fix panic when marshaling interface field containing typed nil value

Makefile

@@ -43,6 +43,7 @@ generate: build
 		./tests/unknown_fields.go \
 		./tests/type_declaration.go \
 		./tests/members_escaped.go \
+		./tests/interface_type.go \
 		./tests/intern.go \
 		./tests/nocopy.go \
 		./tests/escaping.go \

gen/encoder.go

@@ -264,23 +264,39 @@ func (g *Generator) genTypeEncoderNoCheck(t reflect.Type, in string, tags fieldT
 	case reflect.Interface:
 		if t.NumMethod() != 0 {
 			if g.interfaceIsEasyjsonMarshaller(t) {
-				fmt.Fprintln(g.out, ws+in+".MarshalEasyJSON(out)")
+				fmt.Fprintln(g.out, ws+"  if easyjson.IsNilInterface("+in+") {")
+				fmt.Fprintln(g.out, ws+"    out.RawString(`null`)")
+				fmt.Fprintln(g.out, ws+"  } else {")
+				fmt.Fprintln(g.out, ws+"    "+in+".MarshalEasyJSON(out)")
+				fmt.Fprintln(g.out, ws+"  }")
 			} else if g.interfaceIsJSONMarshaller(t) {
 				fmt.Fprintln(g.out, ws+"if m, ok := "+in+".(easyjson.Marshaler); ok {")
-				fmt.Fprintln(g.out, ws+"  m.MarshalEasyJSON(out)")
+				fmt.Fprintln(g.out, ws+"  if easyjson.IsNilInterface("+in+") {")
+				fmt.Fprintln(g.out, ws+"    out.RawString(`null`)")
+				fmt.Fprintln(g.out, ws+"  } else {")
+				fmt.Fprintln(g.out, ws+"    m.MarshalEasyJSON(out)")
+				fmt.Fprintln(g.out, ws+"  }")
 				fmt.Fprintln(g.out, ws+"} else {")
-				fmt.Fprintln(g.out, ws+in+".MarshalJSON()")
+				fmt.Fprintln(g.out, ws+"  if easyjson.IsNilInterface("+in+") {")
+				fmt.Fprintln(g.out, ws+"    out.RawString(`null`)")
+				fmt.Fprintln(g.out, ws+"  } else {")
+				fmt.Fprintln(g.out, ws+"    "+in+".MarshalJSON()")
+				fmt.Fprintln(g.out, ws+"  }")
 				fmt.Fprintln(g.out, ws+"}")
 			} else {
 				return fmt.Errorf("interface type %v not supported: only interface{} and interfaces that implement json or easyjson Marshaling are allowed", t)
 			}
 		} else {
-			fmt.Fprintln(g.out, ws+"if m, ok := "+in+".(easyjson.Marshaler); ok {")
-			fmt.Fprintln(g.out, ws+"  m.MarshalEasyJSON(out)")
-			fmt.Fprintln(g.out, ws+"} else if m, ok := "+in+".(json.Marshaler); ok {")
-			fmt.Fprintln(g.out, ws+"  out.Raw(m.MarshalJSON())")
-			fmt.Fprintln(g.out, ws+"} else {")
-			fmt.Fprintln(g.out, ws+"  out.Raw(json.Marshal("+in+"))")
+			fmt.Fprintln(g.out, ws+"if easyjson.IsNilInterface("+in+") {")
+			fmt.Fprintln(g.out, ws+"  out.RawString(`null`)")
+			fmt.Fprintln(g.out, ws+"  } else {")
+			fmt.Fprintln(g.out, ws+"  if m, ok := "+in+".(easyjson.Marshaler); ok {")
+			fmt.Fprintln(g.out, ws+"    m.MarshalEasyJSON(out)")
+			fmt.Fprintln(g.out, ws+"  } else if m, ok := "+in+".(json.Marshaler); ok {")
+			fmt.Fprintln(g.out, ws+"    out.Raw(m.MarshalJSON())")
+			fmt.Fprintln(g.out, ws+"  } else {")
+			fmt.Fprintln(g.out, ws+"    out.Raw(json.Marshal("+in+"))")
+			fmt.Fprintln(g.out, ws+"  }")
 			fmt.Fprintln(g.out, ws+"}")
 		}
 	default:
@@ -419,7 +435,6 @@ func (g *Generator) genStructEncoder(t reflect.Type) error {
 	firstCondition := true
 	for i, f := range fs {
 		firstCondition, err = g.genStructFieldEncoder(t, f, i == 0, firstCondition)
-
 		if err != nil {
 			return err
 		}

helpers.go

@@ -3,7 +3,6 @@ package easyjson
 
 import (
 	"io"
-	"io/ioutil"
 	"net/http"
 	"strconv"
 	"unsafe"
@@ -43,14 +42,14 @@ type UnknownsMarshaler interface {
 	MarshalUnknowns(w *jwriter.Writer, first bool)
 }
 
