checker: disallow string to voidptr cast entirely (#20351)

Author: Delta456

vlib/builtin/js/builtin.v

@@ -37,7 +37,7 @@ pub fn (err IError) str() string {
 		else {
 			// >> Hack to allow old style custom error implementations
 			// TODO: remove once deprecation period for `IError` methods has ended
-			old_error_style := unsafe { voidptr(&err.msg) != voidptr(&err.code) } // if fields are not defined (new style) they don't have an offset between them
+			old_error_style := unsafe { voidptr(&err.msg.str) != voidptr(&err.code.str) } // if fields are not defined (new style) they don't have an offset between them
 			if old_error_style {
 				'${err.type_name()}: ${err.msg}'
 			} else {

vlib/v/checker/checker.v

@@ -3180,6 +3180,9 @@ fn (mut c Checker) cast_expr(mut node ast.CastExpr) ast.Type {
 		snexpr := node.expr.str()
 		tt := c.table.type_to_str(to_type)
 		c.error('cannot cast string to `${tt}`, use `${snexpr}[index]` instead.', node.pos)
+	} else if final_from_sym.kind == .string && to_type.is_voidptr()
+		&& !node.expr_type.has_flag(.generic) && !from_type.is_ptr() {
+		c.error('cannot cast string to `voidptr`, use voidptr(s.str) instead', node.pos)
 	} else if final_from_sym.kind == .string && to_type.is_pointer() && !c.inside_unsafe {
 		tt := c.table.type_to_str(to_type)
 		c.error('cannot cast string to `${tt}` outside `unsafe`, use ${tt}(s.str) instead',

vlib/v/checker/tests/invalid_string_cast_to_pointers_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/invalid_string_cast_to_pointers_err.vv:2:9: error: cannot cast string to `voidptr` outside `unsafe`, use voidptr(s.str) instead
+vlib/v/checker/tests/invalid_string_cast_to_pointers_err.vv:2:9: error: cannot cast string to `voidptr`, use voidptr(s.str) instead
     1 | test := 'test'
     2 | println(voidptr(test))
       |         ~~~~~~~~~~~~~