internal/core/adt: use a named vertexStatus constant rather than 0

"unprocessed" is much clearer than 0, and since it's not an untyped
integer constant, it cannot be mixed up for other types.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: I1664686a59c8a69e9e3c493bc785de072babaf89
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198625
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>

Author: mvdan

internal/core/adt/disjunct.go

@@ -502,7 +502,7 @@ func clone(v Vertex) Vertex {
 			case finalized:
 				v.Arcs[i] = arc
 
-			case 0:
+			case unprocessed:
 				a := *arc
 				v.Arcs[i] = &a
 

internal/core/adt/eval.go

@@ -194,7 +194,7 @@ func (c *OpContext) unify(v *Vertex, flags combinedFlags) {
 		return
 
 	case evaluatingArcs:
-		Assertf(c, v.status > 0, "unexpected status %d", v.status)
+		Assertf(c, v.status > unprocessed, "unexpected status %d", v.status)
 		return
 
 	case 0:
@@ -1787,7 +1787,7 @@ func (n *nodeContext) addVertexConjuncts(c Conjunct, arc *Vertex, inline bool) {
 		closeInfo = closeInfo.SpawnRef(arc, IsDef(x), x)
 	}
 
-	if arc.status == 0 && !inline {
+	if arc.status == unprocessed && !inline {
 		// This is a rare condition, but can happen in certain
 		// evaluation orders. Unfortunately, adding this breaks
 		// resolution of cyclic mutually referring disjunctions. But it