encoding/json/v2: remove `unknown` tag option and DiscardUnknownMembers

WARNING: This commit contains breaking changes
for those already using GOEXPERIMENT=jsonv2.

This removes support for the `unknown` tag option and
the DiscardUnknownMembers marshal option.

The `unknown` tag option semantics are a bit too subtle
even for experienced Go programmers to understand.
Remove support for it. The exact same feature (or something similar)
can be added back into a future release of json/v2.

We already support the `inline` tag option,
which can handle most cases of what someone might want to do
with unknown fields (such as preserve them).

Fixes #77271
Updates #76444

Change-Id: I875952f0755e58aac4c571869b2cdb56e75cfda9
Reviewed-on: https://go-review.googlesource.com/c/go/+/741320
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

8 files changed, 54 insertions, 206 deletions

diff --git a/src/encoding/json/internal/jsonflags/flags.go b/src/encoding/json/internal/jsonflags/flags.go
index a53dfe7985..3a581a08df 100644
--- a/src/encoding/json/internal/jsonflags/flags.go
+++ b/src/encoding/json/internal/jsonflags/flags.go
@@ -120,7 +120,6 @@ const (
 	FormatNilSliceAsNull      // marshal only
 	OmitZeroStructFields      // marshal only
 	MatchCaseInsensitiveNames // marshal or unmarshal
-	DiscardUnknownMembers     // marshal only
 	RejectUnknownMembers      // unmarshal only
 	Marshalers                // marshal only; non-boolean flag
 	Unmarshalers              // unmarshal only; non-boolean flag
@@ -151,7 +150,7 @@ const (
 )
 
 // bitsUsed is the number of bits used in the 64-bit boolean flags
-const bitsUsed = 42
+const bitsUsed = 41
 
