From a4bfd559f1671f3bf9f0a8961d90bbd978cd3f08 Mon Sep 17 00:00:00 2001
From: Sergei Zimmerman <xokdvium@proton.me>
Date: Mon, 19 May 2025 19:53:20 +0000
Subject: [PATCH] libexpr: Use `attrs.alreadySorted()` in primop_functionArgs

Formals are already sorted as it's an invariant of `Formals`.
---
 src/libexpr/include/nix/expr/nixexpr.hh |  3 +++
 src/libexpr/primops.cc                  | 12 +++++++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/libexpr/include/nix/expr/nixexpr.hh b/src/libexpr/include/nix/expr/nixexpr.hh
index a5ce0fd89..6ede91948 100644
--- a/src/libexpr/include/nix/expr/nixexpr.hh
+++ b/src/libexpr/include/nix/expr/nixexpr.hh
@@ -306,6 +306,9 @@ struct Formal
 struct Formals
 {
     typedef std::vector<Formal> Formals_;
+    /**
+     * @pre Sorted according to predicate (std::tie(a.name, a.pos) < std::tie(b.name, b.pos)).
+     */
     Formals_ formals;
     bool ellipsis;
 
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index d13314ffb..c38eed267 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -3148,10 +3148,16 @@ static void prim_functionArgs(EvalState & state, const PosIdx pos, Value * * arg
         return;
     }
 
-    auto attrs = state.buildBindings(args[0]->payload.lambda.fun->formals->formals.size());
-    for (auto & i : args[0]->payload.lambda.fun->formals->formals)
+    const auto &formals = args[0]->payload.lambda.fun->formals->formals;
+    auto attrs = state.buildBindings(formals.size());
+    for (auto & i : formals)
         attrs.insert(i.name, state.getBool(i.def), i.pos);
-    v.mkAttrs(attrs);
+    /* Optimization: avoid sorting bindings. `formals` must already be sorted according to
+       (std::tie(a.name, a.pos) < std::tie(b.name, b.pos)) predicate, so the following assertion
+       always holds:
+       assert(std::is_sorted(attrs.alreadySorted()->begin(), attrs.alreadySorted()->end()));
+       .*/
+    v.mkAttrs(attrs.alreadySorted());
 }
 
 static RegisterPrimOp primop_functionArgs({