types/views: remove alloc in hot path

Signed-off-by: Maisem Ali <maisem@tailscale.com>

types/views/views.go

@@ -9,8 +9,6 @@
 import (
 	"encoding/json"
 	"errors"
-	"fmt"
-	"reflect"
 
 	"inet.af/netaddr"
 	"tailscale.com/net/tsaddr"
@@ -100,11 +98,8 @@ type Slice[T any] struct {
 }
 
 // SliceOf returns a Slice for the provided slice for immutable values.
-// It panics if the value type contains pointers.
+// It is the caller's responsibility to make sure V is immutable.
 func SliceOf[T any](x []T) Slice[T] {
-	if ev := reflect.TypeOf(x).Elem(); containsMutable(ev) {
-		panic(fmt.Sprintf("slice value type %q has pointers", ev.Name()))
-	}
 	return Slice[T]{x}
 }
 
@@ -194,56 +189,13 @@ func (v *IPPrefixSlice) UnmarshalJSON(b []byte) error {
 	return v.ж.UnmarshalJSON(b)
 }
 
-// containsMutable reports whether the provided type has anything mutable.
-func containsMutable(t reflect.Type) bool {
-	switch x := fmt.Sprintf("%v.%v", t.PkgPath(), t.Name()); x {
-	case "time.Time",
-		"inet.af/netaddr.IP":
-		return false
-	}
-	k := t.Kind()
-	switch k {
-	case reflect.Bool,
-		reflect.Int,
-		reflect.Int8,
-		reflect.Int16,
-		reflect.Int32,
-		reflect.Int64,
-		reflect.Uint,
-		reflect.Uint8,
-		reflect.Uint16,
-		reflect.Uint32,
-		reflect.Uint64,
-		reflect.Float32,
-		reflect.Float64,
-		reflect.Complex64,
-		reflect.Complex128,
-		reflect.String:
-		return false
-	case reflect.Array: // Not a slice.
-		return containsMutable(t.Elem()) && t.Len() > 0
-	case reflect.Struct:
-		for i := 0; i < t.NumField(); i++ {
-			f := t.Field(i)
-			if containsMutable(f.Type) {
-				return true
-			}
-		}
-		return false
-	}
-	return true
-}
-
-// MapOf returns a read-only view over m for immutable values.
-// It panics if the value type contains pointers.
+// MapOf returns a view over m. It is the caller's responsibility to make sure K
+// and V is immutable, if this is being used to provide a read-only view over m.
 func MapOf[K comparable, V comparable](m map[K]V) Map[K, V] {
-	if ev := reflect.TypeOf(m).Elem(); containsMutable(ev) {
-		panic(fmt.Sprintf("map value type %q has pointers", ev.Name()))
-	}
 	return Map[K, V]{m}
 }
 
-// Map is a read-only accessor over a map whose values are immutable.
+// Map is a view over a map whose values are immutable.
 type Map[K comparable, V any] struct {
 	// ж is the underlying mutable value, named with a hard-to-type
 	// character that looks pointy like a pointer.

types/views/views_test.go

@@ -10,43 +10,10 @@
 	"reflect"
 	"strings"
 	"testing"
-	"time"
 
-	"go4.org/mem"
 	"inet.af/netaddr"
-	"tailscale.com/types/structs"
 )
 
-func TestContainsPointers(t *testing.T) {
-	tests := []struct {
-		name string
-		in   any
-		want bool
-	}{
-		{name: "string", in: "foo", want: false},
-		{name: "int", in: 42, want: false},
-		{name: "struct", in: struct{ string }{"foo"}, want: false},
-		{name: "mem.RO", in: mem.B([]byte{1}), want: false},
-		{name: "time.Time", in: time.Now(), want: false},
-		{name: "netaddr.IP", in: netaddr.MustParseIP("1.1.1.1"), want: false},
-		{name: "netaddr.IPPrefix", in: netaddr.MustParseIP("1.1.1.1"), want: false},
-		{name: "structs.Incomparable", in: structs.Incomparable{}, want: false},
-
-		{name: "*int", in: (*int)(nil), want: true},
-		{name: "*string", in: (*string)(nil), want: true},
-		{name: "struct-with-pointer", in: struct{ X *string }{}, want: true},
-		{name: "slice-with-pointer", in: []struct{ X *string }{}, want: true},
-		{name: "slice-of-struct", in: []struct{ string }{}, want: true},
-	}
-	for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
-			if containsMutable(reflect.TypeOf(tc.in)) != tc.want {
-				t.Errorf("containsPointers %T; want %v", tc.in, tc.want)
-			}
-		})
-	}
-}
-
 func TestViewsJSON(t *testing.T) {
 	mustCIDR := func(cidrs ...string) (out []netaddr.IPPrefix) {
 		for _, cidr := range cidrs {