-func isNilInterface(i interface{}) bool {
+func IsNilInterface(i interface{}) bool {
 	return (*[2]uintptr)(unsafe.Pointer(&i))[1] == 0
 }
 
 // Marshal returns data as a single byte slice. Method is suboptimal as the data is likely to be copied
 // from a chain of smaller chunks.
 func Marshal(v Marshaler) ([]byte, error) {
-	if isNilInterface(v) {
+	if IsNilInterface(v) {
 		return nullBytes, nil
 	}
 
@@ -61,7 +60,7 @@ func Marshal(v Marshaler) ([]byte, error) {
 
 // MarshalToWriter marshals the data to an io.Writer.
 func MarshalToWriter(v Marshaler, w io.Writer) (written int, err error) {
-	if isNilInterface(v) {
+	if IsNilInterface(v) {
 		return w.Write(nullBytes)
 	}
 
@@ -75,7 +74,7 @@ func MarshalToWriter(v Marshaler, w io.Writer) (written int, err error) {
 // false if an error occurred before any http.ResponseWriter methods were actually
 // invoked (in this case a 500 reply is possible).
 func MarshalToHTTPResponseWriter(v Marshaler, w http.ResponseWriter) (started bool, written int, err error) {
-	if isNilInterface(v) {
+	if IsNilInterface(v) {
 		w.Header().Set("Content-Type", "application/json")
 		w.Header().Set("Content-Length", strconv.Itoa(len(nullBytes)))
 		written, err = w.Write(nullBytes)
@@ -104,7 +103,7 @@ func Unmarshal(data []byte, v Unmarshaler) error {
 
 // UnmarshalFromReader reads all the data in the reader and decodes as JSON into the object.
 func UnmarshalFromReader(r io.Reader, v Unmarshaler) error {
-	data, err := ioutil.ReadAll(r)
+	data, err := io.ReadAll(r)
 	if err != nil {
 		return err
 	}

helpers_test.go

@@ -5,7 +5,7 @@ import "testing"
 func BenchmarkNilCheck(b *testing.B) {
 	var a *int
 	for i := 0; i < b.N; i++ {
-		if !isNilInterface(a) {
+		if !IsNilInterface(a) {
 			b.Fatal("expected it to be nil")
 		}
 	}

tests/interface_test.go

@@ -0,0 +1,72 @@
+package tests
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestInterfaceMarshal(t *testing.T) {
+	inner := InterfaceType{
+		Field1: 1,
+	}
+	wrapper := WrapperType{
+		Inner: inner,
+	}
+
+	data, err := wrapper.MarshalJSON()
+	if err != nil {
+		t.Fatalf("MarshalJSON failed: %v", err)
+	}
+
+	expected := []byte(`{"Inner":{"Field1":1}}`)
+	if !bytes.Equal(data, expected) {
+		t.Fatalf("MarshalJSON failed: got=%s, expected=%s", string(data), string(expected))
+	}
+}
+
+func TestInterfaceUnmarshal(t *testing.T) {
+	data := []byte(`{"Inner":{"Field1":1}}`)
+
+	var inner InterfaceType
+	wrapper := WrapperType{Inner: &inner}
+	err := wrapper.UnmarshalJSON(data)
+	if err != nil {
+		t.Fatalf("UnmarshalJSON failed: %v", err)
+	}
+
+	if inner.Field1 != 1 {
+		t.Fatalf("UnmarshalJSON failed: got=%d, expected=%d", inner.Field1, 1)
+	}
+}
+
+func TestInterfaceMarshalNil(t *testing.T) {
+	var inner *InterfaceType
+	wrapper := WrapperType{
+		Inner: inner,
+	}
+
+	data, err := wrapper.MarshalJSON()
+	if err != nil {
+		t.Fatalf("MarshalJSON failed: %v", err)
+	}
+
+	expected := []byte(`{"Inner":null}`)
+	if !bytes.Equal(data, expected) {
+		t.Fatalf("MarshalJSON failed: got=%s, expected=%s", string(data), string(expected))
+	}
+}
+
+func TestInterfaceUnmarshalNil(t *testing.T) {
+	data := []byte(`{"Inner":null}`)
+
+	var inner InterfaceType
+	wrapper := WrapperType{Inner: &inner}
+	err := wrapper.UnmarshalJSON(data)
+	if err != nil {
+		t.Fatalf("UnmarshalJSON failed: %v", err)
+	}
+
+	if inner.Field1 != 0 {
+		t.Fatalf("UnmarshalJSON failed: got=%d, expected=0", inner.Field1)
+	}
+}

tests/interface_type.go

@@ -0,0 +1,11 @@
+package tests
+
+// easyjson:json
+type WrapperType struct {
+	Inner any `json:"Inner"`
+}
+
+// easyjson:json
+type InterfaceType struct {
+	Field1 int `json:"Field1"`
+}