internal/impl: simplify getMessageInfo

Our specific protoreflect.Message implementations have a special
ProtoMessageInfo method to obtain the *MessageInfo for v1 compatibility.
Use that instead to implement getMessageInfo.

Change-Id: I6cab9aeaa93714be73bd812c3d9a3be0ec86dd52
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/187777
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/codec_field.go b/internal/impl/codec_field.go
index 94b7d6a..9d587d2 100644
--- a/internal/impl/codec_field.go
+++ b/internal/impl/codec_field.go
@@ -79,19 +79,19 @@
 }
 
 func makeMessageFieldCoder(fd pref.FieldDescriptor, ft reflect.Type) pointerCoderFuncs {
-	if fi, ok := getMessageInfo(ft); ok {
+	if mi := getMessageInfo(ft); mi != nil {
 		return pointerCoderFuncs{
 			size: func(p pointer, tagsize int, opts marshalOptions) int {
-				return sizeMessageInfo(p, fi, tagsize, opts)
+				return sizeMessageInfo(p, mi, tagsize, opts)
 			},
 			marshal: func(b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-				return appendMessageInfo(b, p, wiretag, fi, opts)
+				return appendMessageInfo(b, p, wiretag, mi, opts)
 			},
 			unmarshal: func(b []byte, p pointer, wtyp wire.Type, opts unmarshalOptions) (int, error) {
-				return consumeMessageInfo(b, p, fi, wtyp, opts)
+				return consumeMessageInfo(b, p, mi, wtyp, opts)
 			},
 			isInit: func(p pointer) error {
-				return fi.isInitializedPointer(p.Elem())
+				return mi.isInitializedPointer(p.Elem())
 			},
 		}
 	} else {
@@ -200,19 +200,19 @@
 
 func makeGroupFieldCoder(fd pref.FieldDescriptor, ft reflect.Type) pointerCoderFuncs {
 	num := fd.Number()
-	if fi, ok := getMessageInfo(ft); ok {
+	if mi := getMessageInfo(ft); mi != nil {
 		return pointerCoderFuncs{
 			size: func(p pointer, tagsize int, opts marshalOptions) int {
-				return sizeGroupType(p, fi, tagsize, opts)
+				return sizeGroupType(p, mi, tagsize, opts)
 			},
 			marshal: func(b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-				return appendGroupType(b, p, wiretag, fi, opts)
+				return appendGroupType(b, p, wiretag, mi, opts)
 			},
 			unmarshal: func(b []byte, p pointer, wtyp wire.Type, opts unmarshalOptions) (int, error) {
-				return consumeGroupType(b, p, fi, num, wtyp, opts)
+				return consumeGroupType(b, p, mi, num, wtyp, opts)
 			},
 			isInit: func(p pointer) error {
-				return fi.isInitializedPointer(p.Elem())
+				return mi.isInitializedPointer(p.Elem())
 			},
 		}
 	} else {
@@ -303,19 +303,19 @@
 }
 
 func makeMessageSliceFieldCoder(fd pref.FieldDescriptor, ft reflect.Type) pointerCoderFuncs {
-	if fi, ok := getMessageInfo(ft); ok {
+	if mi := getMessageInfo(ft); mi != nil {
 		return pointerCoderFuncs{
 			size: func(p pointer, tagsize int, opts marshalOptions) int {
-				return sizeMessageSliceInfo(p, fi, tagsize, opts)
+				return sizeMessageSliceInfo(p, mi, tagsize, opts)
 			},
 			marshal: func(b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-				return appendMessageSliceInfo(b, p, wiretag, fi, opts)
+				return appendMessageSliceInfo(b, p, wiretag, mi, opts)
 			},
 			unmarshal: func(b []byte, p pointer, wtyp wire.Type, opts unmarshalOptions) (int, error) {
-				return consumeMessageSliceInfo(b, p, fi, wtyp, opts)
+				return consumeMessageSliceInfo(b, p, mi, wtyp, opts)
 			},
 			isInit: func(p pointer) error {
-				return isInitMessageSliceInfo(p, fi)
+				return isInitMessageSliceInfo(p, mi)
 			},
 		}
 	}
@@ -471,19 +471,19 @@
 
 func makeGroupSliceFieldCoder(fd pref.FieldDescriptor, ft reflect.Type) pointerCoderFuncs {
 	num := fd.Number()
-	if fi, ok := getMessageInfo(ft); ok {
+	if mi := getMessageInfo(ft); mi != nil {
 		return pointerCoderFuncs{
 			size: func(p pointer, tagsize int, opts marshalOptions) int {
-				return sizeGroupSliceInfo(p, fi, tagsize, opts)
+				return sizeGroupSliceInfo(p, mi, tagsize, opts)
 			},
 			marshal: func(b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-				return appendGroupSliceInfo(b, p, wiretag, fi, opts)
+				return appendGroupSliceInfo(b, p, wiretag, mi, opts)
 			},
 			unmarshal: func(b []byte, p pointer, wtyp wire.Type, opts unmarshalOptions) (int, error) {
-				return consumeGroupSliceInfo(b, p, num, wtyp, fi, opts)
+				return consumeGroupSliceInfo(b, p, num, wtyp, mi, opts)
 			},
 			isInit: func(p pointer) error {
-				return isInitMessageSliceInfo(p, fi)
+				return isInitMessageSliceInfo(p, mi)
 			},
 		}
 	}