Why do try!() and ? not compile when used in a function that doesn't return Option or Result?

Question

Why does this code not compile?

use std::{fs, path::Path};

fn main() {
    let dir = Path::new(\"../FileSystem\");

    if !dir.is_dir() {
        println!(         


        

Answer 1

As of Rust 1.26, Rust supports a return value from main(), and thus supports the use of the error-check operator ? (or equivalently the try!() macro) in main() when main() is defined to return a Result:

extern crate failure;
use failure::Error;
use std::fs::File;

type Result = std::result::Result;

fn main() -> Result<()> {
    let mut _file = File::open("foo.txt")?; // does not exist; returns error
    println!("the file is open!");
    Ok(())
}

The above compiles and returns a file not found error (assuming foo.txt does not exist in the local path).

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