Heavily restricted format macro for const str

format_a0.rs

macro_rules! format_a0 {
  { a0 = $s:expr ; $x:expr } 
  => 
  { $x };

  { a0 = $s:expr ; $x:expr , a0 } 
  => 
  { concat!( $x, $s ) };

  { a0 = $s:expr ; $x:expr , $(a0 , $y:expr),* } 
  => 
  { concat!( $x, $($s , $y),* ) };
}

const USAGE: &str = format_a0!(a0 = "foo";
"Tool `", a0, "`, a super-duper fantastic one.
Usage:
  ", a0, " -h
  ", a0, " <file>
");

fn main() {
  println!("{}", USAGE);
}