Enable pedantic clippy lints in serde_derive

serde_derive/src/internals/attr.rs

@@ -777,9 +777,7 @@ impl Field {
         };
 
         let variant_borrow = attrs
-            .map(|variant| &variant.borrow)
-            .unwrap_or(&None)
-            .as_ref()
+            .map_or(None, |variant| variant.borrow.as_ref())
             .map(|borrow| vec![Meta(borrow.clone())]);
 
         for meta_items in field