Support compile-time strings and compile-time format string compilati…

…on in module

Make just the necessary parts available for lookup from client context.
Shorter linker symbols are a nice sideeffect.

include/fmt/compile.h

@@ -164,7 +164,7 @@ struct is_compiled_string : std::is_base_of<compiled_string, S> {};
 #endif
 
 #if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
-template <typename Char, size_t N, fixed_string<Char, N> Str>
+template <typename Char, size_t N, fmt::ct::str<Char, N> Str>
 struct udl_compiled_string : compiled_string {
   using char_type = Char;
   constexpr operator basic_string_view<char_type>() const {
@@ -622,7 +622,7 @@ void print(const S& format_str, const Args&... args) {
 
 #if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
 inline namespace literals {
-template <detail::fixed_string Str>
+template <ct::str Str>
 constexpr detail::udl_compiled_string<
     remove_cvref_t<decltype(Str.data[0])>,
     sizeof(Str.data) / sizeof(decltype(Str.data[0])), Str>

include/fmt/format.h

@@ -785,6 +785,33 @@ FMT_INLINE auto make_args_checked(const S& fmt,
   return {args...};
 }
 
+// compile-time support
+namespace ct {
+#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+template <typename Char, size_t N> struct str {
+  using char_type = Char;
+  constexpr str(const Char (&s)[N]) {
+    detail::copy_str<Char, const Char*, Char*>(static_cast<const Char*>(s),
+                                               s + N, data);
+  }
+  Char data[N]{};
+};
+#endif
+
+// Converts a compile-time string to basic_string_view.
+template <typename Char, size_t N>
+constexpr auto string_to_view(const Char (&s)[N]) -> basic_string_view<Char> {
+  // Remove trailing NUL character if needed. Won't be present if this is used
+  // with a raw character array (i.e. not defined as a string).
+  return {s, N - (std::char_traits<Char>::to_int_type(s[N - 1]) == 0 ? 1 : 0)};
+}
+template <typename Char>
+constexpr auto string_to_view(detail::std_string_view<Char> s)
+    -> basic_string_view<Char> {
+  return {s.data(), s.size()};
+}
+}  // namespace ct
+
 FMT_BEGIN_DETAIL_NAMESPACE
 
 inline void throw_format_error(const char* message) {
@@ -2105,20 +2132,6 @@ FMT_CONSTEXPR void handle_dynamic_spec(int& value,
   }
 }
 
-// Converts a compile-time string to basic_string_view.
-template <typename Char, size_t N>
-constexpr auto compile_string_to_view(const Char (&s)[N])
-    -> basic_string_view<Char> {
-  // Remove trailing NUL character if needed. Won't be present if this is used
-  // with a raw character array (i.e. not defined as a string).
-  return {s, N - (std::char_traits<Char>::to_int_type(s[N - 1]) == 0 ? 1 : 0)};
-}
-template <typename Char>
-constexpr auto compile_string_to_view(std_string_view<Char> s)
-    -> basic_string_view<Char> {
-  return {s.data(), s.size()};
-}
-
 #define FMT_STRING_IMPL(s, base, explicit)                                 \
   [] {                                                                     \
     /* Use the hidden visibility as a workaround for a GCC bug (#1973). */ \
@@ -2127,7 +2140,7 @@ constexpr auto compile_string_to_view(std_string_view<Char> s)
       using char_type = fmt::remove_cvref_t<decltype(s[0])>;               \
       FMT_MAYBE_UNUSED FMT_CONSTEXPR explicit                              \
       operator fmt::basic_string_view<char_type>() const {                 \
-        return fmt::detail::compile_string_to_view<char_type>(s);          \
+        return fmt::ct::string_to_view<char_type>(s);                      \
       }                                                                    \
     };                                                                     \
     return FMT_COMPILE_STRING();                                           \
@@ -2145,16 +2158,6 @@ constexpr auto compile_string_to_view(std_string_view<Char> s)
  */
 #define FMT_STRING(s) FMT_STRING_IMPL(s, fmt::compile_string, )
 
-#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
-template <typename Char, size_t N> struct fixed_string {
-  constexpr fixed_string(const Char (&str)[N]) {
-    copy_str<Char, const Char*, Char*>(static_cast<const Char*>(str), str + N,
-                                       data);
-  }
-  Char data[N]{};
-};
-#endif
-
 #if FMT_USE_USER_DEFINED_LITERALS
 template <typename Char> struct udl_formatter {
   basic_string_view<Char> str;
@@ -2166,22 +2169,22 @@ template <typename Char> struct udl_formatter {
 };
 
 #  if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
-template <typename T, typename Char, size_t N, fixed_string<Char, N> Str>
+template <typename T, typename Char, size_t N, fmt::ct::str<Char, N> Str>
 struct statically_named_arg : view {
   static constexpr auto name = Str.data;
 
   const T& value;
   statically_named_arg(const T& v) : value(v) {}
 };
 
-template <typename T, typename Char, size_t N, fixed_string<Char, N> Str>
+template <typename T, typename Char, size_t N, fmt::ct::str<Char, N> Str>
 struct is_named_arg<statically_named_arg<T, Char, N, Str>> : std::true_type {};
 
-template <typename T, typename Char, size_t N, fixed_string<Char, N> Str>
+template <typename T, typename Char, size_t N, fmt::ct::str<Char, N> Str>
 struct is_statically_named_arg<statically_named_arg<T, Char, N, Str>>
     : std::true_type {};
 
-template <typename Char, size_t N, fixed_string<Char, N> Str> struct udl_arg {
+template <typename Char, size_t N, fmt::ct::str<Char, N> Str> struct udl_arg {
   template <typename T> auto operator=(T&& value) const {
     return statically_named_arg<T, Char, N, Str>(std::forward<T>(value));
   }
@@ -2728,7 +2731,7 @@ inline namespace literals {
   \endrst
  */
 #if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
-template <detail::fixed_string Str>
+template <ct::str Str>
 constexpr auto operator""_a()
     -> detail::udl_arg<remove_cvref_t<decltype(Str.data[0])>,
                        sizeof(Str.data) / sizeof(decltype(Str.data[0])), Str> {

src/fmt.cc

@@ -1,4 +1,8 @@
 module;
+#ifndef __cpp_modules
+#  error Module not supported.
+#endif
+
 // put all implementation-provided headers into the global module fragment
 // to prevent attachment to this module
 #if !defined(_CRT_SECURE_NO_WARNINGS) && defined(_MSC_VER)
@@ -76,10 +80,6 @@ export module fmt;
   }                              \
   export {
 
-#if defined(_MSC_FULL_VER) && _MSC_FULL_VER > 192930036
-#define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 0
-#endif
-
 // all library-provided declarations and definitions
 // must be in the module purview to be exported
 #include "fmt/args.h"

test/module-test.cc

@@ -445,3 +445,11 @@ TEST(module_test, has_formatter) {
 TEST(module_test, is_formattable) {
   EXPECT_FALSE(fmt::is_formattable<disabled_formatter>::value);
 }
+
+TEST(module_test, compile_format_string) {
+  using namespace fmt::literals;
+  EXPECT_EQ("42", fmt::format("{0:x}"_cf, 0x42));
+  EXPECT_EQ(L"42", fmt::format(L"{:}"_cf, 42));
+  EXPECT_EQ("4.2", fmt::format("{arg:3.1f}"_cf, "arg"_a = 4.2));
+  EXPECT_EQ(L" 42", fmt::format(L"{arg:>3}"_cf, L"arg"_a = L"42"));
+}