add support for proteus:",(protobuf field id)" in golang tag

.travis.yml

@@ -1,8 +1,8 @@
 language: go
 
 go:
-  - 1.7
-  - 1.8
+  - 1.10.x
+  - 1.11.x
   - tip
 
 matrix:

example/categories/categories.go

@@ -5,4 +5,9 @@ package categories
 type CategoryOptions struct {
 	ShowPrices bool
 	CanBuy     bool
+	// The next field was deleted.
+	// Field3 int
+
+	// 4th field, use `proteus:",4"` forward compatible
+	Field4 int `proteus:",4"`
 }

protobuf/transform.go

@@ -256,6 +256,12 @@ func (t *Transformer) transformField(pkg *Package, msg *Message, field *scanner.
 		Repeated: repeated,
 	}
 
+	// If this field has set pos in tag,
+	// use customize protobuf positin.
+	if field.Pos != 0 {
+		f.Pos = field.Pos
+	}
+
 	// []byte is the only repeated type that maps to
 	// a non-repeated type in protobuf, so we handle
 	// it a bit differently.

protobuf/transform_test.go

@@ -270,6 +270,7 @@ func (s *TransformerSuite) TestTransformField() {
 		name     string
 		typ      scanner.Type
 		expected *Field
+		pos      int
 	}{
 		{
 			"Foo",
@@ -279,6 +280,7 @@ func (s *TransformerSuite) TestTransformField() {
 				Type:    NewBasic("int64"),
 				Options: Options{"(gogoproto.casttype)": NewStringValue("int")},
 			},
+			0,
 		},
 		{
 			"Bar",
@@ -288,6 +290,7 @@ func (s *TransformerSuite) TestTransformField() {
 				Type:    NewBasic("bytes"),
 				Options: Options{},
 			},
+			0,
 		},
 		{
 			"BazBar",
@@ -298,6 +301,7 @@ func (s *TransformerSuite) TestTransformField() {
 				Repeated: true,
 				Options:  Options{"(gogoproto.casttype)": NewStringValue("int")},
 			},
+			0,
 		},
 		{
 			"CustomID",
@@ -310,6 +314,7 @@ func (s *TransformerSuite) TestTransformField() {
 					"(gogoproto.casttype)":   NewStringValue("int"),
 				},
 			},
+			0,
 		},
 		{
 			"NullableType",
@@ -319,6 +324,7 @@ func (s *TransformerSuite) TestTransformField() {
 				Type:    NewNamed("my.pckg", "hello"),
 				Options: Options{},
 			},
+			0,
 		},
 		{
 			"NonNullableType",
@@ -330,11 +336,13 @@ func (s *TransformerSuite) TestTransformField() {
 					"(gogoproto.nullable)": NewLiteralValue("false"),
 				},
 			},
+			0,
 		},
 		{
 			"Invalid",
 			scanner.NewBasic("complex64"),
 			nil,
+			0,
 		},
 		{
 			"MyEnum",
@@ -344,6 +352,7 @@ func (s *TransformerSuite) TestTransformField() {
 				Type:    NewNamed("my.pckg", "MyEnum"),
 				Options: Options{},
 			},
+			0,
 		},
 		{
 			"MyAlias",
@@ -361,6 +370,7 @@ func (s *TransformerSuite) TestTransformField() {
 					"(gogoproto.casttype)": NewStringValue("my/pckg.MyAlias"),
 				},
 			},
+			0,
 		},
 		{
 			"MyRepeatedAlias",
@@ -376,6 +386,18 @@ func (s *TransformerSuite) TestTransformField() {
 				),
 				Options: Options{},
 			},
+			0,
+		},
+		{
+			"ProtoID",
+			scanner.NewBasic("int64"),
+			&Field{
+				Name:    "proto_id",
+				Type:    NewBasic("int64"),
+				Options: Options{"(gogoproto.customname)": NewStringValue("ProtoID")},
+				Pos:     101,
+			},
+			101,
 		},
 	}
 
@@ -387,13 +409,15 @@ func (s *TransformerSuite) TestTransformField() {
 		f := s.t.transformField(&Package{}, &Message{}, &scanner.Field{
 			Name: c.name,
 			Type: c.typ,
+			Pos:  c.pos,
 		}, 0)
 		if c.expected == nil {
 			s.Nil(f, c.name)
 		} else {
 			s.Equal(c.expected.Name, f.Name, fmt.Sprintf("Name in %s", c.name))
 			s.assertType(c.expected.Type, f.Type, c.name)
 			s.Equal(c.expected.Options, f.Options, fmt.Sprintf("Options in %s", c.name))
+			s.Equal(c.expected.Pos, f.Pos, fmt.Sprintf("Proto id in %d", c.pos))
 		}
 	}
 }

scanner/package.go

@@ -302,6 +302,7 @@ type Field struct {
 	Docs
 	Name string
 	Type Type
+	Pos  int
 }
 
 // Func is either a function or a method. Receiver will be nil in functions,

scanner/scanner.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 
@@ -294,6 +295,7 @@ func scanStruct(s *Struct, elem *types.Struct) *Struct {
 		f := &Field{
 			Name: v.Name(),
 			Type: scanType(v.Type()),
+			Pos:  getProtoID(v, tags),
 		}
 		if f.Type == nil {
 			continue
@@ -390,6 +392,17 @@ func isIgnoredField(f *types.Var, tags []string) bool {
 	return !f.Exported() || (len(tags) > 0 && tags[0] == "-")
 }
 
+func getProtoID(f *types.Var, tags []string) int {
+	if len(tags) < 2 {
+		return 0
+	}
+	i, err := strconv.Atoi(tags[1])
+	if err != nil {
+		return 0
+	}
+	return i
+}
+
 func objectsInScope(scope *types.Scope) (objs []types.Object) {
 	for _, n := range scope.Names() {
 		obj := scope.Lookup(n)

scanner/scanner_test.go

@@ -157,6 +157,36 @@ func TestScanStruct(t *testing.T) {
 				},
 			},
 		},
+		{
+			"struct with proto field id tag",
+			types.NewStruct(
+				[]*types.Var{
+					mkField("Foo", types.Typ[types.Int], false),
+				},
+				[]string{`proteus:",101"`},
+			),
+			&Struct{
+				Fields: []*Field{
+					{Name: "Foo", Type: NewBasic("int"), Pos: 101},
+				},
+			},
+		},
+		{
+			"invalid struct with proto field id tag",
+			types.NewStruct(
+				[]*types.Var{
+					mkField("Foo", types.Typ[types.Int], false),
+					mkField("Bar", types.Typ[types.Int], false),
+				},
+				[]string{`proteus:","`, `proteus:",lol"`},
+			),
+			&Struct{
+				Fields: []*Field{
+					{Name: "Foo", Type: NewBasic("int"), Pos: 0},
+					{Name: "Bar", Type: NewBasic("int"), Pos: 0},
+				},
+			},
+		},
 		{
 			"struct with unsupported type",
 			types.NewStruct(