‘`for<'r，'r，'r> ...`不能安全地在线程之间发送

Question

我学习了Rust，并试图构建一个建立在hyper之上的微路由系统(这只是为了学习目的，我知道框架是存在的)。

我不知道如何与hyper::server::Handler共享“复杂”类型。我阅读了错误消息，但不幸的是，我不知道如何修复它(大多数情况下，生锈编译器只是说要修复什么，现在我还不确定是否理解)。

下面是我尝试过的一个(非)工作过度简化的例子：

extern crate hyper;
use std::sync::Mutex;

use hyper::*;

type Route = (method::Method, String, Box<Fn(server::Request, server::Response)>);

struct MyHandler {
    routes: Mutex<Vec<Route>>
}

impl server::Handler for MyHandler {
    fn handle(&self, req: server::Request, mut res: server::Response) {
        // This is not important
    }
}

fn main() {
    // This is not important
}

错误是：

error: the trait bound `for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static: std::marker::Send` is not satisfied [--explain E0277]
  --> src/main.rs:12:10
   |>
12 |>     impl server::Handler for MyHandler {
   |>          ^^^^^^^^^^^^^^^
note: `for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static` cannot be sent between threads safely
note: required because it appears within the type `Box<for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static>`
note: required because it appears within the type `(hyper::method::Method, std::string::String, Box<for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static>)`
note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<(hyper::method::Method, std::string::String, Box<for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static>)>`
note: required because it appears within the type `alloc::raw_vec::RawVec<(hyper::method::Method, std::string::String, Box<for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static>)>`
note: required because it appears within the type `std::vec::Vec<(hyper::method::Method, std::string::String, Box<for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static>)>`
note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Mutex<std::vec::Vec<(hyper::method::Method, std::string::String, Box<for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static>)>>`
note: required because it appears within the type `MyHandler`
note: required by `hyper::server::Handler`

如果我使用一个简单的整数，但不使用Route类型，它就能工作。

因此，这种特性和“不能安全地在线程之间发送”的特性存在问题。阅读hyper文档后，我添加了一个Mutex，但我一定很笨，我不知道自己在做什么，不知道是该停下来学习Rust，还是继续尝试。

Answer 1

你就快到了。

错误是说MyHandler没有实现Send特性，这意味着可以安全地将类型发送到其他线程：

note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Mutex<std::vec::Vec<(hyper::method::Method, std::string::String, Box<for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static>)>>`
note: required because it appears within the type `MyHandler`
note: required by `hyper::server::Handler`

这个错误确实指向了正确的地方，但它有点让人难以抗拒。第一行是：

note: `for<'r, 'r, 'r> std::ops::Fn(hyper::server::Request<'r, 'r>, hyper::server::Response<'r>) + 'static` cannot be sent between threads safely

也就是说，Fn类型不实现Send。这是您的Route类型中的一个：

type Route = (method::Method, String, Box<Fn(server::Request, server::Response)>);

如果Fn闭包捕获的所有变量也都是Send，则它的闭包仅为Send，但在这里我们不知道传入的任何闭包是否合适。

解决方案很简单:将Fn类型约束为Send

type Route = (method::Method, String, Box<Fn(server::Request, server::Response)+Send>);