Macro fail 

macro_rules! fail {
    ($msg:literal) => { ... };
    (($msg:literal), ($context:literal)) => { ... };
    ($fmtstr:literal, $($args:tt)*) => { ... };
    (($fmtstr:literal, $($args:tt)*), ($context:literal)) => { ... };
    (($fmtstr:literal, $($args:tt)*), ($contextfmt:literal, $($contextargs:tt)*)) => { ... };
}