proto: use fixed UnmarshalState API

Change-Id: I08abfaed1bafd0d650988883be31faefbd312331
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/222679
Reviewed-by: Damien Neil <dneil@google.com>

go.mod

@@ -4,5 +4,5 @@ go 1.9
 
 require (
 	github.com/google/go-cmp v0.4.0
-	google.golang.org/protobuf v1.20.0
+	google.golang.org/protobuf v1.20.1-0.20200309200217-e05f789c0967
 )

go.sum

@@ -9,7 +9,6 @@ golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IV
 golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
 google.golang.org/protobuf v0.0.0-20200109180630-ec00e32a8dfd/go.mod h1:DFci5gLYBciE7Vtevhsrf46CRTquxDuWsQurQQe4oz8=
 google.golang.org/protobuf v0.0.0-20200221191635-4d8936d0db64/go.mod h1:kwYJMbMJ01Woi6D6+Kah6886xMZcty6N08ah7+eCXa0=
-google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60 h1:qkfzMNEf79BNs1//mQGZjYHIXAOv+AOdvPnMsU6R+1I=
 google.golang.org/protobuf v0.0.0-20200228230310-ab0ca4ff8a60/go.mod h1:cfTl7dwQJ+fmap5saPgwCLgHXTUD7jkjRqWcaiX5VyM=
-google.golang.org/protobuf v1.20.0 h1:SsQNHvKpk2VTiWoQ5Pqkt3Go/c2ly77C+v2Lggu5Qek=
-google.golang.org/protobuf v1.20.0/go.mod h1:A+miEFZTKqfCUM6K7xSMQL9OKL/b6hQv+e19PK+JZNE=
+google.golang.org/protobuf v1.20.1-0.20200309200217-e05f789c0967 h1:DwkfSP6tZMxKX50J0dBSqEgJvJdFYP1Gvzbjtvkmrug=
+google.golang.org/protobuf v1.20.1-0.20200309200217-e05f789c0967/go.mod h1:A+miEFZTKqfCUM6K7xSMQL9OKL/b6hQv+e19PK+JZNE=

proto/wire.go

@@ -64,8 +64,9 @@ func UnmarshalMerge(b []byte, m Message) error {
 	out, err := protoV2.UnmarshalOptions{
 		AllowPartial: true,
 		Merge:        true,
-	}.UnmarshalState(mi, protoiface.UnmarshalInput{
-		Buf: b,
+	}.UnmarshalState(protoiface.UnmarshalInput{
+		Buf:     b,
+		Message: mi.ProtoReflect(),
 	})
 	if err != nil {
 		return err