Add helper methods for storing USD prim paths into mjsElements.

This way we fix a previously uncaught casting error due to using different types.

PiperOrigin-RevId: 765177079
Change-Id: I4f817f12c92fe578b25e8c53a94b3c91e22a3db0

src/experimental/usd/utils.cc

@@ -0,0 +1,44 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "third_party/mujoco/src/experimental/usd/utils.h"
+
+#include <mujoco/mujoco.h>
+#include <pxr/usd/sdf/path.h>
+
+namespace mujoco {
+namespace usd {
+
+constexpr const char* kUsdPrimPathKey = "usd_primpath";
+
+void SetUsdPrimPathUserValue(mjsElement* element,
+                             const pxr::SdfPath& prim_path) {
+  // The value is a pointer to a newly allocated SdfPath, which will be deleted
+  // when the mjsElement is deleted.
+  const pxr::SdfPath* usd_primpath = new pxr::SdfPath(prim_path);
+  mjs_setUserValueWithCleanup(
+      element, kUsdPrimPathKey, usd_primpath,
+      [](const void* data) { delete static_cast<const pxr::SdfPath*>(data); });
+}
+
+pxr::SdfPath GetUsdPrimPathUserValue(mjsElement* element) {
+  const void* user_data = mjs_getUserValue(element, kUsdPrimPathKey);
+  if (user_data) {
+    return *static_cast<const pxr::SdfPath*>(user_data);
+  }
+  return pxr::SdfPath();
+}
+
+}  // namespace usd
+}  // namespace mujoco

src/experimental/usd/utils.h

@@ -0,0 +1,35 @@
+// Copyright 2025 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef MUJOCO_SRC_EXPERIMENTAL_USD_UTILS_H_
+#define MUJOCO_SRC_EXPERIMENTAL_USD_UTILS_H_
+
+#include <mujoco/mujoco.h>
+#include <pxr/usd/sdf/path.h>
+
+namespace mujoco {
+namespace usd {
+
+// Sets a user value on an mjsElement with the key "usd_primpath".
+void SetUsdPrimPathUserValue(mjsElement* element,
+                             const pxr::SdfPath& prim_path);
+
+// Gets the user value associated with the key "usd_primpath" from an
+// mjsElement. Returns empty pxr::SdfPath() if the value is not found.
+pxr::SdfPath GetUsdPrimPathUserValue(mjsElement* element);
+
+}  // namespace usd
+}  // namespace mujoco
+
+#endif  // MUJOCO_SRC_EXPERIMENTAL_USD_UTILS_H_