reflect/protoreflect: add MessageFieldTypes

The MessageFieldTypes interface (if implemented by a MessageType)
provides Go type information about the fields if they are
an enum or message type.

Change-Id: I68b20f5726377f6b0f2c20a8b6e45f9802b43f67
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/236777
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/testing/prototest/message.go b/testing/prototest/message.go
new file mode 100644
index 0000000..e495628
--- /dev/null
+++ b/testing/prototest/message.go
@@ -0,0 +1,894 @@
+// Copyright 2019 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 prototest exercises protobuf reflection.
+package prototest
+
+import (
+	"bytes"
+	"fmt"
+	"math"
+	"reflect"
+	"sort"
+	"strings"
+	"testing"
+
+	"google.golang.org/protobuf/encoding/prototext"
+	"google.golang.org/protobuf/encoding/protowire"
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/reflect/protoreflect"
+	pref "google.golang.org/protobuf/reflect/protoreflect"
+	"google.golang.org/protobuf/reflect/protoregistry"
+)
+
+// TODO: Test invalid field descriptors or oneof descriptors.
+// TODO: This should test the functionality that can be provided by fast-paths.
+
+// Message tests a message implemention.
+type Message struct {
+	// Resolver is used to determine the list of extension fields to test with.
+	// If nil, this defaults to using protoregistry.GlobalTypes.
+	Resolver interface {
+		FindExtensionByName(field pref.FullName) (pref.ExtensionType, error)
+		FindExtensionByNumber(message pref.FullName, field pref.FieldNumber) (pref.ExtensionType, error)
+		RangeExtensionsByMessage(message pref.FullName, f func(pref.ExtensionType) bool)
+	}
+}
+
+// Test performs tests on a MessageType implementation.
+func (test Message) Test(t testing.TB, mt pref.MessageType) {
+	testType(t, mt)
+
+	md := mt.Descriptor()
+	m1 := mt.New()
+	for i := 0; i < md.Fields().Len(); i++ {
+		fd := md.Fields().Get(i)
+		testField(t, m1, fd)
+	}
+	if test.Resolver == nil {
+		test.Resolver = protoregistry.GlobalTypes
+	}
+	var extTypes []pref.ExtensionType
+	test.Resolver.RangeExtensionsByMessage(md.FullName(), func(e pref.ExtensionType) bool {
+		extTypes = append(extTypes, e)
+		return true
+	})
+	for _, xt := range extTypes {
+		testField(t, m1, xt.TypeDescriptor())
+	}
+	for i := 0; i < md.Oneofs().Len(); i++ {
+		testOneof(t, m1, md.Oneofs().Get(i))
+	}
+	testUnknown(t, m1)
+
+	// Test round-trip marshal/unmarshal.
+	m2 := mt.New().Interface()
+	populateMessage(m2.ProtoReflect(), 1, nil)
+	for _, xt := range extTypes {
+		m2.ProtoReflect().Set(xt.TypeDescriptor(), newValue(m2.ProtoReflect(), xt.TypeDescriptor(), 1, nil))
+	}
+	b, err := proto.MarshalOptions{
+		AllowPartial: true,
+	}.Marshal(m2)
+	if err != nil {
+		t.Errorf("Marshal() = %v, want nil\n%v", err, prototext.Format(m2))
+	}
+	m3 := mt.New().Interface()
+	if err := (proto.UnmarshalOptions{
+		AllowPartial: true,
+		Resolver:     test.Resolver,
+	}.Unmarshal(b, m3)); err != nil {
+		t.Errorf("Unmarshal() = %v, want nil\n%v", err, prototext.Format(m2))
+	}
+	if !proto.Equal(m2, m3) {
+		t.Errorf("round-trip marshal/unmarshal did not preserve message\nOriginal:\n%v\nNew:\n%v", prototext.Format(m2), prototext.Format(m3))
+	}
+}
+
+func testType(t testing.TB, mt pref.MessageType) {
+	m := mt.New().Interface()
+	want := reflect.TypeOf(m)
+	if got := reflect.TypeOf(m.ProtoReflect().Interface()); got != want {
+		t.Errorf("type mismatch: reflect.TypeOf(m) != reflect.TypeOf(m.ProtoReflect().Interface()): %v != %v", got, want)
+	}
+	if got := reflect.TypeOf(m.ProtoReflect().New().Interface()); got != want {
+		t.Errorf("type mismatch: reflect.TypeOf(m) != reflect.TypeOf(m.ProtoReflect().New().Interface()): %v != %v", got, want)
+	}
+	if got := reflect.TypeOf(m.ProtoReflect().Type().Zero().Interface()); got != want {
+		t.Errorf("type mismatch: reflect.TypeOf(m) != reflect.TypeOf(m.ProtoReflect().Type().Zero().Interface()): %v != %v", got, want)
