Allow converting between nil big.Int and nil uint256.Int

conversion.go

@@ -41,7 +41,11 @@ var (
 )
 
 // ToBig returns a big.Int version of z.
+// Return `nil` if z is nil
 func (z *Int) ToBig() *big.Int {
+	if z == nil {
+		return nil
+	}
 	b := new(big.Int)
 	switch maxWords { // Compile-time check.
 	case 4: // 64-bit architectures.
@@ -61,15 +65,24 @@ func (z *Int) ToBig() *big.Int {
 
 // FromBig is a convenience-constructor from big.Int.
 // Returns a new Int and whether overflow occurred.
+// OBS: If b is `nil`, this method returns `nil, false`
 func FromBig(b *big.Int) (*Int, bool) {
+	if b == nil {
+		return nil, false
+	}
 	z := &Int{}
 	overflow := z.SetFromBig(b)
 	return z, overflow
 }
 
 // MustFromBig is a convenience-constructor from big.Int.
 // Returns a new Int and panics if overflow occurred.
+// OBS: If b is `nil`, this method does _not_ panic, but 
+// instead returns `nil`
 func MustFromBig(b *big.Int) *Int {
+	if b == nil {
+		return nil
+	}
 	z := &Int{}
 	if z.SetFromBig(b) {
 		panic("overflow")

conversion_test.go

@@ -19,15 +19,27 @@ var (
 )
 
 func TestFromBig(t *testing.T) {
-	a := new(big.Int)
+	var a *big.Int
 	b, o := FromBig(a)
+	if o {
+		t.Fatalf("nil conversion overflowed! big.Int %x", b)
+	}
+	if b != nil {
+		t.Fatalf("got %x, exp %v", b, nil)
+	}
+	b2 := MustFromBig(a)
+	if b2 != nil {
+		t.Fatalf("got %x, exp %v", b2, nil)
+	}
+	a = new(big.Int)
+	b, o = FromBig(a)
 	if o {
 		t.Fatalf("conversion overflowed! big.Int %x", a.Bytes())
 	}
 	if exp, got := a.Bytes(), b.Bytes(); !bytes.Equal(got, exp) {
 		t.Fatalf("got %x exp %x", got, exp)
 	}
-	b2 := MustFromBig(a)
+	b2 = MustFromBig(a)
 	if exp, got := a.Bytes(), b2.Bytes(); !bytes.Equal(got, exp) {
 		t.Fatalf("got %x exp %x", got, exp)
 	}
@@ -211,7 +223,10 @@ func TestFromBigOverflow(t *testing.T) {
 }
 
 func TestToBig(t *testing.T) {
-
+	var uint256Nil *Int
+	if bigNil := uint256Nil.ToBig(); bigNil != nil {
+		t.Errorf("want big.Int <nil>, have %x", bigNil)
+	}
 	if bigZero := new(Int).ToBig(); bigZero.Cmp(new(big.Int)) != 0 {
 		t.Errorf("expected big.Int 0, got %x", bigZero)
 	}