Support custom IsZero() methods with omitzero tag (#1020)

The omitzero tag now respects custom IsZero() methods on types,
similar to how encoding/json handles this. Previously, only
reflect.Value.IsZero() was used, which ignores user-defined
implementations.

Fixes #1003

Co-authored-by: Claude <noreply@anthropic.com>

marshaler.go

@@ -390,7 +390,28 @@ func shouldOmitEmpty(options valueOptions, v reflect.Value) bool {
 }
 
 func shouldOmitZero(options valueOptions, v reflect.Value) bool {
-	return options.omitzero && v.IsZero()
+	if !options.omitzero {
+		return false
+	}
+
+	// Check if the type implements isZeroer interface (has a custom IsZero method).
+	if v.Type().Implements(isZeroerType) {
+		return v.Interface().(isZeroer).IsZero()
+	}
+
+	// Check if pointer type implements isZeroer.
+	if reflect.PointerTo(v.Type()).Implements(isZeroerType) {
+		if v.CanAddr() {
+			return v.Addr().Interface().(isZeroer).IsZero()
+		}
+		// Create a temporary addressable copy to call the pointer receiver method.
+		pv := reflect.New(v.Type())
+		pv.Elem().Set(v)
+		return pv.Interface().(isZeroer).IsZero()
+	}
+
+	// Fall back to reflect's IsZero for types without custom IsZero method.
+	return v.IsZero()
 }
 
 func (enc *Encoder) encodeKv(b []byte, ctx encoderCtx, options valueOptions, v reflect.Value) ([]byte, error) {

marshaler_test.go

@@ -1195,6 +1195,286 @@ IP = '192.168.178.35'
 	assert.Equal(t, expected, string(b))
 }
 
