Feature: allow to unmarshal json.Number

Both go native encoding/json json.Unmarshal and ffjson's UnmarshalJSON methods could unmarshal both json-encoded strings:
`{"number": 10}`
`{"number": "20"}`
to struct {
  Number json.Number `json:"number"`
}
But easyjson got error on unmarshaling second string.

gen/decoder.go

@@ -48,6 +48,10 @@ var primitiveStringDecoders = map[reflect.Kind]string{
 	reflect.Uint64: "in.Uint64Str()",
 }
 
+var customDecoders = map[string]string{
+	"json.Number":    "in.JsonNumber()",
+}
+
 // genTypeDecoder generates decoding code for the type t, but uses unmarshaler interface if implemented by t.
 func (g *Generator) genTypeDecoder(t reflect.Type, out string, tags fieldTags, indent int) error {
 	ws := strings.Repeat("  ", indent)
@@ -82,7 +86,10 @@ func (g *Generator) genTypeDecoder(t reflect.Type, out string, tags fieldTags, i
 func (g *Generator) genTypeDecoderNoCheck(t reflect.Type, out string, tags fieldTags, indent int) error {
 	ws := strings.Repeat("  ", indent)
 	// Check whether type is primitive, needs to be done after interface check.
-	if dec := primitiveStringDecoders[t.Kind()]; dec != "" && tags.asString {
+	if dec := customDecoders[t.String()]; dec != ""  {
+		fmt.Fprintln(g.out, ws+out+" = "+dec)
+		return nil
+	} else if dec := primitiveStringDecoders[t.Kind()]; dec != "" && tags.asString {
 		fmt.Fprintln(g.out, ws+out+" = "+g.getType(t)+"("+dec+")")
 		return nil
 	} else if dec := primitiveDecoders[t.Kind()]; dec != "" {

jlexer/lexer.go

@@ -6,6 +6,7 @@ package jlexer
 
 import (
 	"encoding/base64"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
@@ -1043,6 +1044,28 @@ func (r *Lexer) GetNonFatalErrors() []*LexerError {
 	return r.multipleErrors
 }
 
+// JsonNumber fetches and json.Number from 'encoding/json' package.
+// Both int, float or string, contains them are valid values
+func (r *Lexer) JsonNumber() json.Number {
+	if r.token.kind == tokenUndef && r.Ok() {
+		r.FetchToken()
+	}
+	if !r.Ok() {
+		r.errInvalidToken("json.Number")
+		return json.Number("0")
+	}
+
+	switch r.token.kind {
+	case tokenString:
+		return json.Number(r.String())
+	case tokenNumber:
+		return json.Number(r.Raw())
+	default:
+		r.errSyntax()
+		return json.Number("0")
+	}
+}
+
 // Interface fetches an interface{} analogous to the 'encoding/json' package.
 func (r *Lexer) Interface() interface{} {
 	if r.token.kind == tokenUndef && r.Ok() {

jlexer/lexer_test.go

@@ -2,6 +2,7 @@ package jlexer
 
 import (
 	"bytes"
+	"encoding/json"
 	"reflect"
 	"testing"
 )
@@ -249,3 +250,62 @@ func TestConsumed(t *testing.T) {
 		}
 	}
 }
+
+func TestJsonNumber(t *testing.T) {
+	for i, test := range []struct {
+		toParse        string
+		want           json.Number
+		wantLexerError bool
+		wantValue      interface{}
+		wantValueError bool
+	}{
+		{toParse: `10`, want: json.Number("10"), wantValue: int64(10)},
+		{toParse: `0`, want: json.Number("0"), wantValue: int64(0)},
+		{toParse: `0.12`, want: json.Number("0.12"), wantValue: 0.12},
+		{toParse: `25E-4`, want: json.Number("25E-4"), wantValue: 25E-4},
+
+		{toParse: `"10"`, want: json.Number("10"), wantValue: int64(10)},
+		{toParse: `"0"`, want: json.Number("0"), wantValue: int64(0)},
+		{toParse: `"0.12"`, want: json.Number("0.12"), wantValue: 0.12},
+		{toParse: `"25E-4"`, want: json.Number("25E-4"), wantValue: 25E-4},
+
+		{toParse: `"a""`, wantValueError: true},
+
+		{toParse: `[1]`, wantLexerError: true},
+		{toParse: `{}`, wantLexerError: true},
+		{toParse: `a`, wantLexerError: true},
+	} {
+		l := Lexer{Data: []byte(test.toParse)}
+
+		got := l.JsonNumber()
+		if got != test.want && !test.wantLexerError && !test.wantValueError {
+			t.Errorf("[%d, %q] JsonNumber() = %v; want %v", i, test.toParse, got, test.want)
+		}
+
+		err := l.Error()
+		if err != nil && !test.wantLexerError {
+			t.Errorf("[%d, %q] JsonNumber() lexer error: %v", i, test.toParse, err)
+		} else if err == nil && test.wantLexerError {
+			t.Errorf("[%d, %q] JsonNumber() ok; want lexer error", i, test.toParse)
+		}
+
+		var valueErr error
+		var gotValue interface{}
+		switch test.wantValue.(type) {
+		case float64:
+			gotValue, valueErr = got.Float64()
+		default:
+			gotValue, valueErr = got.Int64()
+		}
+
+		if !reflect.DeepEqual(gotValue, test.wantValue) && !test.wantLexerError && !test.wantValueError {
+			t.Errorf("[%d, %q] JsonNumber() = %v; want %v", i, test.toParse, gotValue, test.wantValue)
+		}
+
+		if valueErr != nil && !test.wantValueError {
+			t.Errorf("[%d, %q] JsonNumber() value error: %v", i, test.toParse, err)
+		} else if valueErr == nil && test.wantValueError {
+			t.Errorf("[%d, %q] JsonNumber() ok; want value error", i, test.toParse)
+		}
+	}
+}