feat(gnovm): support constant evaluation of len and cap on arrays

gnovm/pkg/gnolang/preprocess.go

@@ -3369,15 +3369,36 @@ func getResultTypedValues(cx *CallExpr) []TypedValue {
 func evalConst(store Store, last BlockNode, x Expr) *ConstExpr {
 	// TODO: some check or verification for ensuring x
 	// is constant?  From the machine?
-	m := NewMachine(".dontcare", store)
-	m.PreprocessorMode = true
+	var cx *ConstExpr
+	if clx, ok := x.(*CallExpr); ok {
+		ar, ok := evalStaticTypeOf(store, last, clx.Args[0]).(*ArrayType)
+		if ok {
+			fv := clx.Func.(*ConstExpr).V.(*FuncValue)
+			switch {
+			case fv.Name == "cap":
+				fallthrough
+			case fv.Name == "len":
+				tv := TypedValue{T: IntType}
+				tv.SetInt(ar.Len)
+				cx = &ConstExpr{
+					Source:     x,
+					TypedValue: tv,
+				}
+			default:
+				panic(fmt.Sprintf("unexpected const func %s", fv.Name))
+			}
+		}
+	}
 
-	cv := m.EvalStatic(last, x)
-	m.PreprocessorMode = false
-	m.Release()
-	cx := &ConstExpr{
-		Source:     x,
-		TypedValue: cv,
+	if cx == nil {
+		m := NewMachine(".dontcare", store)
+		cv := m.EvalStatic(last, x)
+		m.PreprocessorMode = false
+		m.Release()
+		cx = &ConstExpr{
+			Source:     x,
+			TypedValue: cv,
+		}
 	}
 	cx.SetLine(x.GetLine())
 	cx.SetAttribute(ATTR_PREPROCESSED, true)

gnovm/tests/files/const51.gno

@@ -0,0 +1,14 @@
+package main
+
+type T struct {
+	x [2]string
+}
+
+func main() {
+	t := T{x: [2]string{"a", "b"}}
+	const v = len(t.x)
+	println(v)
+}
+
+// Output:
+// 2