Removes special format handling for fmt. (#2736)

* Removes special format handling for fmt. Regains test compatibility with fmt
1.10.0.

fixes #2735

* reverted std::vector back to filename_t and used pointer to array start likewise as fmt's implementation uses

* calc_filename buffer increase softened, exception is throw if buffer exceeds 4k, filename parameter renamed to match intend.

* calc_filetime based on std::put_time for simpler implementation

include/spdlog/sinks/daily_file_sink.h

@@ -13,6 +13,9 @@
 #include <spdlog/details/circular_q.h>
 #include <spdlog/details/synchronous_factory.h>
 
+#include <iostream>
+#include <sstream>
+#include <iomanip>
 #include <chrono>
 #include <cstdio>
 #include <ctime>
@@ -46,46 +49,15 @@ struct daily_filename_calculator
  */
 struct daily_filename_format_calculator
 {
-    static filename_t calc_filename(const filename_t &filename, const tm &now_tm)
+    static filename_t calc_filename(const filename_t &file_path, const tm &now_tm)
     {
-#ifdef SPDLOG_USE_STD_FORMAT
-        // adapted from fmtlib: https://github.com/fmtlib/fmt/blob/8.0.1/include/fmt/chrono.h#L522-L546
-
-        filename_t tm_format;
-        tm_format.append(filename);
-        // By appending an extra space we can distinguish an empty result that
-        // indicates insufficient buffer size from a guaranteed non-empty result
-        // https://github.com/fmtlib/fmt/issues/2238
-        tm_format.push_back(' ');
-
-        const size_t MIN_SIZE = 10;
-        filename_t buf;
-        buf.resize(MIN_SIZE);
-        for (;;)
-        {
-            size_t count = strftime(buf.data(), buf.size(), tm_format.c_str(), &now_tm);
-            if (count != 0)
-            {
-                // Remove the extra space.
-                buf.resize(count - 1);
-                break;
-            }
-            buf.resize(buf.size() * 2);
-        }
-
-        return buf;
+#if defined(_WIN32) && defined(SPDLOG_WCHAR_FILENAMES)
+      std::wstringstream stream;  
 #else
-        // generate fmt datetime format string, e.g. {:%Y-%m-%d}.
-        filename_t fmt_filename = fmt::format(SPDLOG_FMT_STRING(SPDLOG_FILENAME_T("{{:{}}}")), filename);
-
-        // MSVC doesn't allow fmt::runtime(..) with wchar, with fmtlib versions < 9.1.x
-#    if defined(_MSC_VER) && defined(SPDLOG_WCHAR_FILENAMES) && FMT_VERSION < 90101
-        return fmt::format(fmt_filename, now_tm);
-#    else
-        return fmt::format(SPDLOG_FMT_RUNTIME(fmt_filename), now_tm);
-#    endif
-
+      std::stringstream stream;
 #endif
+      stream << std::put_time(&now_tm, file_path.c_str()); 
+      return stream.str();
     }
 
 private: