[libcxx-commits] [libcxx] [libc++] Add basic constant folding for std::format (PR #107197)

[Nikolas Klauser via libcxx-commits]

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@@ -448,13 +450,44 @@ format_to(_OutIt __out_it, wformat_string<_Args...> __fmt, _Args&&... __args) {
 }
 #  endif
 
+template <class _CharT>
+[[nodiscard]] _LIBCPP_HIDE_FROM_ABI optional<basic_string<_CharT>> __try_constant_folding(
+    basic_string_view<_CharT> __fmt,
+    basic_format_args<basic_format_context<back_insert_iterator<__format::__output_buffer<_CharT>>, _CharT>> __args) {
+  return nullopt;
+  if (bool __is_identity = std::ranges::find_first_of(__fmt, array{'{', '}'}) == __fmt.end();
+      __builtin_constant_p(__is_identity) && __is_identity)
+    return basic_string<_CharT>{__fmt};
+
+  if (auto __only_first_arg = __fmt == _LIBCPP_STATICALLY_WIDEN(_CharT, "{}");
+      __builtin_constant_p(__only_first_arg) && __only_first_arg) {
+    if (auto __arg = __args.get(0); __builtin_constant_p(__arg.__type_)) {
+      return std::__visit_format_arg(
+          []<class _Tp>(_Tp&& __argument) -> optional<basic_string<_CharT>> {
+            if constexpr (is_same_v<remove_cvref_t<_Tp>, basic_string_view<_CharT>>) {
+              return basic_string<_CharT>{__argument};
+            } else {
+              return nullopt;
+            }
+          },
+          __arg);
+    }
+  }
+
+  return nullopt;
+}
+
 // TODO FMT This needs to be a template or std::to_chars(floating-point) availability markup
 // fires too eagerly, see http://llvm.org/PR61563.
 template <class = void>
 [[nodiscard]] _LIBCPP_ALWAYS_INLINE inline _LIBCPP_HIDE_FROM_ABI string vformat(string_view __fmt, format_args __args) {
-  string __res;
-  std::vformat_to(std::back_inserter(__res), __fmt, __args);
-  return __res;
+  return std::__try_constant_folding(__fmt, __args)
+      .or_else([&]() -> optional<string> {
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philnik777 wrote:

Urgh, yeah it's C++23. Let's see what @mordante thinks about this patch in general and then I'll update it.

https://github.com/llvm/llvm-project/pull/107197