Add XML compiler/saveinertial flag. Fixes #2405

PiperOrigin-RevId: 743607654
Change-Id: I1f2e61be89b79c1995e079f36aa285096f9fb441

test/xml/xml_native_writer_test.cc

@@ -22,7 +22,7 @@
 #include <array>
 #include <clocale>
 #include <cstdio>
-#include <filesystem>
+#include <filesystem>  // NOLINT(build/c++17)
 #include <string>
 #include <vector>
 
@@ -32,7 +32,6 @@
 #include <mujoco/mjmodel.h>
 #include <mujoco/mjtnum.h>
 #include <mujoco/mujoco.h>
-#include "src/cc/array_safety.h"
 #include "src/xml/xml_numeric_format.h"
 #include "test/fixture.h"
 
@@ -135,6 +134,23 @@ TEST_F(XMLWriterTest, SavesDisableSensor) {
   mj_deleteModel(model);
 }
 
+TEST_F(XMLWriterTest, SavesInertial) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <compiler saveinertial="true"/>
+    <worldbody>
+      <body>
+        <geom type="box" size=".05 .05 .05"/>
+      </body>
+    </worldbody>
+  </mujoco>
+  )";
+  mjModel* model = LoadModelFromString(xml);
+  std::string saved_xml = SaveAndReadXml(model);
+  EXPECT_THAT(saved_xml, HasSubstr("mass=\"1\""));
+  mj_deleteModel(model);
+}
+
 TEST_F(XMLWriterTest, EmptyUserSensor) {
   static constexpr char xml[] = R"(
   <mujoco>
@@ -957,8 +973,10 @@ TEST_F(XMLWriterTest, WritesSkin) {
   ASSERT_THAT(model, NotNull());
   EXPECT_THAT(model->nskin, 1);
 
-  mjModel* mtemp = LoadModelFromString(SaveAndReadXml(model));
-  ASSERT_THAT(mtemp, NotNull());
+  char error[1024];
+  mjModel* mtemp = LoadModelFromString(SaveAndReadXml(model),
+                                       error, sizeof(error));
+  ASSERT_THAT(mtemp, NotNull()) << error;
   EXPECT_THAT(mtemp->nskin, 1);
 
   mj_deleteModel(model);