Save <compiler {boundmass/boundinertia}> if nonzero.

Fixes #928

PiperOrigin-RevId: 560967976
Change-Id: Idc8c043278362d8925ce41061c65f3e678ad50ce

src/xml/xml_native_writer.cc

@@ -768,6 +768,12 @@ void mjXWriter::Compiler(XMLElement* root) {
   if (model->exactmeshinertia) {
     WriteAttrTxt(section, "exactmeshinertia", "true");
   }
+  if (model->boundmass) {
+    WriteAttr(section, "boundmass", 1, &model->boundmass);
+  }
+  if (model->boundinertia) {
+    WriteAttr(section, "boundinertia", 1, &model->boundinertia);
+  }
   // always enable autolimits. limited attributes will be written appropriately
   // TODO(b/245077553): Remove this when the default is true.
   WriteAttrTxt(section, "autolimits", "true");

test/xml/xml_native_writer_test.cc

@@ -208,6 +208,34 @@ TEST_F(XMLWriterTest, NotAddsInertial) {
   mj_deleteModel(model);
 }
 
+TEST_F(XMLWriterTest, KeepsBoundMassInertia) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <compiler boundmass="0.1" boundinertia="0.2"/>
+  </mujoco>
+  )";
+  mjModel* model = LoadModelFromString(xml);
+  ASSERT_THAT(model, NotNull());
+  std::string saved_xml = SaveAndReadXml(model);
+  EXPECT_THAT(saved_xml, HasSubstr("boundmass=\"0.1\""));
+  EXPECT_THAT(saved_xml, HasSubstr("boundinertia=\"0.2\""));
+  mj_deleteModel(model);
+}
+
+TEST_F(XMLWriterTest, DropsZeroBoundMassInertia) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <compiler boundmass="0" boundinertia="0"/>
+  </mujoco>
+  )";
+  mjModel* model = LoadModelFromString(xml);
+  ASSERT_THAT(model, NotNull());
+  std::string saved_xml = SaveAndReadXml(model);
+  EXPECT_THAT(saved_xml, Not(HasSubstr("boundmass")));
+  EXPECT_THAT(saved_xml, Not(HasSubstr("boundinertia")));
+  mj_deleteModel(model);
+}
+
 TEST_F(XMLWriterTest, DropsInertialIfFromGeom) {
   static constexpr char xml[] = R"(
   <mujoco>