+	}
+	if mt, ok := mt.(pref.MessageFieldTypes); ok {
+		testFieldTypes(t, mt)
+	}
+}
+
+func testFieldTypes(t testing.TB, mt pref.MessageFieldTypes) {
+	descName := func(d pref.Descriptor) pref.FullName {
+		if d == nil {
+			return "<nil>"
+		}
+		return d.FullName()
+	}
+	typeName := func(mt pref.MessageType) pref.FullName {
+		if mt == nil {
+			return "<nil>"
+		}
+		return mt.Descriptor().FullName()
+	}
+	adjustExpr := func(idx int, expr string) string {
+		expr = strings.Replace(expr, "fd.", "md.Fields().Get(i).", -1)
+		expr = strings.Replace(expr, "(fd)", "(md.Fields().Get(i))", -1)
+		expr = strings.Replace(expr, "mti.", "mt.Message(i).", -1)
+		expr = strings.Replace(expr, "(i)", fmt.Sprintf("(%d)", idx), -1)
+		return expr
+	}
+	checkEnumDesc := func(idx int, gotExpr, wantExpr string, got, want protoreflect.EnumDescriptor) {
+		if got != want {
+			t.Errorf("descriptor mismatch: %v != %v: %v != %v", adjustExpr(idx, gotExpr), adjustExpr(idx, wantExpr), descName(got), descName(want))
+		}
+	}
+	checkMessageDesc := func(idx int, gotExpr, wantExpr string, got, want protoreflect.MessageDescriptor) {
+		if got != want {
+			t.Errorf("descriptor mismatch: %v != %v: %v != %v", adjustExpr(idx, gotExpr), adjustExpr(idx, wantExpr), descName(got), descName(want))
+		}
+	}
+	checkMessageType := func(idx int, gotExpr, wantExpr string, got, want protoreflect.MessageType) {
+		if got != want {
+			t.Errorf("type mismatch: %v != %v: %v != %v", adjustExpr(idx, gotExpr), adjustExpr(idx, wantExpr), typeName(got), typeName(want))
+		}
+	}
+
+	fds := mt.Descriptor().Fields()
+	m := mt.New()
+	for i := 0; i < fds.Len(); i++ {
+		fd := fds.Get(i)
+		switch {
+		case fd.IsList():
+			if fd.Enum() != nil {
+				checkEnumDesc(i,
+					"mt.Enum(i).Descriptor()", "fd.Enum()",
+					mt.Enum(i).Descriptor(), fd.Enum())
+			}
+			if fd.Message() != nil {
+				checkMessageDesc(i,
+					"mt.Message(i).Descriptor()", "fd.Message()",
+					mt.Message(i).Descriptor(), fd.Message())
+				checkMessageType(i,
+					"mt.Message(i)", "m.NewField(fd).List().NewElement().Message().Type()",
+					mt.Message(i), m.NewField(fd).List().NewElement().Message().Type())
+			}
+		case fd.IsMap():
+			mti := mt.Message(i)
+			if m := mti.New(); m != nil {
+				checkMessageDesc(i,
+					"m.Descriptor()", "fd.Message()",
+					m.Descriptor(), fd.Message())
+			}
+			if m := mti.Zero(); m != nil {
+				checkMessageDesc(i,
+					"m.Descriptor()", "fd.Message()",
+					m.Descriptor(), fd.Message())
+			}
+			checkMessageDesc(i,
+				"mti.Descriptor()", "fd.Message()",
+				mti.Descriptor(), fd.Message())
+			if mti := mti.(pref.MessageFieldTypes); mti != nil {
+				if fd.MapValue().Enum() != nil {
+					checkEnumDesc(i,
+						"mti.Enum(fd.MapValue().Index()).Descriptor()", "fd.MapValue().Enum()",
+						mti.Enum(fd.MapValue().Index()).Descriptor(), fd.MapValue().Enum())
+				}
+				if fd.MapValue().Message() != nil {
+					checkMessageDesc(i,
+						"mti.Message(fd.MapValue().Index()).Descriptor()", "fd.MapValue().Message()",
+						mti.Message(fd.MapValue().Index()).Descriptor(), fd.MapValue().Message())
+					checkMessageType(i,
+						"mti.Message(fd.MapValue().Index())", "m.NewField(fd).Map().NewValue().Message().Type()",
+						mti.Message(fd.MapValue().Index()), m.NewField(fd).Map().NewValue().Message().Type())
+				}
+			}
+		default:
+			if fd.Enum() != nil {
+				checkEnumDesc(i,
+					"mt.Enum(i).Descriptor()", "fd.Enum()",
+					mt.Enum(i).Descriptor(), fd.Enum())
+			}
+			if fd.Message() != nil {
+				checkMessageDesc(i,
+					"mt.Message(i).Descriptor()", "fd.Message()",
+					mt.Message(i).Descriptor(), fd.Message())
+				checkMessageType(i,
+					"mt.Message(i)", "m.NewField(fd).Message().Type()",
+					mt.Message(i), m.NewField(fd).Message().Type())
+			}
+		}
+	}
+}
+
+// testField exercises set/get/has/clear of a field.
