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>> {
   ...
}

std::process::exit(code: i32) is the way to exit with a code.

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Rust does it this way so that there is a consistent explicit interface for returning a value from a program, wherever it is set from. If main starts a series of tasks then any of these can set the return value, even if main has exited.

Rust does have a way to write a main function that returns a value, however it is normally abstracted within stdlib. See the documentation on writing an executable without stdlib for details.

The reddit thread on this has a "why" explanation:

Rust certainly could be designed to do this. It used to, in fact.

But because of the task model Rust uses, the fn main task could start a bunch of other tasks and then exit! But one of those other tasks may want to set the OS exit code after main has gone away.

Calling set_exit_status is explicit, easy, and doesn't require you to always put a 0 at the bottom of main when you otherwise don't care.