Internally tagged enums duplicate tag when struct also has it assigned

[kossnocorp]

Update: I've found a StackOverflow anwer that helped me to come up with a solution. While it works, I think my proposal is still relevant. It will be great to automatically detect that structs are tagged or at least update the documentation. I'm willing to do either. Cheers!

---

If enums and structs all have tags assigned, serde_json ends up serializing it twice:

{"type":"post","type":"post","text":"post text"}

Here's the code:

extern crate serde;
extern crate serde_json;

use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(tag = "type")]
enum Content {
    #[serde(rename = "post")]
    Post(Post),
    #[serde(rename = "comment")]
    Comment(Comment)
}

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(tag = "type", rename = "post")]
struct Post {
    text: String
}

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(tag = "type", rename = "comment")]
struct Comment {
    text: String
}

fn main() {
    let post = Post { text: "post text".to_owned() };
    println!("Serialized post:             {}", serde_json::to_string(&post).unwrap());

    let post = Content::Post(post);
    println!("Serialized post via enum:    {}", serde_json::to_string(&post).unwrap());

    let post_str = r#"{"type": "post", "text": "post text"}"#;
    let post : Content = serde_json::from_str(post_str).expect("Failed to parse post");
    println!("Parsed post:                 {:?}", post);

    println!("---");

    let comment = Comment { text: "comment text".to_owned() };
    println!("Serialized comment:          {}", serde_json::to_string(&comment).unwrap());

    let comment = Content::Comment(comment);
    println!("Serialized comment via enum: {}", serde_json::to_string(&comment).unwrap());

    let comment_str = r#"{"type": "comment", "text": "comment text"}"#;
    let comment : Content = serde_json::from_str(comment_str).expect("Failed to parse comment");
    println!("Parsed comment:              {:?}", comment);
}

Output:

Serialized post:             {"type":"post","text":"post text"}
Serialized post via enum:    {"type":"post","type":"post","text":"post text"}
Parsed post:                 Post(Post { text: "post text" })
---
Serialized comment:          {"type":"comment","text":"comment text"}
Serialized comment via enum: {"type":"comment","type":"comment","text":"comment text"}
Parsed comment:              Comment(Comment { text: "comment text" })

Playground link

I expect that serde_json (or serde?) would recognize that structs have tags assigned and use them, allowing code like this:

#[derive(Serialize, Deserialize, Debug, Clone)]
enum Content {
    Post(Post),
    Comment(Comment)
}

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(tag = "type", rename = "post")]
struct Post {
    text: String
}

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(tag = "type", rename = "comment")]
struct Comment {
    text: String
}

However, it fails with:

thread 'main' panicked at src/main.rs:32:57:
Failed to parse post: Error("unknown variant `type`, expected `Post` or `Comment`", line: 1, column: 7)

I need to assign tags to structs, as it can be parsed or serialized as a standalone structure or part of an enum, so I must ensure type is always present. This is what I get when I don't add it:

Serialized post:             {"text":"post text"}
Serialized post via enum:    {"type":"post","text":"post text"}
Parsed post:                 Post(Post { text: "post text" })
---
Serialized comment:          {"text":"comment text"}
Serialized comment via enum: {"type":"comment","text":"comment text"}
Parsed comment:              Comment(Comment { text: "comment text" })

You can see that "Serialized post" and "Serialized comment" don't have type.

Is there a way to work around it?

Would a contribution with a fix be welcome?

I didn't look at the code, so maybe it's impossible, but I think consuming struct's tags if there's no tag assigned to the enum would be a nice addition.

Alternatively, deduplicating type can be a solution, albeit not very correct.

[adamchalmers]

Here's another playground showing the issue, with comments explaining it.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1175677900a32540ade949251a701b31

Note that serde_json serializes JSON which it cannot deserialize. In my opinion, "to_string and from_str should always be compatible" is an important property of any derived Serialize/Deserialize impl.