Macro delegate

macro_rules! delegate {
    ( $prefix:path : $(
        $(#[$attr: meta])*
        $(unsafe $($placeholder: lifetime)?)?
        fn $func: ident $(<$(const $generic: ident: $generic_ty: ty),* $(,)?>)?(
            $($arg: ident: $ty: ty),* $(,)?
        ) $(-> $ret: ty)?;
    )*) => { ... };
}