internal/core/adt: ensure emptyNode is inert

Currently, emptyNode is sometimes passed to
operations that request its nodeContext. Because it
was not marked as finalized, this would cause a
nodeContext to be created. This nodeContext would
then be initialized for a certain version.

We now set the global to be "finalized" and add tests
to verify this.

Fixes #3985

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I73313f1ea0af4c7d2032f1166556908871e1beb5
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1217744
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mpvl

cue/testdata/benchmarks/issue2176.txtar

@@ -58,13 +58,13 @@ output: {
 	"2": (datastream & {marker: length: 14}).marker.position
 }
 -- out/evalalpha/stats --
-Leaks:  449
+Leaks:  448
 Freed:  0
 Reused: 0
-Allocs: 449
+Allocs: 448
 Retain: 0
 
-Unifications: 377
+Unifications: 376
 Conjuncts:    1333
 Disjuncts:    8
 
@@ -87,13 +87,13 @@ diff old new
 -
 -CloseIDElems: 0
 -NumCloseIDs: 12
-+Leaks:  449
++Leaks:  448
 +Freed:  0
 +Reused: 0
-+Allocs: 449
++Allocs: 448
 +Retain: 0
 +
-+Unifications: 377
++Unifications: 376
 +Conjuncts:    1333
 +Disjuncts:    8
 +

internal/core/adt/adt_test.go

@@ -0,0 +1,34 @@
+// Copyright 2025 CUE Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package adt
+
+import (
+	"testing"
+)
+
+// TestEmptyNodeInert checks that getting a node for an empty struct does not
+// cause a nodeContext to be created.
+func TestEmptyNodeInert(t *testing.T) {
+	// We run the evaluator twice with different versions of the evaluator. Each
+	// results in the use of emptyNode. Ensure that the it does not get
+	// assigned a nodeContext.
+
+	ctx := &OpContext{}
+
+	s := emptyNode.getBareState(ctx)
+	if s != nil {
+		t.Fatal("expected nil state for empty struct")
+	}
+}

internal/core/adt/context.go

@@ -1091,7 +1091,7 @@ func (c *OpContext) typeErrorAs(v Value, k Kind, as interface{}) {
 	}
 }
 
-var emptyNode = &Vertex{}
+var emptyNode = &Vertex{status: finalized}
 
 func pos(x Node) token.Pos {
 	if x.Source() == nil {

internal/core/adt/eval_test.go

@@ -195,6 +195,15 @@ func runEvalTest(t *cuetxtar.Test, version internal.EvaluatorVersion, flags cued
 	return
 }
 
+func TestIssue3985(t *testing.T) {
+	// We run the evaluator twice with different versions of the evaluator. Each
+	// results in the use of emptyNode. Ensure that the it does not get
+	// assigned a nodeContext.
+	cuecontext.New(cuecontext.EvaluatorVersion(cuecontext.EvalV3)).CompileString(`a!: _, b: [for c in a if a != _|_ {}]`)
+
+	cuecontext.New(cuecontext.EvaluatorVersion(cuecontext.EvalV2)).CompileString(`matchN(0, [_|_]) & []`)
+}
+
 // TestX is for debugging. Do not delete.
 func TestX(t *testing.T) {
 	adt.DebugDeps = true