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

Veedrac's answer helped me too, although the OP's question is slightly different. While reading the Rust documentation, I saw this snippet:

use std::fs::File;
use std::io::prelude::*;

let mut file = File::open("foo.txt")?;
let mut contents = String::new();
file.read_to_string(&mut contents)?;
assert_eq!(contents, "Hello, world!");

Though in the Rust Book they point out the centrality of the main function, if you run this inside it you'll get a similar error. If you wrap the code inside a function handling the errors the aforementioned snippet works:

use std::error::Error;
use std::io::prelude::*;
use std::fs::File;

fn print_file_content() -> Result> {
    let mut f = File::open("foo.txt")?;
    let mut contents = String::new();

    f.read_to_string(&mut contents)?;

    println!("The content: {:?}", contents);

    Ok("Done".into())
}

fn main() {
    match print_file_content() {
        Ok(s) => println!("{}", s),
        Err(e) => println!("Error: {}", e.to_string()),
    }
}

P.S. I'm learning Rust so these snippets are not intended as good Rust coding :)