Why does Rust not have a return value in the main function, and how to return a value anyway?

As of Rust 1.26, main can return a Result:

use std::fs::File;

fn main() -> Result<(), std::io::Error> {
    let f = File::open("bar.txt")?;

    Ok(())
}

The returned error code in this case is 1 in case of an error. With File::open("bar.txt").expect("file not found"); instead, an error value of 101 is returned (at least on my machine).

Also, if you want to return a more generic error, use:

use std::error::Error;
...

fn main() -> Result<(), Box<dyn Error>> {
   ...
}