cmd/cloner, cmd/viewer: handle named map/slice types with Clone/View methods

The cloner and viewer code generators didn't handle named types
with basic underlying types (map/slice) that have their own Clone
or View methods. For example, a type like:

    type Map map[string]any
    func (m Map) Clone() Map { ... }
    func (m Map) View() MapView { ... }

When used as a struct field, the cloner would descend into the
underlying map[string]any and fail because it can't clone the any
(interface{}) value type. Similarly, the viewer would try to create
a MapFnOf view and fail.

Fix the cloner to check for a Clone method on the named type
before falling through to the underlying type handling.

Fix the viewer to check for a View method on named map/slice types,
so the type author can provide a purpose-built safe view that
doesn't leak raw any values. Named map/slice types without a View
method fall through to normal handling, which correctly rejects
types like map[string]any as unsupported.

Updates tailscale/corp#39502 (needed by tailscale/corp#39594)

Change-Id: Iaef0192a221e02b4b8e409c99ef8398090327744
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/viewer/tests/tests_view.go

@@ -16,7 +16,7 @@ import (
 	"tailscale.com/types/views"
 )
 
-//go:generate go run tailscale.com/cmd/cloner  -clonefunc=false -type=StructWithPtrs,StructWithoutPtrs,Map,StructWithSlices,OnlyGetClone,StructWithEmbedded,GenericIntStruct,GenericNoPtrsStruct,GenericCloneableStruct,StructWithContainers,StructWithTypeAliasFields,GenericTypeAliasStruct,StructWithMapOfViews
+//go:generate go run tailscale.com/cmd/cloner  -clonefunc=false -type=StructWithPtrs,StructWithoutPtrs,Map,StructWithSlices,OnlyGetClone,StructWithEmbedded,GenericIntStruct,GenericNoPtrsStruct,GenericCloneableStruct,StructWithContainers,StructWithTypeAliasFields,GenericTypeAliasStruct,StructWithMapOfViews,StructWithNamedMap,StructWithNamedSlice
 
 // View returns a read-only view of StructWithPtrs.
 func (p *StructWithPtrs) View() StructWithPtrsView {
@@ -1129,3 +1129,151 @@ func (v StructWithMapOfViewsView) MapOfViews() views.Map[string, StructWithoutPt
 var _StructWithMapOfViewsViewNeedsRegeneration = StructWithMapOfViews(struct {
 	MapOfViews map[string]StructWithoutPtrsView
 }{})
+
+// View returns a read-only view of StructWithNamedMap.
+func (p *StructWithNamedMap) View() StructWithNamedMapView {
+	return StructWithNamedMapView{ж: p}
+}
+
+// StructWithNamedMapView provides a read-only view over StructWithNamedMap.
+//
+// Its methods should only be called if `Valid()` returns true.
+type StructWithNamedMapView 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 *StructWithNamedMap
+}
+
+// Valid reports whether v's underlying value is non-nil.
+func (v StructWithNamedMapView) Valid() bool { return v.ж != nil }
+
+// AsStruct returns a clone of the underlying value which aliases no memory with
+// the original.
+func (v StructWithNamedMapView) AsStruct() *StructWithNamedMap {
+	if v.ж == nil {
+		return nil
+	}
+	return v.ж.Clone()
+}
+
+// MarshalJSON implements [jsonv1.Marshaler].
+func (v StructWithNamedMapView) MarshalJSON() ([]byte, error) {
+	return jsonv1.Marshal(v.ж)
+}
+
+// MarshalJSONTo implements [jsonv2.MarshalerTo].
+func (v StructWithNamedMapView) MarshalJSONTo(enc *jsontext.Encoder) error {
+	return jsonv2.MarshalEncode(enc, v.ж)
+}
+
+// UnmarshalJSON implements [jsonv1.Unmarshaler].
+func (v *StructWithNamedMapView) UnmarshalJSON(b []byte) error {
+	if v.ж != nil {
+		return errors.New("already initialized")
+	}
+	if len(b) == 0 {
+		return nil
+	}
+	var x StructWithNamedMap
+	if err := jsonv1.Unmarshal(b, &x); err != nil {
+		return err
+	}
+	v.ж = &x
+	return nil
+}
+
+// UnmarshalJSONFrom implements [jsonv2.UnmarshalerFrom].
+func (v *StructWithNamedMapView) UnmarshalJSONFrom(dec *jsontext.Decoder) error {
+	if v.ж != nil {
+		return errors.New("already initialized")
+	}
+	var x StructWithNamedMap
+	if err := jsonv2.UnmarshalDecode(dec, &x); err != nil {
+		return err
+	}
+	v.ж = &x
+	return nil
+}
+
+func (v StructWithNamedMapView) Attrs() NamedMapView { return v.ж.Attrs.View() }
+
+// A compilation failure here means this code must be regenerated, with the command at the top of this file.
+var _StructWithNamedMapViewNeedsRegeneration = StructWithNamedMap(struct {
+	Attrs NamedMap
+}{})
+
+// View returns a read-only view of StructWithNamedSlice.
+func (p *StructWithNamedSlice) View() StructWithNamedSliceView {
+	return StructWithNamedSliceView{ж: p}
+}
+
+// StructWithNamedSliceView provides a read-only view over StructWithNamedSlice.
+//
+// Its methods should only be called if `Valid()` returns true.
+type StructWithNamedSliceView 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 *StructWithNamedSlice
+}
+
+// Valid reports whether v's underlying value is non-nil.
+func (v StructWithNamedSliceView) Valid() bool { return v.ж != nil }
+
+// AsStruct returns a clone of the underlying value which aliases no memory with
+// the original.
+func (v StructWithNamedSliceView) AsStruct() *StructWithNamedSlice {
+	if v.ж == nil {
+		return nil
+	}
+	return v.ж.Clone()
+}
+
+// MarshalJSON implements [jsonv1.Marshaler].
+func (v StructWithNamedSliceView) MarshalJSON() ([]byte, error) {
+	return jsonv1.Marshal(v.ж)
+}
+
+// MarshalJSONTo implements [jsonv2.MarshalerTo].
+func (v StructWithNamedSliceView) MarshalJSONTo(enc *jsontext.Encoder) error {
+	return jsonv2.MarshalEncode(enc, v.ж)
+}
+
+// UnmarshalJSON implements [jsonv1.Unmarshaler].
+func (v *StructWithNamedSliceView) UnmarshalJSON(b []byte) error {
+	if v.ж != nil {
+		return errors.New("already initialized")
+	}
+	if len(b) == 0 {
+		return nil
+	}
+	var x StructWithNamedSlice
+	if err := jsonv1.Unmarshal(b, &x); err != nil {
+		return err
+	}
+	v.ж = &x
+	return nil
+}
+
+// UnmarshalJSONFrom implements [jsonv2.UnmarshalerFrom].
+func (v *StructWithNamedSliceView) UnmarshalJSONFrom(dec *jsontext.Decoder) error {
+	if v.ж != nil {
+		return errors.New("already initialized")
+	}
+	var x StructWithNamedSlice
+	if err := jsonv2.UnmarshalDecode(dec, &x); err != nil {
+		return err
+	}
+	v.ж = &x
+	return nil
+}
+
+func (v StructWithNamedSliceView) Items() NamedSliceView { return v.ж.Items.View() }
+
+// A compilation failure here means this code must be regenerated, with the command at the top of this file.
+var _StructWithNamedSliceViewNeedsRegeneration = StructWithNamedSlice(struct {
+	Items NamedSlice
+}{})