encoding/json: add omitzero option

doc/next/6-stdlib/99-minor/encoding/json/45669.md

@@ -0,0 +1,7 @@
+When marshaling, a struct field with the new `omitzero` option in the struct field
+tag will be omitted if its value is zero. If the field type has an `IsZero() bool`
+method, that will be used to determine whether the value is zero. Otherwise, the
+value is zero if it is [the zero value for its type](/ref/spec#The_zero_value).
+
+If both `omitempty` and `omitzero` are specified, the field will be omitted if the
+value is either empty or zero (or both).

src/encoding/json/encode.go

@@ -99,6 +99,17 @@ import (
 //	// Field appears in JSON as key "-".
 //	Field int `json:"-,"`
 //
+// The "omitzero" option specifies that the field should be omitted
+// from the encoding if the field has a zero value, according to rules:
+//
+// 1) If the field type has an "IsZero() bool" method, that will be used to
+// determine whether the value is zero.
+//
+// 2) Otherwise, the value is zero if it is the zero value for its type.
+//
+// If both "omitempty" and "omitzero" are specified, the field will be omitted
+// if the value is either empty or zero (or both).
+//
 // The "string" option signals that a field is stored as JSON inside a
 // JSON-encoded string. It applies only to fields of string, floating point,
 // integer, or boolean types. This extra level of encoding is sometimes used
@@ -701,7 +712,8 @@ FieldLoop:
 			fv = fv.Field(i)
 		}
 
-		if f.omitEmpty && isEmptyValue(fv) {
+		if (f.omitEmpty && isEmptyValue(fv)) ||
+			(f.omitZero && (f.isZero == nil && fv.IsZero() || (f.isZero != nil && f.isZero(fv)))) {
 			continue
 		}
 		e.WriteByte(next)
@@ -1048,11 +1060,19 @@ type field struct {
 	index     []int
 	typ       reflect.Type
 	omitEmpty bool
+	omitZero  bool
+	isZero    func(reflect.Value) bool
 	quoted    bool
 
 	encoder encoderFunc
 }
 
+type isZeroer interface {
+	IsZero() bool
+}
+
+var isZeroerType = reflect.TypeFor[isZeroer]()
+
 // typeFields returns a list of fields that JSON should recognize for the given type.
 // The algorithm is breadth-first search over the set of structs to include - the top struct
 // and then any reachable anonymous structs.
@@ -1154,6 +1174,7 @@ func typeFields(t reflect.Type) structFields {
 						index:     index,
 						typ:       ft,
 						omitEmpty: opts.Contains("omitempty"),
+						omitZero:  opts.Contains("omitzero"),
 						quoted:    quoted,
 					}
 					field.nameBytes = []byte(field.name)
@@ -1163,6 +1184,40 @@ func typeFields(t reflect.Type) structFields {
 					field.nameEscHTML = `"` + string(nameEscBuf) + `":`
 					field.nameNonEsc = `"` + field.name + `":`
 
+					if field.omitZero {
+						t := sf.Type
+						// Provide a function that uses a type's IsZero method.
+						switch {
+						case t.Kind() == reflect.Interface && t.Implements(isZeroerType):
+							field.isZero = func(v reflect.Value) bool {
+								// Avoid panics calling IsZero on a nil interface or
+								// non-nil interface with nil pointer.
+								return v.IsNil() ||
+									(v.Elem().Kind() == reflect.Pointer && v.Elem().IsNil()) ||
+									v.Interface().(isZeroer).IsZero()
+							}
+						case t.Kind() == reflect.Pointer && t.Implements(isZeroerType):
+							field.isZero = func(v reflect.Value) bool {
+								// Avoid panics calling IsZero on nil pointer.
+								return v.IsNil() || v.Interface().(isZeroer).IsZero()
+							}
+						case t.Implements(isZeroerType):
+							field.isZero = func(v reflect.Value) bool {
+								return v.Interface().(isZeroer).IsZero()
+							}
+						case reflect.PointerTo(t).Implements(isZeroerType):
+							field.isZero = func(v reflect.Value) bool {
+								if !v.CanAddr() {
+									// Temporarily box v so we can take the address.
+									v2 := reflect.New(v.Type()).Elem()
+									v2.Set(v)
+									v = v2
+								}
+								return v.Addr().Interface().(isZeroer).IsZero()
+							}
+						}
+					}
+
 					fields = append(fields, field)
 					if count[f.typ] > 1 {
 						// If there were multiple instances, add a second,

src/encoding/json/encode_test.go

@@ -15,9 +15,10 @@ import (
 	"runtime/debug"
 	"strconv"
 	"testing"
+	"time"
 )
 
