Macro const_fn

macro_rules! const_fn {
    ($(
                $(#[$attr:meta])*
                $({$constness:ident})* fn $i:ident($($arg:ident: $argty:ty),* $(,)*) -> $ret:ty
                    $body:block
            )*) => { ... };
}

A nonpublic function that is const as long as libc_const_extern_fn is enabled.