()
                  (T0, T1)
                  (T0, T1, T2)
                  (T0, T1, T2, T3)
                  (T0, T1, T2, T3, T4)
                  (T0, T1, T2, T3, T4, T5)
                and 248 others
    note: required for `PhantomDataWrapper<X>` to implement `Serialize`
       --> test_suite/tests/test_gen.rs:262:14
        |
    262 |     #[derive(Serialize, Deserialize)]
        |              ^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
    263 |     //#[serde(bound = "")]
    264 |     struct PhantomDataWrapper<T> {
        |            ^^^^^^^^^^^^^^^^^^^^^
    note: required by a bound in `assert`
       --> test_suite/tests/test_gen.rs:767:14
        |
    767 | fn assert<T: Serialize + DeserializeOwned>() {}
        |              ^^^^^^^^^ required by this bound in `assert`
        = note: this error originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0277]: the trait bound `X: Deserialize<'_>` is not satisfied
       --> test_suite/tests/test_gen.rs:268:14
        |
    268 |     assert::<PhantomDataWrapper<X>>();
        |              ^^^^^^^^^^^^^^^^^^^^^ the trait `Deserialize<'_>` is not implemented for `X`
        |
        = help: the following other types implement trait `Deserialize<'de>`:
                  <&'a Path as Deserialize<'de>>
                  <&'a [u8] as Deserialize<'de>>
                  <&'a str as Deserialize<'de>>
                  <() as Deserialize<'de>>
                  <(T0, T1) as Deserialize<'de>>
                  <(T0, T1, T2) as Deserialize<'de>>
                  <(T0, T1, T2, T3) as Deserialize<'de>>
                  <(T0, T1, T2, T3, T4) as Deserialize<'de>>
                and 331 others
    note: required for `PhantomDataWrapper<X>` to implement `for<'de> Deserialize<'de>`
       --> test_suite/tests/test_gen.rs:262:25
        |
    262 |     #[derive(Serialize, Deserialize)]
        |                         ^^^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
    263 |     //#[serde(bound = "")]
    264 |     struct PhantomDataWrapper<T> {
        |            ^^^^^^^^^^^^^^^^^^^^^
        = note: required for `PhantomDataWrapper<X>` to implement `DeserializeOwned`
    note: required by a bound in `assert`
       --> test_suite/tests/test_gen.rs:767:26
        |
    767 | fn assert<T: Serialize + DeserializeOwned>() {}
        |                          ^^^^^^^^^^^^^^^^ required by this bound in `assert`
        = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)

    error[E0277]: the trait bound `X: Default` is not satisfied
       --> test_suite/tests/test_gen.rs:268:14
        |
    268 |     assert::<PhantomDataWrapper<X>>();
        |              ^^^^^^^^^^^^^^^^^^^^^ the trait `Default` is not implemented for `X`
        |
    note: required for `PhantomDataWrapper<X>` to implement `for<'de> Deserialize<'de>`
       --> test_suite/tests/test_gen.rs:262:25
        |
    262 |     #[derive(Serialize, Deserialize)]
        |                         ^^^^^^^^^^^ unsatisfied trait bound introduced in this `derive` macro
    263 |     //#[serde(bound = "")]
    264 |     struct PhantomDataWrapper<T> {
        |            ^^^^^^^^^^^^^^^^^^^^^
        = note: required for `PhantomDataWrapper<X>` to implement `DeserializeOwned`
    note: required by a bound in `assert`
       --> test_suite/tests/test_gen.rs:767:26
        |
    767 | fn assert<T: Serialize + DeserializeOwned>() {}
        |                          ^^^^^^^^^^^^^^^^ required by this bound in `assert`
        = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)
    help: consider annotating `X` with `#[derive(Default)]`
        |
    779 | #[derive(Default)]
        |

test_suite/tests/test_gen.rs

@@ -257,6 +257,16 @@ fn test_gen() {
     }
     assert::<VariantWithTraits2<X, X>>();
 
+    type PhantomDataAlias<T> = PhantomData<T>;
+
+    #[derive(Serialize, Deserialize)]
+    #[serde(bound = "")]
+    struct PhantomDataWrapper<T> {
+        #[serde(default)]
+        field: PhantomDataAlias<T>,
+    }
+    assert::<PhantomDataWrapper<X>>();
+
     #[derive(Serialize, Deserialize)]
     struct CowStr<'a>(Cow<'a, str>);
     assert::<CowStr>();