How do I overcome match arms with incompatible types for structs implementing same trait?

Question

I am attempting to write the cat command to learn Rust, but I can\'t seem to convert command line arguments into reader structs.

use std::{env,          


        

Answer 1

The coalesce crate provides a way to do this without boxing and in a way that is less verbose than my other answer. The idea is to use a simple enum that can hold the concrete type corresponding to each arm and a macro (coalesce!) that expands to a match where the body expression is the same for each arm.

#[macro_use]
extern crate coalesce;

use std::io::{self, Read};
use std::fs::File;
use std::path::Path;
use coalesce::Coalesce2;

fn main() {
    if let Some(arg) = std::env::args().nth(1).as_ref() {
        let reader = match arg.as_ref() {
            "-"  => Coalesce2::A(io::stdin()),
            path => Coalesce2::B(File::open(&Path::new(path)).unwrap()),
        };

        let reader = coalesce!(2 => |ref reader| reader as &Read);

        // the previous line is equivalent to:
        let reader = match reader {
            Coalesce2::A(ref reader) => reader as &Read,
            Coalesce2::B(ref reader) => reader as &Read,
        };
    }
}