use a dummy const to support rustc <= 1.31

Author: mystor

src/expand.rs

@@ -11,8 +11,10 @@ pub fn derive(input: &DeriveInput) -> Result<TokenStream> {
     }?;
 
     let helpers = specialization();
+    let dummy_const = format_ident!("_DERIVE_Display_FOR_{}", input.ident);
     Ok(quote! {
-        const _: () = {
+        #[allow(non_upper_case_globals, unused_attributes, unused_qualifications)]
+        const #dummy_const: () = {
             #helpers
             #impls
         };