encoding/json/v2: reduce allocations when checking IsZero

src/encoding/json/v2/bench_test.go

@@ -278,6 +278,12 @@ var arshalTestdata = []struct {
 	raw:  []byte(`"2006-01-02T22:04:05Z"`),
 	val:  addr(time.Unix(1136239445, 0).UTC()),
 	new:  func() any { return new(time.Time) },
+}, {
+	name:   "OmitZeroMethod",
+	raw:    []byte(`{}`),
+	val:    new(omitZeroStruct),
+	new:    func() any { return new(omitZeroStruct) },
+	skipV1: true,
 }}
 
 type textArshaler struct{ _ [4]int }
@@ -317,6 +323,25 @@ func (*jsonArshalerV2) UnmarshalJSONFrom(dec *jsontext.Decoder) error {
 	return err
 }
 
+type omitZeroStruct struct {
+	A zeroAlways    `json:"a,omitzero"`
+	B zeroAlwaysPtr `json:"b,omitzero"`
+}
+
+type zeroAlways struct {
+	// Just so the struct has some size to allocate
+	A int
+}
+
+func (zeroAlways) IsZero() bool { return true }
+
+type zeroAlwaysPtr struct {
+	// Just so the struct has some size to allocate
+	A int
+}
+
+func (*zeroAlwaysPtr) IsZero() bool { return true }
+
 func TestBenchmarkUnmarshal(t *testing.T) { runUnmarshal(t) }
 func BenchmarkUnmarshal(b *testing.B)     { runUnmarshal(b) }
 

src/encoding/json/v2/fields.go

@@ -229,10 +229,10 @@ func makeStructFields(root reflect.Type) (fs structFields, serr *SemanticError)
 						// Avoid panics calling IsZero on nil pointer.
 						return va.IsNil() || va.Interface().(isZeroer).IsZero()
 					}
-				case sf.Type.Implements(isZeroerType):
-					f.isZero = func(va addressableValue) bool { return va.Interface().(isZeroer).IsZero() }
 				case reflect.PointerTo(sf.Type).Implements(isZeroerType):
 					f.isZero = func(va addressableValue) bool { return va.Addr().Interface().(isZeroer).IsZero() }
+				case sf.Type.Implements(isZeroerType):
+					f.isZero = func(va addressableValue) bool { return va.Interface().(isZeroer).IsZero() }
 				}
 
 				// Provide a function that can determine whether the value would