-type Optionals struct {
+type OptionalsEmpty struct {
 	Sr string `json:"sr"`
 	So string `json:"so,omitempty"`
 	Sw string `json:"-"`
@@ -45,7 +46,7 @@ type Optionals struct {
 }
 
 func TestOmitEmpty(t *testing.T) {
-	var want = `{
+	const want = `{
  "sr": "",
  "omitempty": 0,
  "slr": null,
@@ -56,7 +57,7 @@ func TestOmitEmpty(t *testing.T) {
  "str": {},
  "sto": {}
 }`
-	var o Optionals
+	var o OptionalsEmpty
 	o.Sw = "something"
 	o.Mr = map[string]any{}
 	o.Mo = map[string]any{}
@@ -70,6 +71,187 @@ func TestOmitEmpty(t *testing.T) {
 	}
 }
 
+type NonZeroStruct struct{}
+
+func (nzs NonZeroStruct) IsZero() bool {
+	return false
+}
+
+type NoPanicStruct struct {
+	Int int `json:"int,omitzero"`
+}
+
+func (nps *NoPanicStruct) IsZero() bool {
+	return nps.Int != 0
+}
+
+type OptionalsZero struct {
+	Sr string `json:"sr"`
+	So string `json:"so,omitzero"`
+	Sw string `json:"-"`
+
+	Ir int `json:"omitzero"` // actually named omitzero, not an option
+	Io int `json:"io,omitzero"`
+
+	Slr       []string `json:"slr,random"`
+	Slo       []string `json:"slo,omitzero"`
+	SloNonNil []string `json:"slononnil,omitzero"`
+
+	Mr  map[string]any `json:"mr"`
+	Mo  map[string]any `json:",omitzero"`
+	Moo map[string]any `json:"moo,omitzero"`
+
+	Fr   float64    `json:"fr"`
+	Fo   float64    `json:"fo,omitzero"`
+	Foo  float64    `json:"foo,omitzero"`
+	Foo2 [2]float64 `json:"foo2,omitzero"`
+
+	Br bool `json:"br"`
+	Bo bool `json:"bo,omitzero"`
+
+	Ur uint `json:"ur"`
+	Uo uint `json:"uo,omitzero"`
+
+	Str struct{} `json:"str"`
+	Sto struct{} `json:"sto,omitzero"`
+
+	Time      time.Time     `json:"time,omitzero"`
+	TimeLocal time.Time     `json:"timelocal,omitzero"`
+	Nzs       NonZeroStruct `json:"nzs,omitzero"`
+
+	NilIsZeroer    isZeroer       `json:"niliszeroer,omitzero"`    // nil interface
+	NonNilIsZeroer isZeroer       `json:"nonniliszeroer,omitzero"` // non-nil interface
+	NoPanicStruct0 isZeroer       `json:"nps0,omitzero"`           // non-nil interface with nil pointer
+	NoPanicStruct1 isZeroer       `json:"nps1,omitzero"`           // non-nil interface with non-nil pointer
+	NoPanicStruct2 *NoPanicStruct `json:"nps2,omitzero"`           // nil pointer
+	NoPanicStruct3 *NoPanicStruct `json:"nps3,omitzero"`           // non-nil pointer
+	NoPanicStruct4 NoPanicStruct  `json:"nps4,omitzero"`           // concrete type
+}
+
+func TestOmitZero(t *testing.T) {
+	const want = `{
+ "sr": "",
+ "omitzero": 0,
+ "slr": null,
+ "slononnil": [],
+ "mr": {},
+ "Mo": {},
+ "fr": 0,
+ "br": false,
+ "ur": 0,
+ "str": {},
+ "nzs": {},
+ "nps1": {},
+ "nps3": {},
+ "nps4": {}