 // Static compile check that bitsUsed and maxArshalV1Flag are in sync.
 const _ = uint64((1<<bitsUsed)-maxArshalV1Flag) + uint64(maxArshalV1Flag-(1<<bitsUsed))
diff --git a/src/encoding/json/v2/arshal_default.go b/src/encoding/json/v2/arshal_default.go
index 33931af17e..6af5a37260 100644
--- a/src/encoding/json/v2/arshal_default.go
+++ b/src/encoding/json/v2/arshal_default.go
@@ -1198,7 +1198,7 @@ func makeStructArshaler(t reflect.Type) *arshaler {
 			}
 			prevIdx = f.id
 		}
-		if fields.inlinedFallback != nil && !(mo.Flags.Get(jsonflags.DiscardUnknownMembers) && fields.inlinedFallback.unknown) {
+		if fields.inlinedFallback != nil {
 			var insertUnquotedName func([]byte) bool
 			if !mo.Flags.Get(jsonflags.AllowDuplicateNames) {
 				insertUnquotedName = func(name []byte) bool {
@@ -1270,7 +1270,7 @@ func makeStructArshaler(t reflect.Type) *arshaler {
 						}
 					}
 					if f == nil {
-						if uo.Flags.Get(jsonflags.RejectUnknownMembers) && (fields.inlinedFallback == nil || fields.inlinedFallback.unknown) {
+						if uo.Flags.Get(jsonflags.RejectUnknownMembers) && fields.inlinedFallback == nil {
 							err := newUnmarshalErrorAfter(dec, t, ErrUnknownName)
 							if !uo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
 								return err
diff --git a/src/encoding/json/v2/arshal_test.go b/src/encoding/json/v2/arshal_test.go
index dc15c5a5f5..1a7ed4b886 100644
--- a/src/encoding/json/v2/arshal_test.go
+++ b/src/encoding/json/v2/arshal_test.go
@@ -416,14 +416,9 @@ type (
 		X            *structInlinedL2 `json:",inline"`
 		StructEmbed1 `json:",inline"`
 	}
-	structInlinedL2        struct{ A, B, C string }
-	StructEmbed1           struct{ C, D, E string }
-	StructEmbed2           struct{ E, F, G string }
-	structUnknownTextValue struct {
-		A int            `json:",omitzero"`
-		X jsontext.Value `json:",unknown"`
-		B int            `json:",omitzero"`
-	}
+	structInlinedL2       struct{ A, B, C string }
+	StructEmbed1          struct{ C, D, E string }
+	StructEmbed2          struct{ E, F, G string }
 	structInlineTextValue struct {
 		A int            `json:",omitzero"`
 		X jsontext.Value `json:",inline"`
@@ -2745,33 +2740,6 @@ func TestMarshal(t *testing.T) {
 		in:   structInlineMapNamedStringAny{X: map[namedString]any{"fizz": 3.14159}},
 		want: `{"fizz":"3.14159"}`,
 	}, {
-		name: jsontest.Name("Structs/InlinedFallback/DiscardUnknownMembers"),
-		opts: []Options{DiscardUnknownMembers(true)},
-		in: structInlineTextValue{
-			A: 1,
-			X: jsontext.Value(` { "fizz" : "buzz" } `),
-			B: 2,
-		},
-		// NOTE: DiscardUnknownMembers has no effect since this is "inline".
-		want: `{"A":1,"B":2,"fizz":"buzz"}`,
-	}, {
-		name: jsontest.Name("Structs/UnknownFallback/DiscardUnknownMembers"),
-		opts: []Options{DiscardUnknownMembers(true)},
-		in: structUnknownTextValue{
-			A: 1,
-			X: jsontext.Value(` { "fizz" : "buzz" } `),
-			B: 2,
-		},
-		want: `{"A":1,"B":2}`,
-	}, {
-		name: jsontest.Name("Structs/UnknownFallback"),
-		in: structUnknownTextValue{
-			A: 1,
-			X: jsontext.Value(` { "fizz" : "buzz" } `),
-			B: 2,
-		},
-		want: `{"A":1,"B":2,"fizz":"buzz"}`,
-	}, {
 		name: jsontest.Name("Structs/DuplicateName/NoCaseInlineTextValue/Other"),
 		in: structNoCaseInlineTextValue{
 			X: jsontext.Value(`{"dupe":"","dupe":""}`),
@@ -6999,29 +6967,12 @@ func TestUnmarshal(t *testing.T) {
 		opts:  []Options{RejectUnknownMembers(true)},
 		inBuf: `{"A":1,"fizz":"buzz","B":2}`,
 		inVal: new(structInlineTextValue),
-		// NOTE: DiscardUnknownMembers has no effect since this is "inline".
 		want: addr(structInlineTextValue{
 			A: 1,
 			X: jsontext.Value(`{"fizz":"buzz"}`),
 			B: 2,
 		}),
 	}, {
-		name:    jsontest.Name("Structs/UnknownFallback/RejectUnknownMembers"),
-		opts:    []Options{RejectUnknownMembers(true)},
-		inBuf:   `{"A":1,"fizz":"buzz","B":2}`,
-		inVal:   new(structUnknownTextValue),
-		want:    addr(structUnknownTextValue{A: 1}),
-		wantErr: EU(ErrUnknownName).withPos(`{"A":1,`, "/fizz").withType('"', T[structUnknownTextValue]()),
-	}, {
-		name:  jsontest.Name("Structs/UnknownFallback"),
-		inBuf: `{"A":1,"fizz":"buzz","B":2}`,
-		inVal: new(structUnknownTextValue),
-		want: addr(structUnknownTextValue{
-			A: 1,
-			X: jsontext.Value(`{"fizz":"buzz"}`),
-			B: 2,
-		}),
-	}, {
 		name:  jsontest.Name("Structs/UnknownIgnored"),
 		opts:  []Options{RejectUnknownMembers(false)},
 		inBuf: `{"unknown":"fizzbuzz"}`,
diff --git a/src/encoding/json/v2/doc.go b/src/encoding/json/v2/doc.go
index 8179f8ab17..adbb200c70 100644
--- a/src/encoding/json/v2/doc.go
+++ b/src/encoding/json/v2/doc.go
@@ -121,17 +121,6 @@
 //     while many non-fallback fields may be specified. This option
 //     must not be specified with any other option (including the JSON name).
 //
-//   - unknown: The "unknown" option is a specialized variant
-//     of the inlined fallback to indicate that this Go struct field
-//     contains any number of unknown JSON object members. The field type must
-//     be a [jsontext.Value], map[~string]T, or an unnamed pointer to such types.
-//     If [DiscardUnknownMembers] is specified when marshaling,
-//     the contents of this field are ignored.
-//     If [RejectUnknownMembers] is specified when unmarshaling,
-//     any unknown object members are rejected regardless of whether
-//     an inlined fallback with the "unknown" option exists. This option
-//     must not be specified with any other option (including the JSON name).
-//
 //   - format: The "format" option specifies a format flag
 //     used to specialize the formatting of the field value.
 //     The option is a key-value pair specified as "format:value" where
diff --git a/src/encoding/json/v2/example_test.go b/src/encoding/json/v2/example_test.go
index dc1f06674c..684ca9c6a2 100644
--- a/src/encoding/json/v2/example_test.go
+++ b/src/encoding/json/v2/example_test.go
@@ -8,7 +8,6 @@ package json_test
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"log"
 	"math"
@@ -338,67 +337,6 @@ func Example_inlinedFields() {
 	// }
 }
 
