encoding/json: use reflect.TypeAssert

Updates #62121

Change-Id: Ic3c4fe84a5dacfd8270aba0d5dd59f83f0a9030f
Reviewed-on: https://go-review.googlesource.com/c/go/+/701955
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

6 files changed, 30 insertions, 27 deletions

diff --git a/src/encoding/json/decode.go b/src/encoding/json/decode.go
index fc29296c0f..46f9ed2da9 100644
--- a/src/encoding/json/decode.go
+++ b/src/encoding/json/decode.go
@@ -482,11 +482,11 @@ func indirect(v reflect.Value, decodingNull bool) (Unmarshaler, encoding.TextUnm
 			v.Set(reflect.New(v.Type().Elem()))
 		}
 		if v.Type().NumMethod() > 0 && v.CanInterface() {
-			if u, ok := v.Interface().(Unmarshaler); ok {
+			if u, ok := reflect.TypeAssert[Unmarshaler](v); ok {
 				return u, nil, reflect.Value{}
 			}
 			if !decodingNull {
-				if u, ok := v.Interface().(encoding.TextUnmarshaler); ok {
+				if u, ok := reflect.TypeAssert[encoding.TextUnmarshaler](v); ok {
 					return nil, u, reflect.Value{}
 				}
 			}
diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index 29fcc91fd7..1b7942dd3a 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -475,7 +475,7 @@ func marshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 		e.WriteString("null")
 		return
 	}
-	m, ok := v.Interface().(Marshaler)
+	m, ok := reflect.TypeAssert[Marshaler](v)
 	if !ok {
 		e.WriteString("null")
 		return
@@ -498,7 +498,7 @@ func addrMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 		e.WriteString("null")
 		return
 	}
-	m := va.Interface().(Marshaler)
+	m, _ := reflect.TypeAssert[Marshaler](va)
 	b, err := m.MarshalJSON()
 	if err == nil {
 		e.Grow(len(b))
@@ -516,7 +516,7 @@ func textMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 		e.WriteString("null")
 		return
 	}
-	m, ok := v.Interface().(encoding.TextMarshaler)
+	m, ok := reflect.TypeAssert[encoding.TextMarshaler](v)
 	if !ok {
 		e.WriteString("null")
 		return
@@ -534,7 +534,7 @@ func addrTextMarshalerEncoder(e *encodeState, v reflect.Value, opts encOpts) {
 		e.WriteString("null")
 		return
 	}
-	m := va.Interface().(encoding.TextMarshaler)
+	m, _ := reflect.TypeAssert[encoding.TextMarshaler](va)
 	b, err := m.MarshalText()
 	if err != nil {
 		e.error(&MarshalerError{v.Type(), err, "MarshalText"})
@@ -991,7 +991,7 @@ func resolveKeyName(k reflect.Value) (string, error) {
 	if k.Kind() == reflect.String {
 		return k.String(), nil
 	}
-	if tm, ok := k.Interface().(encoding.TextMarshaler); ok {
+	if tm, ok := reflect.TypeAssert[encoding.TextMarshaler](k); ok {
 		if k.Kind() == reflect.Pointer && k.IsNil() {
 			return "", nil
 		}
diff --git a/src/encoding/json/v2/arshal_funcs.go b/src/encoding/json/v2/arshal_funcs.go
index 5986c54732..673caf3c37 100644
--- a/src/encoding/json/v2/arshal_funcs.go
+++ b/src/encoding/json/v2/arshal_funcs.go
@@ -177,7 +177,8 @@ func MarshalFunc[T any](fn func(T) ([]byte, error)) *Marshalers {
 	typFnc := typedMarshaler{
 		typ: t,
 		fnc: func(enc *jsontext.Encoder, va addressableValue, mo *jsonopts.Struct) error {
-			val, err := fn(va.castTo(t).Interface().(T))
+			v, _ := reflect.TypeAssert[T](va.castTo(t))
+			val, err := fn(v)
 			if err != nil {
 				err = wrapSkipFunc(err, "marshal function of type func(T) ([]byte, error)")
 				if mo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
@@ -222,7 +223,8 @@ func MarshalToFunc[T any](fn func(*jsontext.Encoder, T) error) *Marshalers {
 			xe := export.Encoder(enc)
 			prevDepth, prevLength := xe.Tokens.DepthLength()
 			xe.Flags.Set(jsonflags.WithinArshalCall | 1)
-			err := fn(enc, va.castTo(t).Interface().(T))
+			v, _ := reflect.TypeAssert[T](va.castTo(t))
+			err := fn(enc, v)
 			xe.Flags.Set(jsonflags.WithinArshalCall | 0)
 			currDepth, currLength := xe.Tokens.DepthLength()
 			if err == nil && (prevDepth != currDepth || prevLength+1 != currLength) {
@@ -269,7 +271,8 @@ func UnmarshalFunc[T any](fn func([]byte, T) error) *Unmarshalers {
 			if err != nil {
 				return err // must be a syntactic or I/O error
 			}
-			err = fn(val, va.castTo(t).Interface().(T))
+			v, _ := reflect.TypeAssert[T](va.castTo(t))
+			err = fn(val, v)
 			if err != nil {
 				err = wrapSkipFunc(err, "unmarshal function of type func([]byte, T) error")
 				if uo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
@@ -304,7 +307,8 @@ func UnmarshalFromFunc[T any](fn func(*jsontext.Decoder, T) error) *Unmarshalers
 			xd := export.Decoder(dec)
 			prevDepth, prevLength := xd.Tokens.DepthLength()
 			xd.Flags.Set(jsonflags.WithinArshalCall | 1)
-			err := fn(dec, va.castTo(t).Interface().(T))
+			v, _ := reflect.TypeAssert[T](va.castTo(t))
+			err := fn(dec, v)
 			xd.Flags.Set(jsonflags.WithinArshalCall | 0)
 			currDepth, currLength := xd.Tokens.DepthLength()
 			if err == nil && (prevDepth != currDepth || prevLength+1 != currLength) {
diff --git a/src/encoding/json/v2/arshal_inlined.go b/src/encoding/json/v2/arshal_inlined.go
index a253144502..d911bfa1c0 100644
--- a/src/encoding/json/v2/arshal_inlined.go
+++ b/src/encoding/json/v2/arshal_inlined.go
@@ -50,8 +50,7 @@ func marshalInlinedFallbackAll(enc *jsontext.Encoder, va addressableValue, mo *j
 	}
 
