compile error with required group proto2 message

[kristopherbullinger]

// protos/group.proto
syntax = "proto2";
message Foo {
  required group Data = 201 { // <------- `required group`
    required int32 group_int32 = 202;
    required uint32 group_uint32 = 203;
  }
}

// build.rs
fn main() {
    println!("cargo:rerun-if-changed=protos/group.proto");
    protobuf_codegen::Codegen::new()
        .pure()
        .includes(&["protos"])
        .input("protos/oneof.proto")
        .cargo_out_dir("protos")
        .run_from_script();
}

// src/main.rs
mod protos {
    include!(concat!(env!("OUT_DIR"), "/protos/mod.rs"));
}
use protos::group;
fn main() {}

Produces compile error:

error[E0609]: no field `data` on type `&Foo`
  --> /protobuf-build-test/target/debug/build/oneof-7aa1b7e9780d5143/out/protos/group.rs:63:17
   |
63 |         if self.data.is_none() {
   |                 ^^^^ unknown field
   |
   = note: available field is: `special_fields`

It seems that a group is merely rendered as a comment in the generated output struct:

// @@protoc_insertion_point(message:Foo)
#[derive(PartialEq,Clone,Default,Debug)]
pub struct Foo {
    // message fields
    // data: <group>    <------------------------------
    // special fields
    // @@protoc_insertion_point(special_field:Foo.special_fields)
    pub special_fields: ::protobuf::SpecialFields,
}

Perhaps it should be rendered as a data: Option<foo::Data>, ?