Add ArgInserter::str().

format_test.cc

@@ -789,6 +789,17 @@ TEST(TempFormatterTest, Example) {
   ReportError("File not found: {0}") << path;
 }
 
+template <typename T>
+std::string str(const T &value) {
+  return (fmt::Format("{0}") << value).str();
+}
+
+TEST(StrTest, Convert) {
+  EXPECT_EQ("42", str(42));
+  std::string s = str(Date(2012, 12, 9));
+  EXPECT_EQ("2012-12-9", s);
+}
+
 int main(int argc, char **argv) {
 #ifdef _WIN32
   // Disable message boxes on assertion failures.