prost-build: more reuse with CodeGenerator::boxed

The bit in CodeGenerator::append_oneof was pretty much the same,
except the configuration is looked up for the oneof name.

Rearrange the logic so that intermediate values are bound when needed.
The repeated fields will never be boxed even if the configuration
says they should be (e.g. by a prefix match).

prost-build/src/code_generator.rs

@@ -367,7 +367,7 @@ impl<'a> CodeGenerator<'a> {
         let repeated = field.label == Some(Label::Repeated as i32);
         let deprecated = self.deprecated(&field);
         let optional = self.optional(&field);
-        let boxed = self.boxed(&field, fq_message_name);
+        let boxed = self.boxed(&field, fq_message_name, None);
         let ty = self.resolve_type(&field, fq_message_name);
 
         debug!(
@@ -576,9 +576,9 @@ impl<'a> CodeGenerator<'a> {
         self.path.pop();
         self.path.pop();
 
-        let oneof_name = format!("{}.{}", fq_message_name, oneof.name());
-        self.append_type_attributes(&oneof_name);
-        self.append_enum_attributes(&oneof_name);
+        let fq_oneof_name = format!("{}.{}", fq_message_name, oneof.name());
+        self.append_type_attributes(&fq_oneof_name);
+        self.append_enum_attributes(&fq_oneof_name);
         self.push_indent();
         self.buf
             .push_str("#[allow(clippy::derive_partial_eq_without_eq)]\n");
@@ -595,8 +595,6 @@ impl<'a> CodeGenerator<'a> {
         self.path.push(2);
         self.depth += 1;
         for (field, idx) in fields {
-            let type_ = field.r#type();
-
             self.path.push(idx as i32);
             self.append_doc(fq_message_name, Some(field.name()));
             self.path.pop();
@@ -608,20 +606,12 @@ impl<'a> CodeGenerator<'a> {
                 ty_tag,
                 field.number()
             ));
-            self.append_field_attributes(&oneof_name, field.name());
+            self.append_field_attributes(&fq_oneof_name, field.name());
 
             self.push_indent();
             let ty = self.resolve_type(&field, fq_message_name);
 
-            let boxed = ((type_ == Type::Message || type_ == Type::Group)
-                && self
-                    .message_graph
-                    .is_nested(field.type_name(), fq_message_name))
-                || (self
-                    .config
-                    .boxed
-                    .get_first_field(&oneof_name, field.name())
-                    .is_some());
+            let boxed = self.boxed(&field, fq_message_name, Some(oneof.name()));
 
             debug!(
                 "    oneof: {:?}, type: {:?}, boxed: {}",
@@ -1026,18 +1016,31 @@ impl<'a> CodeGenerator<'a> {
         }
     }
 
-    fn boxed(&self, field: &FieldDescriptorProto, fq_message_name: &str) -> bool {
+    fn boxed(
+        &self,
+        field: &FieldDescriptorProto,
+        fq_message_name: &str,
+        oneof: Option<&str>,
+    ) -> bool {
+        if field.label == Some(Label::Repeated as i32) {
+            return false;
+        }
         let fd_type = field.r#type();
-        field.label != Some(Label::Repeated as i32)
-            && ((fd_type == Type::Message || fd_type == Type::Group)
-                && self
-                    .message_graph
-                    .is_nested(field.type_name(), fq_message_name))
-            || (self
-                .config
-                .boxed
-                .get_first_field(fq_message_name, field.name())
-                .is_some())
+        if (fd_type == Type::Message || fd_type == Type::Group)
+            && self
+                .message_graph
+                .is_nested(field.type_name(), fq_message_name)
+        {
+            return true;
+        }
+        let config_path = match oneof {
+            None => Cow::Borrowed(fq_message_name),
+            Some(ooname) => Cow::Owned(format!("{fq_message_name}.{ooname}")),
+        };
+        self.config
+            .boxed
+            .get_first_field(&config_path, field.name())
+            .is_some()
     }
 
     /// Returns `true` if the field options includes the `deprecated` option.