From 6dd271b7b4a679ce3c2a8d84a39909bb9b60bf9f Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Thu, 6 Jan 2022 15:11:37 +0100
Subject: [PATCH] withBuffer: avoid allocating a std::function

---
 src/libstore/derivations.cc | 2 +-
 src/libutil/util.hh         | 5 ++++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/libstore/derivations.cc b/src/libstore/derivations.cc
index 21ea84dbf..f8fe6a59c 100644
--- a/src/libstore/derivations.cc
+++ b/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)
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index 840acd67b..407505be9 100644
--- a/src/libutil/util.hh
+++ b/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);