Allow skipping invalid fields when marshalling (#2967)

Currently unexported fields refuse to marshal or unmarshal, even if
they're told to skip. Now, they can be skipped. By default, they are
still errors.

go/marshal/decode.go

@@ -254,13 +254,13 @@ func structDecoder(t reflect.Type) decoderFunc {
 	fields := make([]decField, 0, t.NumField())
 	for i := 0; i < t.NumField(); i++ {
 		f := t.Field(i)
-		validateField(f, t)
-
 		tags := getTags(f)
 		if tags.skip {
 			continue
 		}
 
+		validateField(f, t)
+
 		fields = append(fields, decField{
 			name:      tags.name,
 			decoder:   typeDecoder(f.Type, tags),

go/marshal/decode_test.go

@@ -813,3 +813,18 @@ func TestDecodeOriginalReceiveTypeError(t *testing.T) {
 	assert.Error(err)
 	assert.Equal(`Cannot unmarshal struct S {} into Go value of type marshal.S, field with tag "original" must have type Struct`, err.Error())
 }
+
+func TestDecodeCanSkipUnexportedField(t *testing.T) {
+	assert := assert.New(t)
+
+	type S struct {
+		Abc         int
+		notExported bool `noms:"-"`
+	}
+	var s S
+	err := Unmarshal(types.NewStruct("S", types.StructData{
+		"abc": types.Number(42),
+	}), &s)
+	assert.NoError(err)
+	assert.Equal(S{42, false}, s)
+}

go/marshal/encode.go

@@ -356,17 +356,17 @@ func typeFields(t reflect.Type, parentStructTypes []reflect.Type) (fields fieldS
 	canComputeStructType := true
 	for i := 0; i < t.NumField(); i++ {
 		f := t.Field(i)
+		tags := getTags(f)
+		if tags.skip {
+			continue
+		}
+
 		validateField(f, t)
 		nt := nomsType(f.Type, parentStructTypes)
 		if nt == nil {
 			canComputeStructType = false
 		}
 
-		tags := getTags(f)
-		if tags.skip {
-			continue
-		}
-
 		if tags.omitEmpty {
 			canComputeStructType = false
 		}

go/marshal/encode_test.go

@@ -632,6 +632,21 @@ func TestInvalidTag(t *testing.T) {
 	assert.Equal(t, `Unrecognized tag: omitEmpty`, err.Error())
 }
 
+func TestEncodeCanSkipUnexportedField(t *testing.T) {
+	assert := assert.New(t)
+
+	type S struct {
+		Abc         int
+		notExported bool `noms:"-"`
+	}
+	s := S{42, true}
+	v, err := Marshal(s)
+	assert.NoError(err)
+	assert.True(types.NewStruct("S", types.StructData{
+		"abc": types.Number(42),
+	}).Equals(v))
+}
+
 type TestInterface interface {
 	M()
 }