From 985afeeb4d92f878b785f7906dd34679e7ba7688 Mon Sep 17 00:00:00 2001
From: eldritch horrors <pennae@lix.systems>
Date: Wed, 27 Nov 2024 02:09:08 +0100
Subject: [PATCH] libexpr: allocate debug state only when debugger is active

Change-Id: Id30f388264c5d1e472e3bdce5078db3914f3b475
---
 lix/libcmd/command.cc     |  2 +-
 lix/libcmd/repl.cc        | 41 +++++++++++----------
 lix/libexpr/eval-error.cc | 21 ++++++-----
 lix/libexpr/eval.cc       | 22 ++++++------
 lix/libexpr/eval.hh       | 10 ++++--
 lix/libexpr/nixexpr.cc    | 76 +++++++++++++++++++--------------------
 lix/libexpr/primops.cc    | 33 +++++++++--------
 7 files changed, 108 insertions(+), 97 deletions(-)

diff --git a/lix/libcmd/command.cc b/lix/libcmd/command.cc
index f02c3bbb5..10b5a6472 100644
--- a/lix/libcmd/command.cc
+++ b/lix/libcmd/command.cc
@@ -118,7 +118,7 @@ ref<EvalState> EvalCommand::getEvalState()
         evalState->repair = repair;
 
         if (startReplOnEvalErrors) {
-            evalState->debug.repl = &AbstractNixRepl::runSimple;
+            evalState->debug = std::make_unique<DebugState>(&AbstractNixRepl::runSimple);
         };
     }
     return ref<EvalState>(evalState);
diff --git a/lix/libcmd/repl.cc b/lix/libcmd/repl.cc
index 2b9eaa122..599086118 100644
--- a/lix/libcmd/repl.cc
+++ b/lix/libcmd/repl.cc
@@ -285,7 +285,7 @@ ReplExitStatus NixRepl::mainLoop()
 {
     if (isFirstRepl) {
         std::string_view debuggerNotice = "";
-        if (state.debug.inDebugger) {
+        if (state.debug && state.debug->inDebugger) {
             debuggerNotice = " debugger";
         }
         notice("Lix %1%%2%\nType :? for help.", nixVersion, debuggerNotice);
@@ -310,7 +310,9 @@ ReplExitStatus NixRepl::mainLoop()
         // number of chars as the prompt.
         if (!interacter->getLine(input, input.empty() ? ReplPromptType::ReplPrompt : ReplPromptType::ContinuationPrompt)) {
             // Ctrl-D should exit the debugger.
-            state.debug.stop = false;
+            if (state.debug) {
+                state.debug->stop = false;
+            }
             logger->cout("");
             // TODO: Should Ctrl-D exit just the current debugger session or
             // the entire program?
@@ -366,7 +368,7 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
             }
         }
 
-        if (state.debug.inDebugger) {
+        if (state.debug && state.debug->inDebugger) {
             for (auto const & colonCmd : this->DEBUG_COMMANDS) {
                 if (colonCmd.starts_with(prefix)) {
                     completions.insert(std::string(colonCmd));
@@ -416,9 +418,8 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
         }
     } else {
         /* Temporarily disable the debugger, to avoid re-entering readline. */
-        auto debug_repl = state.debug.repl;
-        state.debug.repl = nullptr;
-        Finally restoreDebug([&]() { state.debug.repl = debug_repl; });
+        auto debug = std::move(state.debug);
+        Finally restoreDebug([&]() { state.debug = std::move(debug); });
         try {
             /* This is an expression that should evaluate to an
                attribute set.  Evaluate it to get the names of the
@@ -485,7 +486,7 @@ void NixRepl::loadDebugTraceEnv(DebugTrace & dt)
 {
     initEnv();
 
-    auto se = state.debug.staticEnvFor(dt.expr);
+    auto se = state.debug->staticEnvFor(dt.expr);
     if (se) {
         auto vm = mapStaticEnvBindings(state.symbols, *se.get(), dt.env);
 
