internal/core/adt: use Vertex.Indirect in more places

This prepares for several things, including the
implementation of disjunctions and structure sharing.

Issue #2851
Issue #2884
Issue #2854

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I831d5419e6b035667aae6f254f45456b34dfd93c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1190799
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

internal/core/adt/composite.go

@@ -908,6 +908,11 @@ func (v *Vertex) IsList() bool {
 func (v *Vertex) Lookup(f Feature) *Vertex {
 	for _, a := range v.Arcs {
 		if a.Label == f {
+			// TODO(share): this indirection should ultimately be eliminated:
+			// the original node may have useful information (like original
+			// conjuncts) that are eliminated after indirection.
+			// We should leave it up to the user of Lookup at what point an
+			// indirection is necessary.
 			a = a.Indirect()
 			return a
 		}

internal/core/adt/context.go

@@ -703,6 +703,9 @@ func (c *OpContext) evalState(v Expr, state combinedFlags) (result Value) {
 		if arc == nil {
 			return nil
 		}
+		// TODO: consider moving this after markCycle, depending on how we
+		// implement markCycle.
+		arc = arc.Indirect()
 
 		// Save the old CloseInfo and restore after evaluate to avoid detecting
 		// spurious cycles.
@@ -823,6 +826,9 @@ func (c *OpContext) unifyNode(v Expr, state combinedFlags) (result Value) {
 		if v == nil {
 			return nil
 		}
+		// TODO: consider moving this after markCycle, depending on how we
+		// implement markCycle.
+		v = v.Indirect()
 
 		if c.isDevVersion() {
 			if n := v.getState(c); n != nil {

internal/core/adt/tasks.go

@@ -89,6 +89,9 @@ func processResolver(ctx *OpContext, t *task, mode runMode) {
 		// TODO: yield instead?
 		return
 	}
+	// TODO: consider moving after markCycle or removing.
+	arc = arc.Indirect()
+
 	// A reference that points to itself indicates equality. In that case
 	// we are done computing and we can return the arc as is.
 	ci, skip := t.node.markCycle(arc, t.env, r, t.id)

internal/core/adt/unify.go

@@ -122,6 +122,17 @@ func (v *Vertex) unify(c *OpContext, needs condition, mode runMode) bool {
 
 	nodeOnlyNeeds := needs &^ (subFieldsProcessed)
 	n.process(nodeOnlyNeeds, mode)
+	w := v.Indirect() // Dereference the disjunction result.
+	if w != v {
+		if w.Closed {
+			// Should resolve with dereference.
+			v.Closed = true
+		}
+		v.ArcType = w.ArcType
+		v.ChildErrors = CombineErrors(nil, v.ChildErrors, w.ChildErrors)
+		v.Arcs = nil
+		return w.state.meets(needs)
+	}
 	n.updateScalar()
 
 	// First process all but the subfields.
@@ -431,6 +442,8 @@ func (v *Vertex) lookup(c *OpContext, pos token.Pos, f Feature, flags combinedFl
 	needs := flags.conditions()
 	runMode := flags.runMode()
 
+	v = v.Indirect()
+
 	c.Logf(c.vertex, "LOOKUP %v", f)
 
 	state := v.getState(c)

internal/core/debug/debug.go

@@ -161,6 +161,7 @@ func (w *printer) interpolation(x *adt.Interpolation) {
 func (w *printer) node(n adt.Node) {
 	switch x := n.(type) {
 	case *adt.Vertex:
+		x = x.Indirect()
 		var kind adt.Kind
 		if x.BaseValue != nil {
 			kind = x.BaseValue.Kind()