Inconsistency in adjacently tagged enums

[baloo]

When using with=/deserialize_with=, it appears serde expects adjacently tagged enums to be in the following form:

use serde::Deserialize;

mod bar {
    use serde::de::Deserializer;

    use super::Foo;

    pub fn deserialize<'de, D>(_de: D) -> Result<Foo, D::Error>
    where
        D: Deserializer<'de>,
    {
        todo!()
    }
}

struct Foo;

#[derive(Deserialize)]
#[serde(tag="type", content="foo")]
enum Bar {
    #[serde(deserialize_with = "bar::deserialize")]
    Baz(Foo),
}

And that when the meta is placed within the variant, you get an error:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=72a2138a26f9d31c419d808261245156

error[E0277]: the trait bound `Foo: serde::Deserialize<'de>` is not satisfied

Meanwhile, serde-with appears to expect the meta next to the field of the variant:

use serde::{Deserialize, Serialize};
use serde_with::serde_as;

#[serde_as]
#[derive(Serialize, Deserialize)]
#[serde(tag = "type", content = "content")]
enum Foo {
    #[serde_as(as = "Base64")]
    Bar(Vec<u8>),
}

error: serde_as attribute is not allowed on enum variants
  --> src/lib.rs:10:5
   |
10 |     #[serde_as(as = "Base64")]
   |     ^

Using:

enum Foo {
    #[serde(with = "serde_with::As::<Base64>")]
    Bar(Vec<u8>),
}

appears to work around the immediate issue.

I'm not sure whether that is a bug in serde itself, or in serde-with, but I thought you might have an opinion!

[jonasbb]

Hi, thanks for the report. I wasn't aware of this limitation in the serde derives. I intentionally didn't add support for serde_as on variants, since I assumed that to be unnecessary. In most cases, putting the attributes within the variant, on the Foo, works fine. It just seems to be this newtype variant pattern that is an issue.

#[serde(tag="type")] works fine. No serde(tag=...) works fine. Extending the variant to a full tuple works fine Baz(#[serde(with = "bar")] Foo, usize),.

To fix this the serde_with_macros must be adjusted there. Where this error error: serde_as attribute is not allowed on enum variants is thrown, needs to be changed to emit the correct attribute