core/adt: Combine Vertex.Structs when inserting Conjunct for evalv3

Two issues:

1. The old evalv2 code, in nodeContext.addValueConjunct, does combine
   Vertex.Structs if the value being merged in is a Vertex. But the new
   evalv3 code, in nodeContext.insertValueConjunct, did not do that. Add
   the necessary code.

2. Even with (1), Vertex Structs from core/convert were being trimmed
   out because the StructLits were not initialised. So make sure whenever
   we create an adt.StructLit, in convert, we Initialize it.

Outcome:

Vertices that are returned from builtin functions, such as time.Split,
which have StructInfos/StructLits which themselves contain field
ordering information, are no longer lost. Consequently, toposort orders
such vertex fields correctly, matching the field order in the Go struct
declaration.

Fixes #3554. Better than before.

Signed-off-by: Matthew Sackman <[email protected]>
Change-Id: I5bb7ad09c328495758ed454fd539cf20fd550ef8
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1208079
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

internal/core/adt/conjunct.go

@@ -638,6 +638,7 @@ func (n *nodeContext) insertValueConjunct(env *Environment, v Value, id CloseInf
 				c := MakeConjunct(nil, a, id)
 				n.insertArc(a.Label, a.ArcType, c, id, true)
 			}
+			n.node.Structs = append(n.node.Structs, x.Structs...)
 
 		case Value:
 			n.insertValueConjunct(env, v, id)

internal/core/convert/go.go

@@ -416,6 +416,7 @@ func (c *goConverter) convertRec(nilIsTop bool, x interface{}) (result adt.Value
 
 		case reflect.Struct:
 			sl := &adt.StructLit{Src: c.setNextPos(ast.NewStruct())}
+			sl.Init(c.ctx)
 			v := &adt.Vertex{}
 
 			t := value.Type()
@@ -476,6 +477,7 @@ func (c *goConverter) convertRec(nilIsTop bool, x interface{}) (result adt.Value
 
 		case reflect.Map:
 			obj := &adt.StructLit{Src: c.setNextPos(ast.NewStruct())}
+			obj.Init(c.ctx)
 			v := &adt.Vertex{}
 
 			t := value.Type()

pkg/time/testdata/gen.txtar

@@ -41,12 +41,12 @@ parse: {
 }
 split: {
 	t1: {
+		year:       2017
+		month:      7
 		day:        14
 		hour:       2
 		minute:     40
-		month:      7
-		nanosecond: 123456
 		second:     0
-		year:       2017
+		nanosecond: 123456
 	}
 }