手写的泛型类型边界

当为具有泛型类型参数的结构派生 Serialize 和 Deserialize 实现时，大多数时候 Serde 能够在没有程序员帮助的情况下推断出正确的特征边界 。它使用几种启发式方法来猜测右边界，但最重要的是，它将 T: Serialize 的一个界限放在每个类型参数 T 上，这是一个序列化字段的一部分，T: Deserialize 的界限是每个类型参数 T，它是反序列化字段的一部分。与大多数启发式方法一样，这并不总是正确的，Serde 提供了一个逃生舱口，用程序员编写的一个替换自动生成的界限。

extern crate serde;
extern crate serde_json;
#[macro_use] extern crate serde_derive;

use serde::de::{self, Deserialize, Deserializer};

use std::fmt::Display;
use std::str::FromStr;

#[derive(Deserialize, Debug)]
struct Outer<'a, S, T: 'a + ?Sized> {
    // When deriving the Deserialize impl, Serde would want to generate a bound
    // `S: Deserialize` on the type of this field. But we are going to use the
    // type's `FromStr` impl instead of its `Deserialize` impl by going through
    // `deserialize_from_str`, so we override the automatically generated bound
    // by the one required for `deserialize_from_str`.
    #[serde(deserialize_with = "deserialize_from_str")]
    #[serde(bound(deserialize = "S: FromStr, S::Err: Display"))]
    s: S,

    // Here Serde would want to generate a bound `T: Deserialize`. That is a
    // stricter condition than is necessary. In fact, the `main` function below
    // uses T=str which does not implement Deserialize. We override the
    // automatically generated bound by a looser one.
    #[serde(bound(deserialize = "Ptr<'a, T>: Deserialize"))]
    ptr: Ptr<'a, T>,
}

/// Deserialize a type `S` by deserializing a string, then using the `FromStr`
/// impl of `S` to create the result. The generic type `S` is not required to
/// implement `Deserialize`.
fn deserialize_from_str<S, D>(deserializer: D) -> Result<S, D::Error>
    where S: FromStr,
          S::Err: Display,
          D: Deserializer
{
    let s: String = try!(Deserialize::deserialize(deserializer));
    S::from_str(&s).map_err(|e| de::Error::custom(e.to_string()))
}

/// A pointer to `T` which may or may not own the data. When deserializing we
/// always want to produce owned data.
#[derive(Debug)]
enum Ptr<'a, T: 'a + ?Sized> {
    Ref(&'a T),
    Owned(Box<T>),
}

impl<'a, T: 'a + ?Sized> Deserialize for Ptr<'a, T>
    where Box<T>: Deserialize
{
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where D: Deserializer
    {
        let box_t = try!(Deserialize::deserialize(deserializer));
        Ok(Ptr::Owned(box_t))
    }
}

fn main() {
    let j = r#"
        {
          "s": "1234567890",
          "ptr": "owned"
        }
    "#;

    let result: Outer<u64, str> = serde_json::from_str(j).unwrap();

    // result = Outer { s: 1234567890, ptr: Owned("owned") }
    println!("result = {:?}", result);
}