使用rustc_serialize反序列化JSON对象：为什么我需要实现PartialEq？

Question

I have a file containing a JSON object with unknown keys. I would like to decode this object into a structure but don't understand how to declare this structure.

extern crate rustc_serialize;
use rustc_serialize::json;
use std::collections::BTreeMap;

#[derive(RustcDecodable, Debug)]
struct MyStruct {
    foo: u8,
    bar: Vec<String>,
}

let raw_json = r#"{
    "A": {
        "foo": 2,
        "bar": ["a", "b"],
    },
    "C": {
        "foo": 1,
        "bar": ["c", "d"],
    },
    :
}"#;

let j: BTreeMap<String, MyStruct> = json::decode(&raw_json).unwrap();
println!("{:?}", j.get("A").unwrap());

The following error occurs:

error: the trait `core::cmp::PartialEq` is not implemented for the type `MyStruct` [E0277]
    let j: BTreeMap<String, MyStruct> = json::decode(&raw_json).unwrap();
                                        ^~~~~~~~~~~~

Would I have to implement Decodable for MyStruct myself then?

Answer 1

json::decode is defined as:

pub fn decode<T: Decodable>(s: &str) -> DecodeResult<T>

This means that given a string slice, it will attempt to be decoded into a type specified by the user, so long as that type implements Decodable . On the page for Decodable , you can see all the implementations of it, including the one for BTreeMap :

impl<K: Decodable + PartialEq + Ord,
     V: Decodable + PartialEq>
Decodable for BTreeMap<K, V>

This shows that in order to decode to a BTreeMap , both the key and value in the map need to be PartialEq . However, I wasn't clear on why that is actually needed. BTreeMap should only require that the key is Ord and not care about the value at all. To that end, I've opened a pull request to remove those bounds, and it was accepted! ^_^ I guess that means that the bounds might have just been a typo originally.

Answer 2

@Shepmaster提示的简单答案是要么实现PartialEq特性要么导出它，因为错误消息表明：

#[derive(RustcDecodable, PartialEq, Debug)]