Macro cfg_if

macro_rules! cfg_if {
    ($(
        if #[cfg($meta:meta)] { $($tokens:tt)* }
    ) else * else {
        $($tokens2:tt)*
    }) => { ... };
    (
        if #[cfg($i_met:meta)] { $($i_tokens:tt)* }
        $(
            else if #[cfg($e_met:meta)] { $($e_tokens:tt)* }
        )*
    ) => { ... };
    (@__items ($($not:meta,)*) ; ) => { ... };
    (@__items ($($not:meta,)*) ; ( ($($m:meta),*) ($($tokens:tt)*) ), $($rest:tt)*) => { ... };
    (@__identity $($tokens:tt)*) => { ... };
}

The main macro provided by this crate. See crate documentation for more information.