Macro f 

macro_rules! f {
    ($(
        $(#[$attr:meta])*
        // Less than ideal hack to match either `fn` or `const fn`.
        pub $(fn $i:ident)? $(const fn $const_i:ident)?
        ($($arg:ident: $argty:ty),* $(,)*) -> $ret:ty
            $body:block
    )+) => { ... };
}

Define a unsafe function.