strings: align Clone output with string([]byte(""))

Add a comment how strings of length 0 are treated and
that they always will result in the return of a string
equal to the constant string "".

The previous implementation would return a string header that uses
runtime.zerobase as the backing array pointer while the string constant
"" has 0 as pointer value.

Using 0 has the backing array pointer is also the behaviour of
string([]byte(input)) which makes the new behaviour a better drop in
replacement.

Change-Id: Ic5460e9494b6708edbdfa4361e878d50db54ba10
Reviewed-on: https://go-review.googlesource.com/c/go/+/360255
Trust: Martin Möhrmann <martin@golang.org>
Run-TryBot: Martin Möhrmann <martin@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

2 files changed, 15 insertions, 1 deletions

diff --git a/src/strings/clone.go b/src/strings/clone.go
index 6097c6cc88..edd1497d9e 100644
--- a/src/strings/clone.go
+++ b/src/strings/clone.go
@@ -16,7 +16,12 @@ import (
 // overuse of Clone can make programs use more memory.
 // Clone should typically be used only rarely, and only when
 // profiling indicates that it is needed.
+// For strings of length zero the string "" will be returned
+// and no allocation is made.
 func Clone(s string) string {
+	if len(s) == 0 {
+		return ""
+	}
 	b := make([]byte, len(s))
 	copy(b, s)
 	return *(*string)(unsafe.Pointer(&b))
diff --git a/src/strings/clone_test.go b/src/strings/clone_test.go
index 5396771047..a9ba8add23 100644
--- a/src/strings/clone_test.go
+++ b/src/strings/clone_test.go
@@ -11,9 +11,13 @@ import (
 	"unsafe"
 )
 
+var emptyString string
+
 func TestClone(t *testing.T) {
 	var cloneTests = []string{
 		"",
+		strings.Clone(""),
+		strings.Repeat("a", 42)[:0],
 		"short",
 		strings.Repeat("a", 42),
 	}
@@ -25,9 +29,14 @@ func TestClone(t *testing.T) {
 
 		inputHeader := (*reflect.StringHeader)(unsafe.Pointer(&input))
 		cloneHeader := (*reflect.StringHeader)(unsafe.Pointer(&clone))
-		if inputHeader.Data == cloneHeader.Data {
+		if len(input) != 0 && cloneHeader.Data == inputHeader.Data {
 			t.Errorf("Clone(%q) return value should not reference inputs backing memory.", input)
 		}
+
+		emptyHeader := (*reflect.StringHeader)(unsafe.Pointer(&emptyString))
+		if len(input) == 0 && cloneHeader.Data != emptyHeader.Data {
+			t.Errorf("Clone(%#v) return value should be equal to empty string.", inputHeader)
+		}
 	}
 }