encoding/json: rely on reflect.Value.SetZero

v.SetZero() is faster than v.Set(reflect.Zero(v.Type()))
and was recently added in Go 1.20.

Benchmark numbers are largely unchanged since this mainly
affects the unmarshaling of large numbers of JSON nulls,
which our benchmarks do not heavily exercise.

Change-Id: I464f60f63c9027e63a99fd5da92e7ab782018329
Reviewed-on: https://go-review.googlesource.com/c/go/+/471195
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

src/encoding/json/decode.go

@@ -577,13 +577,11 @@ func (d *decodeState) array(v reflect.Value) error {
 
 	if i < v.Len() {
 		if v.Kind() == reflect.Array {
-			// Array. Zero the rest.
-			z := reflect.Zero(v.Type().Elem())
 			for ; i < v.Len(); i++ {
-				v.Index(i).Set(z)
+				v.Index(i).SetZero() // zero remainder of array
 			}
 		} else {
-			v.SetLen(i)
+			v.SetLen(i) // truncate the slice
 		}
 	}
 	if i == 0 && v.Kind() == reflect.Slice {
@@ -688,7 +686,7 @@ func (d *decodeState) object(v reflect.Value) error {
 			if !mapElem.IsValid() {
 				mapElem = reflect.New(elemType).Elem()
 			} else {
-				mapElem.Set(reflect.Zero(elemType))
+				mapElem.SetZero()
 			}
 			subv = mapElem
 		} else {
@@ -902,7 +900,7 @@ func (d *decodeState) literalStore(item []byte, v reflect.Value, fromQuoted bool
 		}
 		switch v.Kind() {
 		case reflect.Interface, reflect.Pointer, reflect.Map, reflect.Slice:
-			v.Set(reflect.Zero(v.Type()))
+			v.SetZero()
 			// otherwise, ignore null for primitives/string
 		}
 	case 't', 'f': // true, false