lib/fmt/ToBuffer: new library

src/system/FmtError.cxx

@@ -28,8 +28,7 @@
  */
 
 #include "FmtError.hxx"
-
-#include <fmt/format.h>
+#include "lib/fmt/ToBuffer.hxx"
 
 #include <array>
 
@@ -37,11 +36,8 @@ std::system_error
 VFmtSystemError(std::error_code code,
 		fmt::string_view format_str, fmt::format_args args) noexcept
 {
-	std::array<char, 512> buffer;
-	auto [p, _] = fmt::vformat_to_n(buffer.begin(), buffer.size() - 1,
-					format_str, args);
-	*p = 0;
-	return std::system_error{code, buffer.data()};
+	const auto msg = VFmtBuffer<512>(format_str, args);
+	return std::system_error{code, msg};
 }
 
 #ifdef _WIN32