Add a simple format string compilation API

include/fmt/compile.h

@@ -15,6 +15,17 @@
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
+// A compile-time string which is compiled into fast formatting code.
+class compiled_string {};
+
+template <typename S>
+struct is_compiled_string : std::is_base_of<compiled_string, S> {};
+
+#define FMT_COMPILE(s) FMT_STRING_IMPL(s, fmt::detail::compiled_string)
+
+template <typename T, typename... Tail>
+const T& first(const T& value, const Tail&...) { return value; }
+
 // Part of a compiled format string. It can be either literal text or a
 // replacement field.
 template <typename Char> struct format_part {
@@ -381,31 +392,6 @@ constexpr text<Char> make_text(basic_string_view<Char> s, size_t pos,
   return {{&s[pos], size}};
 }
 
-template <typename Char, typename OutputIt, typename T,
-          std::enable_if_t<std::is_integral_v<T>, int> = 0>
-OutputIt format_default(OutputIt out, T value) {
-  // TODO: reserve
-  format_int fi(value);
-  return std::copy(fi.data(), fi.data() + fi.size(), out);
-}
-
-template <typename Char, typename OutputIt>
-OutputIt format_default(OutputIt out, double value) {
-  return detail::write(out, value);
-}
-
-template <typename Char, typename OutputIt>
-OutputIt format_default(OutputIt out, Char value) {
-  *out++ = value;
-  return out;
-}
-
-template <typename Char, typename OutputIt>
-OutputIt format_default(OutputIt out, const Char* value) {
-  auto length = std::char_traits<Char>::length(value);
-  return copy_str<Char>(value, value + length, out);
-}
-
 // A replacement field that refers to argument N.
 template <typename Char, typename T, int N> struct field {
   using char_type = Char;
@@ -414,7 +400,7 @@ template <typename Char, typename T, int N> struct field {
   OutputIt format(OutputIt out, const Args&... args) const {
     // This ensures that the argument type is convertile to `const T&`.
     const T& arg = get<N>(args...);
-    return format_default<Char>(out, arg);
+    return write<Char>(out, arg);
   }
 };
 
@@ -456,7 +442,8 @@ constexpr auto compile_format_string(S format_str);
 
 template <typename Args, size_t POS, int ID, typename T, typename S>
 constexpr auto parse_tail(T head, S format_str) {
-  if constexpr (POS != to_string_view(format_str).size()) {
+  if constexpr (POS !=
+                basic_string_view<typename S::char_type>(format_str).size()) {
     constexpr auto tail = compile_format_string<Args, POS, ID>(format_str);
     if constexpr (std::is_same<remove_cvref_t<decltype(tail)>,
                                unknown_format>())
@@ -506,7 +493,8 @@ constexpr auto compile_format_string(S format_str) {
 #if FMT_USE_CONSTEXPR
 #  ifdef __cpp_if_constexpr
 template <typename... Args, typename S,
-          FMT_ENABLE_IF(is_compile_string<S>::value)>
+          FMT_ENABLE_IF(is_compile_string<S>::value ||
+                        detail::is_compiled_string<S>::value)>
 constexpr auto compile(S format_str) {
   constexpr basic_string_view<typename S::char_type> str = format_str;
   if constexpr (str.size() == 0) {
@@ -529,7 +517,8 @@ template <typename CompiledFormat, typename... Args,
           FMT_ENABLE_IF(detail::is_compiled_format<CompiledFormat>::value)>
 std::basic_string<Char> format(const CompiledFormat& cf, const Args&... args) {
   basic_memory_buffer<Char> buffer;
-  cf.format(std::back_inserter(buffer), args...);
+  detail::buffer<Char>& base = buffer;
+  cf.format(std::back_inserter(base), args...);
   return to_string(buffer);
 }
 
@@ -569,6 +558,16 @@ std::basic_string<Char> format(const CompiledFormat& cf, const Args&... args) {
   return to_string(buffer);
 }
 