 	if v.Type() == jsontextValueType {
-		// TODO(https://go.dev/issue/62121): Use reflect.Value.AssertTo.
-		b := *v.Addr().Interface().(*jsontext.Value)
+		b, _ := reflect.TypeAssert[jsontext.Value](v.Value)
 		if len(b) == 0 { // TODO: Should this be nil? What if it were all whitespace?
 			return nil
 		}
@@ -174,7 +173,7 @@ func unmarshalInlinedFallbackNext(dec *jsontext.Decoder, va addressableValue, uo
 	v = v.indirect(true)
 
 	if v.Type() == jsontextValueType {
-		b := v.Addr().Interface().(*jsontext.Value)
+		b, _ := reflect.TypeAssert[*jsontext.Value](v.Addr())
 		if len(*b) == 0 { // TODO: Should this be nil? What if it were all whitespace?
 			*b = append(*b, '{')
 		} else {
diff --git a/src/encoding/json/v2/arshal_methods.go b/src/encoding/json/v2/arshal_methods.go
index 099be298c2..664c3927cc 100644
--- a/src/encoding/json/v2/arshal_methods.go
+++ b/src/encoding/json/v2/arshal_methods.go
@@ -111,7 +111,7 @@ func makeMethodArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 				(needAddr && va.forcedAddr) {
 				return prevMarshal(enc, va, mo)
 			}
-			marshaler := va.Addr().Interface().(encoding.TextMarshaler)
+			marshaler, _ := reflect.TypeAssert[encoding.TextMarshaler](va.Addr())
 			if err := export.Encoder(enc).AppendRaw('"', false, func(b []byte) ([]byte, error) {
 				b2, err := marshaler.MarshalText()
 				return append(b, b2...), err
@@ -137,7 +137,7 @@ func makeMethodArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 				(needAddr && va.forcedAddr) {
 				return prevMarshal(enc, va, mo)
 			}
-			appender := va.Addr().Interface().(encoding.TextAppender)
+			appender, _ := reflect.TypeAssert[encoding.TextAppender](va.Addr())
 			if err := export.Encoder(enc).AppendRaw('"', false, appender.AppendText); err != nil {
 				err = wrapSkipFunc(err, "append method")
 				if mo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
@@ -160,7 +160,7 @@ func makeMethodArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 				((needAddr && va.forcedAddr) || export.Encoder(enc).Tokens.Last.NeedObjectName()) {
 				return prevMarshal(enc, va, mo)
 			}
-			marshaler := va.Addr().Interface().(Marshaler)
+			marshaler, _ := reflect.TypeAssert[Marshaler](va.Addr())
 			val, err := marshaler.MarshalJSON()
 			if err != nil {
 				err = wrapSkipFunc(err, "marshal method")
@@ -194,7 +194,8 @@ func makeMethodArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 			xe := export.Encoder(enc)
 			prevDepth, prevLength := xe.Tokens.DepthLength()
 			xe.Flags.Set(jsonflags.WithinArshalCall | 1)
-			err := va.Addr().Interface().(MarshalerTo).MarshalJSONTo(enc)
+			marshaler, _ := reflect.TypeAssert[MarshalerTo](va.Addr())
+			err := marshaler.MarshalJSONTo(enc)
 			xe.Flags.Set(jsonflags.WithinArshalCall | 0)
 			currDepth, currLength := xe.Tokens.DepthLength()
 			if (prevDepth != currDepth || prevLength+1 != currLength) && err == nil {
@@ -233,7 +234,7 @@ func makeMethodArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 				return newUnmarshalErrorAfter(dec, t, errNonStringValue)
 			}
 			s := jsonwire.UnquoteMayCopy(val, flags.IsVerbatim())
-			unmarshaler := va.Addr().Interface().(encoding.TextUnmarshaler)
+			unmarshaler, _ := reflect.TypeAssert[encoding.TextUnmarshaler](va.Addr())
 			if err := unmarshaler.UnmarshalText(s); err != nil {
 				err = wrapSkipFunc(err, "unmarshal method")
 				if uo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
@@ -260,7 +261,7 @@ func makeMethodArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 			if err != nil {
 				return err // must be a syntactic or I/O error
 			}
-			unmarshaler := va.Addr().Interface().(Unmarshaler)
+			unmarshaler, _ := reflect.TypeAssert[Unmarshaler](va.Addr())
 			if err := unmarshaler.UnmarshalJSON(val); err != nil {
 				err = wrapSkipFunc(err, "unmarshal method")
 				if uo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
@@ -284,7 +285,8 @@ func makeMethodArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 			xd := export.Decoder(dec)
 			prevDepth, prevLength := xd.Tokens.DepthLength()
 			xd.Flags.Set(jsonflags.WithinArshalCall | 1)
-			err := va.Addr().Interface().(UnmarshalerFrom).UnmarshalJSONFrom(dec)
+			unmarshaler, _ := reflect.TypeAssert[UnmarshalerFrom](va.Addr())
+			err := unmarshaler.UnmarshalJSONFrom(dec)
 			xd.Flags.Set(jsonflags.WithinArshalCall | 0)
 			currDepth, currLength := xd.Tokens.DepthLength()
 			if (prevDepth != currDepth || prevLength+1 != currLength) && err == nil {
diff --git a/src/encoding/json/v2/arshal_time.go b/src/encoding/json/v2/arshal_time.go
index ac29abe502..70dca8a294 100644
--- a/src/encoding/json/v2/arshal_time.go
+++ b/src/encoding/json/v2/arshal_time.go
@@ -57,8 +57,7 @@ func makeTimeArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 				return newMarshalErrorBefore(enc, t, errors.New("no default representation (see https://go.dev/issue/71631); specify an explicit format"))
 			}
 
