encoding/json: remove legacy option to EscapeInvalidUTF8

In the presence of invalid UTF-8, the AllowInvalidUTF8 option
allows such bytes to be present, but silently mangles them
using the Unicode replacement character.

The v2 default is to emit the replacement character verbatim
(which is valid UTF-8 and exactly what it is for).

However, the v1 behavior has historically been to emit
the escaped form of the replacement character.
This behavior was introduced in https://go.dev/cl/11211045
where the documentation says that it is:

    replacing invalid bytes with the Unicode replacement rune U+FFFD

but the implementation actually replaces it with
the escaped form of the Unicode replacement rune.
Given that the documentation differs from the implementation,
the actual behavior is likely an oversight.

Given how esoteric of behavior this is,
we change the v1in2 behavior to avoid the unnecesary escaping
and drop support for EscapeInvalidUTF8.

This does not violate the Go compatibility agreement since
we do not document what the exact syntactic output is.
Also, there has already been prior precedence for changing the output:

* [encoding/json: encode \b and \f as '\b' and '\f' in JSON strings](https://go.dev/cl/521675)
* [encoding/json: encode \n in strings as "\n", not "\u000A"](https://go.dev/cl/4678046)
* [encoding/json: encode \t as \t instead of \u0009](https://go.dev/cl/162340043)
* [encoding/json: use standard ES6 formatting for numbers during marshal](https://go.dev/cl/30371)

Fixes #74551

Change-Id: Ib59a873c44713d302f1f6ab103ffba2520d63276
Reviewed-on: https://go-review.googlesource.com/c/go/+/687116
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

src/encoding/json/internal/jsonflags/flags.go

@@ -52,7 +52,6 @@ const (
 		AllowInvalidUTF8 |
 		EscapeForHTML |
 		EscapeForJS |
-		EscapeInvalidUTF8 |
 		PreserveRawStrings |
 		Deterministic |
 		FormatNilMapAsNull |
@@ -77,7 +76,7 @@ const (
 	WhitespaceFlags = AnyWhitespace | Indent | IndentPrefix
 
 	// AnyEscape is the set of flags related to escaping in a JSON string.
-	AnyEscape = EscapeForHTML | EscapeForJS | EscapeInvalidUTF8
+	AnyEscape = EscapeForHTML | EscapeForJS
 
 	// CanonicalizeNumbers is the set of flags related to raw number canonicalization.
 	CanonicalizeNumbers = CanonicalizeRawInts | CanonicalizeRawFloats
@@ -97,7 +96,6 @@ const (
 	ReorderRawObjects     // encode only
 	EscapeForHTML         // encode only
 	EscapeForJS           // encode only
-	EscapeInvalidUTF8     // encode only; only exposed in v1
 	Multiline             // encode only
 	SpaceAfterColon       // encode only
 	SpaceAfterComma       // encode only

src/encoding/json/internal/jsonwire/encode.go

@@ -92,11 +92,7 @@ func AppendQuote[Bytes ~[]byte | ~string](dst []byte, src Bytes, flags *jsonflag
 			case isInvalidUTF8(r, rn):
 				hasInvalidUTF8 = true
 				dst = append(dst, src[i:n-rn]...)
-				if flags.Get(jsonflags.EscapeInvalidUTF8) {
-					dst = append(dst, `\ufffd`...)
-				} else {
-					dst = append(dst, "\ufffd"...)
-				}
+				dst = append(dst, "\ufffd"...)
 				i = n
 			case (r == '\u2028' || r == '\u2029') && flags.Get(jsonflags.EscapeForJS):
 				dst = append(dst, src[i:n-rn]...)

src/encoding/json/v2_decode_test.go

@@ -1249,12 +1249,12 @@ func TestMarshalInvalidUTF8(t *testing.T) {
 		in   string
 		want string
 	}{
-		{Name(""), "hello\xffworld", `"hello\ufffdworld"`},
+		{Name(""), "hello\xffworld", "\"hello\ufffdworld\""},
 		{Name(""), "", `""`},
-		{Name(""), "\xff", `"\ufffd"`},
-		{Name(""), "\xff\xff", `"\ufffd\ufffd"`},
-		{Name(""), "a\xffb", `"a\ufffdb"`},
-		{Name(""), "\xe6\x97\xa5\xe6\x9c\xac\xff\xaa\x9e", `"日本\ufffd\ufffd\ufffd"`},
+		{Name(""), "\xff", "\"\ufffd\""},
+		{Name(""), "\xff\xff", "\"\ufffd\ufffd\""},
+		{Name(""), "a\xffb", "\"a\ufffdb\""},
+		{Name(""), "\xe6\x97\xa5\xe6\x9c\xac\xff\xaa\x9e", "\"日本\ufffd\ufffd\ufffd\""},
 	}
 	for _, tt := range tests {
 		t.Run(tt.Name, func(t *testing.T) {

src/encoding/json/v2_diff_test.go

@@ -786,8 +786,8 @@ func TestInvalidUTF8(t *testing.T) {
 			switch {
 			case json.Version == "v1" && err != nil:
 				t.Fatalf("json.Marshal error: %v", err)
-			case json.Version == "v1" && string(got) != `"\ufffd"`:
-				t.Fatalf(`json.Marshal = %s, want "\ufffd"`, got)
+			case json.Version == "v1" && string(got) != "\"\ufffd\"":
+				t.Fatalf(`json.Marshal = %s, want %q`, got, "\ufffd")
 			case json.Version == "v2" && err == nil:
 				t.Fatal("json.Marshal error is nil, want non-nil")
 			}

src/encoding/json/v2_options.go

@@ -204,7 +204,6 @@ type Options = jsonopts.Options
 // It is equivalent to the following boolean options being set to true:
 //
 //   - [CallMethodsWithLegacySemantics]
-//   - [EscapeInvalidUTF8]
 //   - [FormatBytesWithLegacySemantics]
 //   - [FormatTimeWithLegacySemantics]
 //   - [MatchCaseSensitiveDelimiter]
@@ -279,23 +278,6 @@ func CallMethodsWithLegacySemantics(v bool) Options {
 	}
 }
 
-// EscapeInvalidUTF8 specifies that when encoding a [jsontext.String]
-// with bytes of invalid UTF-8, such bytes are escaped as
-// a hexadecimal Unicode codepoint (i.e., \ufffd).
-// In contrast, the v2 default is to use the minimal representation,
-// which is to encode invalid UTF-8 as the Unicode replacement rune itself
-// (without any form of escaping).
-//
-// This only affects encoding and is ignored when decoding.
-// The v1 default is true.
-func EscapeInvalidUTF8(v bool) Options {
-	if v {
-		return jsonflags.EscapeInvalidUTF8 | 1
-	} else {
-		return jsonflags.EscapeInvalidUTF8 | 0
-	}
-}
-
 // FormatBytesWithLegacySemantics specifies that handling of
 // []~byte and [N]~byte types follow legacy semantics:
 //