runtime: zero tmpbuf between len and cap

Zero the entire buffer so we don't need to
lower its capacity upon return.  This lets callers
do some appending without allocation.

Zeroing is cheap, the byte buffer requires only
4 extra instructions.

Fixes #14235

Change-Id: I970d7badcef047dafac75ac17130030181f18fe2
Reviewed-on: https://go-review.googlesource.com/22424
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/string.go

@@ -139,7 +139,8 @@ func slicebytetostringtmp(b []byte) string {
 func stringtoslicebyte(buf *tmpBuf, s string) []byte {
 	var b []byte
 	if buf != nil && len(s) <= len(buf) {
-		b = buf[:len(s):len(s)]
+		*buf = tmpBuf{}
+		b = buf[:len(s)]
 	} else {
 		b = rawbyteslice(len(s))
 	}
@@ -171,7 +172,8 @@ func stringtoslicerune(buf *[tmpStringBufSize]rune, s string) []rune {
 	}
 	var a []rune
 	if buf != nil && n <= len(buf) {
-		a = buf[:n:n]
+		*buf = [tmpStringBufSize]rune{}
+		a = buf[:n]
 	} else {
 		a = rawruneslice(n)
 	}

src/runtime/string_test.go

@@ -238,17 +238,35 @@ func TestRangeStringCast(t *testing.T) {
 	}
 }
 
+func isZeroed(b []byte) bool {
+	for _, x := range b {
+		if x != 0 {
+			return false
+		}
+	}
+	return true
+}
+
+func isZeroedR(r []rune) bool {
+	for _, x := range r {
+		if x != 0 {
+			return false
+		}
+	}
+	return true
+}
+
 func TestString2Slice(t *testing.T) {
 	// Make sure we don't return slices that expose
 	// an unzeroed section of stack-allocated temp buf
 	// between len and cap. See issue 14232.
 	s := "foož"
 	b := ([]byte)(s)
-	if cap(b) != 5 {
-		t.Errorf("want cap of 5, got %d", cap(b))
+	if !isZeroed(b[len(b):cap(b)]) {
+		t.Errorf("extra bytes not zeroed")
 	}
 	r := ([]rune)(s)
-	if cap(r) != 4 {
-		t.Errorf("want cap of 4, got %d", cap(r))
+	if !isZeroedR(r[len(r):cap(r)]) {
+		t.Errorf("extra runes not zeroed")
 	}
 }