The main function was incorrectly declared.

Erroneous code example:

fn main(x: i32) { // error: main function has wrong type
    println!("{}", x);
}

The main function prototype should never take arguments. Example:

fn main() {
    // your code
}

If you want to get command-line arguments, use std::env::args. To exit with a specified exit code, use std::process::exit.