How can I store enums in "adjacently-tagged" style?

[inklesspen]

Serde offers the adjacently-tagged representation for enums. As an example:

#[derive(Debug, Serialize, Deserialize)]
#[serde(tag = "t", content = "c")]
pub enum ThingKind {
    Animal { friendly: bool, name: String },
    Vegetable,
    Mineral { hardness: u8 },
}

let dog = ThingKind::Animal {
    friendly: true,
    name: "Rover".to_string(),
};
let cabbage = ThingKind::Vegetable;
let diamond = ThingKind::Mineral { hardness: 10 };

serde-json serializes these three instances as:

{"t":"Animal","c":{"friendly":true,"name":"Rover"}}
{"t":"Vegetable"}
{"t":"Mineral","c":{"hardness":10}}

What I would like to do is to split off the tag and content: that is, I would like to store an enum like this as a pair of varchar and jsonb columns. (Let's say that the majority of the time, I only care about whether something is an animal, vegetable, or mineral, and not what the thing's friendliness or hardness may be. But sometimes I do need the full enum variant.)

pub struct Agent {
    username: String,
    favorite_thing: ThingKind,
}

CREATE TABLE agents (
    username VARCHAR PRIMARY KEY,
    favorite_thing VARCHAR NOT NULL,
    favorite_thing_payload JSONB
);
INSERT INTO agents (username, favorite_thing, favorite_thing_payload)
    VALUES ('alice', 'Animal', '{"friendly":true,"name":"Rover"}'::jsonb);
INSERT INTO agents (username, favorite_thing, favorite_thing_payload)
    VALUES ('bob', 'Vegetable', NULL);
INSERT INTO agents (username, favorite_thing, favorite_thing_payload)
    VALUES ('carol', 'Mineral', '{"hardness":10}'::jsonb);

I know that I could query this using an intermediate struct that has separate fields for the enum variant name and the "payload", and then I could use serde_json::Value to put those fields together and deserialize them into a ThingKind instance. But that is a lot of stuff to do by hand.

(Note that I want to use the query macros, so the FromRow trait is not going to be helpful here at all.)