rust - 在 Rust 中的两个异步函数之间进行选择的变量 - 不兼容的 ARM 类型

Question

我有一个变量可以控制哪个函数是我的 Web 应用程序的默认行为，但是这两个函数都是 async 并且它不会让我这样做，因为它们是不同的闭包。像这样的东西会重现同样的错误: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=db0269ef4852a94438e6d4925ca83d3b

现在我的问题是:

• 如何解决这个问题才能使匹配正常进行？
• 是否有更好的方法可以根据用户 CLI 标志在 actix 上以编程方式设置默认服务器行为？对于上下文，这或多或少是我所拥有的:

main.rs

// Matches comes from the command line
let a = match matches.occurrences_of("default"){
        1 => handlers::forward,
        _ => handlers::resource_not_found,
    };

... setting up db, auth etc

HttpServer::new(move || {
            App::new()
            ... lots of routes 
            .default_service(web::route().to(a))

处理程序.rs

pub async fn resource_not_found(
) -> Result<HttpResponse, Error> {
   Ok(HttpResponse::NotFound().body("Resource does not exist"))
}
pub async fn forward(
) -> Result<HttpResponse, Error> {
   Ok(HttpResponse::Ok().body("Resource does exist"))
}

Answer 1

How do I solve this so that the match works

您可以创建两个都返回装箱 future 的闭包，并将它们强制转换为函数指针:

use std::pin::Pin;
use std::future::Future;

let a: fn() -> Pin<Box<dyn Future<Output = Result<HttpResponse, Error>>>> =
    match matches.occurrences_of("default") {
        1 => || Box::pin(handlers::forward()),
        _ => || Box::pin(handlers::resource_not_found()),
    };

如果函数真的那么简单，那么你可以避免装箱并返回 std::future::Ready:

use std::pin::Pin;
use std::future::{Future, Ready, ready};

let a: fn() -> Ready<Result<HttpResponse, Error>> =
    match matches.occurrences_of("default") {
        1 => || handlers::forward,
        _ => || handlers::resource_not_found,
    };

// handlers.rs

pub fn resource_not_found(
) -> Ready<Result<HttpResponse, Error>> {
   ready(Ok(HttpResponse::NotFound().body("Resource does not exist")))
}

pub fn forward(
) -> Ready<Result<HttpResponse, Error>> {
   ready(Ok(HttpResponse::Ok().body("Resource does exist")))
}

Is there a better way to set default server behavior programmatically on actix based on user CLI flags

您可以在新处理程序内部而不是外部进行匹配:

let x = matches.occurrences_of("default");
let a = move || async move {
    match x {
        1 => handlers::forward().await,
        _ => handlers::resource_not_found().await,
    }
};

现在，您只需匹配一个整数，而不是每次调用都进行额外的分配和动态分派(dispatch)。