Extra fields treated as belonging to flattened unit struct rather than being ignored

[ss2165]

serde_json example, but have checked the same problem occurs with serde_yaml:

    #[derive(Serialize, Deserialize)]
    struct Unit;

    #[derive(Serialize, Deserialize)]
    enum UnitOrNot {
        Unit(Unit),
        NotUnit { field: String },
    }
    #[derive(Serialize, Deserialize)]
    struct Nested {
        #[serde(flatten)]
        inner: UnitOrNot,
    }

    let data = r#"
    [
        {
            "NotUnit": {
                "field": "value",
                "ignored": "bar"
            }
        },
        {
            "Unit": {
                "ignored": "foo"
            }
        }
    ]"#;

    let _: Vec<Nested> = serde_json::from_str(data).unwrap();

The first element of the array successfully deserializes and ignores the extra "ignored" field. The second fails with:

Error: Error("invalid type: map, expected unit struct Unit", line: 11, column: 13)

(i.e. the extra field is now treated as belonging to the unit struct).

The code succeeds if you change the definition of Unit to

struct Unit {}