问 错误:无法在tokio中找到main或io，无效的返回类型“`impl Future”

我正准备从ML家族转到Rust，但我发现在一些陌生的地方，我不习惯遇到问题。

我试图使用 hyper 进行http处理，但似乎无法让 tokio 工作。

我试过复制粘贴这个 示例

use hyper::{body::HttpBody as _, Client};
use tokio::io::{self, AsyncWriteExt as _};
type Result<T> = std::result::Result<T, Box<dyn std::error::Error + Send + Sync>>;
#[tokio::main]
async fn main() -> Result<()> {
    // ...
    fetch_url(url).await
async fn fetch_url(url: hyper::Uri) -> Result<()> {
    // ...
    Ok(())
}

这是我的 Cargo.toml

[package]
name = "projectname"
version = "0.1.0"
authors = ["username"]
edition = "2018"
[dependencies]
hyper = "0.14.4"
tokio = "1.2.0"

它在抱怨它找不到 io 机箱， main 有一个无效的 impl Future 类型，并且在 tokio 中找不到 main

error[E0433]: failed to resolve: could not find `main` in `tokio`
 --> src/main.rs:9:10
9 | #[tokio::main]
  |          ^^^^ could not find `main` in `tokio`
error[E0277]: `main` has invalid return type `impl Future`
  --> src/main.rs:10:20
10 | async fn main() -> Result<()> {
   |                    ^^^^^^^^^^ `main` can only return types that implement `Termination`
error[E0432]: unresolved import `hyper::Client`
 --> src/main.rs:3:34
3 | use hyper::{body::HttpBody as _, Client};
  |                                  ^^^^^^ no `Client` in the root
error[E0425]: cannot find function `stdout` in module `io`
  --> src/main.rs:45:13
45 |         io::stdout().write_all(&chunk).await?;
   |             ^^^^^^ not found in `io`
error[E0432]: unresolved import `tokio::io::AsyncWriteExt`
 --> src/main.rs:4:23