+template <typename S, typename... Args,
+          FMT_ENABLE_IF(detail::is_compiled_string<S>::value)>
+FMT_INLINE std::basic_string<typename S::char_type> format(S, Args&&... args) {
+  constexpr basic_string_view<typename S::char_type> str = S();
+  if (str.size() == 2 && str[0] == '{' && str[1] == '}')
+    return fmt::to_string(detail::first(args...));
+  constexpr auto compiled = compile<Args...>(S());
+  return format(compiled, std::forward<Args...>(args...));
+}
+
 template <typename OutputIt, typename CompiledFormat, typename... Args,
           FMT_ENABLE_IF(std::is_base_of<detail::basic_compiled_format,
                                         CompiledFormat>::value)>

include/fmt/core.h

@@ -481,7 +481,7 @@ inline basic_string_view<Char> to_string_view(detail::std_string_view<Char> s) {
 }
 
 // A base class for compile-time strings. It is defined in the fmt namespace to
-// make formatting functions visible via ADL, e.g. format(fmt("{}"), 42).
+// make formatting functions visible via ADL, e.g. format(FMT_STRING("{}"), 42).
 struct compile_string {};
 
 template <typename S>
@@ -1745,8 +1745,8 @@ template <typename..., typename S, FMT_ENABLE_IF(!is_compile_string<S>::value)>
 FMT_INLINE void check_format_string(const S&) {
 #ifdef FMT_ENFORCE_COMPILE_STRING
   static_assert(is_compile_string<S>::value,
-                "FMT_ENFORCE_COMPILE_STRING requires all format strings to "
-                "utilize FMT_STRING() or fmt().");
+                "FMT_ENFORCE_COMPILE_STRING requires all format strings to use "
+                "FMT_STRING.");
 #endif
 }
 template <typename..., typename S, FMT_ENABLE_IF(is_compile_string<S>::value)>

include/fmt/format.h

@@ -700,6 +700,10 @@ void basic_memory_buffer<T, SIZE, Allocator>::grow(size_t size) {
 using memory_buffer = basic_memory_buffer<char>;
 using wmemory_buffer = basic_memory_buffer<wchar_t>;
 
+template <typename T, size_t SIZE, typename Allocator>
+struct is_contiguous<basic_memory_buffer<T, SIZE, Allocator>> : std::true_type {
+};
+
 /** A formatting error such as invalid format string. */
 FMT_CLASS_API
 class FMT_API format_error : public std::runtime_error {
@@ -2746,12 +2750,12 @@ FMT_CONSTEXPR basic_string_view<Char> compile_string_to_view(
   return {s.data(), s.size()};
 }
 
-#define FMT_STRING_IMPL(s, ...)                                   \
+#define FMT_STRING_IMPL(s, base)                                  \
   [] {                                                            \
     /* Use a macro-like name to avoid shadowing warnings. */      \
-    struct FMT_COMPILE_STRING : fmt::compile_string {             \
+    struct FMT_COMPILE_STRING : base {                            \
       using char_type = fmt::remove_cvref_t<decltype(s[0])>;      \
-      FMT_MAYBE_UNUSED __VA_ARGS__ FMT_CONSTEXPR                  \
+      FMT_MAYBE_UNUSED FMT_CONSTEXPR                              \
       operator fmt::basic_string_view<char_type>() const {        \
         return fmt::detail::compile_string_to_view<char_type>(s); \
       }                                                           \
@@ -2769,7 +2773,7 @@ FMT_CONSTEXPR basic_string_view<Char> compile_string_to_view(
     std::string s = format(FMT_STRING("{:d}"), "foo");
   \endrst
  */
-#define FMT_STRING(s) FMT_STRING_IMPL(s, )
+#define FMT_STRING(s) FMT_STRING_IMPL(s, fmt::compile_string)
 
 template <typename... Args, typename S,
           enable_if_t<(is_compile_string<S>::value), int>>
@@ -3344,9 +3348,14 @@ join(const Range& range, wstring_view sep) {
     std::string answer = fmt::to_string(42);
   \endrst
  */
-template <typename T> inline std::string to_string(const T& value) {
+template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
+inline std::string to_string(const T& value) {
   return format("{}", value);
 }
+template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
+inline std::string to_string(T value) {
+  return format_int(value).str();
+}
 
 /**
   Converts *value* to ``std::wstring`` using the default format for type *T*.