From f2e11ddce1ef76d9f653e0c32659d46ff7d6cafa Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Fri, 8 Mar 2024 09:30:33 +0100
Subject: [PATCH] Merge pull request #9914 from 9999years/debugger-on-trace

Enter debugger on `builtins.trace` with an option

(cherry picked from commit 774e7ca5847ebc392eac2a124a8f12b24da4f65a)
Change-Id: If01e2110b3a128e639b05143227e365227d149f1
---
 doc/manual/rl-next/debugger-on-trace.md |  9 +++++++++
 src/libexpr/eval-settings.hh            | 10 ++++++++++
 src/libexpr/primops.cc                  | 10 ++++++++++
 3 files changed, 29 insertions(+)
 create mode 100644 doc/manual/rl-next/debugger-on-trace.md

diff --git a/doc/manual/rl-next/debugger-on-trace.md b/doc/manual/rl-next/debugger-on-trace.md
new file mode 100644
index 000000000..721928550
--- /dev/null
+++ b/doc/manual/rl-next/debugger-on-trace.md
@@ -0,0 +1,9 @@
+---
+synopsis: Enter the `--debugger` when `builtins.trace` is called if `debugger-on-trace` is set
+prs: 9914
+---
+
+If the `debugger-on-trace` option is set and `--debugger` is given,
+`builtins.trace` calls will behave similarly to `builtins.break` and will enter
+the debug REPL. This is useful for determining where warnings are being emitted
+from.
diff --git a/src/libexpr/eval-settings.hh b/src/libexpr/eval-settings.hh
index 7a92b9728..c78213255 100644
--- a/src/libexpr/eval-settings.hh
+++ b/src/libexpr/eval-settings.hh
@@ -114,6 +114,16 @@ 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, "debugger-on-trace",
+        R"(
+          If set to true and the `--debugger` flag is given,
+          [`builtins.trace`](@docroot@/language/builtins.md#builtins-trace) will
+          enter the debugger like
+          [`builtins.break`](@docroot@/language/builtins.md#builtins-break).
+
+          This is useful for debugging warnings in third-party Nix code.
+        )"};
 };
 
 extern EvalSettings evalSettings;
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index 30933e191..b5b28efcd 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -980,6 +980,10 @@ static void prim_trace(EvalState & state, const PosIdx pos, Value * * args, Valu
         printError("trace: %1%", args[0]->string.s);
     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];
 }
@@ -991,6 +995,12 @@ static RegisterPrimOp primop_trace({
       Evaluate *e1* and print its abstract syntax representation on
       standard error. Then return *e2*. This function is useful for
       debugging.
+
+      If the
+      [`debugger-on-trace`](@docroot@/command-ref/conf-file.md#conf-debugger-on-trace)
+      option is set to `true` and the `--debugger` flag is given, the
+      interactive debugger will be started when `trace` is called (like
+      [`break`](@docroot@/language/builtins.md#builtins-break)).
     )",
     .fun = prim_trace,
 });