internal/core/adt: do not add arc dependencies for some nodes

Let and root nodes can never depend on their parent,
and as such, we should not add the dependency.

This is currently a no-op, but not doing so will break
once we implement the reduction of closeContext
graph depth optimization.

Issue #2850

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I6ddc07278a7a489a19b8f7237128655cb87c69bb
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1209180
Unity-Result: CUE porcuepine <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>

Author: mpvl

cue/testdata/cycle/issue990.txtar

@@ -87,7 +87,7 @@ Allocs: 1265
 Retain: 0
 
 Unifications: 243
-Conjuncts:    4276
+Conjuncts:    4197
 Disjuncts:    204
 -- out/evalalpha --
 (struct){
@@ -338,7 +338,7 @@ diff old new
 -Conjuncts:    12056
 -Disjuncts:    3258
 +Unifications: 243
-+Conjuncts:    4276
++Conjuncts:    4197
 +Disjuncts:    204
 -- diff/-out/evalalpha<==>+out/eval --
 diff old new

cue/testdata/eval/letjoin.txtar

@@ -139,7 +139,7 @@ Allocs: 222
 Retain: 0
 
 Unifications: 178
-Conjuncts:    1209
+Conjuncts:    1205
 Disjuncts:    36
 -- diff/-out/evalalpha/stats<==>+out/eval/stats --
 diff old new
@@ -161,7 +161,7 @@ diff old new
 -Conjuncts:    486
 -Disjuncts:    325
 +Unifications: 178
-+Conjuncts:    1209
++Conjuncts:    1205
 +Disjuncts:    36
 -- out/eval/stats --
 Leaks:  24

internal/core/adt/composite.go

@@ -335,6 +335,10 @@ func (v *Vertex) rootCloseContext(ctx *OpContext) *closeContext {
 	}
 	v._cc.incDependent(ctx, ROOT, nil) // matched in REF(decrement:nodeDone)
 
+	if f := v.Label; f.IsLet() || f == InvalidLabel {
+		return v._cc
+	}
+
 	if p := v.Parent; p != nil {
 		pcc := p.rootCloseContext(ctx)
 		// pcc.addDependency(ctx, ARC, false, v._cc, v._cc, v._cc)

internal/core/adt/dep.go

@@ -147,6 +147,10 @@ type ccDepRef struct {
 
 // addArc adds a dependent arc to c. If child is an arc, child.src == key
 func (c *closeContext) addArcDependency(ctx *OpContext, matched bool, child *closeContext) {
+	if f := child.src.Label; f.IsLet() || f == InvalidLabel {
+		return
+	}
+
 	root := child.src.cc()
 
 	// NOTE: do not increment