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    /// A macro for defining #[cfg] if-else statements.
    ///
    /// This is similar to the `if/elif` C preprocessor macro by allowing definition
    /// of a cascade of `#[cfg]` cases, emitting the implementation which matches
    /// first.
    ///
    /// This allows you to conveniently provide a long list #[cfg]'d blocks of code
    /// without having to rewrite each clause multiple times.
    macro_rules! cfg_if {
        ($(
            if #[cfg($($meta:meta),*)] { $($it:item)* }
        ) else * else {
            $($it2:item)*
        }) => {
            __cfg_if_items! {
                () ;
                $( ( ($($meta),*) ($($it)*) ), )*
                ( () ($($it2)*) ),
            }
        }
    }

    macro_rules! __cfg_if_items {
        (($($not:meta,)*) ; ) => {};
        (($($not:meta,)*) ; ( ($($m:meta),*) ($($it:item)*) ), $($rest:tt)*) => {
            __cfg_if_apply! { cfg(all(not(any($($not),*)), $($m,)*)), $($it)* }
            __cfg_if_items! { ($($not,)* $($m,)*) ; $($rest)* }
        }
    }

    macro_rules! __cfg_if_apply {
        ($m:meta, $($it:item)*) => {
            $(#[$m] $it)*
        }
    }

    macro_rules! s {
        ($($(#[$attr:meta])* pub struct $i:ident { $($field:tt)* })*) => ($(
            __item! {
                #[repr(C)]
                $(#[$attr])*
                pub struct $i { $($field)* }
            }
            impl ::dox::Copy for $i {}
            impl ::dox::Clone for $i {
                fn clone(&self) -> $i { *self }
            }
        )*)
    }

    macro_rules! f {
        ($(pub fn $i:ident($($arg:ident: $argty:ty),*) -> $ret:ty {
            $($body:stmt);*
        })*) => ($(
            #[inline]
            #[cfg(not(dox))]
            pub unsafe extern fn $i($($arg: $argty),*) -> $ret {
                $($body);*
            }

            #[cfg(dox)]
            #[allow(dead_code)]
            pub unsafe extern fn $i($($arg: $argty),*) -> $ret {
                loop {}
            }
        )*)
    }

    macro_rules! __item {
        ($i:item) => ($i)
    }

    #[cfg(test)]
    mod tests {
        cfg_if! {
            if #[cfg(test)] {
                use std::option::Option as Option2;
                fn works1() -> Option2<u32> { Some(1) }
            } else {
                fn works1() -> Option<u32> { None }
            }
        }

        cfg_if! {
            if #[cfg(foo)] {
                fn works2() -> bool { false }
            } else if #[cfg(test)] {
                fn works2() -> bool { true }
            } else {
                fn works2() -> bool { false }
            }
        }

        cfg_if! {
            if #[cfg(foo)] {
                fn works3() -> bool { false }
            } else {
                fn works3() -> bool { true }
            }
        }

        #[test]
        fn it_works() {
            assert!(works1().is_some());
            assert!(works2());
            assert!(works3());
        }
    }

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