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

print! and println! are convenience methods for writing to standard output. There are other macros with the same formatting features available for different tasks:

• write! and writeln! to write a formatted string to a &mut Writer
• format! to just generate a formatted String

To write to the standard error stream, you can use e.g. writeln! like this:

use std::io::Write;

fn main() {
    let mut stderr = std::io::stderr();
    writeln!(&mut stderr, "Error!").unwrap();
}