How to format a const string

Question

How can I make part of a const string conditional on some flag?

#[cfg(target_os = "macos")]
const OS: &'static str = "OSx";
#[cfg(target_os = "windows")]
const OS: &'static str = "Windows";

const SOME_STRING: &'static str = format!("this os is {}", OS);

This code doesn't compile because the format macro returns a String. I'd like to be able to do this formatting without any allocation. Is it possible to do without making the whole string conditional?

Answer 1

Well, for one, you should be aware of http://doc.rust-lang.org/stable/std/env/consts/constant.OS.html

Second, you can't really do this, exactly. You could use the lazy_static crate, but that's still going to end up giving you an allocation.

In the future, when const fn is stable, this should be easier to do.

Answer 2

You can use the const_format crate to do this.

use const_format::formatcp;

#[cfg(target_os = "macos")]
const OS: &'static str = "OSx";
#[cfg(target_os = "windows")]
const OS: &'static str = "Windows";

const SOME_STRING: &'static str = formatcp!("this os is {}", OS);

pub fn main() {
    println!("{}", SOME_STRING);
}

this os is Windows

(This code is just for example as you could simply copy "this os is" into each of the cfg strings, and also should consider using std::env::const::OS)