-// Due to version skew, the set of JSON object members known at compile-time
-// may differ from the set of members encountered at execution-time.
-// As such, it may be useful to have finer grain handling of unknown members.
-// This package supports preserving, rejecting, or discarding such members.
-func Example_unknownMembers() {
-	const input = `{
-		"Name": "Teal",
-		"Value": "#008080",
-		"WebSafe": false
-	}`
-	type Color struct {
-		Name  string
-		Value string
-
-		// Unknown is a Go struct field that holds unknown JSON object members.
-		// It is marked as having this behavior with the "unknown" tag option.
-		//
-		// The type may be a jsontext.Value or map[string]T.
-		Unknown jsontext.Value `json:",unknown"`
-	}
-
-	// By default, unknown members are stored in a Go field marked as "unknown"
-	// or ignored if no such field exists.
-	var color Color
-	err := json.Unmarshal([]byte(input), &color)
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println("Unknown members:", string(color.Unknown))
-
-	// Specifying RejectUnknownMembers causes Unmarshal
-	// to reject the presence of any unknown members.
-	err = json.Unmarshal([]byte(input), new(Color), json.RejectUnknownMembers(true))
-	serr, ok := errors.AsType[*json.SemanticError](err)
-	if ok && serr.Err == json.ErrUnknownName {
-		fmt.Println("Unmarshal error:", serr.Err, strconv.Quote(serr.JSONPointer.LastToken()))
-	}
-
-	// By default, Marshal preserves unknown members stored in
-	// a Go struct field marked as "unknown".
-	b, err := json.Marshal(color)
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println("Output with unknown members:   ", string(b))
-
-	// Specifying DiscardUnknownMembers causes Marshal
-	// to discard any unknown members.
-	b, err = json.Marshal(color, json.DiscardUnknownMembers(true))
-	if err != nil {
-		log.Fatal(err)
-	}
-	fmt.Println("Output without unknown members:", string(b))
-
-	// Output:
-	// Unknown members: {"WebSafe":false}
-	// Unmarshal error: unknown object member name "WebSafe"
-	// Output with unknown members:    {"Name":"Teal","Value":"#008080","WebSafe":false}
-	// Output without unknown members: {"Name":"Teal","Value":"#008080"}
-}
-
 // The "format" tag option can be used to alter the formatting of certain types.
 func Example_formatFlags() {
 	value := struct {
diff --git a/src/encoding/json/v2/fields.go b/src/encoding/json/v2/fields.go
index 4a02be7327..5c560ec5d5 100644
--- a/src/encoding/json/v2/fields.go
+++ b/src/encoding/json/v2/fields.go
@@ -129,25 +129,16 @@ func makeStructFields(root reflect.Type) (fs structFields, serr *SemanticError)
 					f.inline = true // implied by use of Go embedding without an explicit name
 				}
 			}
-			if f.inline || f.unknown {
+			if f.inline {
 				// Handle an inlined field that serializes to/from
 				// zero or more JSON object members.
 
