How to send output to stderr?

Question

One uses this to send output to stdout:

println!(\"some output\")

I think there is no corresponding macro to do the same for stderr.

Answer 1

After Rust 1.19

As of Rust 1.19, you can use the eprint and eprintln macros:

fn main() {
    eprintln!("This is going to standard error!, {}", "awesome");
}

This was originally proposed in RFC 1896.

Before Rust 1.19

You can see the implementation of println! to dive into exactly how it works, but it was a bit overwhelming when I first read it.

You can format stuff to stderr using similar macros though:

use std::io::Write;

let name = "world";
writeln!(&mut std::io::stderr(), "Hello {}!", name);

This will give you a unused result which must be used warning though, as printing to IO can fail (this is not something we usually think about when printing!). We can see that the existing methods simply panic in this case, so we can update our code to do the same:

use std::io::Write;

let name = "world";
let r = writeln!(&mut std::io::stderr(), "Hello {}!", name);
r.expect("failed printing to stderr");

This is a bit much, so let's wrap it back in a macro:

use std::io::Write;

macro_rules! println_stderr(
    ($($arg:tt)*) => { {
        let r = writeln!(&mut ::std::io::stderr(), $($arg)*);
        r.expect("failed printing to stderr");
    } }
);

fn main() {
    let name = "world";
    println_stderr!("Hello {}!", name)
}