internal/core/export: treat empty data vertices as structs

The current algorithm treated them as `_`

Fixes #1131

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>

Change-Id: Ib8834b7574619c48b1b649551a91d55d2f0b8885
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/528072
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>

Author: mpvl

cue/cuecontext/cuecontext_test.go

@@ -35,6 +35,15 @@ func TestAPI(t *testing.T) {
 			return ctx.BuildExpr(expr)
 		},
 		want: `close({})`,
+	}, {
+		name: "issue1131",
+		fun: func() cue.Value {
+			m := make(map[string]interface{})
+			ctx := New()
+			cv := ctx.Encode(m)
+			return cv
+		},
+		want: "", // empty file.
 	}}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {

internal/core/export/export_test.go

@@ -203,6 +203,22 @@ func TestGenerated(t *testing.T) {
 		},
 		out: `"foo", #enum: 2`,
 		p:   export.All,
+	}, {
+		// Issue #1131
+		in: func(r *adt.OpContext) (adt.Expr, error) {
+			m := make(map[string]interface{})
+			v := ctx.Encode(m)
+			_, x := value.ToInternal(v)
+			return x, nil
+		},
+		out: ``, // empty file
+	}, {
+		in: func(r *adt.OpContext) (adt.Expr, error) {
+			v := &adt.Vertex{}
+			v.SetValue(r, adt.Finalized, &adt.StructMarker{})
+			return v, nil
+		},
+		out: ``, // empty file
 	}}
 	for _, tc := range testCases {
 		t.Run("", func(t *testing.T) {

internal/core/export/expr.go

@@ -173,7 +173,7 @@ func (x *exporter) mergeValues(label adt.Feature, src *adt.Vertex, a []conjunct,
 			if len(e.attrs) > 0 {
 				break
 			}
-			if len(e.structs) > 0 {
+			if len(e.structs) > 0 || e.isData {
 				return e.wrapCloseIfNecessary(s, src)
 			}
 			return ast.NewIdent("_")
@@ -278,6 +278,10 @@ type conjuncts struct {
 	fields      map[adt.Feature]field
 	attrs       []*ast.Attribute
 	hasEllipsis bool
+
+	// A value is a struct if it has a non-zero structs slice or if isData is
+	// set to true. Data vertices may not have conjuncts associated with them.
+	isData bool
 }
 
 func (c *conjuncts) addValueConjunct(src *adt.Vertex, env *adt.Environment, x adt.Expr) {
@@ -376,6 +380,7 @@ func (e *conjuncts) addExpr(env *adt.Environment, src *adt.Vertex, x adt.Expr, i
 
 			case v.IsData():
 				e.structs = append(e.structs, v.Structs...)
+				e.isData = true
 
 				if y, ok := v.BaseValue.(adt.Value); ok {
 					e.addValueConjunct(src, env, y)