rustc E0277 when using async block vs without.

cargo-check.txt

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
  --> src/main.rs:13:21
   |
13 |         let _conn = tokio::net::TcpListener::bind(&addr).await.map_err(|_err| "help")?;
   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
   |
   = help: the trait `std::ops::Try` is not implemented for `()`
   = note: required by `std::ops::Try::from_error`

Cargo.toml

[package]
name = "debug-await-method-chaining"
version = "0.1.0"
authors = ["boxofrox"]
edition = "2018"

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

[dependencies]
tokio = { version = "0.2", features = [ "full" ] }
futures = "0.3.1"

main.rs

 use std::error::Error;

#[tokio::main]
async fn main() {
    fails().await.unwrap();
    works().await.unwrap();
}

async fn fails() -> Result<(), Box<dyn Error>> {
    let addr = "127.0.0.1:8888".parse::<std::net::SocketAddr>()?;

    let server = async {
        let _conn = tokio::net::TcpListener::bind(&addr).await.map_err(|_err| "help")?;
    };

    server.await;

    Ok(())
}

async fn works() -> Result<(), Box<dyn Error>> {
    let addr = "127.0.0.1:8888".parse::<std::net::SocketAddr>()?;
    let _conn = tokio::net::TcpListener::bind(&addr).await.map_err(|_err| "help")?;

    Ok(())
}