Rust 将 any：：Result 与 std：：io：：Result 混合

有人能帮我理解为什么这段代码编译得很好吗

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(())
}

虽然这无法编译:

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
}

错误如下:

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不同？