@@ -541,7 +542,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
              << "                               errors\n"
              << "  :?, :help                    Brings up this help menu\n"
              ;
-        if (state.debug.inDebugger) {
+        if (state.debug && state.debug->inDebugger) {
              std::cout
              << "\n"
              << "        Debug mode commands\n"
@@ -556,15 +557,15 @@ ProcessLineResult NixRepl::processLine(std::string line)
 
     }
 
-    else if (state.debug.inDebugger && (command == ":bt" || command == ":backtrace")) {
-        for (const auto & [idx, i] : enumerate(state.debug.traces)) {
+    else if (state.debug && state.debug->inDebugger && (command == ":bt" || command == ":backtrace")) {
+        for (const auto & [idx, i] : enumerate(state.debug->traces)) {
             std::cout << "\n" << ANSI_BLUE << idx << ANSI_NORMAL << ": ";
             showDebugTrace(std::cout, state.positions, i);
         }
     }
 
-    else if (state.debug.inDebugger && (command == ":env")) {
-        for (const auto & [idx, i] : enumerate(state.debug.traces)) {
+    else if (state.debug && state.debug->inDebugger && (command == ":env")) {
+        for (const auto & [idx, i] : enumerate(state.debug->traces)) {
             if (idx == debugTraceIndex) {
                 printEnvBindings(state, i.expr, i.env);
                 break;
@@ -572,13 +573,13 @@ ProcessLineResult NixRepl::processLine(std::string line)
         }
     }
 
-    else if (state.debug.inDebugger && (command == ":st")) {
+    else if (state.debug && state.debug->inDebugger && (command == ":st")) {
         try {
             // change the DebugTrace index.
             debugTraceIndex = stoi(arg);
         } catch (...) { }
 
-        for (const auto & [idx, i] : enumerate(state.debug.traces)) {
+        for (const auto & [idx, i] : enumerate(state.debug->traces)) {
              if (idx == debugTraceIndex) {
                  std::cout << "\n" << ANSI_BLUE << idx << ANSI_NORMAL << ": ";
                  showDebugTrace(std::cout, state.positions, i);
@@ -590,15 +591,15 @@ ProcessLineResult NixRepl::processLine(std::string line)
         }
     }
 
-    else if (state.debug.inDebugger && (command == ":s" || command == ":step")) {
+    else if (state.debug && state.debug->inDebugger && (command == ":s" || command == ":step")) {
         // set flag to stop at next DebugTrace; exit repl.
-        state.debug.stop = true;
+        state.debug->stop = true;
         return ProcessLineResult::Continue;
     }
 
-    else if (state.debug.inDebugger && (command == ":c" || command == ":continue")) {
+    else if (state.debug && state.debug->inDebugger && (command == ":c" || command == ":continue")) {
         // set flag to run to next breakpoint or end of program; exit repl.
-        state.debug.stop = false;
+        state.debug->stop = false;
         return ProcessLineResult::Continue;
     }
 
@@ -771,7 +772,9 @@ ProcessLineResult NixRepl::processLine(std::string line)
     }
 
     else if (command == ":q" || command == ":quit") {
-        state.debug.stop = false;
+        if (state.debug) {
+            state.debug->stop = false;
+        }
         return ProcessLineResult::Quit;
     }
 
diff --git a/lix/libexpr/eval-error.cc b/lix/libexpr/eval-error.cc
index 1fa43e81e..d32c5734d 100644
--- a/lix/libexpr/eval-error.cc
+++ b/lix/libexpr/eval-error.cc
@@ -45,12 +45,15 @@ EvalErrorBuilder<T> & EvalErrorBuilder<T>::withFrame(const Env & env, const Expr
     // NOTE: This is abusing side-effects.
     // TODO: check compatibility with nested debugger calls.
     // TODO: What side-effects??
-    error.state.debug.traces.push_front(DebugTrace{
-        .pos = error.state.positions[expr.getPos()],
-        .expr = expr,
-        .env = env,
-        .hint = HintFmt("Fake frame for debugging purposes"),
-        .isError = true});
+    if (error.state.debug) {
+        error.state.debug->traces.push_front(DebugTrace{
+            .pos = error.state.positions[expr.getPos()],
+            .expr = expr,
+            .env = env,
+            .hint = HintFmt("Fake frame for debugging purposes"),
+            .isError = true
+        });
+    }
     return *this;
 }
 
@@ -74,11 +77,11 @@ EvalErrorBuilder<T>::addTrace(PosIdx pos, std::string_view formatString, const A
 template<class T>
 void EvalErrorBuilder<T>::debugThrow()
 {
-    if (error.state.debug.repl && !error.state.debug.traces.empty()) {
-        const DebugTrace & last = error.state.debug.traces.front();
+    if (error.state.debug && !error.state.debug->traces.empty()) {
+        const DebugTrace & last = error.state.debug->traces.front();
         const Env * env = &last.env;
         const Expr * expr = &last.expr;
-        error.state.debug.runDebugRepl(error.state, &error, *env, *expr);
+        error.state.debug->runDebugRepl(error.state, &error, *env, *expr);
     }
 
     // `EvalState` is the only class that can construct an `EvalErrorBuilder`,
diff --git a/lix/libexpr/eval.cc b/lix/libexpr/eval.cc
index 7f9eefe74..81b907aea 100644
--- a/lix/libexpr/eval.cc
+++ b/lix/libexpr/eval.cc
@@ -587,7 +587,7 @@ void printEnvBindings(const SymbolTable & st, const StaticEnv & se, const Env &
 void printEnvBindings(const EvalState &es, const Expr & expr, const Env & env)
 {
     // just print the names for now
-    auto se = es.debug.staticEnvFor(expr);
+    auto se = es.debug->staticEnvFor(expr);
     if (se)
         printEnvBindings(es.symbols, *se, env, 0);
 }
@@ -641,7 +641,7 @@ void DebugState::runDebugRepl(
 )
 {
     // Make sure we have a debugger to run and we're not already in a debugger.
-    if (!repl || inDebugger)
+    if (inDebugger)
         return;
 
     auto dts =
@@ -717,9 +717,9 @@ DebugTraceStacker::DebugTraceStacker(EvalState & evalState, DebugTrace t)
     : evalState(evalState)
     , trace(std::move(t))
 {
-    evalState.debug.traces.push_front(trace);
-    if (evalState.debug.stop && evalState.debug.repl)
-        evalState.debug.runDebugRepl(evalState, nullptr, trace.env, trace.expr);
+    evalState.debug->traces.push_front(trace);
+    if (evalState.debug->stop && evalState.debug->repl)
+        evalState.debug->runDebugRepl(evalState, nullptr, trace.env, trace.expr);
 }
 
 inline Value * EvalState::lookupVar(Env * env, const ExprVar & var, bool noEval)
@@ -954,7 +954,7 @@ void EvalState::cacheFile(
     fileParseCache[resolvedPath] = e;
 
     try {
-        auto dts = debug.repl
+        auto dts = debug
             ? makeDebugTraceStacker(
                 *this,
                 *e,
@@ -1172,7 +1172,7 @@ void ExprLet::eval(EvalState & state, Env & env, Value & v)
             *i.second.chooseByKind(&env2, &env, inheritEnv));
     }
 
-    auto dts = state.debug.repl
+    auto dts = state.debug
         ? makeDebugTraceStacker(
             state,
             *this,
@@ -1257,7 +1257,7 @@ void ExprSelect::eval(EvalState & state, Env & env, Value & v)
     }
 
     try {
-        auto dts = state.debug.repl
+        auto dts = state.debug
             ? makeDebugTraceStacker(
                 state,
                 *this,
@@ -1575,7 +1575,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
 
             /* Evaluate the body. */
             try {
-                auto dts = debug.repl
+                auto dts = debug
                     ? makeDebugTraceStacker(
                         *this, *lambda.body, env2, positions[lambda.pos],
                         "while calling %s",
@@ -1714,7 +1714,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
 
 void ExprCall::eval(EvalState & state, Env & env, Value & v)
 {
-    auto dts = state.debug.repl
+    auto dts = state.debug
         ? makeDebugTraceStacker(
             state,
             *this,
@@ -2087,7 +2087,7 @@ void EvalState::forceValueDeep(Value & v)
             for (auto & i : *v.attrs)
                 try {
                     // If the value is a thunk, we're evaling. Otherwise no trace necessary.
-                    auto dts = debug.repl && i.value->isThunk()
+                    auto dts = debug && i.value->isThunk()
                         ? makeDebugTraceStacker(*this, *i.value->thunk.expr, *i.value->thunk.env, positions[i.pos],
                             "while evaluating the attribute '%1%'", symbols[i.name])
                         : nullptr;
diff --git a/lix/libexpr/eval.hh b/lix/libexpr/eval.hh
index 26a43f08b..50075961d 100644
--- a/lix/libexpr/eval.hh
+++ b/lix/libexpr/eval.hh
@@ -161,6 +161,12 @@ struct DebugState
     std::map<const Expr *, const std::shared_ptr<const StaticEnv>> exprEnvs;
     int trylevel = 0;
 
+    explicit DebugState(std::function<ReplExitStatus(EvalState & es, ValMap const & extraEnv)> repl)
+        : repl(repl)
+    {
+        assert(repl);
+    }
+
     void runDebugRepl(
         EvalState & evalState, const EvalError * error, const Env & env, const Expr & expr
     );
@@ -224,7 +230,7 @@ public:
     RootValue vCallFlake = nullptr;
     RootValue vImportedDrvToDerivation = nullptr;
 
-    DebugState debug;
+    std::unique_ptr<DebugState> debug;
 
     template<class T, typename... Args>
     [[nodiscard, gnu::noinline]]
@@ -759,7 +765,7 @@ struct DebugTraceStacker {
     DebugTraceStacker(EvalState & evalState, DebugTrace t);
     ~DebugTraceStacker()
     {
-        evalState.debug.traces.pop_front();
+        evalState.debug->traces.pop_front();
     }
     EvalState & evalState;
     DebugTrace trace;
diff --git a/lix/libexpr/nixexpr.cc b/lix/libexpr/nixexpr.cc
index 16a87b51a..c78b0605e 100644
--- a/lix/libexpr/nixexpr.cc
+++ b/lix/libexpr/nixexpr.cc
@@ -302,32 +302,32 @@ void Expr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env
 
 void ExprInt::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 }
 
 void ExprFloat::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 }
 
 void ExprString::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 }
 
 void ExprPath::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 }
 
 void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     fromWith = nullptr;
 
@@ -364,14 +364,14 @@ void ExprVar::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
 
 void ExprInheritFrom::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 }
 
 void ExprSelect::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     e->bindVars(es, env);
     if (def) def->bindVars(es, env);
@@ -382,8 +382,8 @@ void ExprSelect::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
 
 void ExprOpHasAttr::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     e->bindVars(es, env);
     for (auto & i : attrPath)
@@ -414,8 +414,8 @@ std::shared_ptr<const StaticEnv> ExprAttrs::bindInheritSources(
 
 void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     if (recursive) {
         auto newEnv = [&] () -> std::shared_ptr<const StaticEnv> {
@@ -453,8 +453,8 @@ void ExprAttrs::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
 
 void ExprList::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     for (auto & i : elems)
         i->bindVars(es, env);
@@ -462,8 +462,8 @@ void ExprList::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
 
 void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     auto newEnv = std::make_shared<StaticEnv>(
         nullptr, env.get(),
@@ -489,8 +489,8 @@ void ExprLambda::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
 
 void ExprCall::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     fun->bindVars(es, env);
     for (auto & e : args)
@@ -514,16 +514,16 @@ void ExprLet::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
     for (auto & i : attrs->attrs)
         i.second.e->bindVars(es, i.second.chooseByKind(newEnv, env, inheritFromEnv));
 
