Macro f 

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

Define a function that can be either safe or unsafe and optionally const. This always marks the function inline and adds extern "C".