diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 6539a346a..1c787645d 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1106,9 +1106,7 @@ void EvalState::callPrimOp(Value & fun, Value & arg, Value & v, const Pos & pos)
 
 void EvalState::callFunction(Value & fun, Value & arg, Value & v, const Pos & pos)
 {
-    std::unique_ptr<FunctionCallTrace> trace;
-    if (evalSettings.traceFunctionCalls)
-        trace = std::make_unique<FunctionCallTrace>(pos);
+    auto trace = evalSettings.traceFunctionCalls ? std::make_unique<FunctionCallTrace>(pos) : nullptr;
 
     forceValue(fun, pos);
 
diff --git a/src/libexpr/function-trace.cc b/src/libexpr/function-trace.cc
new file mode 100644
index 000000000..af1486f78
--- /dev/null
+++ b/src/libexpr/function-trace.cc
@@ -0,0 +1,17 @@
+#include "function-trace.hh"
+
+namespace nix {
+
+FunctionCallTrace::FunctionCallTrace(const Pos & pos) : pos(pos) {
+    auto duration = std::chrono::high_resolution_clock::now().time_since_epoch();
+    auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(duration);
+    printMsg(lvlInfo, "function-trace entered %1% at %2%", pos, ns.count());
+}
+
+FunctionCallTrace::~FunctionCallTrace() {
+    auto duration = std::chrono::high_resolution_clock::now().time_since_epoch();
+    auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(duration);
+    printMsg(lvlInfo, "function-trace exited %1% at %2%", pos, ns.count());
+}
+
+}
diff --git a/src/libexpr/function-trace.hh b/src/libexpr/function-trace.hh
index 2c39b7430..472f2045e 100644
--- a/src/libexpr/function-trace.hh
+++ b/src/libexpr/function-trace.hh
@@ -9,17 +9,7 @@ namespace nix {
 struct FunctionCallTrace
 {
     const Pos & pos;
-
-    FunctionCallTrace(const Pos & pos) : pos(pos) {
-        auto duration = std::chrono::high_resolution_clock::now().time_since_epoch();
-        auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(duration);
-        printMsg(lvlInfo, "function-trace entered %1% at %2%", pos, ns.count());
-    }
-
-    ~FunctionCallTrace() {
-        auto duration = std::chrono::high_resolution_clock::now().time_since_epoch();
-        auto ns = std::chrono::duration_cast<std::chrono::nanoseconds>(duration);
-        printMsg(lvlInfo, "function-trace exited %1% at %2%", pos, ns.count());
-    }
+    FunctionCallTrace(const Pos & pos);
+    ~FunctionCallTrace();
 };
 }