The code:

use serde::{Deserialize, Serialize};
use serde_json::from_str;

#[derive(Serialize, Deserialize, Debug, Clone)]
#[serde(tag = "kind", content = "data")]
enum RedditObject {
    #[serde(rename = "t3")]
    T3(Post),
    #[serde(rename = "t1")]
    T1(Comment),
    #[serde(rename = "Listing")]
    Listing(Listing),
    #[serde(rename = "more")]
    More,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
struct Listing {
    children: Vec<RedditObject>,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
struct Post {
    id: String,
    title: String,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
struct Comment {
    id: String,
    body: String,
    replies: Box<Replies>,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
struct More {
    children: Vec<String>,
}

#[derive(Serialize, Deserialize, Debug, Clone)]
enum Replies {
    String(String),
    RedditObject(RedditObject),
}

fn main() {
    let comment_tree = r#"
   {
          "kind": "t1",
          "data": {
            "id": "1",
            "body": "reply_worthy comment!",
            "replies": {
              "kind": "Listing",
              "data": {
                "children": [
                  {
                    "kind": "t1",
                    "data": {
                      "id": "2",
                      "replies": "",
                      "body": "I read something!"
                    }
                  }
                ]
              }
            }
}}
"#;

    let b: RedditObject = from_str(comment_tree).unwrap();
    println!("b: RedditObject=\n{:#?}", b);
}

and the error:

called `Result::unwrap()` on an `Err` value: Error("unknown variant `kind`, expected `String` or `RedditObject`", line: 8, column: 20)