How to concatenate static strings in Rust

Question

I'm trying to concatenate static strings and string literals to build another static string. The following is the best I could come up with, but it doesn't work:

const DESCRIPTION: &'static str = "my program";
const VERSION: &'static str = env!("CARGO_PKG_VERSION");
const VERSION_STRING: &'static str = concat!(DESCRIPTION, " v", VERSION);

Is there any way to do that in Rust or do I have to write the same literal over and over again?

Answer 1

The compiler error is

error: expected a literal

A literal is anything you type directly like "hello" or 5. The moment you start working with constants, you are not using literals anymore, but identifiers. So right now the best you can do is

const VERSION_STRING: &'static str =
    concat!("my program v", env!("CARGO_PKG_VERSION"));

Since the env! macro expands to a literal, you can use it inside concat!.

Answer 2

Since I was essentially trying to emulate C macros, I tried to solve the problem with Rust macros and succeeded:

macro_rules! description {
    () => ( "my program" )
}
macro_rules! version {
    () => ( env!("CARGO_PKG_VERSION") )
}
macro_rules! version_string {
    () => ( concat!(description!(), " v", version!()) )
}

It feels a bit ugly to use macros instead of constants, but it works as expected.