期望一个实现“Fn”特性的闭包，但这个闭包只实现了“FnOnce”答案

【问题标题】：expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`期望一个实现“Fn”特性的闭包，但这个闭包只实现了“FnOnce”
【发布时间】：2018-02-13 10:41:39
【问题描述】：

我想使用 Hyper 来实现 Web 服务。我从the hello world example 复制了代码，它成功了。当我尝试将数据访问对象添加到 HelloWorld 结构时，出现错误，我不知道如何修复它。如何将 trait 成员添加到 Hyper 服务器？

extern crate futures;
extern crate hyper;

use futures::future::Future;
use hyper::header::ContentLength;
use hyper::server::{Http, Request, Response, Service};

trait Dao {}

struct MysqlDao;

impl Dao for MysqlDao {}

struct HelloWorld {
    dao: Box<Dao>,
}

const PHRASE: &'static str = "Hello, World!";

impl Service for HelloWorld {
    // boilerplate hooking up hyper's server types
    type Request = Request;
    type Response = Response;
    type Error = hyper::Error;
    // The future representing the eventual Response your call will
    // resolve to. This can change to whatever Future you need.
    type Future = Box<Future<Item = Self::Response, Error = Self::Error>>;

    fn call(&self, _req: Request) -> Self::Future {
        // We're currently ignoring the Request
        // And returning an 'ok' Future, which means it's ready
        // immediately, and build a Response with the 'PHRASE' body.
        Box::new(futures::future::ok(
            Response::new()
                .with_header(ContentLength(PHRASE.len() as u64))
                .with_body(PHRASE),
        ))
    }
}

fn main() {
    let addr = "127.0.0.1:3000".parse().unwrap();
    let dao = Box::new(MysqlDao);
    let server = Http::new().bind(&addr, || Ok(HelloWorld { dao })).unwrap();
    server.run().unwrap();
}

错误信息：

error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`
  --> src/main.rs:44:42
   |
44 |     let server = Http::new().bind(&addr, || Ok(HelloWorld { dao })).unwrap();
   |                              ----        ^^^^^^^^^^^^^^^^^^^^^^^^^
   |                              |
   |                              the requirement to implement `Fn` derives from here
   |
note: closure is `FnOnce` because it moves the variable `dao` out of its environment
  --> src/main.rs:44:61
   |
44 |     let server = Http::new().bind(&addr, || Ok(HelloWorld { dao })).unwrap();
   |                                                             ^^^

【问题讨论】：

不清楚你到底在问什么。编译器告诉您，您必须提供一种满足特定限制的类型，而您不是。也许您正在寻找When does a closure implement Fn, FnMut and FnOnce？为了在处理程序之间共享东西，也许你想要How do I share a HashMap between Hyper handlers？
@Shepmaster 非常感谢。我查看了您提供的链接，我认为这些不是我的情况。问题是“实现 Fn 特征的闭包”，我不知道如何在 HelloWorld 中修复它！
第二个链接非常接近您的问题，如果您完全理解第一个链接，您应该能够根据您的情况调整第二个链接的解决方案。或者换句话说，第一个链接解释了为什么会出现编译器错误，第二个链接解释了如何修复它。
@SebastianRedl，我仔细阅读了链接，我知道他们所说的内容，但我就是无法让它发挥作用。你能帮我修复我的代码吗？非常感谢！

标签： rust hyper

【解决方案1】：

我对@987654321@ 结构进行了这些更改：

struct HelloWorld<'a> {
    dao: &'a Dao,
}

我还将let server 语句更改为：

let server = Http::new()
    .bind(&addr, move || Ok(HelloWorld { dao: &dao }))
    .unwrap();

整个代码：

extern crate futures;
extern crate hyper;

use futures::future::Future;
use hyper::header::ContentLength;
use hyper::server::{Http, Request, Response, Service};

trait Dao {}

struct MysqlDao;

impl Dao for MysqlDao {}

struct HelloWorld<'a> {
    dao: &'a Dao,
}

const PHRASE: &'static str = "Hello, World!";

impl<'a> Service for HelloWorld<'a> {
    // boilerplate hooking up hyper's server types
    type Request = Request;
    type Response = Response;
    type Error = hyper::Error;
    // The future representing the eventual Response your call will
    // resolve to. This can change to whatever Future you need.
    type Future = Box<Future<Item = Self::Response, Error = Self::Error>>;

    fn call(&self, _req: Request) -> Self::Future {
        // We're currently ignoring the Request
        // And returning an 'ok' Future, which means it's ready
        // immediately, and build a Response with the 'PHRASE' body.
        Box::new(futures::future::ok(
            Response::new()
                .with_header(ContentLength(PHRASE.len() as u64))
                .with_body(PHRASE),
        ))
    }
}

fn main() {
    let addr = "127.0.0.1:3000".parse().unwrap();
    let dao = MysqlDao;
    let server = Http::new()
        .bind(&addr, move || Ok(HelloWorld { dao: &dao }))
        .unwrap();
    server.run().unwrap();
}

【讨论】：

您可以在HelloWorld 中添加&'a Dao 而不是Box<&'a Dao>。没有理由在此处加框引用。