postcard-schema: handling unsupported serde attributes

[Calsign]

There are a bunch of serde attributes that are unsupported by postcard, and supporting some attributes is impossible: #125

There are some attributes that are supported by postcard, but break postcard-schema/postcard-dyn, such as #[serde(skip)] on struct fields. Consider this example:

use postcard_schema::{Schema, schema::owned::OwnedNamedType};
use serde::{Deserialize, Serialize};

#[derive(Serialize, Deserialize, Schema)]
struct Foobar {
    x: u32,
    #[serde(skip)]
    y: u32,
}

fn main() {
    let foobar = Foobar { x: 1, y: 2 };

    let encoded = postcard::to_stdvec(&foobar).unwrap();
    let schema = OwnedNamedType::from(Foobar::SCHEMA);

    // this works
    let _decoded: Foobar = postcard::from_bytes(&encoded).unwrap();

    // this fails
    let _json_dyn = postcard_dyn::from_slice_dyn(&schema, &encoded).unwrap();
}

Output:

thread 'main' panicked at src/bin/ex3.rs:21:69:
called `Result::unwrap()` on an `Err` value: UnexpectedEndOfData

I don't expect postcard-schema to handle all of these attributes correctly. But it occurs to me that unlike postcard, postcard-schema has the ability to have the derive macro fail on unsupported schemas.

Currently the only attribute that postcard-schema supports is #[serde(rename)]. Is it reasonable for #[derive(Schema)] to fail for all other serde attributes? (Possibly with opt-in to ignore?) If more attributes get supported in the future then those could be added without breaking backwards-compatibility.

I desire this because I would like to have confidence that if I derive Schema, the derived schema will actually work correctly.