+func testField(t testing.TB, m pref.Message, fd pref.FieldDescriptor) {
+	name := fd.FullName()
+	num := fd.Number()
+
+	switch {
+	case fd.IsList():
+		testFieldList(t, m, fd)
+	case fd.IsMap():
+		testFieldMap(t, m, fd)
+	case fd.Message() != nil:
+	default:
+		if got, want := m.NewField(fd), fd.Default(); !valueEqual(got, want) {
+			t.Errorf("Message.NewField(%v) = %v, want default value %v", name, formatValue(got), formatValue(want))
+		}
+		if fd.Kind() == pref.FloatKind || fd.Kind() == pref.DoubleKind {
+			testFieldFloat(t, m, fd)
+		}
+	}
+
+	// Set to a non-zero value, the zero value, different non-zero values.
+	for _, n := range []seed{1, 0, minVal, maxVal} {
+		v := newValue(m, fd, n, nil)
+		m.Set(fd, v)
+		wantHas := true
+		if n == 0 {
+			if fd.Syntax() == pref.Proto3 && fd.Message() == nil {
+				wantHas = false
+			}
+			if fd.IsExtension() {
+				wantHas = true
+			}
+			if fd.Cardinality() == pref.Repeated {
+				wantHas = false
+			}
+			if fd.ContainingOneof() != nil {
+				wantHas = true
+			}
+		}
+		if fd.Syntax() == pref.Proto3 && fd.Cardinality() != pref.Repeated && fd.ContainingOneof() == nil && fd.Kind() == pref.EnumKind && v.Enum() == 0 {
+			wantHas = false
+		}
+		if got, want := m.Has(fd), wantHas; got != want {
+			t.Errorf("after setting %q to %v:\nMessage.Has(%v) = %v, want %v", name, formatValue(v), num, got, want)
+		}
+		if got, want := m.Get(fd), v; !valueEqual(got, want) {
+			t.Errorf("after setting %q:\nMessage.Get(%v) = %v, want %v", name, num, formatValue(got), formatValue(want))
+		}
+		found := false
+		m.Range(func(d pref.FieldDescriptor, got pref.Value) bool {
+			if fd != d {
+				return true
+			}
+			found = true
+			if want := v; !valueEqual(got, want) {
+				t.Errorf("after setting %q:\nMessage.Range got value %v, want %v", name, formatValue(got), formatValue(want))
+			}
+			return true
+		})
+		if got, want := wantHas, found; got != want {
+			t.Errorf("after setting %q:\nMessageRange saw field: %v, want %v", name, got, want)
+		}
+	}
+
+	m.Clear(fd)
+	if got, want := m.Has(fd), false; got != want {
+		t.Errorf("after clearing %q:\nMessage.Has(%v) = %v, want %v", name, num, got, want)
+	}
+	switch {
+	case fd.IsList():
+		if got := m.Get(fd); got.List().Len() != 0 {
+			t.Errorf("after clearing %q:\nMessage.Get(%v) = %v, want empty list", name, num, formatValue(got))
+		}
+	case fd.IsMap():
+		if got := m.Get(fd); got.Map().Len() != 0 {
+			t.Errorf("after clearing %q:\nMessage.Get(%v) = %v, want empty map", name, num, formatValue(got))
+		}
+	case fd.Message() == nil:
+		if got, want := m.Get(fd), fd.Default(); !valueEqual(got, want) {
+			t.Errorf("after clearing %q:\nMessage.Get(%v) = %v, want default %v", name, num, formatValue(got), formatValue(want))
+		}
+	}
+
+	// Set to the default value.
+	switch {
+	case fd.IsList() || fd.IsMap():
+		m.Set(fd, m.Mutable(fd))
+		if got, want := m.Has(fd), (fd.IsExtension() && fd.Cardinality() != pref.Repeated) || fd.ContainingOneof() != nil; got != want {
+			t.Errorf("after setting %q to default:\nMessage.Has(%v) = %v, want %v", name, num, got, want)
+		}
+	case fd.Message() == nil:
+		m.Set(fd, m.Get(fd))
+		if got, want := m.Get(fd), fd.Default(); !valueEqual(got, want) {
+			t.Errorf("after setting %q to default:\nMessage.Get(%v) = %v, want default %v", name, num, formatValue(got), formatValue(want))
+		}
+	}
+	m.Clear(fd)
+
+	// Set to the wrong type.
