checker: disallow non ptr struct values to voidptr fields (#16958)

Author: Delta456

vlib/v/checker/struct.v

@@ -467,6 +467,10 @@ fn (mut c Checker) struct_init(mut node ast.StructInit) ast.Type {
 					expr_type = c.check_expr_opt_call(field.expr, expr_type)
 				}
 				expr_type_sym := c.table.sym(expr_type)
+				if field_type_sym.kind == .voidptr && expr_type_sym.kind == .struct_
+					&& !expr_type.is_ptr() {
+					c.error('allocate on the heap for use in other functions', field.pos)
+				}
 				if field_type_sym.kind == .interface_ {
 					if c.type_implements(expr_type, field_info.typ, field.pos) {
 						if !c.inside_unsafe && expr_type_sym.kind != .interface_

vlib/v/checker/tests/struct_voidptr_field_no_ptr_struct_value_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_voidptr_field_no_ptr_struct_value_err.vv:11:3: error: allocate on the heap for use in other functions
+    9 | fn main() {
+   10 |     _ := Foo2{
+   11 |         a: Foo{}
+      |         ~~~~~~~~
+   12 |     }
+   13 | }

vlib/v/checker/tests/struct_voidptr_field_no_ptr_struct_value_err.vv

@@ -0,0 +1,13 @@
+struct Foo {
+	num int
+}
+
+struct Foo2 {
+	a voidptr
+}
+
+fn main() {
+	_ := Foo2{
+		a: Foo{}
+	}
+}