Your First async fn

cargo add tokio --features full
cargo add reqwest --features json

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let body = reqwest::get("https://www.rust-lang.org").await?.text().await?;
    println!("{} bytes", body.len());
    Ok(())
}

The key idea: an async fn returns a Future, and a Future does nothing until it is polled. Calling fetch() without .await builds a value and drops it — no request is made. Rust will warn you (unused implementer of Future), and you should believe the warning.

#[tokio::main] is a macro that wraps your body in a runtime. Without a runtime there is nothing to poll the futures.