From 22e3f0e9875082be7f4eec8e3caeb134a7f1c05f Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Wed, 20 Mar 2024 13:45:36 +0100
Subject: [PATCH] libexpr: unbreak PosTable performance

this was mostly an inconvenience for error reporting, but fully broke
the debugger (because the debugger does *a lot* of eager position
resolution). copying the line offsets into a local and filling that
local when empty without also storing the calculated offsets back does
kind of ... not cache anything.

fixes https://git.lix.systems/lix-project/lix/issues/165

Change-Id: Iccb0ba193ce2f15c832978daecf7b9bebbbe8585
---
 src/libexpr/nixexpr.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libexpr/nixexpr.cc b/src/libexpr/nixexpr.cc
index e0f4def4c..fd380935a 100644
--- a/src/libexpr/nixexpr.cc
+++ b/src/libexpr/nixexpr.cc
@@ -593,7 +593,7 @@ Pos PosTable::operator[](PosIdx p) const
 
     Pos result{0, 0, origin->origin};
     auto lines = this->lines.lock();
-    auto linesForInput = (*lines)[origin->offset];
+    auto & linesForInput = (*lines)[origin->offset];
 
     if (linesForInput.empty()) {
         auto source = result.getSource().value_or("");