reflect/protoreflect: add ExtensionType IsValid{Interface,Value} methods

Add a way to typecheck a Value or interface{} without converting it to
the other form.  This permits implementations which store field values as
a Value (such as dynamicpb, or (soon) extensions in generated messages)
to validate inputs without an unnecessary conversion.

Fixes golang/protobuf#905

Change-Id: I1b78612b22ae832efbb55f81ae420871729e3a02
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/192457
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/convert.go b/internal/impl/convert.go
index 9b74c03..2186b26 100644
--- a/internal/impl/convert.go
+++ b/internal/impl/convert.go
@@ -25,6 +25,12 @@
 	// GoValueOf converts a protoreflect.Value to a reflect.Value.
 	GoValueOf(pref.Value) reflect.Value
 
+	// IsValidPB returns whether a protoreflect.Value is compatible with this type.
+	IsValidPB(pref.Value) bool
+
+	// IsValidGo returns whether a reflect.Value is compatible with this type.
+	IsValidGo(reflect.Value) bool
+
 	// New returns a new field value.
 	// For scalars, it returns the default value of the field.
 	// For composite types, it returns a new mutable value.
@@ -151,6 +157,13 @@
 func (c *boolConverter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(v.Bool()).Convert(c.goType)
 }
+func (c *boolConverter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(bool)
+	return ok
+}
+func (c *boolConverter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *boolConverter) New() pref.Value  { return c.def }
 func (c *boolConverter) Zero() pref.Value { return c.def }
 
@@ -168,6 +181,13 @@
 func (c *int32Converter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(int32(v.Int())).Convert(c.goType)
 }
+func (c *int32Converter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(int32)
+	return ok
+}
+func (c *int32Converter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *int32Converter) New() pref.Value  { return c.def }
 func (c *int32Converter) Zero() pref.Value { return c.def }
 
@@ -185,6 +205,13 @@
 func (c *int64Converter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(int64(v.Int())).Convert(c.goType)
 }
+func (c *int64Converter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(int64)
+	return ok
+}
+func (c *int64Converter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *int64Converter) New() pref.Value  { return c.def }
 func (c *int64Converter) Zero() pref.Value { return c.def }
 
@@ -202,6 +229,13 @@
 func (c *uint32Converter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(uint32(v.Uint())).Convert(c.goType)
 }
+func (c *uint32Converter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(uint32)
+	return ok
+}
+func (c *uint32Converter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *uint32Converter) New() pref.Value  { return c.def }
 func (c *uint32Converter) Zero() pref.Value { return c.def }
 
@@ -219,6 +253,13 @@
 func (c *uint64Converter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(uint64(v.Uint())).Convert(c.goType)
 }
+func (c *uint64Converter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(uint64)
+	return ok
+}
+func (c *uint64Converter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *uint64Converter) New() pref.Value  { return c.def }
 func (c *uint64Converter) Zero() pref.Value { return c.def }
 
@@ -236,6 +277,13 @@
 func (c *float32Converter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(float32(v.Float())).Convert(c.goType)
 }
+func (c *float32Converter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(float32)
+	return ok
+}
+func (c *float32Converter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *float32Converter) New() pref.Value  { return c.def }
 func (c *float32Converter) Zero() pref.Value { return c.def }
 
@@ -253,6 +301,13 @@
 func (c *float64Converter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(float64(v.Float())).Convert(c.goType)
 }
+func (c *float64Converter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(float64)
+	return ok
+}
+func (c *float64Converter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *float64Converter) New() pref.Value  { return c.def }
 func (c *float64Converter) Zero() pref.Value { return c.def }
 
@@ -276,6 +331,13 @@
 	}
 	return reflect.ValueOf(s).Convert(c.goType)
 }
+func (c *stringConverter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(string)
+	return ok
+}
+func (c *stringConverter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *stringConverter) New() pref.Value  { return c.def }
 func (c *stringConverter) Zero() pref.Value { return c.def }
 
@@ -296,6 +358,13 @@
 func (c *bytesConverter) GoValueOf(v pref.Value) reflect.Value {
 	return reflect.ValueOf(v.Bytes()).Convert(c.goType)
 }
+func (c *bytesConverter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().([]byte)
+	return ok
+}
+func (c *bytesConverter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
 func (c *bytesConverter) New() pref.Value  { return c.def }
 func (c *bytesConverter) Zero() pref.Value { return c.def }
 
@@ -325,6 +394,15 @@
 	return reflect.ValueOf(v.Enum()).Convert(c.goType)
 }
 
+func (c *enumConverter) IsValidPB(v pref.Value) bool {
+	_, ok := v.Interface().(pref.EnumNumber)
+	return ok
+}
+
+func (c *enumConverter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
+
 func (c *enumConverter) New() pref.Value {
 	return c.def
 }
@@ -365,6 +443,21 @@
 	return rv
 }
 
+func (c *messageConverter) IsValidPB(v pref.Value) bool {
+	m := v.Message()
+	var rv reflect.Value
+	if u, ok := m.(Unwrapper); ok {
+		rv = reflect.ValueOf(u.ProtoUnwrap())
+	} else {
+		rv = reflect.ValueOf(m.Interface())
+	}
+	return rv.Type() == c.goType
+}
+
+func (c *messageConverter) IsValidGo(v reflect.Value) bool {
+	return v.Type() == c.goType
+}
+
 func (c *messageConverter) New() pref.Value {
 	return c.PBValueOf(reflect.New(c.goType.Elem()))
 }