fix(gnovm): prevent assignment to non-assignable expressions (#2896)

closes: #2889
<!-- please provide a detailed description of the changes made in this
pull request. -->

<details><summary>Contributors' checklist...</summary>

- [ ] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

gnovm/pkg/gnolang/preprocess.go

@@ -1961,6 +1961,7 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 			// TRANS_LEAVE -----------------------
 			case *AssignStmt:
 				n.AssertCompatible(store, last)
+
 				// NOTE: keep DEFINE and ASSIGN in sync.
 				if n.Op == DEFINE {
 					// Rhs consts become default *ConstExprs.
@@ -2291,6 +2292,8 @@ func preprocess1(store Store, ctx BlockNode, n Node) Node {
 
 			// TRANS_LEAVE -----------------------
 			case *ValueDecl:
+				assertValidAssignRhs(store, last, n)
+
 				// evaluate value if const expr.
 				if n.Const {
 					// NOTE: may or may not be a *ConstExpr,

gnovm/pkg/gnolang/type_check.go

@@ -837,6 +837,7 @@ func (x *RangeStmt) AssertCompatible(store Store, last BlockNode) {
 
 func (x *AssignStmt) AssertCompatible(store Store, last BlockNode) {
 	if x.Op == ASSIGN || x.Op == DEFINE {
+		assertValidAssignRhs(store, last, x)
 		if len(x.Lhs) > len(x.Rhs) {
 			if len(x.Rhs) != 1 {
 				panic(fmt.Sprintf("assignment mismatch: %d variables but %d values", len(x.Lhs), len(x.Rhs)))
@@ -997,6 +998,40 @@ func assertValidAssignLhs(store Store, last BlockNode, lx Expr) {
 	}
 }
 
+func assertValidAssignRhs(store Store, last BlockNode, n Node) {
+	var exps []Expr
+	switch x := n.(type) {
+	case *ValueDecl:
+		exps = x.Values
+	case *AssignStmt:
+		exps = x.Rhs
+	default:
+		panic(fmt.Sprintf("unexpected node type %T", n))
+	}
+
+	for _, exp := range exps {
+		tt := evalStaticTypeOfRaw(store, last, exp)
+		if tt == nil {
+			switch x := n.(type) {
+			case *ValueDecl:
+				if x.Type != nil {
+					continue
+				}
+				panic("use of untyped nil in variable declaration")
+			case *AssignStmt:
+				if x.Op != DEFINE {
+					continue
+				}
+				panic("use of untyped nil in assignment")
+			}
+		}
+		if _, ok := tt.(*TypeType); ok {
+			tt = evalStaticType(store, last, exp)
+			panic(fmt.Sprintf("%s (type) is not an expression", tt.String()))
+		}
+	}
+}
+
 func kindString(xt Type) string {
 	if xt != nil {
 		return xt.Kind().String()

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/assign30.gno

@@ -0,0 +1,8 @@
+package main
+
+func main() {
+	t := *struct{}
+}
+
+// Error:
+// main/files/assign30.gno:4:2: *struct{} (type) is not an expression

gnovm/tests/files/assign31.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	a := nil
+	println(a)
+}
+
+// Error:
+// main/files/assign31.gno:4:2: use of untyped nil in assignment

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

gnovm/tests/files/var32.gno

@@ -0,0 +1,8 @@
+package main
+
+func main() {
+	var t = nil
+}
+
+// Error:
+// main/files/var32.gno:4:6: use of untyped nil in variable declaration

gnovm/tests/files/var33.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	var t *int = nil
+	println("pass")
+}
+
+// Output:
+// pass