Allow flattened untagged Options in struct fields

[alberdingk-thijm]

I have a small use case for #[serde(flatten)] that did not work as I expected due to the intermediary presence of an Option type. Here's what I mean:

#[derive(Serialize, Deserialize)]
enum Quux {
    A,
    B,
    C,
}

#[derive(Serialize, Deserialize)]
struct Bar {
    quux: Quux,
}

#[derive(Serialize, Deserialize)]
struct Foo {
    baz: i32,
    #[serde(flatten)]
    bar: Option<Bar>,
}

fn main() {
    let f : Foo = serde_json::from_str(r#"{
        "baz": 10,
        "quux": "A"
    }"#).unwrap();  // panics unwrapping 'Error("can only flatten structs and maps", line: 4, column: 5)'
}

I'm able to get around this problem by using my own untagged Option type.

#[derive(Serialize, Deserialize)]
enum Quux {
    A,
    B,
    C,
}

#[derive(Serialize, Deserialize)]
#[serde(untagged)]
enum OptBar {
    Some(Bar),
    None,
}

#[derive(Serialize, Deserialize)]
struct Bar {
    quux: Quux,
}

#[derive(Serialize, Deserialize)]
struct Foo {
    baz: i32,
    #[serde(flatten)]
    bar: OptBar,
}

fn main() {
    let f : Foo = serde_json::from_str(r#"{
        "n": 10,
        "quux": "A"
    }"#).unwrap();  // returns a 'Foo { n: 10, bar: Some(Bar { quux: A }) }'
}

Is there any way to get around this otherwise? Continuing to use the standard Option type allows some more straightforward checking of Foo.bar, which is quite useful. Worst case scenario I can change my data representation.

[dtolnay]

Thanks, I released Serde 1.0.54 with a fix. Your code should work as written.