-				switch f.fieldOptions {
-				case fieldOptions{name: f.name, quotedName: f.quotedName, inline: true}:
-				case fieldOptions{name: f.name, quotedName: f.quotedName, unknown: true}:
-				case fieldOptions{name: f.name, quotedName: f.quotedName, inline: true, unknown: true}:
-					serr = orErrorf(serr, t, "Go struct field %s cannot have both `inline` and `unknown` specified", sf.Name)
-					f.inline = false // let `unknown` take precedence
-				default:
-					serr = orErrorf(serr, t, "Go struct field %s cannot have any options other than `inline` or `unknown` specified", sf.Name)
+				if f.fieldOptions != (fieldOptions{name: f.name, quotedName: f.quotedName, inline: true}) {
+					serr = orErrorf(serr, t, "Go struct field %s cannot have any options other than `inline` specified", sf.Name)
 					if f.hasName {
 						continue // invalid inlined field; treat as ignored
 					}
-					f.fieldOptions = fieldOptions{name: f.name, quotedName: f.quotedName, inline: f.inline, unknown: f.unknown}
-					if f.inline && f.unknown {
-						f.inline = false // let `unknown` take precedence
-					}
+					f.fieldOptions = fieldOptions{name: f.name, quotedName: f.quotedName, inline: f.inline}
 				}
 
 				// Reject any types with custom serialization otherwise
@@ -160,10 +151,6 @@ func makeStructFields(root reflect.Type) (fs structFields, serr *SemanticError)
 				// Handle an inlined field that serializes to/from
 				// a finite number of JSON object members backed by a Go struct.
 				if tf.Kind() == reflect.Struct {
-					if f.unknown {
-						serr = orErrorf(serr, t, "inlined Go struct field %s of type %s with `unknown` tag must be a Go map of string key or a jsontext.Value", sf.Name, tf)
-						continue // invalid inlined field; treat as ignored
-					}
 					if qe.visitChildren {
 						queue = append(queue, queueEntry{tf, f.index, !seen[tf]})
 					}
@@ -392,7 +379,6 @@ type fieldOptions struct {
 	nameNeedEscape bool
 	casing         int8 // either 0, caseIgnore, or caseStrict
 	inline         bool
-	unknown        bool
 	omitzero       bool
 	omitempty      bool
 	string         bool
@@ -526,8 +512,6 @@ func parseFieldOptions(sf reflect.StructField) (out fieldOptions, ignored bool,
 			}
 		case "inline":
 			out.inline = true
-		case "unknown":
-			out.unknown = true
 		case "omitzero":
 			out.omitzero = true
 		case "omitempty":
@@ -553,7 +537,7 @@ func parseFieldOptions(sf reflect.StructField) (out fieldOptions, ignored bool,
 			// This catches invalid mutants such as "omitEmpty" or "omit_empty".
 			normOpt := strings.ReplaceAll(strings.ToLower(opt), "_", "")
 			switch normOpt {
-			case "case", "inline", "unknown", "omitzero", "omitempty", "string", "format":
+			case "case", "inline", "omitzero", "omitempty", "string", "format":
 				err = cmp.Or(err, fmt.Errorf("Go struct field %s has invalid appearance of `%s` tag option; specify `%s` instead", sf.Name, opt, normOpt))
 			}
 
