strings: avoid utf8.RuneError mangling in Split

Split should only split strings and not perform mangling
of invalid UTF-8 into ut8.RuneError.
The prior behavior is clearly a bug since mangling is not
performed in all other situations (e.g., separator is non-empty).

Fixes #53511

Change-Id: I112a2ef15ee46ddecda015ee14bca04cd76adfbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/413715
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/bytes/bytes_test.go

@@ -755,6 +755,8 @@ var splittests = []SplitTest{
 	{"123", "", 2, []string{"1", "23"}},
 	{"123", "", 17, []string{"1", "2", "3"}},
 	{"bT", "T", math.MaxInt / 4, []string{"b", ""}},
+	{"\xff-\xff", "", -1, []string{"\xff", "-", "\xff"}},
+	{"\xff-\xff", "-", -1, []string{"\xff", "\xff"}},
 }
 
 func TestSplit(t *testing.T) {

src/strings/strings.go

@@ -15,7 +15,7 @@ import (
 
 // explode splits s into a slice of UTF-8 strings,
 // one string per Unicode character up to a maximum of n (n < 0 means no limit).
-// Invalid UTF-8 sequences become correct encodings of U+FFFD.
+// Invalid UTF-8 bytes are sliced individually.
 func explode(s string, n int) []string {
 	l := utf8.RuneCountInString(s)
 	if n < 0 || n > l {
@@ -23,12 +23,9 @@ func explode(s string, n int) []string {
 	}
 	a := make([]string, n)
 	for i := 0; i < n-1; i++ {
-		ch, size := utf8.DecodeRuneInString(s)
+		_, size := utf8.DecodeRuneInString(s)
 		a[i] = s[:size]
 		s = s[size:]
-		if ch == utf8.RuneError {
-			a[i] = string(utf8.RuneError)
-		}
 	}
 	if n > 0 {
 		a[n-1] = s

src/strings/strings_test.go

@@ -406,6 +406,8 @@ var splittests = []SplitTest{
 	{"1 2 3 4", " ", 3, []string{"1", "2", "3 4"}},
 	{"1 2", " ", 3, []string{"1", "2"}},
 	{"", "T", math.MaxInt / 4, []string{""}},
+	{"\xff-\xff", "", -1, []string{"\xff", "-", "\xff"}},
+	{"\xff-\xff", "-", -1, []string{"\xff", "\xff"}},
 }
 
 func TestSplit(t *testing.T) {