-			// TODO(https://go.dev/issue/62121): Use reflect.Value.AssertTo.
-			m.td = *va.Addr().Interface().(*time.Duration)
+			m.td, _ = reflect.TypeAssert[time.Duration](va.Value)
 			k := stringOrNumberKind(!m.isNumeric() || xe.Tokens.Last.NeedObjectName() || mo.Flags.Get(jsonflags.StringifyNumbers))
 			if err := xe.AppendRaw(k, true, m.appendMarshal); err != nil {
 				if !isSyntacticError(err) && !export.IsIOError(err) {
@@ -85,7 +84,7 @@ func makeTimeArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 
 			stringify := !u.isNumeric() || xd.Tokens.Last.NeedObjectName() || uo.Flags.Get(jsonflags.StringifyNumbers)
 			var flags jsonwire.ValueFlags
-			td := va.Addr().Interface().(*time.Duration)
+			td, _ := reflect.TypeAssert[*time.Duration](va.Addr())
 			val, err := xd.ReadValue(&flags)
 			if err != nil {
 				return err
@@ -129,8 +128,7 @@ func makeTimeArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 				}
 			}
 
-			// TODO(https://go.dev/issue/62121): Use reflect.Value.AssertTo.
-			m.tt = *va.Addr().Interface().(*time.Time)
+			m.tt, _ = reflect.TypeAssert[time.Time](va.Value)
 			k := stringOrNumberKind(!m.isNumeric() || xe.Tokens.Last.NeedObjectName() || mo.Flags.Get(jsonflags.StringifyNumbers))
 			if err := xe.AppendRaw(k, !m.hasCustomFormat(), m.appendMarshal); err != nil {
 				if mo.Flags.Get(jsonflags.ReportErrorsWithLegacySemantics) {
@@ -156,7 +154,7 @@ func makeTimeArshaler(fncs *arshaler, t reflect.Type) *arshaler {
 
 			stringify := !u.isNumeric() || xd.Tokens.Last.NeedObjectName() || uo.Flags.Get(jsonflags.StringifyNumbers)
 			var flags jsonwire.ValueFlags
-			tt := va.Addr().Interface().(*time.Time)
+			tt, _ := reflect.TypeAssert[*time.Time](va.Addr())
 			val, err := xd.ReadValue(&flags)
 			if err != nil {
 				return err