[Bug] Postcard does not handle serde attribute skip_serializing_if properly

[tugtugtug]

#[test]
fn test_serialize_flow_context() {
    #[derive(PartialEq, Debug, Default, Deserialize, Serialize, Clone)]
    pub struct Test {
        /// The id of the policy the lookup matched on
        #[serde(default, rename = "pid", skip_serializing_if = "Option::is_none")]
        pub policy_id: Option<String>,
        /// The name of the policy the lookup matched on
        #[serde(default, rename = "pname", skip_serializing_if = "Option::is_none")]
        pub policy_name: Option<String>,
    }
    let binary = postcard::to_allocvec(&Test::default()).unwrap();
    let _: Test = postcard::from_bytes(binary.as_ref()).unwrap();
}

This fails, and if I remove the skip_serializing_if attributes from the fields, then it works.

[jamesmunns]

In general, postcard does not handle many of serde's attributes that only apply to self describing formats. See also #29.

Docs PRs welcome that make this more clear.

[tugtugtug]

@jamesmunns thanks for your comments, and I kinda agree that postcard may not support all attributes, that's fine. But is there anything postcard can do to ensure the consistency of the behaviour between its own serialization and the deserialization?

[elrafoon]

I just hit the same problem.

And it's much worse if the same type needs to be serializes/deserialized with postcard and some other serializer - json, cbor, ...

Then it may be not possible to just adjust serde attributes to the postcard's taste, because it would break formatting for other serializer (that i.e. must fullfil some json schema).

[jamesmunns]

@tugtugtug:

But is there anything postcard can do to ensure the consistency of the behaviour between its own serialization and the deserialization

I am not aware of any way to statically "notice" that round-trips will fail if you use any of the relevant serde attributes, nor am I aware of any way of preventing it in the first place.

If you find a way, I am very interested!

[jamesmunns]

I've opened https://github.com/jamesmunns/postcard/issues/125 to track this, please feel free to discuss there if you have potential solutions or ideas.