add DebugTrace for the current error

src/libcmd/command.cc

@@ -84,7 +84,7 @@ ref<EvalState> EvalCommand::getEvalState()
                 if (expr.staticenv)
                 {
                     auto vm = mapStaticEnvBindings(*expr.staticenv.get(), env);
-                    runRepl(evalState,  &error, *vm);
+                    runRepl(evalState,  &error, expr, *vm);
                 }
             };
     }

src/libcmd/command.hh

@@ -312,8 +312,10 @@ void printClosureDiff(
     const StorePath & afterPath,
     std::string_view indent);
 
+
 void runRepl(
     ref<EvalState> evalState,
     const Error *debugError,
+    const Expr &expr,
     const std::map<std::string, Value *> & extraEnv);
 }

src/libcmd/repl.cc

@@ -897,6 +897,7 @@ std::ostream & NixRepl::printValue(std::ostream & str, Value & v, unsigned int m
 void runRepl(
     ref<EvalState> evalState,
     const Error *debugError,
+    const Expr &expr,
     const std::map<std::string, Value *> & extraEnv)
 {
     auto repl = std::make_unique<NixRepl>(evalState);
@@ -905,6 +906,15 @@ void runRepl(
 
     repl->initEnv();
 
+    // tack on a final DebugTrace for the error position.
+    DebugTraceStacker ldts(
+          *evalState,
+          DebugTrace 
+                {.pos = debugError->info().errPos,
+                 .expr = expr,
+                 .hint = debugError->info().msg
+                });
+
     // add 'extra' vars.
     std::set<std::string> names;
     for (auto & [name, value] : extraEnv) {

src/libexpr/eval.cc

@@ -38,21 +38,6 @@ namespace nix {
 
 std::function<void(const Error & error, const Env & env, const Expr & expr)> debuggerHook;
 
-class DebugTraceStacker {
-  public:
-   DebugTraceStacker(EvalState &evalState, DebugTrace t)
-    :evalState(evalState), trace(t)
-    {
-        evalState.debugTraces.push_front(t);
-    }
-   ~DebugTraceStacker() {
-     // assert(evalState.debugTraces.front() == trace);
-     evalState.debugTraces.pop_front();
-   }
-   EvalState &evalState;
-   DebugTrace trace;
-};
-
 static char * dupString(const char * s)
 {
     char * t;

src/libexpr/eval.hh

@@ -76,11 +76,10 @@ std::shared_ptr<RegexCache> makeRegexCache();
 
 struct DebugTrace {
     std::optional<ErrPos> pos;
-    Expr &expr;
+    const Expr &expr;
     hintformat hint;
 };
 
-
 class EvalState
 {
 public:
@@ -406,6 +405,21 @@ private:
     friend void prim_match(EvalState & state, const Pos & pos, Value * * args, Value & v);
 };
 
+class DebugTraceStacker {
+    public:
+        DebugTraceStacker(EvalState &evalState, DebugTrace t)
+        :evalState(evalState), trace(t)
+        {
+            evalState.debugTraces.push_front(t);
+        }
+        ~DebugTraceStacker() 
+        {
+            // assert(evalState.debugTraces.front() == trace);
+            evalState.debugTraces.pop_front();
+        }
+        EvalState &evalState;
+        DebugTrace trace;
+};
 
 /* Return a string representing the type of the value `v'. */
 string showType(ValueType type);