slices: prevent Clone keeping alive the array when cloning empty slices

Fixes #68488

Change-Id: I39aba22cdfe8ca0bbe69db7c64f1bca75fa067fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/598875
Reviewed-by: Keith Randall <khr@golang.org>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/slices/slices.go

@@ -346,8 +346,13 @@ func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
 // The elements are copied using assignment, so this is a shallow clone.
 // The result may have additional unused capacity.
 func Clone[S ~[]E, E any](s S) S {
-	// The s[:0:0] preserves nil in case it matters.
-	return append(s[:0:0], s...)
+	// Preserve nilness in case it matters.
+	if s == nil {
+		return nil
+	}
+	// Avoid s[:0:0] as it leads to unwanted liveness when cloning a
+	// zero-length slice of a large array; see https://go.dev/issue/68488.
+	return append(S{}, s...)
 }
 
 // Compact replaces consecutive runs of equal elements with a single copy.

src/slices/slices_test.go

@@ -12,6 +12,7 @@ import (
 	. "slices"
 	"strings"
 	"testing"
+	"unsafe"
 )
 
 var equalIntTests = []struct {
@@ -1450,3 +1451,12 @@ func TestRepeatPanics(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue68488(t *testing.T) {
+	s := make([]int, 3)
+	clone := Clone(s[1:1])
+	switch unsafe.SliceData(clone) {
+	case &s[0], &s[1], &s[2]:
+		t.Error("clone keeps alive s due to array overlap")
+	}
+}