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

It's done so:

use std::io::Write;

fn main() {
    std::io::stderr().write(b"some output\n");
}

You can test it by sending the program output to /dev/null to ensure it works (I ignore the warning):

$ rustc foo.rs && ./foo > /dev/null
foo.rs:4:5: 4:42 warning: unused result which must be used, #[warn(unused_must_use)] on by default
foo.rs:4     io::stderr().write(b"some output\n");
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
some output

Similarly, one can do the following for stdout:

use std::io::Write;

fn main() {
    std::io::stdout().write(b"some output\n");
}

I think this means println! is just a convenience: it's shorter and it also allows some formatting. As an example of the latter, the following displays 0x400:

println!("0x{:x}", 1024u)