Hide FunctionCallTrace constructor/destructor

This prevents them from being inlined. On gcc 9, this reduces the
stack size needed for

  nix-instantiate '<nixpkgs>' -A texlive.combined.scheme-full --dry-run

from 12.9 MiB to 4.8 MiB.

(cherry picked from commit cb90e382b5b6e177ea902b3909fd1897643ae3cd)

1 files changed, 1 insertions, 3 deletions

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index dc2520229381..330f0191a284 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1096,9 +1096,7 @@ 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);