Serialize/deserialize tagged newtype variant containing unit

[tage64]

To me, it seems that a tagged newtype variant containing () should be serializable/deserializable in the same way as unit variants. Specificly I have the following enum:

#[derive(Serialize, Deserialize, Debug)]
#[serde(tag = "result", rename_all = "snake_case")]
enum Response<T> {
    Success(T),
    Error {
        ...
    },
}

I'd expect Success(()) to pe deserialized from and serialized to { "result": "success"}, but that doesn't seem to be the case. It doesn't work to deserialize it and when I try to serialize it (with serde_json) I get the following error:

Error("cannot serialize tagged newtype variant Response::Success containing unit", line: 0, column: 0)

Is there any reason why this doesn't work, or should we fix this? I'd would be happy to make a PR.

[dtolnay]

It would be fine to make this work I think.

[tage64]

The PR is: #2303