From 137673de5610ce20d03a0a79e242eb64a5c0f615 Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Mon, 4 Mar 2024 07:39:12 +0100
Subject: [PATCH] Merge pull request #9681 from edolstra/eval-optimisations

Optimize empty list constants

(cherry picked from commit 315aade89d00c692715e5953c36a1b7d6528b703)
Change-Id: I0f28ef8a27ccedc45acf44243eec9dc35b733300
---
 src/libexpr/eval.cc    | 11 +++++++++++
 src/libexpr/eval.hh    |  5 +++++
 src/libexpr/nixexpr.hh |  1 +
 3 files changed, 17 insertions(+)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 0fa686701..309b9f103 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -527,6 +527,8 @@ EvalState::EvalState(
 
     static_assert(sizeof(Env) <= 16, "environment must be <= 16 bytes");
 
+    vEmptyList.mkList(0);
+
     /* Initialise the Nix expression search path. */
     if (!evalSettings.pureEval) {
         for (auto & i : _searchPath.elements)
@@ -1405,6 +1407,15 @@ void ExprList::eval(EvalState & state, Env & env, Value & v)
 }
 
 
+Value * ExprList::maybeThunk(EvalState & state, Env & env)
+{
+    if (elems.empty()) {
+        return &state.vEmptyList;
+    }
+    return Expr::maybeThunk(state, env);
+}
+
+
 void ExprVar::eval(EvalState & state, Env & env, Value & v)
 {
     Value * v2 = state.lookupVar(&env, *this, false);
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 6adf6d066..9cf50ba6c 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -218,6 +218,11 @@ public:
 
     Bindings emptyBindings;
 
+    /**
+     * Empty list constant.
+     */
+    Value vEmptyList;
+
     const SourcePath derivationInternal;
 
     /**
diff --git a/src/libexpr/nixexpr.hh b/src/libexpr/nixexpr.hh
index 2b5b72bc0..b38f19b41 100644
--- a/src/libexpr/nixexpr.hh
+++ b/src/libexpr/nixexpr.hh
@@ -298,6 +298,7 @@ struct ExprList : Expr
     std::vector<Expr *> elems;
     ExprList() { };
     COMMON_METHODS
+    Value * maybeThunk(EvalState & state, Env & env) override;
 
     PosIdx getPos() const override
     {