internal/legacy: extract legacy support out from the impl package

The impl package currently supports wrapping legacy v1 enums and messages
so that they implement the v2 reflective APIs. This functionality is necessary
for v1 and v2 to interoperate. However, the existence of this functionality
presents several problems:
	* A significant portion of the complexity in impl is for legacy wrapping.
	* This complexity is linked into a Go binary even if all the other messages
	in the binary natively support v2 reflection.
	* It presents a cyclic dependency when trying to generate descriptor proto.

Suppose you are generating descriptor.proto. The generated code would want to
depend on the impl package because impl is the runtime implementation for
protobuf messages. However, impl currently depends depends on descriptor in
order to wrap legacy enum and messages since it needs the ability to dynamically
create new protobuf descriptor types. In the case of descriptor.proto, it would
presumably be generated with native reflection support, so the legacy wrapping
logic is unneccessary.

To break the dependency of impl on descriptor, we move the legacy support logic
to a different package and instead add hooks in impl so that legacy support could
be dynamically registered at runtime. This is dependency injection.

Change-Id: I01a582908ed5629993f6699e9bf2f4bee93857a4
Reviewed-on: https://go-review.googlesource.com/c/151877
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/legacy/file_test.go b/internal/legacy/file_test.go
new file mode 100644
index 0000000..6d994ac
--- /dev/null
+++ b/internal/legacy/file_test.go
@@ -0,0 +1,440 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package legacy
+
+import (
+	"reflect"
+	"testing"
+
+	pragma "github.com/golang/protobuf/v2/internal/pragma"
+	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
+	ptype "github.com/golang/protobuf/v2/reflect/prototype"
+	cmp "github.com/google/go-cmp/cmp"
+
+	proto2_20160225 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto2.v0.0.0-20160225-2fc053c5"
+	proto2_20160519 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto2.v0.0.0-20160519-a4ab9ec5"
+	proto2_20180125 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto2.v1.0.0-20180125-92554152"
+	proto2_20180430 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto2.v1.1.0-20180430-b4deda09"
+	proto2_20180814 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto2.v1.2.0-20180814-aa810b61"
+	proto2_20181126 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto2.v1.2.1-20181126-8d0c54c1"
+	proto3_20160225 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto3.v0.0.0-20160225-2fc053c5"
+	proto3_20160519 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto3.v0.0.0-20160519-a4ab9ec5"
+	proto3_20180125 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto3.v1.0.0-20180125-92554152"
+	proto3_20180430 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto3.v1.1.0-20180430-b4deda09"
+	proto3_20180814 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto3.v1.2.0-20180814-aa810b61"
+	proto3_20181126 "github.com/golang/protobuf/v2/internal/testprotos/legacy/proto3.v1.2.1-20181126-8d0c54c1"
+)
+
+func mustLoadFileDesc(b []byte, _ []int) pref.FileDescriptor {
+	fd, err := ptype.NewFileFromDescriptorProto(legacyLoadFileDesc(b), nil)
+	if err != nil {
+		panic(err)
+	}
+	return fd
+}
+
+func TestDescriptor(t *testing.T) {
+	var tests []struct{ got, want pref.Descriptor }
+
+	fileDescP2_20160225 := mustLoadFileDesc(new(proto2_20160225.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20160225.SiblingEnum(0))),
+		want: fileDescP2_20160225.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20160225.Message_ChildEnum(0))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.SiblingMessage))),
+		want: fileDescP2_20160225.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_ChildMessage))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message))),
+		want: fileDescP2_20160225.Messages().ByName("Message"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_NamedGroup))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("NamedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_OptionalGroup))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("OptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_RequiredGroup))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("RequiredGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_RepeatedGroup))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("RepeatedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_OneofGroup))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("OneofGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_ExtensionOptionalGroup))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("ExtensionOptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160225.Message_ExtensionRepeatedGroup))),
+		want: fileDescP2_20160225.Messages().ByName("Message").Messages().ByName("ExtensionRepeatedGroup"),
+	}}...)
+
+	fileDescP3_20160225 := mustLoadFileDesc(new(proto3_20160225.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20160225.SiblingEnum(0))),
+		want: fileDescP3_20160225.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20160225.Message_ChildEnum(0))),
+		want: fileDescP3_20160225.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20160225.SiblingMessage))),
+		want: fileDescP3_20160225.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20160225.Message_ChildMessage))),
+		want: fileDescP3_20160225.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20160225.Message))),
+		want: fileDescP3_20160225.Messages().ByName("Message"),
+	}}...)
