rust - 无论如何混合::结果与 std::io::Result

Question

谁能帮我理解为什么这段代码编译得很好:

use actix_web::{App, HttpServer};
use anyhow::Result;

mod error;

#[actix_rt::main]
async fn main() -> Result<()> {
    HttpServer::new(|| App::new())
        .bind("127.0.0.1:8080")?
        .run()
        .await?;

    Ok(())
}

虽然这不编译:

use actix_web::{App, HttpServer};
use anyhow::Result;

mod error;

#[actix_rt::main]
async fn main() -> Result<()> {
    HttpServer::new(|| App::new())
        .bind("127.0.0.1:8080")?
        .run()
        .await
}

有错误:

error[E0308]: mismatched types
 --> src/main.rs:6:1
  |
6 | #[actix_rt::main]
  | ^^^^^^^^^^^^^^^^^ expected struct `anyhow::Error`, found struct `std::io::Error`
7 | async fn main() -> Result<()> {
  |                    ---------- expected `std::result::Result<(), anyhow::Error>` because of return type
  |
  = note: expected enum `std::result::Result<_, anyhow::Error>`
             found enum `std::result::Result<_, std::io::Error>`
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

这两个例子有什么根本区别？

Answer 1

问题是第二个例子返回 Result<(), std::io::Error>这是通过启动服务器返回的，而第二个返回的是 Result<(), anyhow::Error>以 Ok(()) 的形式.

第一个例子起作用的原因是 ?运算符(或 try! 宏)执行从返回的任何错误到函数返回类型的错误的转换。

来自 the docs :

In case of the Err variant, it retrieves the inner error. try! then performs conversion using From. This provides automatic conversion between specialized errors and more general ones.