strings: move Clone to stringslite

Follow-up CL help package like unique use Clone.

Change-Id: Ie64adf7e1a331f47c3cfe178c368d72fc72493ff
GitHub-Last-Rev: 499476cc4a
GitHub-Pull-Request: golang/go#67106
Reviewed-on: https://go-review.googlesource.com/c/go/+/581956
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

src/internal/stringslite/strings.go

@@ -8,7 +8,10 @@
 // Tests for these functions are in the strings package.
 package stringslite
 
-import "internal/bytealg"
+import (
+	"internal/bytealg"
+	"unsafe"
+)
 
 func HasPrefix(s, prefix string) bool {
 	return len(s) >= len(prefix) && s[0:len(prefix)] == prefix
@@ -136,3 +139,12 @@ func TrimSuffix(s, suffix string) string {
 	}
 	return s
 }
+
+func Clone(s string) string {
+	if len(s) == 0 {
+		return ""
+	}
+	b := make([]byte, len(s))
+	copy(b, s)
+	return unsafe.String(&b[0], len(b))
+}

src/strings/clone.go

@@ -5,7 +5,7 @@
 package strings
 
 import (
-	"unsafe"
+	"internal/stringslite"
 )
 
 // Clone returns a fresh copy of s.
@@ -19,10 +19,5 @@ import (
 // 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 unsafe.String(&b[0], len(b))
+	return stringslite.Clone(s)
 }