Add support for MarshalAppend

Expose the more efficient `MarshalAppend` method added in v1.31 of the
protobuf runtime.

binary.go

@@ -53,22 +53,26 @@ func (b *binaryCodec) Unmarshal(binary []byte, msg any) error {
 }
 
 func (b *binaryCodec) Marshal(msg any) ([]byte, error) {
-	return b.marshalBinary(msg, false /* stable */)
+	return b.marshalBinary(nil, msg, false /* stable */)
 }
 
 func (b *binaryCodec) MarshalStable(msg any) ([]byte, error) {
-	return b.marshalBinary(msg, true /* stable */)
+	return b.marshalBinary(nil, msg, true /* stable */)
 }
 
-func (b *binaryCodec) marshalBinary(msg any, stable bool) ([]byte, error) {
+func (b *binaryCodec) MarshalAppend(dst []byte, msg any) ([]byte, error) {
+	return b.marshalBinary(dst, msg, false /* stable */)
+}
+
+func (b *binaryCodec) marshalBinary(dst []byte, msg any, stable bool) ([]byte, error) {
 	pm, ok := msg.(proto.Message)
 	if !ok {
 		return nil, errNotProto(msg)
 	}
 	if stable {
-		return b.stable.Marshal(pm)
+		return b.stable.MarshalAppend(dst, pm)
 	}
-	return b.marshal.Marshal(pm)
+	return b.marshal.MarshalAppend(dst, pm)
 }
 
 type vtBinaryCodec struct {

binary_test.go

@@ -62,6 +62,13 @@ func TestBinaryMarshal(t *testing.T) {
 			attest.True(t, bytes.Equal(out, again), attest.Sprint("MarshalStable produced unstable output"))
 		}
 	})
+	t.Run("append", func(t *testing.T) {
+		out, err := codec.MarshalAppend(make([]byte, 0, 128), dict)
+		attest.Ok(t, err)
+		roundtrip := &structpb.Struct{}
+		attest.Ok(t, proto.Unmarshal(out, roundtrip))
+		attest.Equal(t, roundtrip, dict, attest.Cmp(protocmp.Transform()))
+	})
 	t.Run("not protobuf", func(t *testing.T) {
 		_, err := codec.Marshal(struct{}{})
 		attest.Error(t, err)

go.mod

@@ -5,7 +5,7 @@ go 1.19
 require (
 	github.com/bufbuild/connect-go v1.7.0
 	go.akshayshah.org/attest v1.0.0
-	google.golang.org/protobuf v1.30.0
+	google.golang.org/protobuf v1.31.0
 )
 
 require (

go.sum

@@ -12,3 +12,5 @@ golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8T
 google.golang.org/protobuf v1.26.0-rc.1/go.mod h1:jlhhOSvTdKEhbULTjvd4ARK9grFBp09yW+WbY/TyQbw=
 google.golang.org/protobuf v1.30.0 h1:kPPoIgf3TsEvrm0PFe15JQ+570QVxYzEvvHqChK+cng=
 google.golang.org/protobuf v1.30.0/go.mod h1:HV8QOd/L58Z+nl8r43ehVNZIU/HEI6OcFqwMG9pJV4I=
+google.golang.org/protobuf v1.31.0 h1:g0LDEJHgrBl9N9r17Ru3sqWhkIx2NB67okBHPwC7hs8=
+google.golang.org/protobuf v1.31.0/go.mod h1:HV8QOd/L58Z+nl8r43ehVNZIU/HEI6OcFqwMG9pJV4I=

json.go

@@ -40,11 +40,7 @@ func (j *jsonCodec) Unmarshal(binary []byte, msg any) error {
 }
 
 func (j *jsonCodec) Marshal(msg any) ([]byte, error) {
-	pm, ok := msg.(proto.Message)
-	if !ok {
-		return nil, errNotProto(msg)
-	}
-	return j.marshal.Marshal(pm)
+	return j.MarshalAppend(nil, msg)
 }
 
 func (j *jsonCodec) MarshalStable(message any) ([]byte, error) {
@@ -62,3 +58,11 @@ func (j *jsonCodec) MarshalStable(message any) ([]byte, error) {
 	}
 	return compacted.Bytes(), nil
 }
+
+func (j *jsonCodec) MarshalAppend(dst []byte, msg any) ([]byte, error) {
+	pm, ok := msg.(proto.Message)
+	if !ok {
+		return nil, errNotProto(msg)
+	}
+	return j.marshal.MarshalAppend(dst, pm)
+}

json_test.go

@@ -48,6 +48,11 @@ func TestJSONMarshal(t *testing.T) {
 		attest.Ok(t, err)
 		attest.Equal(t, string(out), `{"name":"Foo"}`)
 	})
+	t.Run("append", func(t *testing.T) {
+		out, err := codec.MarshalAppend(make([]byte, 0, 100), enum)
+		attest.Ok(t, err)
+		attest.Subsequence(t, string(out), "\n\t")
+	})
 	t.Run("not protobuf", func(t *testing.T) {
 		_, err := codec.Marshal(struct{}{})
 		attest.Error(t, err)