Macro error 

macro_rules! error {
    (label: $lbl:tt, $($arg:tt)+) => { ... };
    ($($arg:tt)+) => { ... };
}

Prints a message with the error label (prints to stderr)

error!("An error occurred while deactivating the nuclear core");
// "Boom" will be displayed in red as a label
error!(label: "Boom", "it is too late");