struct - 如何为字段值为另一个结构的结构实现 rustc_serialize::Decodable？

Question

我需要为我的 Herd 结构实现 rustc_serialize::Decoder 特性:

extern crate chrono;
extern crate rustc_serialize;

use chrono::NaiveDate;
use rustc_serialize::Decodable;

struct Herd {
    id: i32,
    breed: String,
    name: String,
    purchase_date: NaiveDate,
}

impl Decodable for Herd {
    fn decode<D: Decoder>(d: &mut D) -> Result<Herd, D::Error> {
        d.read_struct("Herd", 4, |d| {
            let id = try!(d.read_struct_field("id", 0, |d| d.read_i32()));
            let breed = try!(d.read_struct_field("breed", 1, |d| d.read_str()));
            let name = try!(d.read_struct_field("name", 2, |d| d.read_str()));
            let purchase_date = try!(d.read_struct_field("purchase_date", 3, |i| {
                i.read_struct("NaiveDate", 1, |i| {
                    let ymdf = try!(i.read_struct_field("ymdf", 0, |i| i.read_i32()));
                    Ok(NaiveDate { ymdf: ymdf })
                })
            }));

            Ok(Herd {
                id: id,
                breed: breed,
                name: name,
                purchase_date: purchase_date,
            })
        })
    }
}

我无法使用 #[derive(RustcDecodable)] 因为发生了以下错误:

error[E0277]: the trait bound chrono::NaiveDate: rustc_serialize::Decodable is not satisfied

我正在努力手动实现 Decodable，这就是您在上面的代码中看到的内容。 NaiveDate from the rust-chrono crate是一个结构，其中一个字段的数据类型为 i32。

当我现在运行代码时，我收到消息:

field ymdf of struct chrono::NaiveDate is private

如何为 NaiveDate 实现 Decoder？有没有更简单的方法来为我的 Herd 结构实现 Decodable？我是一个全能的初级程序员；也许还有另一种方式来看待这个问题。

我正在使用具有以下依赖项的 Rust 1.12:

• 镍 = "0.9.0"
• postgres = { version = "0.12", features = ["with-chrono"]}
• chrono = "0.2"
• rustc-serialize = "0.3"

Answer 1

NaiveDate 确实实现了 Decodable 但在可选功能 “rustc-serialize” 下。

您应该将此添加到您的 Cargo.toml 中以激活它:

chrono = { version = "0.2", features = ["rustc-serialize"]}