反序列化可能是字符串数组或常量字符串的值？答案

【问题标题】：Deserialize value that may be an array of strings or a constant string?反序列化可能是字符串数组或常量字符串的值？
【发布时间】：2021-12-28 09:21:36
【问题描述】：

我正在使用 Serde 反序列化一些 JSON。我遇到的问题通常是一个字符串数组，但也可以是常量字符串"all"。用 JSON 模式表示，它看起来像这样：

{
    "oneOf": [
        {
            "const": "all" 
        },
        {
            "type": "array",
            "items": {
                "type": "string"
            }
        }
    ]
}

我想把它序列化到这个枚举中：

enum MyList {
    List(Vec<String>),
    All
}

这里有几个例子：

["foo", "bar"]  // Should be MyList::List
"all"           // Should be MyList::All
"foo"           // Should be an error!

问题是，我如何使用 serde 反序列化这个枚举？ container attributes 或 variant attributes 似乎都没有帮助。如果需要，我可能会更改枚举的设计。但是，JSON 的结构超出了我的控制范围。

【问题讨论】：

标签： rust enums deserialization serde

【解决方案1】：

untagged enum representation 和 deserialize_with variant attribute 的组合是可能的：

use serde::{Deserialize, Deserializer};

#[derive(Debug, Deserialize)]
// This annotation "flattens" the enum, allowing one to treat a list of strings
// directly as a List variant, without extra annotations.
// Serde will attempt to deserialize input as each variant in order,
// returning an error if no one matches.
#[serde(untagged)]
enum MyList {
    // This annotation delegates deserialization of this variant to own code,
    // since otherwise Serde wouldn't know what to do with the string.
    #[serde(deserialize_with = "all")]
    All,
    // This variant is deserialized as usual.
    List(Vec<String>),
}

// A custom deserialization function, referred to by `deserialize_with`.
// (reference: https://github.com/serde-rs/serde/issues/1158)
fn all<'de, D>(deserializer: D) -> Result<(), D::Error>
where
    D: Deserializer<'de>,
{
    #[derive(Deserialize)]
    // This enum is, by default, "externally tagged";
    // but, since it consists only of a single unit variant,
    // it means that it can be deserialized only from
    // the corresponding constant string - and that's exactly what we need
    enum Helper {
        #[serde(rename = "all")]
        Variant,
    }
    // We're not interested in the deserialized value (we know what it is),
    // so we can simply map it to (), as required by signature
    Helper::deserialize(deserializer).map(|_| ())
}

fn main() {
    // Trying to parse an array with both variants...
    let json = r#"["all", ["a", "b", "c"]]"#;
    let lists: Vec<MyList> = serde_json::from_str(json).expect("cannot parse");
    // ...we can see that it is indeed parsed correctly...
    println!("{:?}", lists);

    // ...and any unexpected string results in an error
    serde_json::from_str::<MyList>(r#""foo""#).unwrap_err();
}

Playground

【讨论】：