In Rust, what happens if main function returns Err?

Question

According to The Rust Reference,

If a main function is present, (snip), and its return type must be one of the following:

• ()

• Result<(), E> where E: Error

but it doesn't say what happens when main() returns (), Ok(()) or Err(<value>).

As far as I tested,

	()	Ok(())	Err(<value>)
Exit Status	0	0	1
Additional Behavior	-	-	Error: <value> is printed to stderr

Are these behaviors defined, explicitly explained or guaranteed in some documentation? In particular, can I assume

• a program always exits with 1 status when main() returns Err(<value>)?

• the error message displayed when main() returns Err(<value>) is always of the form Error: <value>?

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Notes:

• I want some sort of documented guarantee rather than an empirical explanation. This is why I added #language-lawyer tag.

• This question is not about When should I use () and when should I use Result<(), E>? or such. One can find answers (or at least hints or criteria) to such questions in many documentations or tutorials, as you know.

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Updates:

Termination trait is finally stabilized in Rust 1.61.0 (source).

Answer 1

The behavior of different return values from main is defined by the std::process::Termination trait:

trait std::process::Termination

A trait for implementing arbitrary return types in the main function.

This trait is documented to return libc::EXIT_SUCCESS on success and libc::EXIT_FAILURE on error.

The default implementations are returning libc::EXIT_SUCCESS to indicate a successful execution. In case of a failure, libc::EXIT_FAILURE is returned.

But those values aren't guaranteed to be 0 and 1 on non-POSIX systems.

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As for printing the error message, Termination requires E: Debug and does print the Debug impl to stderr, but I don't believe it's guaranteed to stay exactly the same.

impl<E: fmt::Debug> Termination for Result<!, E> {
    fn report(self) -> ExitCode {
        let Err(err) = self;
        eprintln!("Error: {:?}", err);
        ExitCode::FAILURE.report()
    }
}

Source