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     body->bindVars(es, newEnv);
 }
 
 void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     parentWith = nullptr;
     for (auto * e = env.get(); e && !parentWith; e = e->up)
@@ -548,8 +548,8 @@ void ExprWith::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> &
 
 void ExprIf::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     cond->bindVars(es, env);
     then->bindVars(es, env);
@@ -558,8 +558,8 @@ void ExprIf::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & e
 
 void ExprAssert::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     cond->bindVars(es, env);
     body->bindVars(es, env);
@@ -567,16 +567,16 @@ void ExprAssert::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv>
 
 void ExprOpNot::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     e->bindVars(es, env);
 }
 
 void ExprConcatStrings::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 
     for (auto & i : this->es)
         i.second->bindVars(es, env);
@@ -584,8 +584,8 @@ void ExprConcatStrings::bindVars(EvalState & es, const std::shared_ptr<const Sta
 
 void ExprPos::bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env)
 {
-    if (es.debug.repl)
-        es.debug.exprEnvs.insert(std::make_pair(this, env));
+    if (es.debug)
+        es.debug->exprEnvs.insert(std::make_pair(this, env));
 }
 
 
diff --git a/lix/libexpr/primops.cc b/lix/libexpr/primops.cc
index b6662c638..c59903469 100644
--- a/lix/libexpr/primops.cc
+++ b/lix/libexpr/primops.cc
@@ -566,15 +566,15 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value * * a
 
 static void prim_break(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    if (state.debug.repl && !state.debug.traces.empty()) {
+    if (state.debug && !state.debug->traces.empty()) {
         auto error = EvalError(state, ErrorInfo {
             .level = lvlInfo,
             .msg = HintFmt("breakpoint reached"),
             .pos = state.positions[pos],
         });
 
-        auto & dt = state.debug.traces.front();
-        state.debug.runDebugRepl(state, &error, dt.env, dt.expr);
+        auto & dt = state.debug->traces.front();
+        state.debug->runDebugRepl(state, &error, dt.env, dt.expr);
     }
 
     // Return the value we were passed.
@@ -637,15 +637,14 @@ static void prim_tryEval(EvalState & state, const PosIdx pos, Value * * args, Va
 {
     auto attrs = state.buildBindings(2);
 
-    /* increment state.trylevel, and decrement it when this function returns. */
-    MaintainCount trylevel(state.debug.trylevel);
-
-    std::function<ReplExitStatus(EvalState & es, ValMap const & extraEnv)> savedDebugRepl;
-    if (state.debug.repl && evalSettings.ignoreExceptionsDuringTry)
-    {
-        /* to prevent starting the repl from exceptions withing a tryEval, null it. */
-        savedDebugRepl = state.debug.repl;
-        state.debug.repl = nullptr;
+    std::optional<MaintainCount<int>> trylevel;
+    std::unique_ptr<DebugState> savedDebug;
+    if (state.debug) {
+        trylevel.emplace(state.debug->trylevel);
+        if (evalSettings.ignoreExceptionsDuringTry) {
+            /* to prevent starting the repl from exceptions withing a tryEval, null it. */
+            savedDebug = std::move(state.debug);
+        }
     }
 
     try {
@@ -658,8 +657,8 @@ static void prim_tryEval(EvalState & state, const PosIdx pos, Value * * args, Va
     }
 
     // restore the debugRepl pointer if we saved it earlier.
-    if (savedDebugRepl)
-        state.debug.repl = savedDebugRepl;
+    if (savedDebug)
+        state.debug = std::move(savedDebug);
 
     v.mkAttrs(attrs);
 }
@@ -697,9 +696,9 @@ 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.debug.repl && !state.debug.traces.empty()) {
-        const DebugTrace & last = state.debug.traces.front();
-        state.debug.runDebugRepl(state, nullptr, last.env, last.expr);
+    if (evalSettings.builtinsTraceDebugger && state.debug && !state.debug->traces.empty()) {
+        const DebugTrace & last = state.debug->traces.front();
+        state.debug->runDebugRepl(state, nullptr, last.env, last.expr);
     }
     state.forceValue(*args[1], pos);
     v = *args[1];