stderr is default piped when using output.

src/main.rs

@@ -1,8 +1,8 @@
+use std::env;
 use std::fs;
 use std::io;
 use std::path::{Path, PathBuf};
-use std::process::{Command, Stdio};
-use std::env;
+use std::process::Command;
 
 fn unar_file(path: &Path) -> io::Result<()> {
     println!("UNAR: {}", path.display());
@@ -11,7 +11,6 @@ fn unar_file(path: &Path) -> io::Result<()> {
     let output = Command::new("unar")
         .current_dir(parent_dir)
         .arg(filename)
-        .stderr(Stdio::piped())
         .output();
 
     match output {