Macro trace 

macro_rules! trace {
    (
        target : $target:expr, $($field:ident = $value:expr,)* $fmt:literal
        $(, $($arg:expr),*)?
    ) => { ... };
    (target : $target:expr, $fmt:literal $(, $($arg:expr),*)?) => { ... };
    (target : $target:expr, $($arg:tt)*) => { ... };
    ($($arg:tt)*) => { ... };
}