internal/core/adt: remove special "tentative" mode

The new AllArcs state is sufficient to indicate the
partial evaluation needed for processing comprehensions.
The old special-casing through the tentative mode is thus
no longer needed.

Change-Id: I851b063cf886f053e7f71a0407471f53ceadada9
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8162
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

internal/core/adt/context.go

@@ -166,10 +166,6 @@ type OpContext struct {
 	// structural cycle errors.
 	vertex *Vertex
 
-	// TODO: remove use of tentative. Should be possible if incomplete
-	// handling is done better.
-	tentative int // set during comprehension evaluation
-
 	// These fields are used associate scratch fields for computing closedness
 	// of a Vertex. These fields could have been included in StructInfo (like
 	// Tomabechi's unification algorithm), but we opted for an indirection to
@@ -188,12 +184,6 @@ func (c *OpContext) Impl() Runtime {
 	return c.Runtime
 }
 
-// If IsTentative is set, evaluation of an arc should not finalize
-// to non-concrete values.
-func (c *OpContext) IsTentative() bool {
-	return c.tentative > 0
-}
-
 func (c *OpContext) Pos() token.Pos {
 	if c.src == nil {
 		return token.NoPos
@@ -431,12 +421,8 @@ func (c *OpContext) Validate(check Validator, value Value) *Bottom {
 func (c *OpContext) Yield(env *Environment, y Yielder, f YieldFunc) *Bottom {
 	s := c.PushState(env, y.Source())
 
-	c.tentative++
-
 	y.yield(c, f)
 
-	c.tentative--
-
 	return c.PopState(s)
 
 }

internal/core/adt/eval.go

@@ -152,24 +152,6 @@ func (e *Unifier) evaluate(c *OpContext, v *Vertex, state VertexStatus) Value {
 	switch x := v.BaseValue.(type) {
 	case *Bottom:
 		return x
-	case *Disjunction:
-		if x.NumDefaults == 1 {
-			if c.IsTentative() && isStruct(v) {
-				// TODO(perf): do something more efficient perhaps? This discards
-				// the computed arcs so far. Instead, we could have a separate
-				// marker to accumulate results. As this only happens within
-				// comprehensions, the effect is likely minimal, though.
-				arcs := v.Arcs
-				w := &Vertex{
-					Parent:    v.Parent,
-					BaseValue: &StructMarker{},
-					Arcs:      arcs,
-					Conjuncts: v.Conjuncts,
-				}
-				w.UpdateStatus(v.Status())
-				v = w
-			}
-		}
 
 	case nil:
 		if v.state != nil {
@@ -433,7 +415,7 @@ func (n *nodeContext) postDisjunct(state VertexStatus) {
 
 	default:
 		if n.node.BaseValue == cycle {
-			if !n.done() { // && !ctx.IsTentative() {
+			if !n.done() {
 				// collect incomplete errors.
 				var err *Bottom // n.incomplete
 				for _, d := range n.dynamicFields {