fix: boxed string field

prost-build/src/code_generator.rs

@@ -639,21 +639,21 @@ impl CodeGenerator<'_> {
 
             self.push_indent();
             let ty_tag = self.field_type_tag(&field.descriptor);
-            self.buf.push_str(&format!(
-                "#[prost({}, tag=\"{}\")]\n",
-                ty_tag,
-                field.descriptor.number()
-            ));
-            self.append_field_attributes(&oneof_name, field.descriptor.name());
-
-            self.push_indent();
-            let ty = self.resolve_type(&field.descriptor, fq_message_name);
-
             let boxed = self.boxed(
                 &field.descriptor,
                 fq_message_name,
                 Some(oneof.descriptor.name()),
             );
+            self.buf.push_str(&format!("#[prost({}", ty_tag));
+            if boxed {
+                self.buf.push_str(", boxed");
+            }
+            self.buf
+                .push_str(&format!(", tag=\"{}\")]\n", field.descriptor.number()));
+            self.append_field_attributes(&oneof_name, field.descriptor.name());
+
+            self.push_indent();
+            let ty = self.resolve_type(&field.descriptor, fq_message_name);
 
             debug!(
                 "    oneof: {:?}, type: {:?}, boxed: {}",

prost-derive/src/field/oneof.rs

@@ -3,7 +3,7 @@ use proc_macro2::TokenStream;
 use quote::quote;
 use syn::{parse_str, Expr, ExprLit, Ident, Lit, Meta, MetaNameValue, Path};
 
-use crate::field::{set_option, tags_attr};
+use crate::field::{set_bool, set_option, tags_attr, word_attr};
 
 #[derive(Clone)]
 pub struct Field {
@@ -15,6 +15,8 @@ impl Field {
     pub fn new(attrs: &[Meta]) -> Result<Option<Field>, Error> {
         let mut ty = None;
         let mut tags = None;
+        let mut boxed = false;
+
         let mut unknown_attrs = Vec::new();
 
         for attr in attrs {
@@ -34,6 +36,8 @@ impl Field {
                 set_option(&mut ty, t, "duplicate oneof attribute")?;
             } else if let Some(t) = tags_attr(attr)? {
                 set_option(&mut tags, t, "duplicate tags attributes")?;
+            } else if word_attr("boxed", attr) {
+                set_bool(&mut boxed, "duplicate boxed attribute")?;
             } else {
                 unknown_attrs.push(attr);
             }

tests/build.rs

@@ -169,6 +169,7 @@ fn main() {
 
     prost_build::Config::new()
         .boxed("Foo.bar")
+        .boxed("Foo.oneof_field.box_qux")
         .compile_protos(&[src.join("boxed_field.proto")], includes)
         .unwrap();
 

tests/src/boxed_field.proto

@@ -4,6 +4,10 @@ package boxed_field;
 
 message Foo {
   Bar bar = 1;
+  oneof oneof_field {
+    string baz = 2;
+    Bar box_qux = 3;
+  }
 }
 
 message Bar {

tests/src/boxed_field.rs

@@ -1,10 +1,12 @@
 include!(concat!(env!("OUT_DIR"), "/boxed_field.rs"));
+use foo::OneofField;
 
 #[test]
 /// Confirm `Foo::bar` is boxed by creating an instance
 fn test_bar_is_boxed() {
     use alloc::boxed::Box;
     let _ = Foo {
         bar: Some(Box::new(Bar {})),
+        oneof_field: Some(OneofField::BoxQux(Box::new(Bar {}))),
     };
 }