Flattening an Enum doesn't "consume" the fields

[npmccallum]

Here is a minimal example:

use std::collections::BTreeMap;
use serde::{Serialize, Deserialize};
use serde_json::{Value, from_slice};

#[derive(Serialize, Deserialize)]
#[serde(tag = "fruit")]
enum Fruit {
    #[serde(rename = "grape")]
    Grape {
        seedless: bool,
    },
    
    #[serde(rename = "banana")]
    Banana {
        ripe: bool,
    },
}

#[derive(Serialize, Deserialize)]
struct Basket {
    #[serde(flatten)]
    fruit: Fruit,

    #[serde(flatten)]
    other: BTreeMap<String, Value>
}

fn main() {
    let basket: Basket = from_slice(b"{ \"fruit\": \"banana\", \"ripe\": true, \"bread\": true }").unwrap();
    println!("{:?}", basket.other);
}

Which prints:

{"bread": Bool(true), "fruit": String("banana"), "ripe": Bool(true)}

This behavior differs from flattening other types, such as structs. For example:

use std::collections::BTreeMap;
use serde::{Serialize, Deserialize};
use serde_json::{Value, from_slice};

#[derive(Serialize, Deserialize)]
struct Fruit {
    fruit: String,
    ripe: bool,
}

#[derive(Serialize, Deserialize)]
struct Basket {
    #[serde(flatten)]
    fruit: Fruit,

    #[serde(flatten)]
    other: BTreeMap<String, Value>
}

fn main() {
    let basket: Basket = from_slice(b"{ \"fruit\": \"banana\", \"ripe\": true, \"bread\": true }").unwrap();
    println!("{:?}", basket.other);
}

Which prints:

{"bread": Bool(true)}

[tgross35]

A pretty fatal issue with this is that round trip deserialization->serialization->deserialize does not work. Deserializing creates the key in both the enum and the map. This key appears twice in the serialization (at least using serde_json), then deserializing again fails with a duplicate key error.