+	v := pref.ValueOfString("")
+	if fd.Kind() == pref.StringKind {
+		v = pref.ValueOfInt32(0)
+	}
+	if !panics(func() {
+		m.Set(fd, v)
+	}) {
+		t.Errorf("setting %v to %T succeeds, want panic", name, v.Interface())
+	}
+}
+
+// testFieldMap tests set/get/has/clear of entries in a map field.
+func testFieldMap(t testing.TB, m pref.Message, fd pref.FieldDescriptor) {
+	name := fd.FullName()
+	num := fd.Number()
+
+	// New values.
+	m.Clear(fd) // start with an empty map
+	mapv := m.Get(fd).Map()
+	if mapv.IsValid() {
+		t.Errorf("after clearing field: message.Get(%v).IsValid() = true, want false", name)
+	}
+	if got, want := mapv.NewValue(), newMapValue(fd, mapv, 0, nil); !valueEqual(got, want) {
+		t.Errorf("message.Get(%v).NewValue() = %v, want %v", name, formatValue(got), formatValue(want))
+	}
+	if !panics(func() {
+		m.Set(fd, pref.ValueOfMap(mapv))
+	}) {
+		t.Errorf("message.Set(%v, <invalid>) does not panic", name)
+	}
+	if !panics(func() {
+		mapv.Set(newMapKey(fd, 0), newMapValue(fd, mapv, 0, nil))
+	}) {
+		t.Errorf("message.Get(%v).Set(...) of invalid map does not panic", name)
+	}
+	mapv = m.Mutable(fd).Map() // mutable map
+	if !mapv.IsValid() {
+		t.Errorf("message.Mutable(%v).IsValid() = false, want true", name)
+	}
+	if got, want := mapv.NewValue(), newMapValue(fd, mapv, 0, nil); !valueEqual(got, want) {
+		t.Errorf("message.Mutable(%v).NewValue() = %v, want %v", name, formatValue(got), formatValue(want))
+	}
+
+	// Add values.
+	want := make(testMap)
+	for i, n := range []seed{1, 0, minVal, maxVal} {
+		if got, want := m.Has(fd), i > 0; got != want {
+			t.Errorf("after inserting %d elements to %q:\nMessage.Has(%v) = %v, want %v", i, name, num, got, want)
+		}
+
+		k := newMapKey(fd, n)
+		v := newMapValue(fd, mapv, n, nil)
+		mapv.Set(k, v)
+		want.Set(k, v)
+		if got, want := m.Get(fd), pref.ValueOfMap(want); !valueEqual(got, want) {
+			t.Errorf("after inserting %d elements to %q:\nMessage.Get(%v) = %v, want %v", i, name, num, formatValue(got), formatValue(want))
+		}
+	}
+
+	// Set values.
+	want.Range(func(k pref.MapKey, v pref.Value) bool {
+		nv := newMapValue(fd, mapv, 10, nil)
+		mapv.Set(k, nv)
+		want.Set(k, nv)
+		if got, want := m.Get(fd), pref.ValueOfMap(want); !valueEqual(got, want) {
+			t.Errorf("after setting element %v of %q:\nMessage.Get(%v) = %v, want %v", formatValue(k.Value()), name, num, formatValue(got), formatValue(want))
+		}
+		return true
+	})
+
+	// Clear values.
+	want.Range(func(k pref.MapKey, v pref.Value) bool {
+		mapv.Clear(k)
+		want.Clear(k)
+		if got, want := m.Has(fd), want.Len() > 0; got != want {
+			t.Errorf("after clearing elements of %q:\nMessage.Has(%v) = %v, want %v", name, num, got, want)
+		}
+		if got, want := m.Get(fd), pref.ValueOfMap(want); !valueEqual(got, want) {
+			t.Errorf("after clearing elements of %q:\nMessage.Get(%v) = %v, want %v", name, num, formatValue(got), formatValue(want))
+		}
+		return true
+	})
+	if mapv := m.Get(fd).Map(); mapv.IsValid() {
+		t.Errorf("after clearing all elements: message.Get(%v).IsValid() = true, want false %v", name, formatValue(pref.ValueOfMap(mapv)))
+	}
+
+	// Non-existent map keys.
+	missingKey := newMapKey(fd, 1)
+	if got, want := mapv.Has(missingKey), false; got != want {
+		t.Errorf("non-existent map key in %q: Map.Has(%v) = %v, want %v", name, formatValue(missingKey.Value()), got, want)
+	}
+	if got, want := mapv.Get(missingKey).IsValid(), false; got != want {
+		t.Errorf("non-existent map key in %q: Map.Get(%v).IsValid() = %v, want %v", name, formatValue(missingKey.Value()), got, want)
+	}
+	mapv.Clear(missingKey) // noop
+
+	// Mutable.
