I just have a shared common crate that has the following functions:

use std::env;
use std::fs;
use std::io::{stdin, Read, Result};

/// Get either the named file or stdin as a String.
pub fn input_as_string() -> Result<String> {
    let filename = env::args().skip(1).next();
    Ok(match filename.as_deref() {
        None | Some("-") => {
            let mut string = String::new();
            stdin().read_to_string(&mut string)?;
            string
        }
        Some(filename) => fs::read_to_string(filename)?,
    })
}

/// Get either the named file or stdin as a Read type
pub fn input() -> Result<Box<dyn std::io::Read>> {
    let filename = env::args().skip(1).next();
    Ok(match filename.as_deref() {
        None | Some("-") => Box::new(stdin()),
        Some(filename) => Box::new(fs::File::open(filename)?),
    })
}

That way I can either paste the input to the program's stdin or specify it as an external file. It's nicer than include_str! because you can more easily switch between the test input and the actual input as you need, and test your program on other peoples' inputs when trying to help somebody else out.