nix_api_value: add nix_get_attr_name_byidx get attr names without forcing

src/libexpr/nix_api_value.cc

@@ -266,6 +266,18 @@ Value *nix_get_attr_byidx(nix_c_context *context, const Value *value,
   NIXC_CATCH_ERRS_NULL
 }
 
+const char *nix_get_attr_name_byidx(nix_c_context *context, const Value *value,
+                                    State *state, unsigned int i) {
+  if (context)
+    context->last_err_code = NIX_OK;
+  try {
+    auto &v = check_value_not_null(value);
+    const nix::Attr &a = (*v.attrs)[i];
+    return ((const std::string &)(state->state.symbols[a.name])).c_str();
+  }
+  NIXC_CATCH_ERRS_NULL
+}
+
 nix_err nix_set_bool(nix_c_context *context, Value *value, bool b) {
   if (context)
     context->last_err_code = NIX_OK;

src/libexpr/nix_api_value.h

@@ -207,6 +207,8 @@ bool nix_has_attr_byname(nix_c_context *context, const Value *value,
                          State *state, const char *name);
 
 /** @brief Get an attribute by index in the sorted bindings
+ *
+ * Also gives you the name.
  *
  * Owned by the GC. Use nix_gc_decref when you're done with the pointer
  * @param[out] context Optional, stores error information
@@ -218,6 +220,20 @@ bool nix_has_attr_byname(nix_c_context *context, const Value *value,
  */
 Value *nix_get_attr_byidx(nix_c_context *context, const Value *value,
                           State *state, unsigned int i, const char **name);
+
+/** @brief Get an attribute name by index in the sorted bindings
+ *
+ * Useful when you want the name but want to avoid evaluation.
+ *
+ * Owned by the nix State
+ * @param[out] context Optional, stores error information
+ * @param[in] value Nix value to inspect
+ * @param[in] state nix evaluator state
+ * @param[in] i attribute index
+ * @return name, NULL in case of errors
+ */
+const char *nix_get_attr_name_byidx(nix_c_context *context, const Value *value,
+                                    State *state, unsigned int i);
 /**@}*/
 /** @name Setters
  */