bugfix: empty structs/unions/exceptions failed to compile

gen/field.go

@@ -88,7 +88,9 @@ func (f fieldGroupGenerator) FromWire(g Generator) error {
 		<$v := newVar "v">
 		<$w := newVar "w">
 		func (<$v> *<.Name>) FromWire(<$w> <$wire>.Value) error {
-			var err error
+			<if len .Fields>
+				var err error
+			<end>
 			<$f := newVar "field">
 			for _, <$f> := range <$w>.GetStruct().Fields {
 				switch <$f>.ID {

gen/struct_test.go

@@ -647,3 +647,33 @@ func TestUnionFromWireInconsistencies(t *testing.T) {
 		}
 	}
 }
+
+func TestEmptyStruct(t *testing.T) {
+	var x, y ts.EmptyStruct
+	v := wire.NewValueStruct(wire.Struct{Fields: []wire.Field{}})
+
+	assert.Equal(t, x.ToWire(), v)
+	if assert.NoError(t, y.FromWire(v)) {
+		assert.Equal(t, x, y)
+	}
+}
+
+func TestEmptyUnion(t *testing.T) {
+	var x, y tu.EmptyUnion
+	v := wire.NewValueStruct(wire.Struct{Fields: []wire.Field{}})
+
+	assert.Equal(t, x.ToWire(), v)
+	if assert.NoError(t, y.FromWire(v)) {
+		assert.Equal(t, x, y)
+	}
+}
+
+func TestEmptyException(t *testing.T) {
+	var x, y tx.EmptyException
+	v := wire.NewValueStruct(wire.Struct{Fields: []wire.Field{}})
+
+	assert.Equal(t, x.ToWire(), v)
+	if assert.NoError(t, y.FromWire(v)) {
+		assert.Equal(t, x, y)
+	}
+}

gen/testdata/thrift/enums.thrift

@@ -1,3 +1,5 @@
+enum EmptyEnum {}
+
 enum EnumDefault {
     Foo, Bar, Baz
 }

gen/testdata/thrift/exceptions.thrift

@@ -1,3 +1,5 @@
+exception EmptyException {}
+
 exception DoesNotExistException {
     1: required string key
 }

gen/testdata/thrift/structs.thrift

@@ -1,3 +1,5 @@
+struct EmptyStruct {}
+
 //////////////////////////////////////////////////////////////////////////////
 // Structs with primitives
 

gen/testdata/thrift/unions.thrift

@@ -1,5 +1,7 @@
 include "./typedefs.thrift"
 
+union EmptyUnion {}
+
 union Document {
     1: typedefs.PDF pdf
     2: string plainText