Rust functions return the unit by default

• By default function return nothing, more precisely they return () the empty tuple which is called the unit.
• We need the #[allow(clippy::let_unit_value, clippy::unit_cmp)] in the example to make Clippy, the Rust linter accept this code.

#[allow(clippy::let_unit_value, clippy::unit_cmp)]

fn main() {
    let res = hello_world();
    assert_eq!(res, ());
    println!("{:?}", res);
}

fn hello_world() {
    println!("Hello, world!");
}

Hello, world!
()