use axum::{routing::get, Router};
use tokio::net::TcpListener;

async fn index() -> &'static str {
    "Hello, World!"
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let app = Router::new().route("/", get(index));

    let mut listenfd = listenfd::ListenFd::from_env();
    let listener = match listenfd.take_tcp_listener(0)? {
        Some(listener) => TcpListener::from_std(listener),
        None => TcpListener::bind("0.0.0.0:3000").await,
    }?;

    axum::serve(listener, app).await?;
    Ok(())
}

$ systemfd --no-pid -s http::5555 -- watchexec -r -- cargo run
~> socket http://127.0.0.1:5555/ -> fd #3
[Running: cargo run]
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.02s
     Running `target/debug/axum-test`
[Running: cargo run]
   Compiling axum-test v0.1.0 (/private/tmp/axum-test)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.52s
     Running `target/debug/axum-test`