client/Response: add method Fmt() based on libfmt

src/client/Idle.cxx

@@ -22,6 +22,8 @@
 #include "Response.hxx"
 #include "Idle.hxx"
 
+#include <fmt/format.h>
+
 #include <cassert>
 
 static void
@@ -30,7 +32,7 @@ WriteIdleResponse(Response &r, unsigned flags) noexcept
 	const char *const*idle_names = idle_get_names();
 	for (unsigned i = 0; idle_names[i]; ++i) {
 		if (flags & (1 << i))
-			r.Format("changed: %s\n", idle_names[i]);
+			r.Fmt(FMT_STRING("changed: {}\n"), idle_names[i]);
 	}
 
 	r.Write("OK\n");

src/client/Response.cxx

@@ -22,6 +22,8 @@
 #include "util/FormatString.hxx"
 #include "util/AllocatedString.hxx"
 
+#include <fmt/format.h>
+
 TagMask
 Response::GetTagMask() const noexcept
 {
@@ -56,17 +58,21 @@ Response::Format(const char *fmt, ...) noexcept
 	return success;
 }
 
+bool
+Response::VFmt(fmt::string_view format_str, fmt::format_args args) noexcept
+{
+	fmt::memory_buffer buffer;
+	fmt::vformat_to(buffer, format_str, args);
+	return Write(buffer.data(), buffer.size());
+}
+
 bool
 Response::WriteBinary(ConstBuffer<void> payload) noexcept
 {
 	assert(payload.size <= client.binary_limit);
 
 	return
-#ifdef _WIN32
-		Format("binary: %lu\n", (unsigned long)payload.size) &&
-#else
-		Format("binary: %zu\n", payload.size) &&
-#endif
+		Fmt("binary: {}\n", payload.size) &&
 		Write(payload.data, payload.size) &&
 		Write("\n");
 }

src/client/Response.hxx

@@ -23,6 +23,11 @@
 #include "protocol/Ack.hxx"
 #include "util/Compiler.h"
 
+#include <fmt/core.h>
+#if FMT_VERSION < 70000
+#include <fmt/format.h>
+#endif
+
 #include <cstdarg>
 #include <cstddef>
 
@@ -79,6 +84,21 @@ public:
 	gcc_printf(2,3)
 	bool Format(const char *fmt, ...) noexcept;
 
+	bool VFmt(fmt::string_view format_str, fmt::format_args args) noexcept;
+
+	template<typename S, typename... Args>
+	bool Fmt(const S &format_str, Args&&... args) noexcept {
+#if FMT_VERSION >= 70000
+		return VFmt(fmt::to_string_view(format_str),
+			    fmt::make_args_checked<Args...>(format_str,
+							    args...));
+#else
+		/* expensive fallback for older libfmt versions */
+		const auto result = fmt::format(format_str, args...);
+		return Write(result.data(), result.size());
+#endif
+	}
+
 	/**
 	 * Write a binary chunk; this writes the "binary" line, the
 	 * given chunk and the trailing newline.