fix(gnovm): add missing length check in DeclStmt and AssignStmt (#2206)

Closes #2137 
<!-- 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>

---------

Co-authored-by: Miloš Živković <milos@zmilos.com>

gnovm/pkg/gnolang/preprocess.go

@@ -427,6 +427,8 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 			switch n := n.(type) {
 			// TRANS_ENTER -----------------------
 			case *AssignStmt:
+				checkValDefineMismatch(n)
+
 				if n.Op == DEFINE {
 					for _, lx := range n.Lhs {
 						ln := lx.(*NameExpr).Name
@@ -445,7 +447,6 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 				} else {
 					// nothing defined.
 				}
-
 			// TRANS_ENTER -----------------------
 			case *ImportDecl, *ValueDecl, *TypeDecl, *FuncDecl:
 				// NOTE func decl usually must happen with a
@@ -457,8 +458,12 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 					// skip declarations already predefined
 					// (e.g. through recursion for a dependent)
 				} else {
+					d := n.(Decl)
+					if cd, ok := d.(*ValueDecl); ok {
+						checkValDefineMismatch(cd)
+					}
 					// recursively predefine dependencies.
-					d2, ppd := predefineNow(store, last, n.(Decl))
+					d2, ppd := predefineNow(store, last, d)
 					if ppd {
 						return d2, TRANS_SKIP
 					} else {
@@ -2162,8 +2167,8 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 					// special case if `var a, b, c T? = f()` form.
 					cx := n.Values[0].(*CallExpr)
 					tt := evalStaticTypeOfRaw(store, last, cx).(*tupleType)
-					if len(tt.Elts) != numNames {
-						panic("should not happen")
+					if rLen := len(tt.Elts); rLen != numNames {
+						panic(fmt.Sprintf("assignment mismatch: %d variable(s) but %s returns %d value(s)", numNames, cx.Func.String(), rLen))
 					}
 					if n.Type != nil {
 						// only a single type can be specified.
@@ -2182,8 +2187,16 @@ func Preprocess(store Store, ctx BlockNode, n Node) Node {
 						}
 					}
 				} else if len(n.Values) != 0 && numNames != len(n.Values) {
-					panic("should not happen")
+					panic(fmt.Sprintf("assignment mismatch: %d variable(s) but %d value(s)", numNames, len(n.Values)))
 				} else { // general case
+					for _, v := range n.Values {
+						if cx, ok := v.(*CallExpr); ok {
+							tt, ok := evalStaticTypeOfRaw(store, last, cx).(*tupleType)
+							if ok && len(tt.Elts) != 1 {
+								panic(fmt.Sprintf("multiple-value %s (value of type %s) in single-value context", cx.Func.String(), tt.Elts))
+							}
+						}
+					}
 					// evaluate types and convert consts.
 					if n.Type != nil {
 						// only a single type can be specified.

gnovm/pkg/gnolang/type_check.go

@@ -215,6 +215,70 @@ func assertAssignableTo(xt, dt Type, autoNative bool) {
 	}
 }
 
+// checkValDefineMismatch checks for mismatch between the number of variables and values in a ValueDecl or AssignStmt.
+func checkValDefineMismatch(n Node) {
+	var (
+		valueDecl *ValueDecl
+		assign    *AssignStmt
+		values    []Expr
+		numNames  int
+		numValues int
+	)
+
+	switch x := n.(type) {
+	case *ValueDecl:
+		valueDecl = x
+		numNames = len(valueDecl.NameExprs)
+		numValues = len(valueDecl.Values)
+		values = valueDecl.Values
+	case *AssignStmt:
+		if x.Op != DEFINE {
+			return
+		}
+
+		assign = x
+		numNames = len(assign.Lhs)
+		numValues = len(assign.Rhs)
+		values = assign.Rhs
+	default:
+		panic(fmt.Sprintf("unexpected node type %T", n))
+	}
+
+	if numValues == 0 || numValues == numNames {
+		return
+	}
+
+	// Special case for single value.
+	// If the value is a call expression, type assertion, or index expression,
+	// it can be assigned to multiple variables.
+	if numValues == 1 {
+		switch values[0].(type) {
+		case *CallExpr:
+			return
+		case *TypeAssertExpr:
+			if numNames != 2 {
+				panic(fmt.Sprintf("assignment mismatch: %d variable(s) but %d value(s)", numNames, numValues))
+			}
+			return
+		case *IndexExpr:
+			if numNames != 2 {
+				panic(fmt.Sprintf("assignment mismatch: %d variable(s) but %d value(s)", numNames, numValues))
+			}
+			return
+		}
+	}
+
+	if valueDecl != nil {
+		if numNames > numValues {
+			panic(fmt.Sprintf("missing init expr for %s", valueDecl.NameExprs[numValues].String()))
+		}
+
+		panic(fmt.Sprintf("extra init expr %s", values[numNames].String()))
+	}
+
+	panic(fmt.Sprintf("assignment mismatch: %d variable(s) but %d value(s)", numNames, numValues))
+}
+
 // 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/assign24.gno

