internal/impl: switch to reflect.Value.IsZero

(Go Protobuf currently requires Go 1.21.)

Change-Id: I5b9f783bbd3611fedc544e1367cc3db9f0123494
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/641018
Reviewed-by: Chressie Himpel <chressie@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/impl/message_reflect_field.go

@@ -419,7 +419,7 @@ func fieldInfoForMessage(fd protoreflect.FieldDescriptor, fs reflect.StructField
 			}
 			rv := p.Apply(fieldOffset).AsValueOf(fs.Type).Elem()
 			if fs.Type.Kind() != reflect.Ptr {
-				return !isZero(rv)
+				return !rv.IsZero()
 			}
 			return !rv.IsNil()
 		},
@@ -497,41 +497,3 @@ func makeOneofInfo(od protoreflect.OneofDescriptor, si structInfo, x exporter) *
 	}
 	return oi
 }
-
-// isZero is identical to reflect.Value.IsZero.
-// TODO: Remove this when Go1.13 is the minimally supported Go version.
-func isZero(v reflect.Value) bool {
-	switch v.Kind() {
-	case reflect.Bool:
-		return !v.Bool()
-	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
-		return v.Int() == 0
-	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
-		return v.Uint() == 0
-	case reflect.Float32, reflect.Float64:
-		return math.Float64bits(v.Float()) == 0
-	case reflect.Complex64, reflect.Complex128:
-		c := v.Complex()
-		return math.Float64bits(real(c)) == 0 && math.Float64bits(imag(c)) == 0
-	case reflect.Array:
-		for i := 0; i < v.Len(); i++ {
-			if !isZero(v.Index(i)) {
-				return false
-			}
-		}
-		return true
-	case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Ptr, reflect.Slice, reflect.UnsafePointer:
-		return v.IsNil()
-	case reflect.String:
-		return v.Len() == 0
-	case reflect.Struct:
-		for i := 0; i < v.NumField(); i++ {
-			if !isZero(v.Field(i)) {
-				return false
-			}
-		}
-		return true
-	default:
-		panic(&reflect.ValueError{Method: "reflect.Value.IsZero", Kind: v.Kind()})
-	}
-}