all: refactor extensions, add proto.GetExtension etc.

Change protoiface.ExtensionDescV1 to implement protoreflect.ExtensionType.

ExtensionDescV1's Name field conflicts with the Descriptor Name method,
so change the protoreflect.{Message,Enum,Extension}Type types to no
longer implement the corresponding Descriptor interface. This also leads
to a clearer distinction between the two types.

Introduce a protoreflect.ExtensionTypeDescriptor type which bridges
between ExtensionType and ExtensionDescriptor.

Add extension accessor functions to the proto package:
proto.{Has,Clear,Get,Set}Extension. These functions take a
protoreflect.ExtensionType parameter, which allows writing the
same function call using either the old or new API:

  proto.GetExtension(message, somepb.E_ExtensionFoo)

Fixes golang/protobuf#908

Change-Id: Ibc65d12a46666297849114fd3aefbc4a597d9f08
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189199
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/prototext/encode_test.go b/encoding/prototext/encode_test.go
index c29169b..493a229 100644
--- a/encoding/prototext/encode_test.go
+++ b/encoding/prototext/encode_test.go
@@ -16,7 +16,6 @@
 	pimpl "google.golang.org/protobuf/internal/impl"
 	"google.golang.org/protobuf/proto"
 	preg "google.golang.org/protobuf/reflect/protoregistry"
-	"google.golang.org/protobuf/runtime/protoiface"
 
 	"google.golang.org/protobuf/encoding/testprotos/pb2"
 	"google.golang.org/protobuf/encoding/testprotos/pb3"
@@ -28,11 +27,6 @@
 	detrand.Disable()
 }
 
