Make the cache constant anonymous.

Author: ratmice

lrpar/src/lib/ctbuilder.rs

@@ -644,8 +644,10 @@ where
         self.output_file(&grm, &stable, &derived_mod_name, outp, &quote!{
             // This declaration can be affected by the pretty printer.
             // But we would hope the actual cache string is not.
-            #[allow(unused)]
-            const CACHE_INFO: &str = #cache;
+            //
+            // This is emitted for the purposes of performing the cache check.
+            // on the output source, but is not used by generated parser.
+            const _: &str = #cache;
         })?;
         let conflicts = if stable.conflicts().is_some() {
             Some((grm, sgraph, stable))