slices: Delete clears the tail when j == len(s)

Fixes #65669

Change-Id: Ifd2011dd604fef399e4352b804fc2f6a9e74096e
Reviewed-on: https://go-review.googlesource.com/c/go/+/566237
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Auto-Submit: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: Deleplace

src/slices/slices.go

@@ -258,7 +258,11 @@ func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
 		return Insert(s, i, v...)
 	}
 	if j == len(s) {
-		return append(s[:i], v...)
+		s2 := append(s[:i], v...)
+		if len(s2) < len(s) {
+			clear(s[len(s2):len(s)]) // zero/nil out the obsolete elements, for GC
+		}
+		return s2
 	}
 
 	tot := len(s[:i]) + len(v) + len(s[j:])

src/slices/slices_test.go

@@ -1120,6 +1120,19 @@ func TestReplaceOverlap(t *testing.T) {
 	}
 }
 
+func TestReplaceEndClearTail(t *testing.T) {
+	s := []int{11, 22, 33}
+	v := []int{99}
+	// case when j == len(s)
+	i, j := 1, 3
+	s = Replace(s, i, j, v...)
+
+	x := s[:3][2]
+	if want := 0; x != want {
+		t.Errorf("TestReplaceEndClearTail: obsolete element is %d, want %d", x, want)
+	}
+}
+
 func BenchmarkReplace(b *testing.B) {
 	cases := []struct {
 		name string