-// TODO: Use proto.SetExtension when available.
-func setExtension(m proto.Message, xd *protoiface.ExtensionDescV1, val interface{}) {
-	m.ProtoReflect().Set(xd.Type, xd.Type.ValueOf(val))
-}
-
 func TestMarshal(t *testing.T) {
 	tests := []struct {
 		desc    string
@@ -905,10 +899,10 @@
 				OptBool:   proto.Bool(true),
 				OptInt32:  proto.Int32(42),
 			}
-			setExtension(m, pb2.E_OptExtBool, true)
-			setExtension(m, pb2.E_OptExtString, "extension field")
-			setExtension(m, pb2.E_OptExtEnum, pb2.Enum_TEN)
-			setExtension(m, pb2.E_OptExtNested, &pb2.Nested{
+			proto.SetExtension(m, pb2.E_OptExtBool, true)
+			proto.SetExtension(m, pb2.E_OptExtString, "extension field")
+			proto.SetExtension(m, pb2.E_OptExtEnum, pb2.Enum_TEN)
+			proto.SetExtension(m, pb2.E_OptExtNested, &pb2.Nested{
 				OptString: proto.String("nested in an extension"),
 				OptNested: &pb2.Nested{
 					OptString: proto.String("another nested in an extension"),
@@ -933,7 +927,7 @@
 		desc: "extension field contains invalid UTF-8",
 		input: func() proto.Message {
 			m := &pb2.Extensions{}
-			setExtension(m, pb2.E_OptExtString, "abc\xff")
+			proto.SetExtension(m, pb2.E_OptExtString, "abc\xff")
 			return m
 		}(),
 		wantErr: true,
@@ -941,10 +935,10 @@
 		desc: "extension partial returns error",
 		input: func() proto.Message {
 			m := &pb2.Extensions{}
-			setExtension(m, pb2.E_OptExtPartial, &pb2.PartialRequired{
+			proto.SetExtension(m, pb2.E_OptExtPartial, &pb2.PartialRequired{
 				OptString: proto.String("partial1"),
 			})
-			setExtension(m, pb2.E_ExtensionsContainer_OptExtPartial, &pb2.PartialRequired{
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_OptExtPartial, &pb2.PartialRequired{
 				OptString: proto.String("partial2"),
 			})
 			return m
@@ -962,7 +956,7 @@
 		mo:   prototext.MarshalOptions{AllowPartial: true},
 		input: func() proto.Message {
 			m := &pb2.Extensions{}
-			setExtension(m, pb2.E_OptExtPartial, &pb2.PartialRequired{
+			proto.SetExtension(m, pb2.E_OptExtPartial, &pb2.PartialRequired{
 				OptString: proto.String("partial1"),
 			})
 			return m
@@ -975,9 +969,9 @@
 		desc: "extensions of repeated fields",
 		input: func() proto.Message {
 			m := &pb2.Extensions{}
-			setExtension(m, pb2.E_RptExtEnum, &[]pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
-			setExtension(m, pb2.E_RptExtFixed32, &[]uint32{42, 47})
-			setExtension(m, pb2.E_RptExtNested, &[]*pb2.Nested{
+			proto.SetExtension(m, pb2.E_RptExtEnum, &[]pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
+			proto.SetExtension(m, pb2.E_RptExtFixed32, &[]uint32{42, 47})
+			proto.SetExtension(m, pb2.E_RptExtNested, &[]*pb2.Nested{
 				&pb2.Nested{OptString: proto.String("one")},
 				&pb2.Nested{OptString: proto.String("two")},
 				&pb2.Nested{OptString: proto.String("three")},
@@ -1003,10 +997,10 @@
 		desc: "extensions of non-repeated fields in another message",
 		input: func() proto.Message {
 			m := &pb2.Extensions{}
-			setExtension(m, pb2.E_ExtensionsContainer_OptExtBool, true)
-			setExtension(m, pb2.E_ExtensionsContainer_OptExtString, "extension field")
-			setExtension(m, pb2.E_ExtensionsContainer_OptExtEnum, pb2.Enum_TEN)
-			setExtension(m, pb2.E_ExtensionsContainer_OptExtNested, &pb2.Nested{
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_OptExtBool, true)
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_OptExtString, "extension field")
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_OptExtEnum, pb2.Enum_TEN)
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_OptExtNested, &pb2.Nested{
 				OptString: proto.String("nested in an extension"),
 				OptNested: &pb2.Nested{
 					OptString: proto.String("another nested in an extension"),
@@ -1032,9 +1026,9 @@
 				OptBool:   proto.Bool(true),
 				OptInt32:  proto.Int32(42),
 			}
-			setExtension(m, pb2.E_ExtensionsContainer_RptExtEnum, &[]pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
-			setExtension(m, pb2.E_ExtensionsContainer_RptExtString, &[]string{"hello", "world"})
-			setExtension(m, pb2.E_ExtensionsContainer_RptExtNested, &[]*pb2.Nested{
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtEnum, &[]pb2.Enum{pb2.Enum_TEN, 101, pb2.Enum_ONE})
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtString, &[]string{"hello", "world"})
+			proto.SetExtension(m, pb2.E_ExtensionsContainer_RptExtNested, &[]*pb2.Nested{
 				&pb2.Nested{OptString: proto.String("one")},
 				&pb2.Nested{OptString: proto.String("two")},
 				&pb2.Nested{OptString: proto.String("three")},
@@ -1063,13 +1057,13 @@
 		desc: "MessageSet",
 		input: func() proto.Message {
 			m := &pb2.MessageSet{}
-			setExtension(m, pb2.E_MessageSetExtension_MessageSetExtension, &pb2.MessageSetExtension{
+			proto.SetExtension(m, pb2.E_MessageSetExtension_MessageSetExtension, &pb2.MessageSetExtension{
 				OptString: proto.String("a messageset extension"),
 			})
-			setExtension(m, pb2.E_MessageSetExtension_NotMessageSetExtension, &pb2.MessageSetExtension{
+			proto.SetExtension(m, pb2.E_MessageSetExtension_NotMessageSetExtension, &pb2.MessageSetExtension{
 				OptString: proto.String("not a messageset extension"),
 			})
-			setExtension(m, pb2.E_MessageSetExtension_ExtNested, &pb2.Nested{
+			proto.SetExtension(m, pb2.E_MessageSetExtension_ExtNested, &pb2.Nested{
 				OptString: proto.String("just a regular extension"),
 			})
 			return m
@@ -1089,7 +1083,7 @@
 		desc: "not real MessageSet 1",
 		input: func() proto.Message {
 			m := &pb2.FakeMessageSet{}
-			setExtension(m, pb2.E_FakeMessageSetExtension_MessageSetExtension, &pb2.FakeMessageSetExtension{
+			proto.SetExtension(m, pb2.E_FakeMessageSetExtension_MessageSetExtension, &pb2.FakeMessageSetExtension{
 				OptString: proto.String("not a messageset extension"),
 			})
 			return m
@@ -1103,7 +1097,7 @@
 		desc: "not real MessageSet 2",
 		input: func() proto.Message {
 			m := &pb2.MessageSet{}
-			setExtension(m, pb2.E_MessageSetExtension, &pb2.FakeMessageSetExtension{
+			proto.SetExtension(m, pb2.E_MessageSetExtension, &pb2.FakeMessageSetExtension{
 				OptString: proto.String("another not a messageset extension"),
 			})
 			return m