Method apply

cmp/options.go:221–260

(s *state, vx, vy reflect.Value)

	return nil
}
func (validator) apply(s *state, vx, vy reflect.Value) {
	// Implies missing slice element or map entry.
	if !vx.IsValid() || !vy.IsValid() {
		s.report(vx.IsValid() == vy.IsValid(), 0)
		return
	}

	// Unable to Interface implies unexported field without visibility access.
	if !vx.CanInterface() || !vy.CanInterface() {
		help := "consider using a custom Comparer; if you control the implementation of type, you can also consider using an Exporter, AllowUnexported, or cmpopts.IgnoreUnexported"
		var name string
		if t := s.curPath.Index(-2).Type(); t.Name() != "" {
			// Named type with unexported fields.
			name = fmt.Sprintf("%q.%v", t.PkgPath(), t.Name()) // e.g., "path/to/package".MyType
			isProtoMessage := func(t reflect.Type) bool {
				m, ok := reflect.PointerTo(t).MethodByName("ProtoReflect")
				return ok && m.Type.NumIn() == 1 && m.Type.NumOut() == 1 &&
					m.Type.Out(0).PkgPath() == "google.golang.org/protobuf/reflect/protoreflect" &&
					m.Type.Out(0).Name() == "Message"
			}
			if isProtoMessage(t) {
				help = `consider using "google.golang.org/protobuf/testing/protocmp".Transform to compare proto.Message types`
			} else if _, ok := reflect.New(t).Interface().(error); ok {
				help = "consider using cmpopts.EquateErrors to compare error values"
			} else if t.Comparable() {
				help = "consider using cmpopts.EquateComparable to compare comparable Go types"
			}
		} else {
			// Unnamed type with unexported fields. Derive PkgPath from field.
			var pkgPath string
			for i := 0; i < t.NumField() && pkgPath == ""; i++ {
				pkgPath = t.Field(i).PkgPath
			}
			name = fmt.Sprintf("%q.(%v)", pkgPath, t.String()) // e.g., "path/to/package".(struct { a int })
		}
		panic(fmt.Sprintf("cannot handle unexported field at %#v:\n\t%v\n%s", s.curPath, name, help))
	}

	panic("not reachable")
}

// identRx represents a valid identifier according to the Go specification.
const identRx = `[_\p{L}][_\p{L}\p{N}]*`