Macro __bool 

macro_rules! __bool {
    ( $($whatever:tt)* ) => { ... };
}

(dev) Always expands to false

Used for conditional expansion of match arms in macros. __bool! expands to false so that the arm is never executed.

match $something {
    $(
        _ if __bool!($variable) => unreachable!(),
        $match-arm,
    )?