Macro safe_f 

macro_rules! safe_f {
    ($(
        $(#[$attr:meta])*
        pub $(const $($const_dummy:literal)?)? safe
        fn $i:ident ($($arg:ident: $argty:ty),* $(,)?) -> $ret:ty
            $body:block
    )+) => { ... };
}

Define a safe function.