cmd/viewer, types/views, util/codegen: add viewer support for custom container types

This adds support for container-like types such as Container[T] that
don't explicitly specify a view type for T. Instead, a package implementing
a container type should also implement and export a ContainerView[T, V] type
and a ContainerViewOf(*Container[T]) ContainerView[T, V] function, which
returns a view for the specified container, inferring the element view type V
from the element type T.

Updates #12736

Signed-off-by: Nick Khyl <nickk@tailscale.com>

types/views/views.go

@@ -11,6 +11,7 @@ import (
 	"errors"
 	"fmt"
 	"maps"
+	"reflect"
 	"slices"
 
 	"go4.org/mem"
@@ -111,6 +112,13 @@ type StructView[T any] interface {
 	AsStruct() T
 }
 
+// Cloner is any type that has a Clone function returning a deep-clone of the receiver.
+type Cloner[T any] interface {
+	// Clone returns a deep-clone of the receiver.
+	// It returns nil, when the receiver is nil.
+	Clone() T
+}
+
 // ViewCloner is any type that has had View and Clone funcs generated using
 // tailscale.com/cmd/viewer.
 type ViewCloner[T any, V StructView[T]] interface {
@@ -555,3 +563,46 @@ func (m MapFn[K, T, V]) Range(f MapRangeFn[K, V]) {
 		}
 	}
 }
+
+// ContainsPointers reports whether T contains any pointers,
+// either explicitly or implicitly.
+// It has special handling for some types that contain pointers
+// that we know are free from memory aliasing/mutation concerns.
+func ContainsPointers[T any]() bool {
+	return containsPointers(reflect.TypeFor[T]())
+}
+
+func containsPointers(typ reflect.Type) bool {
+	switch typ.Kind() {
+	case reflect.Pointer, reflect.UnsafePointer:
+		return true
+	case reflect.Chan, reflect.Map, reflect.Slice:
+		return true
+	case reflect.Array:
+		return containsPointers(typ.Elem())
+	case reflect.Interface, reflect.Func:
+		return true // err on the safe side.
+	case reflect.Struct:
+		if isWellKnownImmutableStruct(typ) {
+			return false
+		}
+		for i := range typ.NumField() {
+			if containsPointers(typ.Field(i).Type) {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+func isWellKnownImmutableStruct(typ reflect.Type) bool {
+	switch typ.String() {
+	case "time.Time":
+		// time.Time contains a pointer that does not need copying
+		return true
+	case "netip.Addr", "netip.Prefix", "netip.AddrPort":
+		return true
+	default:
+		return false
+	}
+}