Вот вам актуальный вариант:

use std::io;
use axum::response::{Html, IntoResponse};
use axum::routing::get;
use axum::Router;
use tokio::net::TcpListener;
#[tokio::main]
async fn main() -> io::Result<()> {
    let app = Router::new().route("/", get(get_form));
    let listener = TcpListener::bind("0.0.0.0:3000").await?;
    axum::serve(listener, app).await
}
async fn get_form() -> impl IntoResponse {
    Html(
        r#"
            <title>Калькулятор двух чисел</title>
            <form action="/gcd" method="post">
                <input type="text" name="n"/>
                <input type="text" name="n"/>
                <button type="submit">Ну посчитай</button>
            </form>
        "#,
    )
}