From 37264ed0ad898cdd6880de8ce6e5dda7977eed5f Mon Sep 17 00:00:00 2001
From: Tuomas Tynkkynen <tuomas@tuxera.com>
Date: Fri, 16 Feb 2018 05:13:39 +0200
Subject: [PATCH] libexpr: Avoid an unnecessary string copy in
 prim_derivationStrict

---
 src/libexpr/primops.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index f6c34d525..a800d2429 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -553,7 +553,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
 
     for (auto & i : args[0]->attrs->lexicographicOrder()) {
         if (i->name == state.sIgnoreNulls) continue;
-        string key = i->name;
+        const string & key = i->name;
         vomit("processing attribute '%1%'", key);
 
         auto handleHashMode = [&](const std::string & s) {