compiler: allow to declare slices of compound types

Previously this declarations were ignored which resulted
in runtime errors, because VM's nil is an element of primitive type
and can't be converted to an array.

pkg/compiler/analysis.go

@@ -187,6 +187,22 @@ func isBuiltin(expr ast.Expr) bool {
 	return false
 }
 
+func (c *codegen) isCompoundArrayType(t ast.Expr) bool {
+	switch s := t.(type) {
+	case *ast.ArrayType:
+		return true
+	case *ast.Ident:
+		arr, ok := c.typeInfo.Types[s].Type.Underlying().(*types.Slice)
+		return ok && !isByte(arr.Elem())
+	}
+	return false
+}
+
+func isByte(t types.Type) bool {
+	e, ok := t.(*types.Basic)
+	return ok && e.Kind() == types.Byte
+}
+
 func isByteArray(lit *ast.CompositeLit, tInfo *types.Info) bool {
 	if len(lit.Elts) == 0 {
 		if typ, ok := lit.Type.(*ast.ArrayType); ok {
@@ -199,14 +215,7 @@ func isByteArray(lit *ast.CompositeLit, tInfo *types.Info) bool {
 	}
 
 	typ := tInfo.Types[lit.Elts[0]].Type.Underlying()
-	switch t := typ.(type) {
-	case *types.Basic:
-		switch t.Kind() {
-		case types.Byte:
-			return true
-		}
-	}
-	return false
+	return isByte(typ)
 }
 
 func isSyscall(fun *funcScope) bool {

pkg/compiler/codegen.go

@@ -276,9 +276,16 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 		for _, spec := range n.Specs {
 			switch t := spec.(type) {
 			case *ast.ValueSpec:
-				for i, val := range t.Values {
-					ast.Walk(c, val)
-					l := c.scope.newLocal(t.Names[i].Name)
+				if len(t.Values) != 0 {
+					for i, val := range t.Values {
+						ast.Walk(c, val)
+						l := c.scope.newLocal(t.Names[i].Name)
+						c.emitStoreLocal(l)
+					}
+				} else if c.isCompoundArrayType(t.Type) {
+					emit.Opcode(c.prog.BinWriter, opcode.PUSH0)
+					emit.Opcode(c.prog.BinWriter, opcode.NEWARRAY)
+					l := c.scope.newLocal(t.Names[0].Name)
 					c.emitStoreLocal(l)
 				}
 			}

pkg/compiler/slice_test.go

@@ -3,6 +3,8 @@ package compiler_test
 import (
 	"math/big"
 	"testing"
+
+	"github.com/nspcc-dev/neo-go/pkg/vm"
 )
 
 var sliceTestCases = []testCase{
@@ -128,6 +130,35 @@ var sliceTestCases = []testCase{
 		}`,
 		[]byte{2, 3},
 	},
+	{
+		"declare compound slice",
+		`package foo
+		func Main() []string {
+			var a []string
+			a = append(a, "a")
+			a = append(a, "b")
+			return a
+		}`,
+		[]vm.StackItem{
+			vm.NewByteArrayItem([]byte("a")),
+			vm.NewByteArrayItem([]byte("b")),
+		},
+	},
+	{
+		"declare compound slice alias",
+		`package foo
+		type strs []string
+		func Main() []string {
+			var a strs
+			a = append(a, "a")
+			a = append(a, "b")
+			return a
+		}`,
+		[]vm.StackItem{
+			vm.NewByteArrayItem([]byte("a")),
+			vm.NewByteArrayItem([]byte("b")),
+		},
+	},
 }
 
 func TestSliceOperations(t *testing.T) {