+	if fd.MapValue().Message() == nil {
+		if !panics(func() {
+			mapv.Mutable(newMapKey(fd, 1))
+		}) {
+			t.Errorf("Mutable on %q succeeds, want panic", name)
+		}
+	} else {
+		k := newMapKey(fd, 1)
+		v := mapv.Mutable(k)
+		if got, want := mapv.Len(), 1; got != want {
+			t.Errorf("after Mutable on %q, Map.Len() = %v, want %v", name, got, want)
+		}
+		populateMessage(v.Message(), 1, nil)
+		if !valueEqual(mapv.Get(k), v) {
+			t.Errorf("after Mutable on %q, changing new mutable value does not change map entry", name)
+		}
+		mapv.Clear(k)
+	}
+}
+
+type testMap map[interface{}]pref.Value
+
+func (m testMap) Get(k pref.MapKey) pref.Value     { return m[k.Interface()] }
+func (m testMap) Set(k pref.MapKey, v pref.Value)  { m[k.Interface()] = v }
+func (m testMap) Has(k pref.MapKey) bool           { return m.Get(k).IsValid() }
+func (m testMap) Clear(k pref.MapKey)              { delete(m, k.Interface()) }
+func (m testMap) Mutable(k pref.MapKey) pref.Value { panic("unimplemented") }
+func (m testMap) Len() int                         { return len(m) }
+func (m testMap) NewValue() pref.Value             { panic("unimplemented") }
+func (m testMap) Range(f func(pref.MapKey, pref.Value) bool) {
+	for k, v := range m {
+		if !f(pref.ValueOf(k).MapKey(), v) {
+			return
+		}
+	}
+}
+func (m testMap) IsValid() bool { return true }
+
+// testFieldList exercises set/get/append/truncate of values in a list.
+func testFieldList(t testing.TB, m pref.Message, fd pref.FieldDescriptor) {
+	name := fd.FullName()
+	num := fd.Number()
+
+	m.Clear(fd) // start with an empty list
+	list := m.Get(fd).List()
+	if list.IsValid() {
+		t.Errorf("message.Get(%v).IsValid() = true, want false", name)
+	}
+	if !panics(func() {
+		m.Set(fd, pref.ValueOfList(list))
+	}) {
+		t.Errorf("message.Set(%v, <invalid>) does not panic", name)
+	}
+	if !panics(func() {
+		list.Append(newListElement(fd, list, 0, nil))
+	}) {
+		t.Errorf("message.Get(%v).Append(...) of invalid list does not panic", name)
+	}
+	if got, want := list.NewElement(), newListElement(fd, list, 0, nil); !valueEqual(got, want) {
+		t.Errorf("message.Get(%v).NewElement() = %v, want %v", name, formatValue(got), formatValue(want))
+	}
+	list = m.Mutable(fd).List() // mutable list
+	if !list.IsValid() {
+		t.Errorf("message.Get(%v).IsValid() = false, want true", name)
+	}
+	if got, want := list.NewElement(), newListElement(fd, list, 0, nil); !valueEqual(got, want) {
+		t.Errorf("message.Mutable(%v).NewElement() = %v, want %v", name, formatValue(got), formatValue(want))
+	}
+
+	// Append values.
+	var want pref.List = &testList{}
+	for i, n := range []seed{1, 0, minVal, maxVal} {
+		if got, want := m.Has(fd), i > 0; got != want {
+			t.Errorf("after appending %d elements to %q:\nMessage.Has(%v) = %v, want %v", i, name, num, got, want)
+		}
+		v := newListElement(fd, list, n, nil)
+		want.Append(v)
+		list.Append(v)
+
+		if got, want := m.Get(fd), pref.ValueOfList(want); !valueEqual(got, want) {
+			t.Errorf("after appending %d elements to %q:\nMessage.Get(%v) = %v, want %v", i+1, name, num, formatValue(got), formatValue(want))
+		}
+	}
+
+	// Set values.
+	for i := 0; i < want.Len(); i++ {
+		v := newListElement(fd, list, seed(i+10), nil)
+		want.Set(i, v)
+		list.Set(i, v)
+		if got, want := m.Get(fd), pref.ValueOfList(want); !valueEqual(got, want) {
+			t.Errorf("after setting element %d of %q:\nMessage.Get(%v) = %v, want %v", i, name, num, formatValue(got), formatValue(want))
+		}
+	}
+
+	// Truncate.
+	for want.Len() > 0 {
+		n := want.Len() - 1
+		want.Truncate(n)
+		list.Truncate(n)
+		if got, want := m.Has(fd), want.Len() > 0; got != want {
+			t.Errorf("after truncating %q to %d:\nMessage.Has(%v) = %v, want %v", name, n, num, got, want)
+		}
+		if got, want := m.Get(fd), pref.ValueOfList(want); !valueEqual(got, want) {
+			t.Errorf("after truncating %q to %d:\nMessage.Get(%v) = %v, want %v", name, n, num, formatValue(got), formatValue(want))
+		}
+	}
+
+	// AppendMutable.
