encoding/json: add omitzero option

Fixes golang#45669

Change-Id: Idec483a03968cc671c8da27804589008b10864a1

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

@@ -0,0 +1,8 @@
+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 +99,32 @@ 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:
+//
+// 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.
+//
+// Examples of struct field tags and their meanings:
+//
+//	// Field appears in JSON as key "myName".
+//	Field time.Time `json:"myName"`
+//
+//	// Field appears in JSON as key "myName" and
+//	// the field is omitted from the object if its value is zero,
+//	// as defined above.
+//	Field time.Time `json:"myName,omitzero"`
+//
+//	// Field appears in JSON as key "Field" (the default), but
+//	// the field is skipped if zero.
+//	// Note the leading comma.
+//	Field time.Time `json:",omitzero"`
+//
+// 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
@@ -318,6 +344,15 @@ func isEmptyValue(v reflect.Value) bool {
 	return false
 }
 
+func isZeroValue(v reflect.Value) bool {
+	if z, ok := v.Interface().(interface {
+		IsZero() bool
+	}); ok {
+		return z.IsZero()
+	}
+	return v.IsZero()
+}
+
 func (e *encodeState) reflectValue(v reflect.Value, opts encOpts) {
 	valueEncoder(v)(e, v, opts)
 }
@@ -701,7 +736,8 @@ FieldLoop:
 			fv = fv.Field(i)
 		}
 
-		if f.omitEmpty && isEmptyValue(fv) {
+		if (f.omitEmpty && isEmptyValue(fv)) ||
+			(f.omitZero && isZeroValue(fv)) {
 			continue
 		}
 		e.WriteByte(next)
@@ -1048,6 +1084,7 @@ type field struct {
 	index     []int
 	typ       reflect.Type
 	omitEmpty bool
+	omitZero  bool
 	quoted    bool
 
 	encoder encoderFunc
@@ -1154,6 +1191,7 @@ 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)

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:"-"`
@@ -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 +71,128 @@ func TestOmitEmpty(t *testing.T) {
 	}
 }
 
+type NonZeroStruct struct{}
+
+func (nzs NonZeroStruct) IsZero() bool {
+	return false
+}
+
+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"`
+
+	Fr float64 `json:"fr"`
+	Fo float64 `json:"fo,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"`
+	Nzs  NonZeroStruct `json:"nzs,omitzero"`
+}
+
+func TestOmitZero(t *testing.T) {
+	var want = `{
+ "sr": "",
+ "omitzero": 0,
+ "slr": null,
+ "slononnil": [],
+ "mr": {},
+ "Mo": {},
+ "fr": 0,
+ "br": false,
+ "ur": 0,
+ "str": {},
+ "nzs": {}
+}`
+	var o OptionalsZero
+	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 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,omitzero"`
+}
+
+func TestOmitEmptyZero(t *testing.T) {
+	var 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"`