You pass a lambda that returns the debug message. With javascripty syntax:

    debug( function(){ return "my expensive log message" })

BTW, if your language is lazily evaluated (like Haskell) then you don't need to do this because arguments will only ve evaluated when they are needed.

The downside there is you can still end up allocating a closure. Whereas an expanded macro shouldn't cost anything. (A sufficiently smart compiler might be able to optimize the closure allocation.)

A sufficiently modern language (like, saaaaay, D2) could also give you a type like "closure you don't intend to escape" (let's call this a "scoped closure", or if you will, "scope string delegate()"), and eschew allocation entirely without requiring optimization.