diff --git a/src/encoding/json/v2/fields_test.go b/src/encoding/json/v2/fields_test.go
index ae58182f29..dbfabd4aef 100644
--- a/src/encoding/json/v2/fields_test.go
+++ b/src/encoding/json/v2/fields_test.go
@@ -183,7 +183,7 @@ func TestMakeStructFields(t *testing.T) {
 				X jsontext.Value `json:",inline"`
 			} `json:",inline"`
 			X2 struct {
-				X map[string]any `json:",unknown"`
+				X map[string]any `json:",inline"`
 			} `json:",inline"`
 		}{},
 		want: structFields{},
@@ -194,12 +194,12 @@ func TestMakeStructFields(t *testing.T) {
 				X jsontext.Value `json:",inline"`
 			} `json:",inline"`
 			X2 struct {
-				X map[string]any `json:",unknown"`
+				X map[string]any `json:",inline"`
 			} `json:",inline"`
-			X map[string]jsontext.Value `json:",unknown"`
+			X map[string]jsontext.Value `json:",inline"`
 		}{},
 		want: structFields{
-			inlinedFallback: &structField{id: 0, index: []int{2}, typ: T[map[string]jsontext.Value](), fieldOptions: fieldOptions{name: "X", quotedName: `"X"`, unknown: true}},
+			inlinedFallback: &structField{id: 0, index: []int{2}, typ: T[map[string]jsontext.Value](), fieldOptions: fieldOptions{name: "X", quotedName: `"X"`, inline: true}},
 		},
 	}, {
 		name: jsontest.Name("InlinedFallback/InvalidImplicit"),
@@ -232,17 +232,11 @@ func TestMakeStructFields(t *testing.T) {
 		want:    structFields{flattened: []structField{}},
 		wantErr: errors.New(`Go struct fields A and B conflict over JSON object name "same"`),
 	}, {
-		name: jsontest.Name("BothInlineAndUnknown"),
-		in: struct {
-			A struct{} `json:",inline,unknown"`
-		}{},
-		wantErr: errors.New("Go struct field A cannot have both `inline` and `unknown` specified"),
-	}, {
 		name: jsontest.Name("InlineWithOptions"),
 		in: struct {
 			A struct{} `json:",inline,omitempty"`
 		}{},
-		wantErr: errors.New("Go struct field A cannot have any options other than `inline` or `unknown` specified"),
+		wantErr: errors.New("Go struct field A cannot have any options other than `inline` specified"),
 	}, {
 		name: jsontest.Name("UnknownWithOptions"),
 		in: struct {
@@ -257,7 +251,7 @@ func TestMakeStructFields(t *testing.T) {
 				inline:     true,
 			},
 		}},
-		wantErr: errors.New("Go struct field A cannot have any options other than `inline` or `unknown` specified"),
+		wantErr: errors.New("Go struct field A cannot have any options other than `inline` specified"),
 	}, {
 		name: jsontest.Name("InlineTextMarshaler"),
 		in: struct {
@@ -287,10 +281,18 @@ func TestMakeStructFields(t *testing.T) {
 		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { encoding.TextAppender } must not implement marshal or unmarshal methods`),
 	}, {
-		name: jsontest.Name("UnknownJSONMarshaler"),
+		name: jsontest.Name("InlineJSONMarshaler"),
 		in: struct {
-			A struct{ Marshaler } `json:",unknown"`
+			A struct{ Marshaler } `json:",inline"`
 		}{},
+		want: structFields{flattened: []structField{{
+			index: []int{0, 0},
+			typ:   reflect.TypeFor[Marshaler](),
+			fieldOptions: fieldOptions{
+				name:       "Marshaler",
+				quotedName: `"Marshaler"`,
+			},
+		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { json.Marshaler } must not implement marshal or unmarshal methods`),
 	}, {
 		name: jsontest.Name("InlineJSONMarshalerTo"),
