gob: report an error when encoding a non-empty struct with

no public fields.
Fix a couple of tests caught out by this change.

R=rsc
CC=golang-dev
https://golang.org/cl/4044043

src/pkg/gob/codec_test.go

@@ -1088,11 +1088,11 @@ func (v Vector) Square() int {
 }
 
 type Point struct {
-	a, b int
+	X, Y int
 }
 
 func (p Point) Square() int {
-	return p.a*p.a + p.b*p.b
+	return p.X*p.X + p.Y*p.Y
 }
 
 // A struct with interfaces in it.

src/pkg/gob/encode.go

@@ -264,9 +264,6 @@ func encComplex128(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	}
 }
 
-func encNoOp(i *encInstr, state *encoderState, p unsafe.Pointer) {
-}
-
 // Byte arrays are encoded as an unsigned count followed by the raw bytes.
 func encUint8Array(i *encInstr, state *encoderState, p unsafe.Pointer) {
 	b := *(*[]byte)(p)
@@ -516,16 +513,18 @@ func (enc *Encoder) compileEnc(rt reflect.Type) *encEngine {
 	srt, isStruct := rt.(*reflect.StructType)
 	engine := new(encEngine)
 	if isStruct {
-		engine.instr = make([]encInstr, srt.NumField()+1) // +1 for terminator
-		for fieldnum := 0; fieldnum < srt.NumField(); fieldnum++ {
-			f := srt.Field(fieldnum)
-			op, indir := enc.encOpFor(f.Type)
+		for fieldNum := 0; fieldNum < srt.NumField(); fieldNum++ {
+			f := srt.Field(fieldNum)
 			if !isExported(f.Name) {
-				op = encNoOp
+				continue
 			}
-			engine.instr[fieldnum] = encInstr{op, fieldnum, indir, uintptr(f.Offset)}
+			op, indir := enc.encOpFor(f.Type)
+			engine.instr = append(engine.instr, encInstr{op, fieldNum, indir, uintptr(f.Offset)})
+		}
+		if srt.NumField() > 0 && len(engine.instr) == 0 {
+			errorf("type %s has no exported fields", rt)
 		}
-		engine.instr[srt.NumField()] = encInstr{encStructTerminator, 0, 0, 0}
+		engine.instr = append(engine.instr, encInstr{encStructTerminator, 0, 0, 0})
 	} else {
 		engine.instr = make([]encInstr, 1)
 		op, indir := enc.encOpFor(rt)

src/pkg/gob/encoder_test.go

@@ -220,7 +220,7 @@ func TestSlice(t *testing.T) {
 func TestValueError(t *testing.T) {
 	// Encode a *T, decode a T
 	type Type4 struct {
-		a int
+		A int
 	}
 	t4p := &Type4{3}
 	var t4 Type4 // note: not a pointer.