Rust has two different kinds of macros: declarative macros (declared with macro_rules!) and procedural macros. Both kinds of macros allow you to write code that writes more code and is evaluated at compile time, but they work in very different ways.

As you mentioned, emitted attributes must be attached to an item, making a proc-macro like this impossible:

#[proc_macro_derive(Num)]
pub fn num_derive(_: TokenStream) -> TokenStream {
    let gen = quote! {
        #[derive(Eq, PartialEq, Ord, PartialOrd)]
    };
    gen.into()
}
proc-macros also come with the hassle of having to be defined in a separate crate, so generating them, or creating simple ones for ergonomic reasons is not worth it.