encoding/asn1: document support for *big.Int

Also add basic tests.

R=golang-dev
CC=golang-dev
https://golang.org/cl/5533045

src/pkg/encoding/asn1/asn1.go

@@ -786,7 +786,8 @@ func setDefaultValue(v reflect.Value, params fieldParameters) (ok bool) {
 // Because Unmarshal uses the reflect package, the structs
 // being written to must use upper case field names.
 //
-// An ASN.1 INTEGER can be written to an int, int32 or int64.
+// An ASN.1 INTEGER can be written to an int, int32, int64,
+// or *big.Int (from the math/big package).
 // If the encoded value does not fit in the Go type,
 // Unmarshal returns a parse error.
 //

src/pkg/encoding/asn1/asn1_test.go

@@ -6,6 +6,7 @@ package asn1
 
 import (
 	"bytes"
+	"math/big"
 	"reflect"
 	"testing"
 	"time"
@@ -351,6 +352,10 @@ type TestElementsAfterString struct {
 	A, B int
 }
 
+type TestBigInt struct {
+	X *big.Int
+}
+
 var unmarshalTestData = []struct {
 	in  []byte
 	out interface{}
@@ -369,6 +374,7 @@ var unmarshalTestData = []struct {
 	{[]byte{0x01, 0x01, 0x00}, newBool(false)},
 	{[]byte{0x01, 0x01, 0x01}, newBool(true)},
 	{[]byte{0x30, 0x0b, 0x13, 0x03, 0x66, 0x6f, 0x6f, 0x02, 0x01, 0x22, 0x02, 0x01, 0x33}, &TestElementsAfterString{"foo", 0x22, 0x33}},
+	{[]byte{0x30, 0x05, 0x02, 0x03, 0x12, 0x34, 0x56}, &TestBigInt{big.NewInt(0x123456)}},
 }
 
 func TestUnmarshal(t *testing.T) {

src/pkg/encoding/asn1/marshal_test.go

@@ -7,6 +7,7 @@ package asn1
 import (
 	"bytes"
 	"encoding/hex"
+	"math/big"
 	"testing"
 	"time"
 )
@@ -20,6 +21,10 @@ type twoIntStruct struct {
 	B int
 }
 
+type bigIntStruct struct {
+	A *big.Int
+}
+
 type nestedStruct struct {
 	A intStruct
 }
@@ -65,6 +70,7 @@ var marshalTests = []marshalTest{
 	{-128, "020180"},
 	{-129, "0202ff7f"},
 	{intStruct{64}, "3003020140"},
+	{bigIntStruct{big.NewInt(0x123456)}, "30050203123456"},
 	{twoIntStruct{64, 65}, "3006020140020141"},
 	{nestedStruct{intStruct{127}}, "3005300302017f"},
 	{[]byte{1, 2, 3}, "0403010203"},