Make formatted_size part of the core API

doc/api.rst

@@ -48,6 +48,8 @@ participate in an overload resolution if the latter is not a string.
 .. doxygenfunction:: format(const S&, Args&&...)
 .. doxygenfunction:: vformat(const S&, basic_format_args<buffer_context<type_identity_t<Char>>>)
 
+.. doxygenfunction:: fmt::formatted_size(string_view, Args&&...)
+
 .. _print:
 
 .. doxygenfunction:: print(const S&, Args&&...)
@@ -300,8 +302,6 @@ Utilities
 
 .. doxygentypedef:: fmt::char_t
 
-.. doxygenfunction:: fmt::formatted_size(string_view, const Args&...)
-
 .. doxygenfunction:: fmt::to_string(const T&)
 
 .. doxygenfunction:: fmt::to_wstring(const T&)

include/fmt/core.h

@@ -269,8 +269,7 @@ struct monostate {};
 
 namespace detail {
 
-// A helper function to suppress bogus "conditional expression is constant"
+// A helper function to suppress "conditional expression is constant" warnings.
-// warnings.
 template <typename T> constexpr T const_check(T value) { return value; }
 
 FMT_NORETURN FMT_API void assert_fail(const char* file, int line,
@@ -797,9 +796,25 @@ class iterator_buffer<std::back_insert_iterator<Container>,
   }
 };
 
-template <typename Container>
+// A buffer that counts the number of code units written discarding the output.
-using container_buffer = iterator_buffer<std::back_insert_iterator<Container>,
+template <typename T = char> class counting_buffer : public buffer<T> {
-                                         typename Container::value_type>;
+ private:
+  enum { buffer_size = 256 };
+  T data_[buffer_size];
+  size_t count_ = 0;
+
+ protected:
+  void grow(size_t) final {
+    if (this->size() != buffer_size) return;
+    count_ += this->size();
+    this->clear();
+  }
+
+ public:
+  counting_buffer() : buffer<T>(data_, 0, buffer_size) {}
+
+  size_t count() { return count_ + this->size(); }
+};
 
 // An output iterator that appends to the buffer.
 // It is used to reduce symbol sizes for the common case.
@@ -1957,6 +1972,18 @@ inline OutputIt format_to(OutputIt out, const S& format_str, Args&&... args) {
   return vformat_to(out, to_string_view(format_str), vargs);
 }
 
+/**
+  Returns the number of characters in the output of
+  ``format(format_str, args...)``.
+ */
+template <typename... Args>
+inline size_t formatted_size(string_view format_str, Args&&... args) {
+  const auto& vargs = fmt::make_args_checked<Args...>(format_str, args...);
+  detail::counting_buffer<> buf;
+  detail::vformat_to(buf, format_str, vargs);
+  return buf.count();
+}
+
 template <typename S, typename Char = char_t<S>>
 FMT_INLINE std::basic_string<Char> vformat(
     const S& format_str,

include/fmt/format.h

@@ -3586,15 +3586,6 @@ std::basic_string<Char> detail::vformat(
   return to_string(buffer);
 }
 
-/**
-  Returns the number of characters in the output of
-  ``format(format_str, args...)``.
- */
-template <typename... Args>
-inline size_t formatted_size(string_view format_str, const Args&... args) {
-  return format_to(detail::counting_iterator(), format_str, args...).count();
-}
-
 template <typename Char, FMT_ENABLE_IF(std::is_same<Char, wchar_t>::value)>
 void vprint(std::FILE* f, basic_string_view<Char> format_str,
             wformat_args args) {