+	if fd.Message() == nil {
+		if !panics(func() {
+			list.AppendMutable()
+		}) {
+			t.Errorf("AppendMutable on %q succeeds, want panic", name)
+		}
+	} else {
+		v := list.AppendMutable()
+		if got, want := list.Len(), 1; got != want {
+			t.Errorf("after AppendMutable on %q, list.Len() = %v, want %v", name, got, want)
+		}
+		populateMessage(v.Message(), 1, nil)
+		if !valueEqual(list.Get(0), v) {
+			t.Errorf("after AppendMutable on %q, changing new mutable value does not change list item 0", name)
+		}
+		want.Truncate(0)
+	}
+}
+
+type testList struct {
+	a []pref.Value
+}
+
+func (l *testList) Append(v pref.Value)       { l.a = append(l.a, v) }
+func (l *testList) AppendMutable() pref.Value { panic("unimplemented") }
+func (l *testList) Get(n int) pref.Value      { return l.a[n] }
+func (l *testList) Len() int                  { return len(l.a) }
+func (l *testList) Set(n int, v pref.Value)   { l.a[n] = v }
+func (l *testList) Truncate(n int)            { l.a = l.a[:n] }
+func (l *testList) NewElement() pref.Value    { panic("unimplemented") }
+func (l *testList) IsValid() bool             { return true }
+
+// testFieldFloat exercises some interesting floating-point scalar field values.
+func testFieldFloat(t testing.TB, m pref.Message, fd pref.FieldDescriptor) {
+	name := fd.FullName()
+	num := fd.Number()
+
+	for _, v := range []float64{math.Inf(-1), math.Inf(1), math.NaN(), math.Copysign(0, -1)} {
+		var val pref.Value
+		if fd.Kind() == pref.FloatKind {
+			val = pref.ValueOfFloat32(float32(v))
+		} else {
+			val = pref.ValueOfFloat64(float64(v))
+		}
+		m.Set(fd, val)
+		// Note that Has is true for -0.
+		if got, want := m.Has(fd), true; got != want {
+			t.Errorf("after setting %v to %v: Message.Has(%v) = %v, want %v", name, v, num, got, want)
+		}
+		if got, want := m.Get(fd), val; !valueEqual(got, want) {
+			t.Errorf("after setting %v: Message.Get(%v) = %v, want %v", name, num, formatValue(got), formatValue(want))
+		}
+	}
+}
+
+// testOneof tests the behavior of fields in a oneof.
+func testOneof(t testing.TB, m pref.Message, od pref.OneofDescriptor) {
+	for _, mutable := range []bool{false, true} {
+		for i := 0; i < od.Fields().Len(); i++ {
+			fda := od.Fields().Get(i)
+			if mutable {
+				// Set fields by requesting a mutable reference.
+				if !fda.IsMap() && !fda.IsList() && fda.Message() == nil {
+					continue
+				}
+				_ = m.Mutable(fda)
+			} else {
+				// Set fields explicitly.
+				m.Set(fda, newValue(m, fda, 1, nil))
+			}
+			if got, want := m.WhichOneof(od), fda; got != want {
+				t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
+			}
+			for j := 0; j < od.Fields().Len(); j++ {
+				fdb := od.Fields().Get(j)
+				if got, want := m.Has(fdb), i == j; got != want {
+					t.Errorf("after setting oneof field %q:\nGet(%q) = %v, want %v", fda.FullName(), fdb.FullName(), got, want)
+				}
+			}
+		}
+	}
+}
+
+// testUnknown tests the behavior of unknown fields.
