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>

cmd/viewer/tests/tests.go

@@ -9,10 +9,11 @@ import (
 	"net/netip"
 
 	"golang.org/x/exp/constraints"
+	"tailscale.com/types/ptr"
 	"tailscale.com/types/views"
 )
 
-//go:generate go run tailscale.com/cmd/viewer --type=StructWithPtrs,StructWithoutPtrs,Map,StructWithSlices,OnlyGetClone,StructWithEmbedded,GenericIntStruct,GenericNoPtrsStruct,GenericCloneableStruct --clone-only-type=OnlyGetClone
+//go:generate go run tailscale.com/cmd/viewer --type=StructWithPtrs,StructWithoutPtrs,Map,StructWithSlices,OnlyGetClone,StructWithEmbedded,GenericIntStruct,GenericNoPtrsStruct,GenericCloneableStruct,StructWithContainers --clone-only-type=OnlyGetClone
 
 type StructWithoutPtrs struct {
 	Int int
@@ -114,3 +115,50 @@ type GenericCloneableStruct[T views.ViewCloner[T, V], V views.StructView[T]] str
 	PtrValueMap map[string]*T
 	SliceMap    map[string][]T
 }
+
+// Container is a pre-defined container type, such as a collection, an optional
+// value or a generic wrapper.
+type Container[T any] struct {
+	Item T
+}
+
+func (c *Container[T]) Clone() *Container[T] {
+	if c == nil {
+		return nil
+	}
+	if cloner, ok := any(c.Item).(views.Cloner[T]); ok {
+		return &Container[T]{cloner.Clone()}
+	}
+	if !views.ContainsPointers[T]() {
+		return ptr.To(*c)
+	}
+	panic(fmt.Errorf("%T contains pointers, but is not cloneable", c.Item))
+}
+
+// ContainerView is a pre-defined readonly view of a Container[T].
+type ContainerView[T views.ViewCloner[T, V], V views.StructView[T]] 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 *Container[T]
+}
+
+func (cv ContainerView[T, V]) Item() V {
+	return cv.ж.Item.View()
+}
+
+func ContainerViewOf[T views.ViewCloner[T, V], V views.StructView[T]](c *Container[T]) ContainerView[T, V] {
+	return ContainerView[T, V]{c}
+}
+
+type GenericBasicStruct[T BasicType] struct {
+	Value T
+}
+
+type StructWithContainers struct {
+	IntContainer             Container[int]
+	CloneableContainer       Container[*StructWithPtrs]
+	BasicGenericContainer    Container[GenericBasicStruct[int]]
+	ClonableGenericContainer Container[*GenericNoPtrsStruct[int]]
+}