Rust function to generify a file or stdin for command line apps. If a path is provided, it will return a readable file handle, or stdin if no path is provided.

file_or_stdin.rs

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

fn file_or_stdin<P>(path: Option<P>) -> io::Result<Box<dyn io::Read + 'static>>
where
    P: AsRef<Path>,
{
    match path {
        None => Ok(Box::new(io::BufReader::new(io::stdin()))),
        Some(path) => match fs::File::open(path) {
            Ok(file) => Ok(Box::new(io::BufReader::new(file))),
            Err(err) => {
                return Err(err);
            }
        },
    }
}