macro_rules! put_doc_on_S {
    ($word:ident) => {
        /*
        // does not work:

        #[doc = stringify!($word)]
        pub struct S;
        */

        // works:
        put_doc_on_S_please!(stringify!($word));
    };
}

macro_rules! put_doc_on_S_please {
    ($word:expr) => {
        #[doc = $word]
        pub struct S;
    };
}

put_doc_on_S!(success);