向反序列化的 serde 结构添加默认字符串值失败，特征 Deserialize<'_> 未实现答案

【问题标题】：Adding a default String value to a deserialized serde struct fails with trait Deserialize<'_> not implemented向反序列化的 serde 结构添加默认字符串值失败，特征 Deserialize<'_> 未实现
【发布时间】：2021-11-23 08:35:11
【问题描述】：

我有以下从 XML 文件反序列化的结构。

#[derive(Deserialize, Debug)]
pub struct RawItem {
    id: String,
    title: String,
    description: Option<String>,
    link: String,
    image_link: String,
    availability: String,
    price: String,
}

到目前为止一切顺利，对象已正确反序列化。现在，我的 XML 数据略有变化，价格字段变为可选。我想在丢失时将价格回退到“0.0”，因此我使用了 serde 默认机制：

#[derive(Deserialize, Debug)]
pub struct RawItem {
    id: String,
    title: String,
    description: Option<String>,
    link: String,
    image_link: String,
    availability: String,
    #[serde(default = "0.0")] // Added this new line
    price: String,
}

由于某种原因，它失败并出现以下错误：

error: failed to parse path: "0.0"
  --> /home/xxxxxx/xxxxx/xxxxx/xxxxxx/xxxx/xxxxx/src/xxxx_specifics.rs:81:20
   |
81 |     #[serde(default = "0.0")]
   |                       ^^^^^

error[E0277]: the trait bound `xxxx_specifics::RawItem: Deserialize<'_>` is not satisfied
    --> /home/xxxxxx/xxxxx/xxxxx/xxxxxx/xxxx/xxxxx/src/xxxx_specifics.rs:68:2
     |
68   |     #[serde(rename = "item")]
     |     ^ the trait `Deserialize<'_>` is not implemented for `xxxx_specifics::RawItem`
     |
     = note: required because of the requirements on the impl of `Deserialize<'_>` for `Vec<xxxx_specifics::RawItem>`
note: required by `next_element`
    --> /home/xxxxx/serde-1.0.130/src/de/mod.rs:1703:5
     |
1703 | /     fn next_element<T>(&mut self) -> Result<Option<T>, Self::Error>
1704 | |     where
1705 | |         T: Deserialize<'de>,
     | |____________________________^

error[E0277]: the trait bound `xxxx_specifics::RawItem: Deserialize<'_>` is not satisfied
    --> /home/xxxxxx/xxxxx/xxxxx/xxxxxx/xxxx/xxxxx/src/xxxx_specifics.rs:68:2
     |
68   |     #[serde(rename = "item")]
     |     ^ the trait `Deserialize<'_>` is not implemented for `xxxx_specifics::RawItem`
     |
     = note: required because of the requirements on the impl of `Deserialize<'_>` for `Vec<xxxx_specifics::RawItem>`
note: required by `next_value`
    --> /home/xxxx/serde-1.0.130/src/de/mod.rs:1842:5
     |
1842 | /     fn next_value<V>(&mut self) -> Result<V, Self::Error>
1843 | |     where
1844 | |         V: Deserialize<'de>,
     | |____________________________^

For more information about this error, try `rustc --explain E0277`.
error: could not compile `inko_importer` due to 3 previous errors

serde 默认的任何原因都会因字符串上的简单默认值而失败？

【问题讨论】：

请注意，实际错误包含在错误消息的最第一行中：failed to parse path: "0.0"，其中“path”指的是由标识符组成的路径，这些标识符导致函数，如module::submodule::function_name。其余的只是意味着宏没有完成它的工作，因此，Deserialize 特征没有在 RawItem 上实现，而你的代码的其余部分期望它是这样的。

标签： rust serde

【解决方案1】：

default 属性要求调用一个方法（实际上是一个带有方法路径的字符串）：

use serde::Deserialize;

#[derive(Deserialize, Debug)]
pub struct RawItem {
    #[serde(default = "default_0")] // Added this new line
    price: String
}

fn default_0() -> String {
    "0.0".to_string()
}

Playground

【讨论】：