Allow unused named arguments in Rust's format!() family

Question

Given:

format!(\"{red}{}{reset}\", \"text\", red = \"RED\", blue = \"BLUE\", reset = \"RESET\");

The compilers exits with an error:

Answer 1

If the set of colors are all known, you could "consume" them with zero-length arguments:

macro_rules! log {
    ($fmt:expr, $($arg:tt)*) => {
        println!(concat!($fmt, "{blue:.0}{red:.0}{reset:.0}"),  // <--
                 $($arg)*,
                 blue="BLUE", 
                 red="RED", 
                 reset="RESET")
    }
}

fn main() {
    log!("{red}{}{reset}", "<!>");
    // prints: RED<!>RESET
}

(Docs for concat! macro)

Note that the strings BLUE, RED, RESET will still be sent to the formatting function, so it will incur a minor overhead even nothing will be printed.

---

I think this is quite error prone, since if you forget a {reset} the rest of your console will become red. I wonder why not write something like:

macro_rules! log_red {
    ($fmt:expr, $($arg:tt)*) => {
        println!(concat!("RED", $fmt, "RESET"), $($arg)*);
    }
}
// also define `log_blue!`.

log_red!("{}", "text");