Translating between Result ⟷ exceptions about cxx HOT 1 CLOSED

struct trycatch {
  template <typename T> trycatch(T);
  static char use_default;
};

template <typename Try, typename Fail,
          typename = decltype(trycatch(std::declval<Try>()).use_default)>
static void trycatch(Try &&func, Fail &&fail) noexcept try {
  func();
} catch (const std::exception &e) {
  fail(e.what());
}

// union with the same layout as std::result::Result<T, cxx::Exception>
rust::Result<T> result;

trycatch(
    [&] {
      new (&result) rust::Result<T>(the_function(args...));
    },
    [&](rust::Exception e) noexcept {
      new (&result) rust::Result<T>(std::move(e));
    });

return result;

template <typename Try, typename Fail>
void trycatch(Try &&func, Fail &&fail) noexcept;

template <typename Try, typename Fail>
static void trycatch(Try &&func, Fail &&fail) noexcept try {
  func();
} catch (const std::exception &e) {
  fail(folly::toStdString(folly::exceptionStr(e)));
} catch (...) {
  fail("<unknown exception>");
}