Restore bare_trait_objects lint within serde_derive code

serde_derive/src/internals/ast.rs

@@ -119,7 +119,7 @@ impl<'a> Container<'a> {
 }
 
 impl<'a> Data<'a> {
-    pub fn all_fields(&'a self) -> Box<Iterator<Item = &'a Field<'a>> + 'a> {
+    pub fn all_fields(&'a self) -> Box<dyn Iterator<Item = &'a Field<'a>> + 'a> {
         match self {
             Data::Enum(variants) => {
                 Box::new(variants.iter().flat_map(|variant| variant.fields.iter()))