cmd/cloner, cmd/viewer, util/codegen: add support for generic types and interfaces

This adds support for generic types and interfaces to our cloner and viewer codegens.
It updates these packages to determine whether to make shallow or deep copies based
on the type parameter constraints. Additionally, if a template parameter or an interface
type has View() and Clone() methods, we'll use them for getters and the cloner of the
owning structure.

Updates #12736

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

cmd/viewer/tests/tests.go

@@ -7,9 +7,12 @@ package tests
 import (
 	"fmt"
 	"net/netip"
+
+	"golang.org/x/exp/constraints"
+	"tailscale.com/types/views"
 )
 
-//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,StructWithEmbedded,GenericIntStruct,GenericNoPtrsStruct,GenericCloneableStruct --clone-only-type=OnlyGetClone
 
 type StructWithoutPtrs struct {
 	Int int
@@ -25,12 +28,12 @@ type Map struct {
 	SlicesWithPtrs      map[string][]*StructWithPtrs
 	SlicesWithoutPtrs   map[string][]*StructWithoutPtrs
 	StructWithoutPtrKey map[StructWithoutPtrs]int `json:"-"`
+	StructWithPtr       map[string]StructWithPtrs
 
 	// Unsupported views.
 	SliceIntPtr      map[string][]*int
 	PointerKey       map[*string]int        `json:"-"`
 	StructWithPtrKey map[StructWithPtrs]int `json:"-"`
-	StructWithPtr    map[string]StructWithPtrs
 }
 
 type StructWithPtrs struct {
@@ -50,12 +53,14 @@ type StructWithSlices struct {
 	Values         []StructWithoutPtrs
 	ValuePointers  []*StructWithoutPtrs
 	StructPointers []*StructWithPtrs
-	Structs        []StructWithPtrs
-	Ints           []*int
 
 	Slice    []string
 	Prefixes []netip.Prefix
 	Data     []byte
+
+	// Unsupported views.
+	Structs []StructWithPtrs
+	Ints    []*int
 }
 
 type OnlyGetClone struct {
@@ -66,3 +71,46 @@ type StructWithEmbedded struct {
 	A *StructWithPtrs
 	StructWithSlices
 }
+
+type GenericIntStruct[T constraints.Integer] struct {
+	Value   T
+	Pointer *T
+	Slice   []T
+	Map     map[string]T
+
+	// Unsupported views.
+	PtrSlice    []*T
+	PtrKeyMap   map[*T]string `json:"-"`
+	PtrValueMap map[string]*T
+	SliceMap    map[string][]T
+}
+
+type BasicType interface {
+	~bool | constraints.Integer | constraints.Float | constraints.Complex | ~string
+}
+
+type GenericNoPtrsStruct[T StructWithoutPtrs | netip.Prefix | BasicType] struct {
+	Value   T
+	Pointer *T
+	Slice   []T
+	Map     map[string]T
+
+	// Unsupported views.
+	PtrSlice    []*T
+	PtrKeyMap   map[*T]string `json:"-"`
+	PtrValueMap map[string]*T
+	SliceMap    map[string][]T
+}
+
+type GenericCloneableStruct[T views.ViewCloner[T, V], V views.StructView[T]] struct {
+	Value T
+	Slice []T
+	Map   map[string]T
+
+	// Unsupported views.
+	Pointer     *T
+	PtrSlice    []*T
+	PtrKeyMap   map[*T]string `json:"-"`
+	PtrValueMap map[string]*T
+	SliceMap    map[string][]T
+}