serde_as does not process enum variant attributes

[ljoonal]

In my attempt to update to serde_with@2.0.0 I noticed that nested JSON support is gone, with a changelog stating that:

json::nested can be replaced with #[serde_as(as = "json::JsonString")].

It doesn't seem quite as simple as that though, as I can't figure out a way to achieve what worked with json::nested using JsonString.

Minimal example which contains example JSON files and tests: https://github.com/ljoonal/serde_with_nested_json

running 4 tests
test enum_nested_json ... ok
test struct_json_string ... ok
test enum_json_string ... FAILED
test struct_nested_json ... ok

failures:

---- enum_json_string stdout ----
thread 'enum_json_string' panicked at 'called Result::unwrap() on an Err value: Error("invalid type: string "{\"id\":\"R-5c5d3057-bb97-4c31-a2e0-26ea055b2bcf\"}", expected struct MessagePayload", line: 6, column: 2)', src/lib.rs:32:70

My expectation is that JsonString should work just like json:nested did, but instead it fails for a reason that is beyond my debugging skills. But I've at least narrowed it down to working with the structs but breaking with the enums in the linked example repo.

As to why support for this would be nice, a real world usecase is de-serializing an external JSON API's responses that contain nested json of different types, but also sometimes just plain strings.

[jonasbb]

I think I identified the issue. The problem is that the serde_as macro does not recognize the attribute in this position. Currently, I think it only supports attributes on fields, but not on enum variants.
Since the attribute is not recognized, it will not be translated into something serde understands.

    #[serde_with::serde_as]
    #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
    #[serde(tag = "messageType", content = "content")]
    /// The contents of a message combined with the `MessageType`
    pub enum MessageContentJsonStringEnum {
        /// A normal message
        Text(String),
        /// Fancy object message
        #[serde_as(as = "serde_with::json::JsonString")]
        Object(crate::MessagePayload),
    }

Instead, you can write this.

/// Fancy object message
Object(
	  #[serde_as(as = "serde_with::json::JsonString")]
	  crate::MessagePayload
),

It might be worthwhile to extend the macro to support variant attributes too.

[ljoonal]

Instead, you can write this.
...

Ah that makes sense, than you, that solves the main issue I had 👍

It might be worthwhile to extend the macro to support variant attributes too.

If that basically achieves the same result, a simpler alternative might just be to add an example of the attribute being used "within" the enum definitions to the docs? :)

[jonasbb]

I decided not to support serde_as on enum variants for now. But instead, an error will be issued when this happens. Your idea of showing an example how to use it with enums is good. So I changed one of the examples in the README to use an enum.
The updated example is in #502.