+func testUnknown(t testing.TB, m pref.Message) {
+	var b []byte
+	b = protowire.AppendTag(b, 1000, protowire.VarintType)
+	b = protowire.AppendVarint(b, 1001)
+	m.SetUnknown(pref.RawFields(b))
+	if got, want := []byte(m.GetUnknown()), b; !bytes.Equal(got, want) {
+		t.Errorf("after setting unknown fields:\nGetUnknown() = %v, want %v", got, want)
+	}
+}
+
+func formatValue(v pref.Value) string {
+	switch v := v.Interface().(type) {
+	case pref.List:
+		var buf bytes.Buffer
+		buf.WriteString("list[")
+		for i := 0; i < v.Len(); i++ {
+			if i > 0 {
+				buf.WriteString(" ")
+			}
+			buf.WriteString(formatValue(v.Get(i)))
+		}
+		buf.WriteString("]")
+		return buf.String()
+	case pref.Map:
+		var buf bytes.Buffer
+		buf.WriteString("map[")
+		var keys []pref.MapKey
+		v.Range(func(k pref.MapKey, v pref.Value) bool {
+			keys = append(keys, k)
+			return true
+		})
+		sort.Slice(keys, func(i, j int) bool {
+			return keys[i].String() < keys[j].String()
+		})
+		for i, k := range keys {
+			if i > 0 {
+				buf.WriteString(" ")
+			}
+			buf.WriteString(formatValue(k.Value()))
+			buf.WriteString(":")
+			buf.WriteString(formatValue(v.Get(k)))
+		}
+		buf.WriteString("]")
+		return buf.String()
+	case pref.Message:
+		b, err := prototext.Marshal(v.Interface())
+		if err != nil {
+			return fmt.Sprintf("<%v>", err)
+		}
+		return fmt.Sprintf("%v{%v}", v.Descriptor().FullName(), string(b))
+	case string:
+		return fmt.Sprintf("%q", v)
+	default:
+		return fmt.Sprint(v)
+	}
+}
+
+func valueEqual(a, b pref.Value) bool {
+	ai, bi := a.Interface(), b.Interface()
+	switch ai.(type) {
+	case pref.Message:
+		return proto.Equal(
+			a.Message().Interface(),
+			b.Message().Interface(),
+		)
+	case pref.List:
+		lista, listb := a.List(), b.List()
+		if lista.Len() != listb.Len() {
+			return false
+		}
+		for i := 0; i < lista.Len(); i++ {
+			if !valueEqual(lista.Get(i), listb.Get(i)) {
+				return false
+			}
+		}
+		return true
+	case pref.Map:
+		mapa, mapb := a.Map(), b.Map()
+		if mapa.Len() != mapb.Len() {
+			return false
+		}
+		equal := true
+		mapa.Range(func(k pref.MapKey, v pref.Value) bool {
+			if !valueEqual(v, mapb.Get(k)) {
+				equal = false
+				return false
+			}
+			return true
+		})
+		return equal
+	case []byte:
+		return bytes.Equal(a.Bytes(), b.Bytes())
+	case float32:
+		// NaNs are equal, but must be the same NaN.
+		return math.Float32bits(ai.(float32)) == math.Float32bits(bi.(float32))
+	case float64:
+		// NaNs are equal, but must be the same NaN.
+		return math.Float64bits(ai.(float64)) == math.Float64bits(bi.(float64))
+	default:
+		return ai == bi
+	}
+}
+
+// A seed is used to vary the content of a value.
+//
+// A seed of 0 is the zero value. Messages do not have a zero-value; a 0-seeded messages
+// is unpopulated.
+//
+// A seed of minVal or maxVal is the least or greatest value of the value type.
+type seed int
+
+const (
+	minVal seed = -1
+	maxVal seed = -2
+)
+
+// newSeed creates new seed values from a base, for example to create seeds for the
+// elements in a list. If the input seed is minVal or maxVal, so is the output.
+func newSeed(n seed, adjust ...int) seed {
+	switch n {
+	case minVal, maxVal:
+		return n
+	}
+	for _, a := range adjust {
+		n = 10*n + seed(a)
+	}
+	return n
+}
+
+// newValue returns a new value assignable to a field.
+//
+// The stack parameter is used to avoid infinite recursion when populating circular
+// data structures.
