encoding/json: encode \b and \f as '\b' and '\f' in JSON strings

According to RFC 8259, there are exactly 5 control characters
that have a shorter escape sequence than the generic \uXXXX format.

Over the years, we added ad-hoc support for the short sequences:
* https://go.dev/cl/4678046 supports \r and \n
* https://go.dev/cl/162340043 supports \t

This CL completes the set by supporting \b and \f.

This may change the encoding of strings in relatively rare cases,
but is a permissible change since the Go 1 compatibility document does
not guarantee that "json" produces byte-for-byte identical outputs.

In fact, we have made even more observable output changes in the past
such as with https://go.dev/cl/30371 which changes the representation
of many JSON numbers.

This change is to prepare the path forward for a potential
v2 "json" package, which has more consistent encoding of JSON strings.

Change-Id: I11102a0602dfb1a0c14eaad82ed23e8df7553c6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/521675
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/encoding/json/encode.go

@@ -969,6 +969,10 @@ func appendString[Bytes []byte | string](dst []byte, src Bytes, escapeHTML bool)
 			switch b {
 			case '\\', '"':
 				dst = append(dst, '\\', b)
+			case '\b':
+				dst = append(dst, '\\', 'b')
+			case '\f':
+				dst = append(dst, '\\', 'f')
 			case '\n':
 				dst = append(dst, '\\', 'n')
 			case '\r':

src/encoding/json/encode_test.go

@@ -113,7 +113,7 @@ func TestRoundtripStringTag(t *testing.T) {
 				"BoolStr": "false",
 				"IntStr": "0",
 				"UintptrStr": "0",
-				"StrStr": "\"\\u0008\\u000c\\n\\r\\t\\\"\\\\\"",
+				"StrStr": "\"\\b\\f\\n\\r\\t\\\"\\\\\"",
 				"NumberStr": "0"
 			}`,
 		},
@@ -795,11 +795,11 @@ var encodeStringTests = []struct {
 	{"\x05", `"\u0005"`},
 	{"\x06", `"\u0006"`},
 	{"\x07", `"\u0007"`},
-	{"\x08", `"\u0008"`},
+	{"\x08", `"\b"`},
 	{"\x09", `"\t"`},
 	{"\x0a", `"\n"`},
 	{"\x0b", `"\u000b"`},
-	{"\x0c", `"\u000c"`},
+	{"\x0c", `"\f"`},
 	{"\x0d", `"\r"`},
 	{"\x0e", `"\u000e"`},
 	{"\x0f", `"\u000f"`},