Rust和serde使用泛型反序列化

Question

我试图使用泛型从文件反序列化结构以与Swagger生成的API一起使用。所以我已经将这几乎一起工作了，但是我无法从“拥有”指针中解压缩外部Struct对象，正如您在测试中看到的那样。

这可能是错误的策略，但问题是我有各种yaml文件，我想读入并反序列化暗示正确的Struct反序列化为。我不想为每个Struct实现一个“readfile”函数，因为有很多。所以我试图使这个通用的lib工作，它应该反序列化到正确的Struct，并与Swagger API一起使用。

它非常接近工作，但我似乎无法将Outer<ExternalStructA>打包成ExternalStructA。

Owned(ExternalStructA { x: 1, y: 2 })
Owned(ExternalStructB { a: 1, b: 2 })

lib.rs：

#[cfg(test)]
mod tests {
    use crate::generics_yaml_deserializer::Outer;
    use serde::{de, Deserialize, Deserializer, Serialize, Serializer};

    #[derive(Debug, Serialize, Deserialize)]
    pub struct ExternalStructA {
        x: u32,
        y: u32,
    }

    #[derive(Debug, Serialize, Deserialize)]
    pub struct ExternalStructB {
        a: u64,
        b: u64,
    }

    #[test]
    fn deserialize() {
        let a = r#"---
ptr:
  x: 1
  y: 2
     "#;

        let b = r#"---
ptr:
  a: 1
  b: 2
        "#;

        let resulta: Outer<ExternalStructA> = serde_yaml::from_str(a).unwrap();
        assert_eq!(1, resulta.ptr.x); // I can't seem to get into ptr ExternalStructA
        let resultb: Outer<ExternalStructB> = serde_yaml::from_str(b).unwrap();
        assert_eq!(1, resultb.ptr.a); // I can't seem to get into ptr ExternalStructB 
    }
}

mod generics_yaml_deserializer {
    use serde::{de, Deserialize, Deserializer, Serialize, Serializer};
    use std::error::Error;

    // empty holding struct which owns a owned ptr
    #[derive(Deserialize, Debug)]
    pub struct Outer<'a, T: 'a + ?Sized> {
        #[serde(bound(deserialize = "Ptr<'a, T>: Deserialize<'de>"))]
        pub ptr: Ptr<'a, T>,
    }

    #[derive(Debug)]
    pub enum Ptr<'a, T: 'a + ?Sized> {
        Ref(&'a T),
        Owned(Box<T>),
    }

    impl<'de, 'a, T: 'a + ?Sized> Deserialize<'de> for Ptr<'a, T>
    where
        Box<T>: Deserialize<'de>,
    {
        fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
            where
                D: Deserializer<'de>,
        {
            Deserialize::deserialize(deserializer).map(Ptr::Owned)
        }
    }
}

货物依赖：

serde = { version = "1.0", features = ["derive"] }
serde_derive = "1.0"
serde_yaml = "0.7.5"
serde_json = "1.0"

更新：

我获得了部分成功的结果：

let resulta: Outer<ExternalStructA> = serde_yaml::from_str(a).unwrap();
    match resulta.ptr {
        Ptr::Owned(e) => {assert_eq!(1, e.x);},
        Ptr::Ref(e) => {println!("error")},
        Ptr::Owned(_) => {println!("error")}
    };
}

但是当我尝试使用泛型类型将其实现为函数时，我会遇到很多错误，主要是：

the trait `for<'de> tests::_IMPL_DESERIALIZE_FOR_ExternalStructA::_serde::Deserialize<'de>` is not implemented for `T`

非工作代码添加到mod generics_yaml_deserializer

fn readfile<T>(filename: String) -> Result<Box<T>, Box<std::error::Error>> {
    let f = std::fs::File::open(filename)?;
    let config_data: Outer<T> = serde_yaml::from_reader(f)?;
    Ok(Box::new(config_data))
}

fn readconfig<T>(filename: String) -> Result<Box<T>, &'static str> {
    // read the config file
    let config_data = readfile(filename);
    match config_data {
        Ok(e) => {
            Ok(Box::new(e))
        },
        Err(_) => {
            Err("nadda")
        }
    }
}

Answer 1

在这里使用像T这样的类型参数：

fn readfile<T>(filename: String) -> Result<Box<T>, Box<std::error::Error>>;

T的具体类型由来电者决定。编译器不只是查看所有可用类型，并猜测有意义的内容。

首先，你需要告诉编译器，这里传递的任何T都会有效。这意味着在兼容的生命周期内将T限制为可反序列化的东西：

// filename should be &str here
fn readfile<'a, T: ?Sized>(filename: &str) -> Result<Box<Outer<'a, T>>, Box<std::error::Error>>
where
    for<'de> T: Deserialize<'de> + 'a
{
    let f = std::fs::File::open(filename)?;
    let config_data: Outer<T> = serde_yaml::from_reader(f)?;
    Ok(Box::new(config_data))
}

// filename should be &str here
fn readconfig<'a, T: ?Sized>(filename: &str) -> Result<Box<Outer<'a, T>>, &'static str>
where
    for<'de> T: Deserialize<'de> + 'a
{
    // read the config file
    let config_data = readfile(filename);
    match config_data {
        Ok(e) => {
            Ok(Box::new(*e)) // need to deref the Box before reboxing
        },
        Err(_) => {
            Err("nadda")
        }
    }
}

接下来，当你调用它时，你需要告诉它一个具体的类型：

let result: Box<Outer<ExternalStructA>> = readconfig("config.yaml")?;

如果无法将输入解析为Box<Outer<ExternalStructA>>，则会失败，在这种情况下，您可以尝试使用Box<Outer<ExternalStructB>>将其解析为Result::or_else。

Answer 2

只是声明T是DeserializeOwned：

fn readfile<T: de::DeserializeOwned>(filename: String) -> Result<Box<T>, Box<std::error::Error>> {
    let f = std::fs::File::open(filename)?;
    let config_data: Outer<T> = serde_yaml::from_reader(f)?;
    match config_data.ptr {
        Ptr::Owned(data) => Ok(data),
        _ => unimplemented!(),
    }
}

与readconfig相同