libexpr: make undefined var errors non-debuggable in the parser

this only affects undefined names outside of with scopes. sending those
errors to the debugger when enabled seems rather less useful; unlike in
the case of with scopes there's nothing meaningful to inspect. avoiding
the debugger also lets us avoid marking the entire parser `NeverAsync`.

cf #761

Change-Id: I3599b826ff5b101acf9a3ba70dcdf9e8e02067f0

lix/libexpr/eval-error.cc

@@ -85,6 +85,12 @@ void EvalErrorBuilder<T>::debugThrow() &&
     throw *error;
 }
 
+template<class T>
+void EvalErrorBuilder<T>::throw_() &&
+{
+    throw *error;
+}
+
 template class EvalErrorBuilder<EvalError>;
 template class EvalErrorBuilder<AssertionError>;
 template class EvalErrorBuilder<ThrownError>;

lix/libexpr/eval-error.hh

@@ -89,9 +89,14 @@ public:
     addTrace(PosIdx pos, std::string_view formatString, const Args &... formatArgs) &&;
 
     /**
-     * Throw the underlying exception.
+     * Throw the underlying exception, invoking the debug state callback.
      */
     [[gnu::noinline, gnu::noreturn]] void debugThrow() &&;
+
+    /**
+     * Throw the underlying exception, bypassing the debug state callback.
+     */
+    [[gnu::noinline, gnu::noreturn]] void throw_() &&;
 };
 
 }

lix/libexpr/nixexpr.cc

@@ -362,7 +362,7 @@ void ExprVar::bindVars(Evaluator & es, const std::shared_ptr<const StaticEnv> &
         es.errors.make<UndefinedVarError>(
             "undefined variable '%1%'",
             es.symbols[name]
-        ).atPos(pos).debugThrow();
+        ).atPos(pos).throw_();
     for (auto * e = env.get(); e && !fromWith; e = e->up)
         fromWith = e->isWith;
     this->level = withLevel;