proto: make InternalMessageInfo functional (#1129)

The InternalMessageInfo type only exists to implement the
XXX methods on generated messages where those methods were
only ever intended to be called by this module itself.

Since v1.4.0, this module no longer relies on the XXX methods,
so the InternalMessageInfo and its implementation is supposed
to be dead code. Unfortunately, there are external usages that
violate our compatibility agreement and either directly call
the XXX methods or indirectly call it because some library
type-asserts to the existence of these methods.

This change adds minimal support for InternalMessageInfo by
just calling out directly to the v2 implementation.

proto/deprecated.go

@@ -9,6 +9,8 @@ import (
 	"errors"
 	"fmt"
 	"strconv"
+
+	protoV2 "google.golang.org/protobuf/proto"
 )
 
 var (
@@ -82,11 +84,30 @@ func UnmarshalJSONEnum(m map[string]int32, data []byte, enumName string) (int32,
 	return val, nil
 }
 
-// Deprecated: Do not use.
+// Deprecated: Do not use; this type existed for intenal-use only.
 type InternalMessageInfo struct{}
 
-func (*InternalMessageInfo) DiscardUnknown(Message)                        { panic("not implemented") }
-func (*InternalMessageInfo) Marshal([]byte, Message, bool) ([]byte, error) { panic("not implemented") }
-func (*InternalMessageInfo) Merge(Message, Message)                        { panic("not implemented") }
-func (*InternalMessageInfo) Size(Message) int                              { panic("not implemented") }
-func (*InternalMessageInfo) Unmarshal(Message, []byte) error               { panic("not implemented") }
+// Deprecated: Do not use; this method existed for intenal-use only.
+func (*InternalMessageInfo) DiscardUnknown(m Message) {
+	DiscardUnknown(m)
+}
+
+// Deprecated: Do not use; this method existed for intenal-use only.
+func (*InternalMessageInfo) Marshal(b []byte, m Message, deterministic bool) ([]byte, error) {
+	return protoV2.MarshalOptions{Deterministic: deterministic}.MarshalAppend(b, MessageV2(m))
+}
+
+// Deprecated: Do not use; this method existed for intenal-use only.
+func (*InternalMessageInfo) Merge(dst, src Message) {
+	protoV2.Merge(MessageV2(dst), MessageV2(src))
+}
+
+// Deprecated: Do not use; this method existed for intenal-use only.
+func (*InternalMessageInfo) Size(m Message) int {
+	return protoV2.Size(MessageV2(m))
+}
+
+// Deprecated: Do not use; this method existed for intenal-use only.
+func (*InternalMessageInfo) Unmarshal(m Message, b []byte) error {
+	return protoV2.UnmarshalOptions{Merge: true}.Unmarshal(b, MessageV2(m))
+}