+
+	fileDescP2_20160519 := mustLoadFileDesc(new(proto2_20160519.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20160519.SiblingEnum(0))),
+		want: fileDescP2_20160519.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20160519.Message_ChildEnum(0))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.SiblingMessage))),
+		want: fileDescP2_20160519.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_ChildMessage))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message))),
+		want: fileDescP2_20160519.Messages().ByName("Message"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_NamedGroup))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("NamedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_OptionalGroup))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("OptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_RequiredGroup))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("RequiredGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_RepeatedGroup))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("RepeatedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_OneofGroup))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("OneofGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_ExtensionOptionalGroup))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("ExtensionOptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20160519.Message_ExtensionRepeatedGroup))),
+		want: fileDescP2_20160519.Messages().ByName("Message").Messages().ByName("ExtensionRepeatedGroup"),
+	}}...)
+
+	fileDescP3_20160519 := mustLoadFileDesc(new(proto3_20160519.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20160519.SiblingEnum(0))),
+		want: fileDescP3_20160519.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20160519.Message_ChildEnum(0))),
+		want: fileDescP3_20160519.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20160519.SiblingMessage))),
+		want: fileDescP3_20160519.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20160519.Message_ChildMessage))),
+		want: fileDescP3_20160519.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20160519.Message))),
+		want: fileDescP3_20160519.Messages().ByName("Message"),
+	}}...)
+
+	fileDescP2_20180125 := mustLoadFileDesc(new(proto2_20180125.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20180125.SiblingEnum(0))),
+		want: fileDescP2_20180125.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20180125.Message_ChildEnum(0))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.SiblingMessage))),
+		want: fileDescP2_20180125.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_ChildMessage))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message))),
+		want: fileDescP2_20180125.Messages().ByName("Message"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_NamedGroup))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("NamedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_OptionalGroup))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("OptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_RequiredGroup))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("RequiredGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_RepeatedGroup))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("RepeatedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_OneofGroup))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("OneofGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_ExtensionOptionalGroup))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("ExtensionOptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180125.Message_ExtensionRepeatedGroup))),
+		want: fileDescP2_20180125.Messages().ByName("Message").Messages().ByName("ExtensionRepeatedGroup"),
+	}}...)
+
+	fileDescP3_20180125 := mustLoadFileDesc(new(proto3_20180125.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20180125.SiblingEnum(0))),
+		want: fileDescP3_20180125.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20180125.Message_ChildEnum(0))),
+		want: fileDescP3_20180125.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180125.SiblingMessage))),
+		want: fileDescP3_20180125.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180125.Message_ChildMessage))),
+		want: fileDescP3_20180125.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180125.Message))),
+		want: fileDescP3_20180125.Messages().ByName("Message"),
+	}}...)
+
+	fileDescP2_20180430 := mustLoadFileDesc(new(proto2_20180430.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20180430.SiblingEnum(0))),
+		want: fileDescP2_20180430.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20180430.Message_ChildEnum(0))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.SiblingMessage))),
+		want: fileDescP2_20180430.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_ChildMessage))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message))),
+		want: fileDescP2_20180430.Messages().ByName("Message"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_NamedGroup))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("NamedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_OptionalGroup))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("OptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_RequiredGroup))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("RequiredGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_RepeatedGroup))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("RepeatedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_OneofGroup))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("OneofGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_ExtensionOptionalGroup))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("ExtensionOptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180430.Message_ExtensionRepeatedGroup))),
+		want: fileDescP2_20180430.Messages().ByName("Message").Messages().ByName("ExtensionRepeatedGroup"),
+	}}...)
+
+	fileDescP3_20180430 := mustLoadFileDesc(new(proto3_20180430.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20180430.SiblingEnum(0))),
+		want: fileDescP3_20180430.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20180430.Message_ChildEnum(0))),
+		want: fileDescP3_20180430.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180430.SiblingMessage))),
+		want: fileDescP3_20180430.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180430.Message_ChildMessage))),
+		want: fileDescP3_20180430.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180430.Message))),
+		want: fileDescP3_20180430.Messages().ByName("Message"),
+	}}...)
+
+	fileDescP2_20180814 := mustLoadFileDesc(new(proto2_20180814.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20180814.SiblingEnum(0))),
+		want: fileDescP2_20180814.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20180814.Message_ChildEnum(0))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.SiblingMessage))),
+		want: fileDescP2_20180814.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_ChildMessage))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message))),
+		want: fileDescP2_20180814.Messages().ByName("Message"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_NamedGroup))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("NamedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_OptionalGroup))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("OptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_RequiredGroup))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("RequiredGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_RepeatedGroup))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("RepeatedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_OneofGroup))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("OneofGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_ExtensionOptionalGroup))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("ExtensionOptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20180814.Message_ExtensionRepeatedGroup))),
+		want: fileDescP2_20180814.Messages().ByName("Message").Messages().ByName("ExtensionRepeatedGroup"),
+	}}...)