+}`
+	var o OptionalsZero
+	o.Sw = "something"
+	o.SloNonNil = make([]string, 0)
+	o.Mr = map[string]any{}
+	o.Mo = map[string]any{}
+
+	o.Foo = -0
+	o.Foo2 = [2]float64{+0, -0}
+
+	o.TimeLocal = time.Time{}.Local()
+
+	o.NonNilIsZeroer = time.Time{}
+	o.NoPanicStruct0 = (*NoPanicStruct)(nil)
+	o.NoPanicStruct1 = &NoPanicStruct{}
+	o.NoPanicStruct3 = &NoPanicStruct{}
+
+	got, err := MarshalIndent(&o, "", " ")
+	if err != nil {
+		t.Fatalf("MarshalIndent error: %v", err)
+	}
+	if got := string(got); got != want {
+		t.Errorf("MarshalIndent:\n\tgot:  %s\n\twant: %s\n", indentNewlines(got), indentNewlines(want))
+	}
+}
+
+func TestOmitZeroMap(t *testing.T) {
+	const want = `{
+ "foo": {
+  "sr": "",
+  "omitzero": 0,
+  "slr": null,
+  "mr": null,
+  "fr": 0,
+  "br": false,
+  "ur": 0,
+  "str": {},
+  "nzs": {},
+  "nps4": {}
+ }
+}`
+	m := map[string]OptionalsZero{"foo": {}}
+	got, err := MarshalIndent(m, "", " ")
+	if err != nil {
+		t.Fatalf("MarshalIndent error: %v", err)
+	}
+	if got := string(got); got != want {
+		fmt.Println(got)
+		t.Errorf("MarshalIndent:\n\tgot:  %s\n\twant: %s\n", indentNewlines(got), indentNewlines(want))
+	}
+}
+
+type OptionalsEmptyZero struct {
+	Sr string `json:"sr"`
+	So string `json:"so,omitempty,omitzero"`
+	Sw string `json:"-"`
+
+	Io int `json:"io,omitempty,omitzero"`
+
+	Slr       []string `json:"slr,random"`
+	Slo       []string `json:"slo,omitempty,omitzero"`
+	SloNonNil []string `json:"slononnil,omitempty,omitzero"`
+
+	Mr map[string]any `json:"mr"`
+	Mo map[string]any `json:",omitempty,omitzero"`
+
+	Fr float64 `json:"fr"`
+	Fo float64 `json:"fo,omitempty,omitzero"`
+
+	Br bool `json:"br"`
+	Bo bool `json:"bo,omitempty,omitzero"`
+
+	Ur uint `json:"ur"`
+	Uo uint `json:"uo,omitempty,omitzero"`
+
+	Str struct{} `json:"str"`
+	Sto struct{} `json:"sto,omitempty,omitzero"`
+
+	Time time.Time     `json:"time,omitempty,omitzero"`
+	Nzs  NonZeroStruct `json:"nzs,omitempty,omitzero"`
+}
+
+func TestOmitEmptyZero(t *testing.T) {
+	const want = `{
+ "sr": "",
+ "slr": null,
+ "mr": {},
+ "fr": 0,
+ "br": false,
+ "ur": 0,
+ "str": {},
+ "nzs": {}
+}`
+	var o OptionalsEmptyZero
+	o.Sw = "something"
+	o.SloNonNil = make([]string, 0)
+	o.Mr = map[string]any{}
+	o.Mo = map[string]any{}
+
+	got, err := MarshalIndent(&o, "", " ")
+	if err != nil {
+		t.Fatalf("MarshalIndent error: %v", err)
+	}
+	if got := string(got); got != want {
+		t.Errorf("MarshalIndent:\n\tgot:  %s\n\twant: %s\n", indentNewlines(got), indentNewlines(want))
+	}
+}
+
 type StringTag struct {
 	BoolStr    bool    `json:",string"`
 	IntStr     int64   `json:",string"`