+func newValue(m pref.Message, fd pref.FieldDescriptor, n seed, stack []pref.MessageDescriptor) pref.Value {
+	switch {
+	case fd.IsList():
+		if n == 0 {
+			return m.New().Mutable(fd)
+		}
+		list := m.NewField(fd).List()
+		list.Append(newListElement(fd, list, 0, stack))
+		list.Append(newListElement(fd, list, minVal, stack))
+		list.Append(newListElement(fd, list, maxVal, stack))
+		list.Append(newListElement(fd, list, n, stack))
+		return pref.ValueOfList(list)
+	case fd.IsMap():
+		if n == 0 {
+			return m.New().Mutable(fd)
+		}
+		mapv := m.NewField(fd).Map()
+		mapv.Set(newMapKey(fd, 0), newMapValue(fd, mapv, 0, stack))
+		mapv.Set(newMapKey(fd, minVal), newMapValue(fd, mapv, minVal, stack))
+		mapv.Set(newMapKey(fd, maxVal), newMapValue(fd, mapv, maxVal, stack))
+		mapv.Set(newMapKey(fd, n), newMapValue(fd, mapv, newSeed(n, 0), stack))
+		return pref.ValueOfMap(mapv)
+	case fd.Message() != nil:
+		return populateMessage(m.NewField(fd).Message(), n, stack)
+	default:
+		return newScalarValue(fd, n)
+	}
+}
+
+func newListElement(fd pref.FieldDescriptor, list pref.List, n seed, stack []pref.MessageDescriptor) pref.Value {
+	if fd.Message() == nil {
+		return newScalarValue(fd, n)
+	}
+	return populateMessage(list.NewElement().Message(), n, stack)
+}
+
+func newMapKey(fd pref.FieldDescriptor, n seed) pref.MapKey {
+	kd := fd.MapKey()
+	return newScalarValue(kd, n).MapKey()
+}
+
+func newMapValue(fd pref.FieldDescriptor, mapv pref.Map, n seed, stack []pref.MessageDescriptor) pref.Value {
+	vd := fd.MapValue()
+	if vd.Message() == nil {
+		return newScalarValue(vd, n)
+	}
+	return populateMessage(mapv.NewValue().Message(), n, stack)
+}
+
+func newScalarValue(fd pref.FieldDescriptor, n seed) pref.Value {
+	switch fd.Kind() {
+	case pref.BoolKind:
+		return pref.ValueOfBool(n != 0)
+	case pref.EnumKind:
+		vals := fd.Enum().Values()
+		var i int
+		switch n {
+		case minVal:
+			i = 0
+		case maxVal:
+			i = vals.Len() - 1
+		default:
+			i = int(n) % vals.Len()
+		}
+		return pref.ValueOfEnum(vals.Get(i).Number())
+	case pref.Int32Kind, pref.Sint32Kind, pref.Sfixed32Kind:
+		switch n {
+		case minVal:
+			return pref.ValueOfInt32(math.MinInt32)
+		case maxVal:
+			return pref.ValueOfInt32(math.MaxInt32)
+		default:
+			return pref.ValueOfInt32(int32(n))
+		}
+	case pref.Uint32Kind, pref.Fixed32Kind:
+		switch n {
+		case minVal:
+			// Only use 0 for the zero value.
+			return pref.ValueOfUint32(1)
+		case maxVal:
+			return pref.ValueOfUint32(math.MaxInt32)
+		default:
+			return pref.ValueOfUint32(uint32(n))
+		}
+	case pref.Int64Kind, pref.Sint64Kind, pref.Sfixed64Kind:
+		switch n {
+		case minVal:
+			return pref.ValueOfInt64(math.MinInt64)
+		case maxVal:
+			return pref.ValueOfInt64(math.MaxInt64)
+		default:
+			return pref.ValueOfInt64(int64(n))
+		}
+	case pref.Uint64Kind, pref.Fixed64Kind:
+		switch n {
+		case minVal:
+			// Only use 0 for the zero value.
+			return pref.ValueOfUint64(1)
+		case maxVal:
+			return pref.ValueOfUint64(math.MaxInt64)
+		default:
+			return pref.ValueOfUint64(uint64(n))
+		}
+	case pref.FloatKind:
+		switch n {
+		case minVal:
+			return pref.ValueOfFloat32(math.SmallestNonzeroFloat32)
+		case maxVal:
+			return pref.ValueOfFloat32(math.MaxFloat32)
+		default:
+			return pref.ValueOfFloat32(1.5 * float32(n))
+		}
+	case pref.DoubleKind:
+		switch n {
+		case minVal:
+			return pref.ValueOfFloat64(math.SmallestNonzeroFloat64)
+		case maxVal:
+			return pref.ValueOfFloat64(math.MaxFloat64)
+		default:
+			return pref.ValueOfFloat64(1.5 * float64(n))
+		}
+	case pref.StringKind:
+		if n == 0 {
+			return pref.ValueOfString("")
+		}
+		return pref.ValueOfString(fmt.Sprintf("%d", n))
+	case pref.BytesKind:
+		if n == 0 {
+			return pref.ValueOfBytes(nil)
+		}
+		return pref.ValueOfBytes([]byte{byte(n >> 24), byte(n >> 16), byte(n >> 8), byte(n)})
+	}
+	panic("unhandled kind")
+}
+
+func populateMessage(m pref.Message, n seed, stack []pref.MessageDescriptor) pref.Value {
+	if n == 0 {
+		return pref.ValueOfMessage(m)
+	}
+	md := m.Descriptor()
+	for _, x := range stack {
+		if md == x {
+			return pref.ValueOfMessage(m)
+		}
+	}
+	stack = append(stack, md)
+	for i := 0; i < md.Fields().Len(); i++ {
+		fd := md.Fields().Get(i)
+		if fd.IsWeak() {
+			continue
+		}
+		m.Set(fd, newValue(m, fd, newSeed(n, i), stack))
+	}
+	return pref.ValueOfMessage(m)
+}
+
+func panics(f func()) (didPanic bool) {
+	defer func() {
+		if err := recover(); err != nil {
+			didPanic = true
+		}
+	}()
+	f()
+	return false
+}