From 91114a6fa48e2eb9399c23938eb12fdbd4fcda42 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Thu, 16 Nov 2023 12:44:10 +0100
Subject: [PATCH] ExprCall::eval: Heap allocate at arity 5+

---
 src/libexpr/eval.cc | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 1425eab97..bfbda52ef 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -1766,8 +1766,13 @@ void ExprCall::eval(EvalState & state, Env & env, Value & v)
     Value vFun;
     fun->eval(state, env, vFun);
 
-
-    boost::container::small_vector<Value *, 64> vArgs(args.size());
+    // Empirical arity of Nixpkgs lambdas by regex e.g. ([a-zA-Z]+:(\s|(/\*.*\/)|(#.*\n))*){5}
+    // 2: over 4000
+    // 3: about 300
+    // 4: about 60
+    // 5: under 10
+    // This excluded attrset lambdas (`{...}:`). Contributions of mixed lambdas appears insignificant at ~150 total.
+    boost::container::small_vector<Value *, 4> vArgs(args.size());
     for (size_t i = 0; i < args.size(); ++i)
         vArgs[i] = args[i]->maybeThunk(state, env);