找不到tokio::main宏？

Question

我正在我的Windows系统上创建一个Rust示例项目，以异步模式通过HTTP GET请求下载文件。

我的代码如下（与Rust Cookbook中提到的the code相同）：

extern crate error_chain;
extern crate tempfile;
extern crate tokio;
extern crate reqwest;

use error_chain::error_chain;
use std::io::copy;
use std::fs::File;
use tempfile::Builder;

error_chain! {
     foreign_links {
         Io(std::io::Error);
         HttpRequest(reqwest::Error);
     }
}

#[tokio::main]
async fn main() -> Result<()> {
    let tmp_dir = Builder::new().prefix("example").tempdir()?;
    let target = "https://www.rust-lang.org/logos/rust-logo-512x512.png";
    let response = reqwest::get(target).await?;

    let mut dest = {
        let fname = response
            .url()
            .path_segments()
            .and_then(|segments| segments.last())
            .and_then(|name| if name.is_empty() { None } else { Some(name) })
            .unwrap_or("tmp.bin");

        println!("file to download: '{}'", fname);
        let fname = tmp_dir.path().join(fname);
        println!("will be located under: '{:?}'", fname);
        File::create(fname)?
    };
    let content =  response.text().await?;
    copy(&mut content.as_bytes(), &mut dest)?;
    Ok(())
}

我的Cargo.toml文件如下：

[package]
name = "abcdef"
version = "0.1.0"
authors = ["xyz"]
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
error-chain = "0.12.4"
tempfile = "3.1.0"
tokio = "0.2.22"
reqwest = "0.10.8"

当我执行cargo run时，会显示以下错误：

error[E0433]: failed to resolve: could not find `main` in `tokio`
  --> src\main.rs:18:10
   |
18 | #[tokio::main]
   |          ^^^^ could not find `main` in `tokio`

error[E0277]: `main` has invalid return type `impl std::future::Future`
  --> src\main.rs:19:20
   |
19 | async fn main() -> Result<()> {
   |                    ^^^^^^^^^^ `main` can only return types that implement `
std::process::Termination`
   |
   = help: consider using `()`, or a `Result`

error[E0752]: `main` function is not allowed to be `async`
  --> src\main.rs:19:1
   |
19 | async fn main() -> Result<()> {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

我从 Cargo.toml 文件中交叉检查了一下，发现 edition = "2018" 已经存在。但我无法找出其他错误。

- user14200418

Answer 1

为了能够使用tokio::main，您需要在tokio中启用一个额外的功能。

尝试在Cargo.toml文件中的tokio依赖项中添加full功能：

[dependencies]
tokio = { version = "0.2.22", features = ["full"] }

这也适用于Tokio的后续版本。

- Ross MacArthur

Answer 2

正如tokio::main文档中所述：

仅在crate features rt和macros中可用。

你需要添加这些功能才能访问tokio::main：

[dependencies]
tokio = { version = "1", features = ["rt", "macros"] }

这将仅允许访问单线程执行器，因此您需要使用#[tokio::main(flavor = "current_thread")]。如果您想使用#[tokio::main]（与#[tokio::main(flavor = "multi_thread")]相同），则需要启用多线程执行器：

仅在crate功能rt-multi-thread上可用。

[dependencies]
tokio = { version = "1", features = ["rt-multi-thread", "macros"] }

另请参阅：

• 如何使用Tokio实现轮询未来的最小功能集？
• 如何启用Rust“crate feature”？

- Shepmaster

---

这是更好的答案。在我的 macOS 12.3.X 上启用“完整”功能会产生错误，提示无法检测到libc。然而，你的建议起作用了。 - dimitarvp