@@ -307,10 +309,18 @@ func TestMakeStructFields(t *testing.T) {
 		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { json.MarshalerTo } must not implement marshal or unmarshal methods`),
 	}, {
-		name: jsontest.Name("UnknownTextUnmarshaler"),
+		name: jsontest.Name("InlineTextUnmarshaler"),
 		in: struct {
-			A *struct{ encoding.TextUnmarshaler } `json:",unknown"`
+			A *struct{ encoding.TextUnmarshaler } `json:",inline"`
 		}{},
+		want: structFields{flattened: []structField{{
+			index: []int{0, 0},
+			typ:   reflect.TypeFor[encoding.TextUnmarshaler](),
+			fieldOptions: fieldOptions{
+				name:       "TextUnmarshaler",
+				quotedName: `"TextUnmarshaler"`,
+			},
+		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { encoding.TextUnmarshaler } must not implement marshal or unmarshal methods`),
 	}, {
 		name: jsontest.Name("InlineJSONUnmarshaler"),
@@ -327,23 +337,23 @@ func TestMakeStructFields(t *testing.T) {
 		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { json.Unmarshaler } must not implement marshal or unmarshal methods`),
 	}, {
-		name: jsontest.Name("UnknownJSONUnmarshalerFrom"),
+		name: jsontest.Name("InlineJSONUnmarshalerFrom"),
 		in: struct {
-			A struct{ UnmarshalerFrom } `json:",unknown"`
+			A struct{ UnmarshalerFrom } `json:",inline"`
 		}{},
+		want: structFields{flattened: []structField{{
+			index: []int{0, 0},
+			typ:   reflect.TypeFor[UnmarshalerFrom](),
+			fieldOptions: fieldOptions{
+				name:       "UnmarshalerFrom",
+				quotedName: `"UnmarshalerFrom"`,
+			},
+		}}},
 		wantErr: errors.New(`inlined Go struct field A of type struct { json.UnmarshalerFrom } must not implement marshal or unmarshal methods`),
 	}, {
-		name: jsontest.Name("UnknownStruct"),
-		in: struct {
-			A struct {
-				X, Y, Z string
-			} `json:",unknown"`
-		}{},
-		wantErr: errors.New("inlined Go struct field A of type struct { X string; Y string; Z string } with `unknown` tag must be a Go map of string key or a jsontext.Value"),
-	}, {
 		name: jsontest.Name("InlineUnsupported/MapIntKey"),
 		in: struct {
-			A map[int]any `json:",unknown"`
+			A map[int]any `json:",inline"`
 		}{},
 		wantErr: errors.New(`inlined Go struct field A of type map[int]interface {} must be a Go struct, Go map of string key, or jsontext.Value`),
 	}, {
@@ -633,9 +643,9 @@ func TestParseTagOptions(t *testing.T) {
 	}, {
 		name: jsontest.Name("SuperfluousCommas"),
 		in: struct {
-			V int `json:",,,,\"\",,inline,unknown,,,,"`
+			V int `json:",,,,\"\",,inline,,,,,"`
 		}{},
-		wantOpts: fieldOptions{name: "V", quotedName: `"V"`, inline: true, unknown: true},
+		wantOpts: fieldOptions{name: "V", quotedName: `"V"`, inline: true},
 		wantErr:  errors.New("Go struct field V has malformed `json` tag: invalid character ',' at start of option (expecting Unicode letter or single quote)"),
 	}, {
 		name: jsontest.Name("CaseAloneOption"),
@@ -684,12 +694,6 @@ func TestParseTagOptions(t *testing.T) {
 		}{},
 		wantOpts: fieldOptions{name: "FieldName", quotedName: `"FieldName"`, inline: true},
 	}, {
-		name: jsontest.Name("UnknownOption"),
-		in: struct {
-			FieldName int `json:",unknown"`
-		}{},
-		wantOpts: fieldOptions{name: "FieldName", quotedName: `"FieldName"`, unknown: true},
