How to do polymorphic IO from either a File or stdin in Rust?

The answer by @A.B. is correct, but it tries to conform to OP’s original program structure. I want to have a more readable alternative for newcomers who stumble upon this question (just like I did).

use std::env;
use std::fs;
use std::io::{self, BufReader, BufRead};

fn main() {
    let input = env::args().nth(1);
    let reader: Box<dyn BufRead> = match input {
        None => Box::new(BufReader::new(io::stdin())),
        Some(filename) => Box::new(BufReader::new(fs::File::open(filename).unwrap()))
    };
    for line in reader.lines() {
        println!("{:?}", line);
    }
}

See the discussion in reddit from which I borrowed the code.

Note the dyn keyword before boxed BufRead. This pattern is called a trait object.