How to enforce a single enum variant with flatten

[dbanty]

At the moment, I can't find a way to enforce that only a single variant of an enum is set for a field when using #[serde(flatten)]. I would expect that if I had a setup like this:

#[derive(Debug, Deserialize)]
enum MultipleOptions {
    FirstThing(HashMap<String, String>),
    SecondThing(HashMap<String, String>),
}

#[derive(Debug, Deserialize)]
struct AThing {
    #[serde(flatten)]
    multiple_options: MultipleOptions
}

Then JSON like this:

{"FirstThing": {}, "SecondThing": {}}

Would be an error when deserializing AThing (as it is when deserializing MultipleOptions). I assume this is because it's treating the second variant it encounters like an unknown field.

Here's a playground that demonstrates this fully

[Mingun]

Adding #[serde(deny_unknown_fields)] to AThing solves this problem:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=4088008fef23f3d7b4eca16aebca5e15