withBuffer: avoid allocating a std::function

src/libstore/derivations.cc

@@ -273,7 +273,7 @@ Derivation parseDerivation(const Store & store, std::string && s, std::string_vi
 static void printString(string & res, std::string_view s)
 {
     size_t bufSize = s.size() * 2 + 2;
-    withBuffer<void, char>(bufSize, [&](char buf[]) {
+    withBuffer(bufSize, [&](char *buf) {
         char * p = buf;
         *p++ = '"';
         for (auto c : s)

src/libutil/util.hh

@@ -671,7 +671,10 @@ template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
 
 std::string showBytes(uint64_t bytes);
 
-template<typename R = void, typename T = char> inline R withBuffer(size_t size, std::function<R (T[])> fun) {
+template<typename T = char, typename Fn>
+inline auto withBuffer(size_t size, Fn fun)
+  -> std::invoke_result_t<Fn, T *>
+{
     if (size < 0x10000) {
         T buf[size];
         return fun(buf);