internal/core/adt: fix dropping of field

Previously, inline struct were reapproriated
by linking them into the destination struct
by setting the parent and label. However,
labels are used to look up fields. This resulted
in fields being dropped.

To keep access to the field, rather than overwriting
it, we now point the parent to the node in which it is
shared, rather than its parent. This introduces an
additional path component upon traversal, which
we subsequently adjust for in the code that generates
paths.

Issue #3601

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I41987d4201ce34b25904f06afaf3e5474fbf926d
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1205232
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Matthew Sackman <matthew@cue.works>

cue/testdata/eval/sharing.txtar

@@ -75,10 +75,7 @@ Disjuncts:    48
         }
       }
       out: (struct){
-        b: (_|_){
-          // [incomplete] issue3601.ok1.out.b: undefined field: a:
-          //     ./in.cue:12:11
-        }
+        b: (_){ _ }
       }
     }
   }
@@ -149,7 +146,7 @@ diff old new
        }
        X: (struct){
          a: (struct){
-@@ -31,47 +24,40 @@
+@@ -31,47 +24,37 @@
            b: (_){ _ }
          }
        }
@@ -161,10 +158,7 @@ diff old new
 -  debug: (_|_){
 -    // [eval]
 +      out: (struct){
-+        b: (_|_){
-+          // [incomplete] issue3601.ok1.out.b: undefined field: a:
-+          //     ./in.cue:12:11
-+        }
++        b: (_){ _ }
 +      }
 +    }
 +  }

internal/core/adt/composite.go

@@ -1479,7 +1479,10 @@ func appendPath(a []Feature, v *Vertex) []Feature {
 		return a
 	}
 	a = appendPath(a, v.Parent)
-	if v.Label != 0 {
+	// Skip if the node is a structure-shared node that has been assingned to
+	// the parent as it's new location: in this case the parent node will
+	// have the desired label.
+	if v.Label != 0 && v.Parent.BaseValue != v {
 		// A Label may be 0 for programmatically inserted nodes.
 		a = append(a, v.Label)
 	}

internal/core/adt/share.go

@@ -79,7 +79,6 @@ func (n *nodeContext) finalizeSharing() {
 		// its assigned location.
 		if v.state != nil && v.state.parent != nil {
 			v.Parent = v.state.parent
-			v.Label = n.node.Label
 
 			// TODO: see if this can be removed and why some errors are not
 			// propagated when removed.
@@ -112,7 +111,7 @@ func (n *nodeContext) share(c Conjunct, arc *Vertex, id CloseInfo) {
 		// Setting the parent in the state instead allows us to defer setting
 		// Parent until it is safe to do so..
 		if s := arc.getState(n.ctx); s != nil {
-			s.parent = n.node.Parent
+			s.parent = n.node
 		}
 	}