Add error message for json read

Author: lixin-wei

include/rfl/json/read.hpp

@@ -32,9 +32,16 @@ auto read(const InputVarType& _obj) {
 template <class T, class... Ps>
 Result<internal::wrap_in_rfl_array_t<T>> read(
     const std::string_view _json_str, const yyjson_read_flag _flag = 0) {
-  yyjson_doc* doc = yyjson_read(_json_str.data(), _json_str.size(), _flag);
+  if (_flag & YYJSON_READ_INSITU) {
+    return error("YYJSON_READ_INSITU is not supported");
+  }
+  yyjson_read_err err;
+  // According to yyjson's doc, it's safe castaway constness as long as
+  // YYJSON_READ_INSITU is not set
+  yyjson_doc* doc = yyjson_read_opts(const_cast<char*>(_json_str.data()),
+                                     _json_str.size(), _flag, NULL, &err);
   if (!doc) {
-    return error("Could not parse document");
+    return error("Could not parse document: " + std::string(err.msg));
   }
   yyjson_val* root = yyjson_doc_get_root(doc);
   const auto r = Reader();

tests/json/test_error_messages.cpp

@@ -43,7 +43,7 @@ TEST(json, test_decode_error_without_exception) {
 
   EXPECT_TRUE(!result.has_value() && true);
 
-  EXPECT_EQ(result.error().what(), "Could not parse document");
+  EXPECT_EQ(result.error().what(), "Could not parse document: unexpected character, expected a comma or a closing brace");
 }
 
 }  // namespace test_error_messages