GDNative: Use VariantWriter for the API JSON generator

Instead of the String representation, which can be finicky to work with.
VariantWriter is more robust since changes to it affects the whole
system thus it's changed less often and it's never ambiguous.

modules/gdnative/nativescript/api_generator.cpp

@@ -38,6 +38,7 @@
 #include "core/object/class_db.h"
 #include "core/string/string_builder.h"
 #include "core/templates/pair.h"
+#include "core/variant/variant_parser.h"
 
 // helper stuff
 
@@ -638,6 +639,7 @@ static List<String> generate_c_api_json(const List<ClassAPI> &p_api) {
 	// I'm sorry for the \t mess
 
 	List<String> source;
+	VariantWriter writer;
 
 	source.push_back("[\n");
 
@@ -682,7 +684,12 @@ static List<String> generate_c_api_json(const List<ClassAPI> &p_api) {
 				source.push_back("\t\t\t\t\t\t\"name\": \"" + e->get().argument_names[i] + "\",\n");
 				source.push_back("\t\t\t\t\t\t\"type\": \"" + e->get().argument_types[i] + "\",\n");
 				source.push_back(String("\t\t\t\t\t\t\"has_default_value\": ") + (e->get().default_arguments.has(i) ? "true" : "false") + ",\n");
-				source.push_back("\t\t\t\t\t\t\"default_value\": \"" + (e->get().default_arguments.has(i) ? (String)e->get().default_arguments[i] : "") + "\"\n");
+				String default_value;
+				if (e->get().default_arguments.has(i)) {
+					writer.write_to_string(e->get().default_arguments[i], default_value);
+					default_value = default_value.replace("\n", "").json_escape();
+				}
+				source.push_back("\t\t\t\t\t\t\"default_value\": \"" + default_value + "\"\n");
 				source.push_back(String("\t\t\t\t\t}") + ((i < e->get().argument_names.size() - 1) ? "," : "") + "\n");
 			}
 			source.push_back("\t\t\t\t]\n");
@@ -708,7 +715,12 @@ static List<String> generate_c_api_json(const List<ClassAPI> &p_api) {
 				source.push_back("\t\t\t\t\t\t\"name\": \"" + e->get().argument_names[i] + "\",\n");
 				source.push_back("\t\t\t\t\t\t\"type\": \"" + e->get().argument_types[i] + "\",\n");
 				source.push_back(String("\t\t\t\t\t\t\"has_default_value\": ") + (e->get().default_arguments.has(i) ? "true" : "false") + ",\n");
-				source.push_back("\t\t\t\t\t\t\"default_value\": \"" + (e->get().default_arguments.has(i) ? (String)e->get().default_arguments[i] : "") + "\"\n");
+				String default_value;
+				if (e->get().default_arguments.has(i)) {
+					writer.write_to_string(e->get().default_arguments[i], default_value);
+					default_value = default_value.replace("\n", "").json_escape();
+				}
+				source.push_back("\t\t\t\t\t\t\"default_value\": \"" + default_value + "\"\n");
 				source.push_back(String("\t\t\t\t\t}") + ((i < e->get().argument_names.size() - 1) ? "," : "") + "\n");
 			}
 			source.push_back("\t\t\t\t]\n");
@@ -756,6 +768,8 @@ static void append_indented(StringBuilder &p_source, const char *p_text) {
 }
 
 static void write_builtin_method(StringBuilder &p_source, const MethodAPI &p_method) {
+	VariantWriter writer;
+
 	append_indented(p_source, vformat(R"("name": "%s",)", p_method.method_name));
 	append_indented(p_source, vformat(R"("return_type": "%s",)", p_method.return_type));
 	append_indented(p_source, vformat(R"("is_const": %s,)", p_method.is_const ? "true" : "false"));
@@ -771,7 +785,12 @@ static void write_builtin_method(StringBuilder &p_source, const MethodAPI &p_met
 		append_indented(p_source, vformat(R"("name": "%s",)", p_method.argument_names[i]));
 		append_indented(p_source, vformat(R"("type": "%s",)", p_method.argument_types[i]));
 		append_indented(p_source, vformat(R"("has_default_value": %s,)", p_method.default_arguments.has(i) ? "true" : "false"));
-		append_indented(p_source, vformat(R"("default_value": "%s")", p_method.default_arguments.has(i) ? p_method.default_arguments[i].operator String() : ""));
+		String default_value;
+		if (p_method.default_arguments.has(i)) {
+			writer.write_to_string(p_method.default_arguments[i], default_value);
+			default_value = default_value.replace("\n", "").json_escape();
+		}
+		append_indented(p_source, vformat(R"("default_value": "%s")", default_value));
 
 		indent_level--;
 		append_indented(p_source, i < p_method.argument_count - 1 ? "}," : "}");