compiler: disallow comparing slices with nil

NEO VM does not distinguish between empty and nil slices. Supporting
this is not easy and requires changing lots of other opcodes.
Pointers are not supported anyway and slices can be checked for
emptiness by inspecting their `len`.

pkg/compiler/codegen.go

@@ -617,25 +617,31 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 			ast.Walk(c, n.X)
 			ast.Walk(c, n.Y)
 
-			switch {
+			switch n.Op {
-			case n.Op == token.ADD:
+			case token.ADD:
 				// VM has separate opcodes for number and string concatenation
 				if isStringType(tinfo.Type) {
 					emit.Opcode(c.prog.BinWriter, opcode.CAT)
 				} else {
 					emit.Opcode(c.prog.BinWriter, opcode.ADD)
 				}
-			case n.Op == token.EQL:
+			case token.EQL, token.NEQ:
-				// VM has separate opcodes for number and string equality
+				if isExprNil(n.X) || isExprNil(n.Y) {
-				op := c.getEqualityOpcode(n.X)
+					c.prog.Err = errors.New("comparison with `nil` is not supported, use `len(..) == 0` instead")
-				emit.Opcode(c.prog.BinWriter, op)
+					return nil
-			case n.Op == token.NEQ:
+				}
-				// VM has separate opcodes for number and string equality
+				if n.Op == token.EQL {
-				if isStringType(c.typeInfo.Types[n.X].Type) {
+					// VM has separate opcodes for number and string equality
-					emit.Opcode(c.prog.BinWriter, opcode.EQUAL)
+					op := c.getEqualityOpcode(n.X)
-					emit.Opcode(c.prog.BinWriter, opcode.NOT)
+					emit.Opcode(c.prog.BinWriter, op)
 				} else {
-					emit.Opcode(c.prog.BinWriter, opcode.NUMNOTEQUAL)
+					// VM has separate opcodes for number and string equality
+					if isStringType(c.typeInfo.Types[n.X].Type) {
+						emit.Opcode(c.prog.BinWriter, opcode.EQUAL)
+						emit.Opcode(c.prog.BinWriter, opcode.NOT)
+					} else {
+						emit.Opcode(c.prog.BinWriter, opcode.NUMNOTEQUAL)
+					}
 				}
 			default:
 				c.convertToken(n.Op)

pkg/compiler/slice_test.go

@@ -1,10 +1,14 @@
 package compiler_test
 
 import (
+	"fmt"
 	"math/big"
+	"strings"
 	"testing"
 
+	"github.com/nspcc-dev/neo-go/pkg/compiler"
 	"github.com/nspcc-dev/neo-go/pkg/vm"
+	"github.com/stretchr/testify/require"
 )
 
 var sliceTestCases = []testCase{
@@ -175,6 +179,31 @@ func TestSliceOperations(t *testing.T) {
 	runTestCases(t, sliceTestCases)
 }
 
+func TestSliceEmpty(t *testing.T) {
+	srcTmpl := `package foo
+	func Main() int {
+		var a []int
+		%s
+		if %s {
+			return 1
+		}
+		return 2
+	}`
+	t.Run("WithNil", func(t *testing.T) {
+		src := fmt.Sprintf(srcTmpl, "", "a == nil")
+		_, err := compiler.Compile(strings.NewReader(src))
+		require.Error(t, err)
+	})
+	t.Run("WithLen", func(t *testing.T) {
+		src := fmt.Sprintf(srcTmpl, "", "len(a) == 0")
+		eval(t, src, big.NewInt(1))
+	})
+	t.Run("NonEmpty", func(t *testing.T) {
+		src := fmt.Sprintf(srcTmpl, "a = []int{1}", "len(a) == 0")
+		eval(t, src, big.NewInt(2))
+	})
+}
+
 func TestJumps(t *testing.T) {
 	src := `
 	package foo