Serde default with only Deserialize wont compile

[ThouCheese]

Hi! I have a bit of a weird issue, when the following two conditions are satisfied:

1. Some struct Outer has a field annotated with #[serde(default)] or #[serde(default = "some_fn")]
2. and that field is of a type Inner, which implements serde::Deserialize and schemars::JsonSchema, but not serde::Serialize.

Then the derivation of schemars::JsonSchema on the struct fails with the following message:

error[E0277]: the trait bound `Inner: Serialize` is not satisfied
   --> src/main.rs:3:30
    |
3   | #[derive(serde::Deserialize, schemars::JsonSchema)]
    |                              ^^^^^^^^^^^^^^^^^^^^ the trait `Serialize` is not implemented for `Inner`
    | 
   ::: /home/luuk.local/.cargo/registry/src/github.com-1ecc6299db9ec823/serde_json-1.0.68/src/value/mod.rs:940:8
    |
940 |     T: Serialize,
    |        --------- required by this bound in `to_value`
    |
    = note: required because of the requirements on the impl of `Serialize` for `std::vec::Vec<Inner>`
    = note: this error originates in the derive macro `schemars::JsonSchema` (in Nightly builds, run with -Z macro-backtrace for more info)

Minimal example

#[derive(serde::Deserialize, schemars::JsonSchema)]
struct Outer {
    #[serde(default)]
    field: Vec<Inner>,
}

#[derive(serde::Deserialize, schemars::JsonSchema)]
struct Inner {
    a: String,
}

[ede1998]

I ran into the same issue. You can work around it by adding a skip_serializing annotation:

#[derive(serde::Deserialize, schemars::JsonSchema)]
struct Outer {
    #[serde(default)]
    #[schemars(skip_serializing)]
    field: Vec<Inner>,
}

#[derive(serde::Deserialize, schemars::JsonSchema)]
struct Inner {
    a: String,
}

At least that's what did the trick for me.

[GREsau]

Thanks for the report, this should be fixed in 0.8.7
https://crates.io/crates/schemars/0.8.7

[ModProg]

I ran into this issue when using # [schemars(with = "SomeTypeWithOnlyDeserialize")] the issue is, when I add #[schemars(skip_serializing)] it makes the field required even when marked as #[serde(default)]