Macro impl_fn_for_zst

macro_rules! impl_fn_for_zst {
    ($(
        $( #[$attr: meta] )*
        struct $Name: ident impl$( <$( $lifetime : lifetime ),+> )? Fn =
            |$( $arg: ident: $ArgTy: ty ),*| -> $ReturnTy: ty
            $body: block;
    )+) => { ... };
}

Creates a zero-size type similar to a closure type, but named.