Store the mjsCompiler -> appended mjSpec map when appending an mjSpec.

Previously, we stored the source `mjSpec` during a copy as a hack for having access to the compiler options, but this is not robust since we cannot guarantee that 1) the source `mjSpec` is not destroyed before we need to look up the compiler options nor 2) that the `mjSpec` was appended without a copy.

While 2) could be solved by simply handling an additional case in `mjCModel::FindSpec`, using a map also solves 1) and it is easier to understand.

PiperOrigin-RevId: 741504264
Change-Id: Iab1bfd9e61299a94fa8caf3a244c067b09d54384

src/user/user_model.h

@@ -215,7 +215,9 @@ class mjCModel : public mjCModel_, private mjSpec {
   mjCTuple* AddTuple();
   mjCKey* AddKey();
   mjCPlugin* AddPlugin();
-  void AppendSpec(mjSpec* spec);
+
+  // append spec to this model, optionally map compiler options to the appended spec
+  void AppendSpec(mjSpec* spec, const mjsCompiler* compiler = nullptr);
 
   // delete elements marked as discard=true
   template <class T> void Delete(std::vector<T*>& elements,
@@ -248,7 +250,7 @@ class mjCModel : public mjCModel_, private mjSpec {
   mjCBase* FindObject(mjtObj type, std::string name) const;         // find object given type and name
   mjCBase* FindTree(mjCBody* body, mjtObj type, std::string name);  // find tree object given name
   mjSpec* FindSpec(std::string name) const;                         // find spec given name
-  mjSpec* FindSpec(const mjsCompiler* compiler_) const;             // find spec given mjsCompiler
+  mjSpec* FindSpec(const mjsCompiler* compiler_);                   // find spec given mjsCompiler
   void ActivatePlugin(const mjpPlugin* plugin, int slot);           // activate plugin
 
   // accessors
@@ -316,9 +318,6 @@ class mjCModel : public mjCModel_, private mjSpec {
   // map from default class name to default class pointer
   std::unordered_map<std::string, mjCDef*> def_map;
 
-  // get the spec from which this model was created
-  mjSpec* GetSourceSpec() const;
-
   // set deepcopy flag
   void SetDeepCopy(bool deepcopy) { deepcopy_ = deepcopy; }
 
@@ -332,9 +331,6 @@ class mjCModel : public mjCModel_, private mjSpec {
   // settings for each defaults class
   std::vector<mjCDef*> defaults_;
 
-  // spec from which this model was created in copy constructor
-  mjSpec* source_spec_;
-
   // list of active plugins
   std::vector<std::pair<const mjpPlugin*, int>> active_plugins_;
 
@@ -453,5 +449,6 @@ class mjCModel : public mjCModel_, private mjSpec {
   bool deepcopy_;     // copy objects when attaching
   bool attached_ = false;  // true if model is attached to a parent model
   int uid_count_ = 0;  // unique id count for all objects
+  std::unordered_map<const mjsCompiler*, mjSpec*> compiler2spec_;  // map from compiler to spec
 };
 #endif  // MUJOCO_SRC_USER_USER_MODEL_H_