From 8b7eb7400b166b1c2ef45a6d66999041f33c40bf Mon Sep 17 00:00:00 2001
From: Rebecca Turner <rbt@sent.as>
Date: Fri, 2 Feb 2024 17:41:34 -0800
Subject: [PATCH] Enter debugger on `builtins.trace` with an option

---
 src/libexpr/eval-settings.hh | 3 +++
 src/libexpr/primops.cc       | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/src/libexpr/eval-settings.hh b/src/libexpr/eval-settings.hh
index 2f6c12d45..757daebc0 100644
--- a/src/libexpr/eval-settings.hh
+++ b/src/libexpr/eval-settings.hh
@@ -127,6 +127,9 @@ struct EvalSettings : Config
 
     Setting<unsigned int> maxCallDepth{this, 10000, "max-call-depth",
         "The maximum function call depth to allow before erroring."};
+
+    Setting<bool> builtinsTraceDebugger{this, false, "builtins-trace-debugger",
+        "Whether to enter the debugger on `builtins.trace` calls."};
 };
 
 extern EvalSettings evalSettings;
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 5e2bbe16f..a24a2d018 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -995,6 +995,10 @@ static void prim_trace(EvalState & state, const PosIdx pos, Value * * args, Valu
         printError("trace: %1%", args[0]->string_view());
     else
         printError("trace: %1%", ValuePrinter(state, *args[0]));
+    if (evalSettings.builtinsTraceDebugger && state.debugRepl && !state.debugTraces.empty()) {
+        const DebugTrace & last = state.debugTraces.front();
+        state.runDebugRepl(nullptr, last.env, last.expr);
+    }
     state.forceValue(*args[1], pos);
     v = *args[1];
 }