如何使用Serde反序列化带有来自读取器的引用的结构？ [英] How do I use Serde to deserialize structs with references from a reader?

问题描述

我有这些结构：

#[derive(Debug, Serialize, Deserialize)]
pub struct GGConf<'a> {
    #[serde(alias = "ssh")]
    #[serde(rename = "ssh")]
    #[serde(default)]
    #[serde(borrow)]
    pub ssh_config: Option<SSHConfig<'a>>,
}

#[derive(Debug, Serialize, Deserialize)]
pub struct SSHConfig<'a> {
    #[serde(alias = "privateKey")]
    #[serde(rename = "privateKey")]
    private_key: &'a str,

    username: &'a str,
}

读取YAML文件时发生反序列化：

let mut config: GGConf = serde_yaml::from_reader(file)?;

编译时，我收到错误：

error: implementation of `conf::_IMPL_DESERIALIZE_FOR_GGConf::_serde::Deserialize` is not general enough
   --> src/conf.rs:50:34
    |
50  |           let mut config: GGConf = serde_yaml::from_reader(file)?;
    |                                    ^^^^^^^^^^^^^^^^^^^^^^^ implementation of `conf::_IMPL_DESERIALIZE_FOR_GGConf::_serde::Deserialize` is not general enough
    |
   ::: /home/ninan/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.98/src/de/mod.rs:524:1
    |
524 | / pub trait Deserialize<'de>: Sized {
525 | |     /// Deserialize this value from the given Serde deserializer.
526 | |     ///
527 | |     /// See the [Implementing `Deserialize`][impl-deserialize] section of the
...   |
562 | |     }
563 | | }
    | |_- trait `conf::_IMPL_DESERIALIZE_FOR_GGConf::_serde::Deserialize` defined here
    |
    = note: `conf::GGConf<'_>` must implement `conf::_IMPL_DESERIALIZE_FOR_GGConf::_serde::Deserialize<'0>`, for any lifetime `'0`...
    = note: ...but `conf::GGConf<'_>` actually implements `conf::_IMPL_DESERIALIZE_FOR_GGConf::_serde::Deserialize<'1>`, for some specific lifetime `'1`

我隐约知道serde反序列化也有生存期'de，编译器会混淆我为它指定的生存期吗？如果我说错了，请纠正我。

当前如何正确地将YAML反序列化为这两个结构？ 我是不是遗漏了什么或误解了什么？

我查看了How do I resolve "implementation of serde::Deserialize is not general enough" with actix-web's Json type?，但我不能使用拥有的类型。我需要它是借来的类型。

我将尝试为此编写一个操场示例。

推荐答案

这是不可能的；您必须使用拥有的数据而不是引用。

这里有一个最小的示例：

use serde::Deserialize; // 1.0.104

#[derive(Debug, Deserialize)]
pub struct SshConfig<'a> {
    username: &'a str,
}

fn example(file: impl std::io::Read) {
    serde_yaml::from_reader::<_, SshConfig>(file);
}

error: implementation of `_IMPL_DESERIALIZE_FOR_SshConfig::_serde::Deserialize` is not general enough
   --> src/lib.rs:9:5
    |
9   |       serde_yaml::from_reader::<_, SshConfig>(file);
    |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `_IMPL_DESERIALIZE_FOR_SshConfig::_serde::Deserialize` is not general enough
    | 
   ::: /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/serde-1.0.104/src/de/mod.rs:531:1
    |
531 | / pub trait Deserialize<'de>: Sized {
532 | |     /// Deserialize this value from the given Serde deserializer.
533 | |     ///
534 | |     /// See the [Implementing `Deserialize`][impl-deserialize] section of the
...   |
569 | |     }
570 | | }
    | |_- trait `_IMPL_DESERIALIZE_FOR_SshConfig::_serde::Deserialize` defined here
    |
    = note: `SshConfig<'_>` must implement `_IMPL_DESERIALIZE_FOR_SshConfig::_serde::Deserialize<'0>`, for any lifetime `'0`...
    = note: ...but `SshConfig<'_>` actually implements `_IMPL_DESERIALIZE_FOR_SshConfig::_serde::Deserialize<'1>`, for some specific lifetime `'1`

如果您查看serde_yaml::from_reader的定义，您会发现它仅限于反序列化所拥有的数据：

pub fn from_reader<R, T>(rdr: R) -> Result<T>
where
    R: Read,
    T: DeserializeOwned,
//     ^^^^^^^^^^^^^^^^ 

serde_json::from_reader和可能任何等效函数都是如此。

只有在有要引用的数据时才能反序列化包含引用的类型。实现Read特征的东西只能保证它可以将一些字节复制到用户提供的缓冲区中。由于from_reader函数不接受该缓冲区作为参数，因此任何缓冲区都将在from_reader退出时被销毁，从而使引用无效。

另请参阅：

• How do I resolve "implementation of serde::Deserialize is not general enough" with actix-web's Json type?

---

如果必须使用引用(在许多情况下并非如此)，则需要：

1. 自己从读取器读取到缓冲区
2. 使用from_str而不是from_reader
3. 保持缓冲区与反序列化数据一样长

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