fix(gnovm): prevent assignment to non-assignable expressions

gnovm/pkg/gnolang/preprocess.go

@@ -457,6 +457,9 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 			case *AssignStmt:
 				checkValDefineMismatch(n)
 
+				for _, rx := range n.Rhs {
+					checkExprIsAssignable(rx)
+				}
 				if n.Op == DEFINE {
 					for _, lx := range n.Lhs {
 						ln := lx.(*NameExpr).Name
@@ -489,6 +492,9 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 					d := n.(Decl)
 					if cd, ok := d.(*ValueDecl); ok {
 						checkValDefineMismatch(cd)
+						for _, rx := range cd.Values {
+							checkExprIsAssignable(rx)
+						}
 					}
 					// recursively predefine dependencies.
 					d2, ppd := predefineNow(store, last, d)

gnovm/pkg/gnolang/type_check.go

@@ -279,6 +279,29 @@ func checkValDefineMismatch(n Node) {
 	panic(fmt.Sprintf("assignment mismatch: %d variable(s) but %d value(s)", numNames, numValues))
 }
 
+func checkExprIsAssignable(exp Expr) {
+	switch exp.(type) {
+	case *NameExpr,
+		*BasicLitExpr,
+		*BinaryExpr,
+		*CallExpr,
+		*IndexExpr,
+		*SelectorExpr,
+		*SliceExpr,
+		*StarExpr,
+		*RefExpr,
+		*TypeAssertExpr,
+		*UnaryExpr,
+		*CompositeLitExpr,
+		*KeyValueExpr,
+		*FuncLitExpr,
+		*ConstExpr:
+		return
+	}
+
+	panic(fmt.Sprintf("%s (type) is not an expression", exp.String()))
+}
+
 // Assert that xt can be assigned as dt (dest type).
 // If autoNative is true, a broad range of xt can match against
 // a target native dt type, if and only if dt is a native type.

gnovm/tests/files/assign29.gno

@@ -0,0 +1,8 @@
+package main
+
+func main() {
+	t := struct{}
+}
+
+// Error:
+// main/files/assign29.gno:4:2: struct {  } (type) is not an expression

gnovm/tests/files/var31.gno

@@ -0,0 +1,8 @@
+package main
+
+func main() {
+	var t = struct{}
+}
+
+// Error:
+// main/files/var31.gno:4:6: struct {  } (type) is not an expression