@@ -0,0 +1,8 @@
+package main
+
+func main() {
+	a, b := 1
+}
+
+// Error:
+// main/files/assign24.gno:4:2: assignment mismatch: 2 variable(s) but 1 value(s)

gnovm/tests/files/assign25.gno

@@ -0,0 +1,14 @@
+package main
+
+func foo() (int, bool) {
+	return 1, true
+}
+
+func main() {
+	a, b, c := 2, foo()
+
+	println(a, b, c)
+}
+
+// Error:
+// main/files/assign25.gno:8:2: assignment mismatch: 3 variable(s) but 2 value(s)

gnovm/tests/files/assign25b.gno

@@ -0,0 +1,14 @@
+package main
+
+func foo() (int, bool) {
+	return 1, true
+}
+
+func main() {
+	a, b, c := 2, 3, 4, foo()
+
+	println(a, b, c)
+}
+
+// Error:
+// main/files/assign25b.gno:8:2: assignment mismatch: 3 variable(s) but 4 value(s)

gnovm/tests/files/assign26.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	var i interface{} = 1
+	a, b, c := i.(int)
+}
+
+// Error:
+// main/files/assign26.gno:5:2: assignment mismatch: 3 variable(s) but 1 value(s)

gnovm/tests/files/assign27.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	s := []string{"1", "2"}
+	a, b, c := s[0]
+}
+
+// Error:
+// main/files/assign27.gno:5:2: assignment mismatch: 3 variable(s) but 1 value(s)

gnovm/tests/files/assign28.gno

@@ -0,0 +1,11 @@
+package main
+
+import "fmt"
+
+func main() {
+	a, c := 1, 2, 3
+	fmt.Println(a, c)
+}
+
+// Error:
+// main/files/assign28.gno:6:2: assignment mismatch: 2 variable(s) but 3 value(s)

gnovm/tests/files/var18.gno

@@ -1,8 +1,8 @@
 package main
 
 func main() {
-	a, b, c := 1, 2
+	var a, b, c = 1, 2
 }
 
 // Error:
-// main/files/var18.gno:4:2: assignment mismatch: 3 variables but 2 values
+// main/files/var18.gno:4:6: missing init expr for c<VPUverse(0)>

gnovm/tests/files/var19.gno

@@ -0,0 +1,11 @@
+package main
+
+func main() {
+	var a, b, c = 1, a+1
+	println(a)
+	println(b)
+	println(c)
+}
+
+// Error:
+// main/files/var19.gno:4:6: missing init expr for c<VPUverse(0)>

gnovm/tests/files/var20.gno

@@ -0,0 +1,12 @@
+package main
+
+func r() int {
+	return 1
+}
+
+func main() {
+	var a, b, c = r()
+}
+
+// Error:
+// main/files/var20.gno:8:6: assignment mismatch: 3 variable(s) but r<VPBlock(3,0)> returns 1 value(s)

gnovm/tests/files/var21.gno

@@ -0,0 +1,14 @@
+package main
+
+func foo() (int, bool) {
+	return 1, true
+}
+
+func main() {
+	var a, b = 2, foo()
+
+	println(a, b)
+}
+
+// Error:
+// main/files/var21.gno:8:6: multiple-value foo<VPBlock(3,0)> (value of type [int bool]) in single-value context

gnovm/tests/files/var22.gno

@@ -0,0 +1,14 @@
+package main
+
+func foo() (int, bool) {
+	return 1, true
+}
+
+func main() {
+	var a, b, c = 2, foo()
+
+	println(a, b, c)
+}
+
+// Error:
+// main/files/var22.gno:8:6: missing init expr for c<VPUverse(0)>

gnovm/tests/files/var22b.gno

@@ -0,0 +1,14 @@
+package main
+
+func foo() (int, bool) {
+	return 1, true
+}
+
+func main() {
+	var a, b, c = 2, 3, 4, foo()
+
+	println(a, b, c)
+}
+
+// Error:
+// main/files/var22b.gno:8:6: extra init expr foo<VPUverse(0)>()

gnovm/tests/files/var23.gno

@@ -0,0 +1,9 @@
+package main
+
+var a, b, c = 1, 2
+
+func main() {
+}
+
+// Error:
+// main/files/var23.gno:3:5: assignment mismatch: 3 variable(s) but 2 value(s)

gnovm/tests/files/var24.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	var i interface{} = 1
+	var a, b, c = i.(int)
+}
+
+// Error:
+// main/files/var24.gno:5:6: assignment mismatch: 3 variable(s) but 1 value(s)

gnovm/tests/files/var25.gno

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	s := []string{"1", "2"}
+	var a, b, c = s[0]
+}
+
+// Error:
+// main/files/var25.gno:5:6: assignment mismatch: 3 variable(s) but 1 value(s)

gnovm/tests/files/var26.gno

@@ -0,0 +1,11 @@
+package main
+
+import "fmt"
+
+func main() {
+	var a, c = 1, 2, 3
+	fmt.Println(a, c)
+}
+
+// Error:
+// main/files/var26.gno:6:6: extra init expr 3