Union discriminator does not work with enums of vec

[dadepo]

Actual Behavior

Given this enum

#[derive(Union, Serialize)]
#[oai(discriminator_name = "type")]
pub enum EqExpr {
    #[serde(rename = "@eq")]
    #[oai(mapping = "@eq")]
    Eq(PathInfo),
    #[serde(rename = "@neq")]
    #[oai(mapping = "@neq")]
    NotEq(PathInfo)
}

Serialising with both serde and poem

        let value = EqExpr::Eq(PathInfo {
            path: "age".to_string(),
            value: json!(20),
        });
        // Serialise with serde
        dbg!(serde_json::to_string(&value).unwrap());
        // Prints: "{\"@eq\":{\"path\":\"age\",\"value\":20}}"
        
        // Serialise with poem
        dbg!(&value.to_json_string());
        // Prints "{\"path\":\"age\",\"type\":\"@eq\",\"value\":20}"

        let value = EqExpr::NotEq(PathInfo {
            path: "age".to_string(),
            value: json!(20),
        });
        // Serialise with serde
        dbg!(serde_json::to_string(&value).unwrap());
        // Prints "{\"@neq\":{\"path\":\"age\",\"value\":20}}"
        
        // Serialise with poem
        dbg!(&value.to_json_string());
        // Prints "{\"path\":\"age\",\"type\":\"@neq\",\"value\":20}"

This is good and dandy, poem does not support externally tagged enums, but the "type": "..." acts as the discriminator.

Now if the enum is changed to the following:

#[derive(Union, Serialize)]
#[oai(discriminator_name = "type")]
pub enum EqExpr {
    #[serde(rename = "@eq")]
    #[oai(mapping = "@eq")]
    Eq(Vec<PathInfo>),
    #[serde(rename = "@neq")]
    #[oai(mapping = "@neq")]
    NotEq(Vec<PathInfo>)
}

Then serialised like this

        let value = EqExpr::Eq(vec![PathInfo {
            path: "age".to_string(),
            value: json!(20),
        }]);
        // Serialise with serde
        dbg!(serde_json::to_string(&value).unwrap());
        // Serialise with poem
        dbg!(&value.to_json_string());

        let value = EqExpr::NotEq(vec![PathInfo {
            path: "age".to_string(),
            value: json!(20),
        }]);

        // Serialise with serde
        dbg!(serde_json::to_string(&value).unwrap());
        // Serialise with poem
        dbg!(&value.to_json_string());

it prints:

[crates/src/lib.rs:500] serde_json::to_string(&value).unwrap() = "{\"@eq\":[{\"path\":\"age\",\"value\":20}]}"
[crates/src/lib.rs:502] &value.to_json_string() = "[{\"path\":\"age\",\"value\":20}]"
[crates/src/lib.rs:510] serde_json::to_string(&value).unwrap() = "{\"@neq\":[{\"path\":\"age\",\"value\":20}]}"
[crates/src/lib.rs:512] &value.to_json_string() = "[{\"path\":\"age\",\"value\":20}]"

As can be seen, with serde the externally discrimination of the enums, makes it possible to differentiate between the two enums, but with poem, both prints [{\"path\":\"age\",\"value\":20}].

Expected Behavior

poem should not serialise both of the enum variants into the same json string representation.

Because of this, it is also impossible to deserialise into the rust enums.

Specifications

• Version:

poem = { version = "2.0.0" }
poem-openapi = { version = "4.0.0", features = ["swagger-ui", "uuid"] }

• Platform:

~ uname -a
Darwin DeeMBP2773 23.2.0 Darwin Kernel Version 23.2.0: Wed Nov 15 21:53:18 PST 2023; root:xnu-10002.61.3~2/RELEASE_ARM64_T6000 arm64

[sunli829]

The members in the enumeration must be an Object, and I will update Union macro to recognize this error at compile time.