找不到 tokio::main 宏？

Question

我正在我的 Windows 系统中创建一个示例 Rust 项目，以在异步模式下通过 HTTP GET 请求下载文件。

我的代码如下（与Rust Cookbook中提到的代码相同）：

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 文件是：

[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"

当我执行时cargo run，显示以下错误：

[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"

我从 Cargo.toml 文件中进行了交叉检查，并且edition = "2018"已经在那里了。我无法弄清楚其他错误。

Answer 1

如文档中所述tokio::main：

仅适用于板条箱功能rtmacros。

您需要添加这些功能才能访问tokio::main：

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

但是，这将只允许访问单线程执行程序，因此您必须使用#[tokio::main(flavor = "current_thread")]. 如果你想使用#[tokio::main]（与 相同#[tokio::main(flavor = "multi_thread")]，那么你需要启用多线程执行器：

仅适用于板条箱功能rt-multi-thread。

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

也可以看看：

• 启用 Tokio 投票未来的最小功能集是什么？
• 如何启用 Rust“板条箱功能”？

Answer 2

您需要启用额外的功能tokio才能使用tokio::main.

尝试将该full功能添加到tokioCargo.toml 文件中的依赖项：

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

这也适用于更高版本的 Tokio。