-	}, {
 		name: jsontest.Name("OmitZeroOption"),
 		in: struct {
 			FieldName int `json:",omitzero"`
@@ -743,14 +747,13 @@ func TestParseTagOptions(t *testing.T) {
 	}, {
 		name: jsontest.Name("AllOptions"),
 		in: struct {
-			FieldName int `json:",case:ignore,inline,unknown,omitzero,omitempty,string,format:format"`
+			FieldName int `json:",case:ignore,inline,omitzero,omitempty,string,format:format"`
 		}{},
 		wantOpts: fieldOptions{
 			name:       "FieldName",
 			quotedName: `"FieldName"`,
 			casing:     caseIgnore,
 			inline:     true,
-			unknown:    true,
 			omitzero:   true,
 			omitempty:  true,
 			string:     true,
@@ -759,14 +762,13 @@ func TestParseTagOptions(t *testing.T) {
 	}, {
 		name: jsontest.Name("AllOptionsQuoted"),
 		in: struct {
-			FieldName int `json:",'case':'ignore','inline','unknown','omitzero','omitempty','string','format':'format'"`
+			FieldName int `json:",'case':'ignore','inline','omitzero','omitempty','string','format':'format'"`
 		}{},
 		wantOpts: fieldOptions{
 			name:       "FieldName",
 			quotedName: `"FieldName"`,
 			casing:     caseIgnore,
 			inline:     true,
-			unknown:    true,
 			omitzero:   true,
 			omitempty:  true,
 			string:     true,
@@ -783,7 +785,7 @@ func TestParseTagOptions(t *testing.T) {
 	}, {
 		name: jsontest.Name("AllOptionsSpaceSensitive"),
 		in: struct {
-			FieldName int `json:", case:ignore , inline , unknown , omitzero , omitempty , string , format:format "`
+			FieldName int `json:", case:ignore , inline , omitzero , omitempty , string , format:format "`
 		}{},
 		wantOpts: fieldOptions{name: "FieldName", quotedName: `"FieldName"`},
 		wantErr:  errors.New("Go struct field FieldName has malformed `json` tag: invalid character ' ' at start of option (expecting Unicode letter or single quote)"),
diff --git a/src/encoding/json/v2/options.go b/src/encoding/json/v2/options.go
index 9685f20f9f..ce6517c901 100644
--- a/src/encoding/json/v2/options.go
+++ b/src/encoding/json/v2/options.go
@@ -65,7 +65,6 @@ import (
 //   - [FormatNilMapAsNull] affects marshaling only
 //   - [OmitZeroStructFields] affects marshaling only
 //   - [MatchCaseInsensitiveNames] affects marshaling and unmarshaling
-//   - [DiscardUnknownMembers] affects marshaling only
 //   - [RejectUnknownMembers] affects unmarshaling only
 //   - [WithMarshalers] affects marshaling only
 //   - [WithUnmarshalers] affects unmarshaling only
@@ -200,22 +199,8 @@ func MatchCaseInsensitiveNames(v bool) Options {
 	}
 }
 
-// DiscardUnknownMembers specifies that marshaling should ignore any
-// JSON object members stored in Go struct fields dedicated to storing
-// unknown JSON object members.
-//
-// This only affects marshaling and is ignored when unmarshaling.
-func DiscardUnknownMembers(v bool) Options {
-	if v {
-		return jsonflags.DiscardUnknownMembers | 1
-	} else {
-		return jsonflags.DiscardUnknownMembers | 0
-	}
-}
-
 // RejectUnknownMembers specifies that unknown members should be rejected
-// when unmarshaling a JSON object, regardless of whether there is a field
-// to store unknown members.
+// when unmarshaling a JSON object.
 //
 // This only affects unmarshaling and is ignored when marshaling.
 func RejectUnknownMembers(v bool) Options {