cmd/cloner,*: optimize nillable slice cloner

A wild @josharian appears with a good suggestion for a refactor, thanks
Josh!

Updates #9410
Signed-off-by: James Tucker <james@tailscale.com>

2 files changed, 13 insertions, 23 deletions

diff --git a/cmd/cloner/cloner.go b/cmd/cloner/cloner.go
index 5a94fa97d..25d796dbf 100644
--- a/cmd/cloner/cloner.go
+++ b/cmd/cloner/cloner.go
@@ -122,8 +122,7 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 		case *types.Slice:
 			if codegen.ContainsPointers(ft.Elem()) {
 				n := it.QualifiedName(ft.Elem())
-				writef("if src.%s != nil {", fname)
-				writef("dst.%s = make([]%s, len(src.%s))", fname, n, fname)
+				writef("dst.%s = append([]%s(nil), make([]%s, len(src.%s))...)", fname, n, n, fname)
 				writef("for i := range dst.%s {", fname)
 				if ptr, isPtr := ft.Elem().(*types.Pointer); isPtr {
 					if _, isBasic := ptr.Elem().Underlying().(*types.Basic); isBasic {
@@ -138,7 +137,6 @@ func gen(buf *bytes.Buffer, it *codegen.ImportTracker, typ *types.Named) {
 					writef("\tdst.%s[i] = *src.%s[i].Clone()", fname, fname)
 				}
 				writef("}")
-				writef("}")
 			} else {
 				writef("dst.%s = append(src.%s[:0:0], src.%s...)", fname, fname, fname)
 			}
diff --git a/cmd/viewer/tests/tests_clone.go b/cmd/viewer/tests/tests_clone.go
index 2b41639fd..35721430e 100644
--- a/cmd/viewer/tests/tests_clone.go
+++ b/cmd/viewer/tests/tests_clone.go
@@ -136,29 +136,21 @@ func (src *StructWithSlices) Clone() *StructWithSlices {
 	dst := new(StructWithSlices)
 	*dst = *src
 	dst.Values = append(src.Values[:0:0], src.Values...)
-	if src.ValuePointers != nil {
-		dst.ValuePointers = make([]*StructWithoutPtrs, len(src.ValuePointers))
-		for i := range dst.ValuePointers {
-			dst.ValuePointers[i] = src.ValuePointers[i].Clone()
-		}
+	dst.ValuePointers = append([]*StructWithoutPtrs(nil), make([]*StructWithoutPtrs, len(src.ValuePointers))...)
+	for i := range dst.ValuePointers {
+		dst.ValuePointers[i] = src.ValuePointers[i].Clone()
 	}
-	if src.StructPointers != nil {
-		dst.StructPointers = make([]*StructWithPtrs, len(src.StructPointers))
-		for i := range dst.StructPointers {
-			dst.StructPointers[i] = src.StructPointers[i].Clone()
-		}
+	dst.StructPointers = append([]*StructWithPtrs(nil), make([]*StructWithPtrs, len(src.StructPointers))...)
+	for i := range dst.StructPointers {
+		dst.StructPointers[i] = src.StructPointers[i].Clone()
 	}
-	if src.Structs != nil {
-		dst.Structs = make([]StructWithPtrs, len(src.Structs))
-		for i := range dst.Structs {
-			dst.Structs[i] = *src.Structs[i].Clone()
-		}
+	dst.Structs = append([]StructWithPtrs(nil), make([]StructWithPtrs, len(src.Structs))...)
+	for i := range dst.Structs {
+		dst.Structs[i] = *src.Structs[i].Clone()
 	}
-	if src.Ints != nil {
-		dst.Ints = make([]*int, len(src.Ints))
-		for i := range dst.Ints {
-			dst.Ints[i] = ptr.To(*src.Ints[i])
-		}
+	dst.Ints = append([]*int(nil), make([]*int, len(src.Ints))...)
+	for i := range dst.Ints {
+		dst.Ints[i] = ptr.To(*src.Ints[i])
 	}
 	dst.Slice = append(src.Slice[:0:0], src.Slice...)
 	dst.Prefixes = append(src.Prefixes[:0:0], src.Prefixes...)