+// customZeroType has a custom IsZero method that returns true
+// when Value is less than 10.
+type customZeroType struct {
+	Value int
+}
+
+func (c customZeroType) IsZero() bool {
+	return c.Value < 10
+}
+
+// customZeroPointerType has a custom IsZero method on the pointer receiver.
+type customZeroPointerType struct {
+	Value int
+}
+
+func (c *customZeroPointerType) IsZero() bool {
+	return c.Value < 10
+}
+
+func TestEncoderOmitzeroCustomIsZero(t *testing.T) {
+	type doc struct {
+		Custom customZeroType `toml:",omitzero"`
+		Normal int            `toml:",omitzero"`
+	}
+
+	// Custom.Value = 5, which is < 10, so custom IsZero returns true
+	d := doc{
+		Custom: customZeroType{Value: 5},
+		Normal: 0,
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Both fields should be omitted: Custom because custom IsZero returns true,
+	// Normal because its reflect zero value is true.
+	expected := ``
+
+	assert.Equal(t, expected, string(b))
+}
+
+func TestEncoderOmitzeroCustomIsZeroNotZero(t *testing.T) {
+	type doc struct {
+		Custom customZeroType `toml:",omitzero"`
+		Normal int            `toml:",omitzero"`
+	}
+
+	// Custom.Value = 15, which is >= 10, so custom IsZero returns false
+	d := doc{
+		Custom: customZeroType{Value: 15},
+		Normal: 42,
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Both fields should be present
+	expected := `Normal = 42
+
+[Custom]
+Value = 15
+`
+
+	assert.Equal(t, expected, string(b))
+}
+
+func TestEncoderOmitzeroCustomIsZeroPointerReceiver(t *testing.T) {
+	type doc struct {
+		Custom customZeroPointerType `toml:",omitzero"`
+	}
+
+	// Custom.Value = 5, which is < 10, so custom IsZero returns true
+	d := doc{
+		Custom: customZeroPointerType{Value: 5},
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Field should be omitted because custom IsZero returns true
+	expected := ``
+
+	assert.Equal(t, expected, string(b))
+}
+
+func TestEncoderOmitzeroCustomIsZeroPointerReceiverNotZero(t *testing.T) {
+	type doc struct {
+		Custom customZeroPointerType `toml:",omitzero"`
+	}
+
+	// Custom.Value = 15, which is >= 10, so custom IsZero returns false
+	d := doc{
+		Custom: customZeroPointerType{Value: 15},
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Field should be present
+	expected := `[Custom]
+Value = 15
+`
+
+	assert.Equal(t, expected, string(b))
+}
+
+// TestEncoderOmitzeroCustomIsZeroPointerReceiverAddressable tests the v.CanAddr() path
+// by marshaling a pointer to a struct, which makes fields addressable.
+func TestEncoderOmitzeroCustomIsZeroPointerReceiverAddressable(t *testing.T) {
+	type doc struct {
+		Custom customZeroPointerType `toml:",omitzero"`
+	}
+
+	// Custom.Value = 5, which is < 10, so custom IsZero returns true
+	d := &doc{
+		Custom: customZeroPointerType{Value: 5},
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Field should be omitted because custom IsZero returns true
+	expected := ``
+
+	assert.Equal(t, expected, string(b))
+}
+
+// TestEncoderOmitzeroCustomIsZeroPointerReceiverAddressableNotZero tests the v.CanAddr() path
+// when custom IsZero returns false.
+func TestEncoderOmitzeroCustomIsZeroPointerReceiverAddressableNotZero(t *testing.T) {
+	type doc struct {
+		Custom customZeroPointerType `toml:",omitzero"`
+	}
+
+	// Custom.Value = 15, which is >= 10, so custom IsZero returns false
+	d := &doc{
+		Custom: customZeroPointerType{Value: 15},
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Field should be present
+	expected := `[Custom]
+Value = 15
+`
+
+	assert.Equal(t, expected, string(b))
+}
+
+// TestEncoderOmitzeroCustomIsZeroInlineTable tests omitzero with inline tables.
+func TestEncoderOmitzeroCustomIsZeroInlineTable(t *testing.T) {
+	type doc struct {
+		Custom customZeroType `toml:",omitzero,inline"`
+	}
+
+	// Custom.Value = 5, which is < 10, so custom IsZero returns true
+	d := doc{
+		Custom: customZeroType{Value: 5},
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Field should be omitted
+	expected := ``
+
+	assert.Equal(t, expected, string(b))
+}
+
+// TestEncoderOmitzeroCustomIsZeroInlineTableNotZero tests omitzero with inline tables when not zero.
+func TestEncoderOmitzeroCustomIsZeroInlineTableNotZero(t *testing.T) {
+	type doc struct {
+		Custom customZeroType `toml:",omitzero,inline"`
+	}
+
+	// Custom.Value = 15, which is >= 10, so custom IsZero returns false
+	d := doc{
+		Custom: customZeroType{Value: 15},
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Field should be present as inline table
+	expected := `Custom = {Value = 15}
+`
+
+	assert.Equal(t, expected, string(b))
+}
+
+// TestEncoderOmitzeroCustomIsZeroMixedTypes tests omitzero with a mix of custom and regular types.
+func TestEncoderOmitzeroCustomIsZeroMixedTypes(t *testing.T) {
+	type doc struct {
+		Custom  customZeroType `toml:",omitzero"`
+		Regular int            `toml:",omitzero"`
+		NoOmit  customZeroType `toml:""`
+		Pointer *int           `toml:",omitzero"`
+	}
+
+	d := doc{
+		Custom:  customZeroType{Value: 5}, // IsZero returns true
+		Regular: 0,                        // zero value
+		NoOmit:  customZeroType{Value: 5}, // not omitted (no omitzero tag)
+		Pointer: nil,                      // nil pointer
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Custom is omitted (custom IsZero true), Regular is omitted (zero value),
+	// NoOmit is present (no omitzero tag), Pointer is omitted (nil)
+	expected := `[NoOmit]
+Value = 5
+`
+
+	assert.Equal(t, expected, string(b))
+}
+
+// TestEncoderOmitzeroCustomIsZeroSlice tests omitzero with slices containing custom types.
+func TestEncoderOmitzeroCustomIsZeroSlice(t *testing.T) {
+	type doc struct {
+		Items []customZeroType `toml:",omitzero"`
+	}
+
+	// Nil slice should be omitted (IsZero returns true for nil slices)
+	d := doc{
+		Items: nil,
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	expected := ``
+
+	assert.Equal(t, expected, string(b))
+
+	// Empty but non-nil slice is NOT zero, so it's included
+	d2 := doc{
+		Items: []customZeroType{},
+	}
+
+	b2, err := toml.Marshal(d2)
+	assert.NoError(t, err)
+
+	expected2 := `Items = []
+`
+
+	assert.Equal(t, expected2, string(b2))
+}
+
+// TestEncoderOmitzeroCustomIsZeroNestedStruct tests omitzero with nested structs.
+func TestEncoderOmitzeroCustomIsZeroNestedStruct(t *testing.T) {
+	type inner struct {
+		Custom customZeroType `toml:",omitzero"`
+		Value  int            `toml:",omitzero"`
+	}
+	type doc struct {
+		Inner inner `toml:",omitzero"`
+	}
+
+	// Inner struct has all zero fields, but the struct itself is not zero
+	// (reflect.Value.IsZero checks if all fields are zero)
+	d := doc{
+		Inner: inner{
+			Custom: customZeroType{Value: 5}, // custom IsZero returns true
+			Value:  0,                        // zero value
+		},
+	}
+
+	b, err := toml.Marshal(d)
+	assert.NoError(t, err)
+
+	// Inner is present but its fields are omitted
+	expected := `[Inner]
+`
+
+	assert.Equal(t, expected, string(b))
+}
+
 func TestEncoderTagFieldName(t *testing.T) {
 	type doc struct {
 		String string `toml:"hello"`

types.go

@@ -6,10 +6,17 @@ import (
 	"time"
 )
 
+// isZeroer is used to check if a type has a custom IsZero method.
+// This allows custom types to define their own zero-value semantics.
+type isZeroer interface {
+	IsZero() bool
+}
+
 var (
 	timeType               = reflect.TypeOf((*time.Time)(nil)).Elem()
 	textMarshalerType      = reflect.TypeOf((*encoding.TextMarshaler)(nil)).Elem()
 	textUnmarshalerType    = reflect.TypeOf((*encoding.TextUnmarshaler)(nil)).Elem()
+	isZeroerType           = reflect.TypeOf((*isZeroer)(nil)).Elem()
 	mapStringInterfaceType = reflect.TypeOf(map[string]interface{}(nil))
 	sliceInterfaceType     = reflect.TypeOf([]interface{}(nil))
 	stringType             = reflect.TypeOf("")