cmd/viewer: add view-only option

This PR adds a --view-only-type flag to viewer to allow
a type to be generated without a cloner.
This is made available to make the few cases where writing
a custom cloner is more practical, for example if the field
is an interface.

Updates corp/#16789

Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>

cmd/viewer/tests/tests.go

@@ -9,7 +9,7 @@ import (
 	"net/netip"
 )
 
-//go:generate go run tailscale.com/cmd/viewer --type=StructWithPtrs,StructWithoutPtrs,Map,StructWithSlices,OnlyGetClone,StructWithEmbedded --clone-only-type=OnlyGetClone
+//go:generate go run tailscale.com/cmd/viewer --type=StructWithPtrs,StructWithoutPtrs,Map,StructWithSlices,OnlyGetClone,OnlyGetView,StructWithEmbedded --clone-only-type=OnlyGetClone --view-only-type=OnlyGetView
 
 type StructWithoutPtrs struct {
 	Int int
@@ -62,6 +62,17 @@ type OnlyGetClone struct {
 	SinViewerPorFavor bool
 }
 
+type OnlyGetView struct {
+	SinClonerPorFavor bool
+}
+
+// Custom cloner func
+func (ogv *OnlyGetView) Clone() *OnlyGetView {
+	return &OnlyGetView{
+		SinClonerPorFavor: ogv.SinClonerPorFavor,
+	}
+}
+
 type StructWithEmbedded struct {
 	A *StructWithPtrs
 	StructWithSlices

cmd/viewer/tests/tests_view.go

@@ -325,6 +325,58 @@ var _StructWithSlicesViewNeedsRegeneration = StructWithSlices(struct {
 	Data           []byte
 }{})
 
+// View returns a readonly view of OnlyGetView.
+func (p *OnlyGetView) View() OnlyGetViewView {
+	return OnlyGetViewView{ж: p}
+}
+
+// OnlyGetViewView provides a read-only view over OnlyGetView.
+//
+// Its methods should only be called if `Valid()` returns true.
+type OnlyGetViewView struct {
+	// ж is the underlying mutable value, named with a hard-to-type
+	// character that looks pointy like a pointer.
+	// It is named distinctively to make you think of how dangerous it is to escape
+	// to callers. You must not let callers be able to mutate it.
+	ж *OnlyGetView
+}
+
+// Valid reports whether underlying value is non-nil.
+func (v OnlyGetViewView) Valid() bool { return v.ж != nil }
+
+// AsStruct returns a clone of the underlying value which aliases no memory with
+// the original.
+func (v OnlyGetViewView) AsStruct() *OnlyGetView {
+	if v.ж == nil {
+		return nil
+	}
+	return v.ж.Clone()
+}
+
+func (v OnlyGetViewView) MarshalJSON() ([]byte, error) { return json.Marshal(v.ж) }
+
+func (v *OnlyGetViewView) UnmarshalJSON(b []byte) error {
+	if v.ж != nil {
+		return errors.New("already initialized")
+	}
+	if len(b) == 0 {
+		return nil
+	}
+	var x OnlyGetView
+	if err := json.Unmarshal(b, &x); err != nil {
+		return err
+	}
+	v.ж = &x
+	return nil
+}
+
+func (v OnlyGetViewView) SinClonerPorFavor() bool { return v.ж.SinClonerPorFavor }
+
+// A compilation failure here means this code must be regenerated, with the command at the top of this file.
+var _OnlyGetViewViewNeedsRegeneration = OnlyGetView(struct {
+	SinClonerPorFavor bool
+}{})
+
 // View returns a readonly view of StructWithEmbedded.
 func (p *StructWithEmbedded) View() StructWithEmbeddedView {
 	return StructWithEmbeddedView{ж: p}

cmd/viewer/viewer.go

@@ -40,7 +40,7 @@ func (v {{.ViewName}}) Valid() bool { return v.ж != nil }
 
 // AsStruct returns a clone of the underlying value which aliases no memory with
 // the original.
-func (v {{.ViewName}}) AsStruct() *{{.StructName}}{ 
+func (v {{.ViewName}}) AsStruct() *{{.StructName}}{
 	if v.ж == nil {
 		return nil
 	}
@@ -324,6 +324,7 @@ var (
 	flagCloneFunc = flag.Bool("clonefunc", false, "add a top-level Clone func")
 
 	flagCloneOnlyTypes = flag.String("clone-only-type", "", "comma-separated list of types (a subset of --type) that should only generate a go:generate clone line and not actual views")
+	flagViewOnlyTypes  = flag.String("view-only-type", "", "comma-separated list of types (a subset of --type) that should only generate views and not be added to go:generate for cloner")
 )
 
 func main() {
@@ -336,10 +337,21 @@ func main() {
 	}
 	typeNames := strings.Split(*flagTypes, ",")
 
+	viewOnlyTypes := map[string]bool{}
+	for _, t := range strings.Split(*flagViewOnlyTypes, ",") {
+		viewOnlyTypes[t] = true
+	}
+
 	var flagArgs []string
 	flagArgs = append(flagArgs, fmt.Sprintf("-clonefunc=%v", *flagCloneFunc))
 	if *flagTypes != "" {
-		flagArgs = append(flagArgs, "-type="+*flagTypes)
+		var cloneTypes []string
+		for _, t := range strings.Split(*flagTypes, ",") {
+			if _, ok := viewOnlyTypes[t]; !ok {
+				cloneTypes = append(cloneTypes, t)
+			}
+		}
+		flagArgs = append(flagArgs, "-type="+strings.Join(cloneTypes, ","))
 	}
 	if *flagBuildTags != "" {
 		flagArgs = append(flagArgs, "-tags="+*flagBuildTags)
@@ -373,7 +385,7 @@ func main() {
 				break
 			}
 		}
-		if !hasClone {
+		if !hasClone && !viewOnlyTypes[typeName] {
 			runCloner = true
 		}
 		genView(buf, it, typ, pkg.Types)