checker: check struct init with pointer field (fix #18485) (#18501)

vlib/v/checker/struct.v

@@ -544,6 +544,12 @@ fn (mut c Checker) struct_init(mut node ast.StructInit, is_field_zero_struct_ini
 						&& field.expr.str() != '0' && !exp_type.has_flag(.option) {
 						c.error('reference field must be initialized with reference',
 							field.pos)
+					} else if exp_type.is_pointer() && !got_type.is_any_kind_of_pointer()
+						&& !got_type.is_int() {
+						got_typ_str := c.table.type_to_str(got_type)
+						exp_typ_str := c.table.type_to_str(exp_type)
+						c.error('cannot assign to field `${field_info.name}`: expected a pointer `${exp_typ_str}`, but got `${got_typ_str}`',
+							field.pos)
 					}
 				}
 				node.fields[i].typ = got_type

vlib/v/checker/tests/struct_voidptr_field_init_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/struct_voidptr_field_init_err.vv:7:3: error: cannot assign to field `example`: expected a pointer `voidptr`, but got `string`
+    5 | fn main() {
+    6 |     println(Example{
+    7 |         example: get()
+      |         ~~~~~~~~~~~~~~
+    8 |     })
+    9 | }

vlib/v/checker/tests/struct_voidptr_field_init_err.vv

@@ -0,0 +1,13 @@
+struct Example {
+	example voidptr
+}
+
+fn main() {
+	println(Example{
+		example: get()
+	})
+}
+
+fn get() string {
+	return 'hello'
+}