reflect/prototype: initial commit

Add the prototype package which provides constructors for
protoreflect.{Enum,Message,Extension}Type.

Switch all usages of the internal/prototype equivalent to the new package.
Switch all custom implementions of {Enum,Message}Type to the new package.

Change-Id: Ia9dae6fed4f2b90e55c123627044a7faf098c4b1
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/178438
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/legacy/export.go b/internal/legacy/export.go
index ec87a68..701b578 100644
--- a/internal/legacy/export.go
+++ b/internal/legacy/export.go
@@ -5,6 +5,7 @@
 package legacy
 
 import (
+	"fmt"
 	"reflect"
 
 	pimpl "google.golang.org/protobuf/internal/impl"
@@ -17,8 +18,8 @@
 // functions that we do not want to appear in godoc.
 type Export struct{}
 
-func (Export) EnumOf(e interface{}) pvalue.LegacyEnum {
-	return wrapEnum(reflect.ValueOf(e)).(pvalue.LegacyEnum)
+func (Export) EnumOf(e interface{}) pref.Enum {
+	return wrapEnum(reflect.ValueOf(e))
 }
 
 func (Export) EnumTypeOf(e interface{}) pref.EnumType {
@@ -29,8 +30,8 @@
 	return LoadEnumDesc(reflect.TypeOf(e))
 }
 
-func (Export) MessageOf(m interface{}) pvalue.LegacyMessage {
-	return wrapMessage(reflect.ValueOf(m)).ProtoReflect().(pvalue.LegacyMessage)
+func (Export) MessageOf(m interface{}) pref.Message {
+	return wrapMessage(reflect.ValueOf(m)).ProtoReflect()
 }
 
 func (Export) MessageTypeOf(m interface{}) pref.MessageType {
@@ -49,6 +50,61 @@
 	return extensionTypeFromDesc(d)
 }
 
+var (
+	enumIfaceV2    = reflect.TypeOf((*pref.Enum)(nil)).Elem()
+	messageIfaceV1 = reflect.TypeOf((*piface.MessageV1)(nil)).Elem()
+	messageIfaceV2 = reflect.TypeOf((*pref.ProtoMessage)(nil)).Elem()
+)
+
+func (Export) NewConverter(t reflect.Type, k pref.Kind) pvalue.Converter {
+	c, _ := newConverter(t, k)
+	return c
+}
+
+func newConverter(t reflect.Type, k pref.Kind) (pvalue.Converter, bool) {
+	switch k {
+	case pref.EnumKind:
+		if t.Kind() == reflect.Int32 && !t.Implements(enumIfaceV2) {
+			return pvalue.Converter{
+				PBValueOf: func(v reflect.Value) pref.Value {
+					if v.Type() != t {
+						panic(fmt.Sprintf("invalid type: got %v, want %v", v.Type(), t))
+					}
+					return pref.ValueOf(pref.EnumNumber(v.Int()))
+				},
+				GoValueOf: func(v pref.Value) reflect.Value {
+					return reflect.ValueOf(v.Enum()).Convert(t)
+				},
+				NewEnum: func(n pref.EnumNumber) pref.Enum {
+					return wrapEnum(reflect.ValueOf(n).Convert(t))
+				},
+			}, true
+		}
+	case pref.MessageKind, pref.GroupKind:
+		if t.Kind() == reflect.Ptr && t.Implements(messageIfaceV1) && !t.Implements(messageIfaceV2) {
+			return pvalue.Converter{
+				PBValueOf: func(v reflect.Value) pref.Value {
+					if v.Type() != t {
+						panic(fmt.Sprintf("invalid type: got %v, want %v", v.Type(), t))
+					}
+					return pref.ValueOf(Export{}.MessageOf(v.Interface()))
+				},
+				GoValueOf: func(v pref.Value) reflect.Value {
+					rv := reflect.ValueOf(v.Message().(pvalue.Unwrapper).ProtoUnwrap())
+					if rv.Type() != t {
+						panic(fmt.Sprintf("invalid type: got %v, want %v", rv.Type(), t))
+					}
+					return rv
+				},
+				NewMessage: func() pref.Message {
+					return wrapMessage(reflect.New(t.Elem())).ProtoReflect()
+				},
+			}, true
+		}
+	}
+	return pvalue.NewConverter(t, k), false
+}
+
 func init() {
 	pimpl.RegisterLegacyWrapper(Export{})
 }