+
+	fileDescP3_20180814 := mustLoadFileDesc(new(proto3_20180814.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20180814.SiblingEnum(0))),
+		want: fileDescP3_20180814.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20180814.Message_ChildEnum(0))),
+		want: fileDescP3_20180814.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180814.SiblingMessage))),
+		want: fileDescP3_20180814.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180814.Message_ChildMessage))),
+		want: fileDescP3_20180814.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20180814.Message))),
+		want: fileDescP3_20180814.Messages().ByName("Message"),
+	}}...)
+
+	fileDescP2_20181126 := mustLoadFileDesc(new(proto2_20181126.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20181126.SiblingEnum(0))),
+		want: fileDescP2_20181126.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto2_20181126.Message_ChildEnum(0))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.SiblingMessage))),
+		want: fileDescP2_20181126.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_ChildMessage))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message))),
+		want: fileDescP2_20181126.Messages().ByName("Message"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_NamedGroup))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("NamedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_OptionalGroup))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("OptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_RequiredGroup))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("RequiredGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_RepeatedGroup))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("RepeatedGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_OneofGroup))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("OneofGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_ExtensionOptionalGroup))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("ExtensionOptionalGroup"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto2_20181126.Message_ExtensionRepeatedGroup))),
+		want: fileDescP2_20181126.Messages().ByName("Message").Messages().ByName("ExtensionRepeatedGroup"),
+	}}...)
+
+	fileDescP3_20181126 := mustLoadFileDesc(new(proto3_20181126.Message).Descriptor())
+	tests = append(tests, []struct{ got, want pref.Descriptor }{{
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20181126.SiblingEnum(0))),
+		want: fileDescP3_20181126.Enums().ByName("SiblingEnum"),
+	}, {
+		got:  legacyLoadEnumDesc(reflect.TypeOf(proto3_20181126.Message_ChildEnum(0))),
+		want: fileDescP3_20181126.Messages().ByName("Message").Enums().ByName("ChildEnum"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20181126.SiblingMessage))),
+		want: fileDescP3_20181126.Messages().ByName("SiblingMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20181126.Message_ChildMessage))),
+		want: fileDescP3_20181126.Messages().ByName("Message").Messages().ByName("ChildMessage"),
+	}, {
+		got:  legacyLoadMessageDesc(reflect.TypeOf(new(proto3_20181126.Message))),
+		want: fileDescP3_20181126.Messages().ByName("Message"),
+	}}...)
+
+	// TODO: We need a test package to compare descriptors.
+	type list interface {
+		Len() int
+		pragma.DoNotImplement
+	}
+	opts := cmp.Options{
+		cmp.Transformer("", func(x list) []interface{} {
+			out := make([]interface{}, x.Len())
+			v := reflect.ValueOf(x)
+			for i := 0; i < x.Len(); i++ {
+				m := v.MethodByName("Get")
+				out[i] = m.Call([]reflect.Value{reflect.ValueOf(i)})[0].Interface()
+			}
+			return out
+		}),
+		cmp.Transformer("", func(x pref.Descriptor) map[string]interface{} {
+			out := make(map[string]interface{})
+			v := reflect.ValueOf(x)
+			for i := 0; i < v.NumMethod(); i++ {
+				name := v.Type().Method(i).Name
+				if m := v.Method(i); m.Type().NumIn() == 0 && m.Type().NumOut() == 1 {
+					switch name {
+					case "Index":
+						// Ignore index since legacy descriptors have no parent.
+					case "Options":
+						// Ignore descriptor options since protos are not cmperable.
+					case "Enums", "Messages", "Extensions":
+						// Ignore nested message and enum declarations since
+						// legacy descriptors are all created standalone.
+					case "OneofType", "ExtendedType", "EnumType", "MessageType":
+						// Avoid descending into a dependency to avoid a cycle.
+						// Just record the full name if available.
+						//
+						// TODO: Cycle support in cmp would be useful here.
+						v := m.Call(nil)[0]
+						if !v.IsNil() {
+							out[name] = v.Interface().(pref.Descriptor).FullName()
+						}
+					default:
+						out[name] = m.Call(nil)[0].Interface()
+					}
+				}
+			}
+			return out
+		}),
+		cmp.Transformer("", func(v pref.Value) interface{} {
+			return v.Interface()
+		}),
+	}
+
+	for _, tt := range tests {
+		t.Run(string(tt.want.FullName()), func(t *testing.T) {
+			if diff := cmp.Diff(&tt.want, &tt.got, opts); diff != "" {
+				t.Errorf("descriptor mismatch (-want, +got):\n%s", diff)
+			}
+		})
+	}
+}