add regression test for OpenOptionsExt downstream compat

tests/ui/std/open-options-ext-compat.rs

@@ -0,0 +1,20 @@
+//@ only-windows
+//@ check-pass
+
+// Regression test for https://github.com/rust-lang/rust/issues/153486
+// Ensures that `OpenOptionsExt` remains implementable by downstream crates
+// without requiring changes when new methods are added to the standard library.
+
+use std::os::windows::fs::OpenOptionsExt;
+
+struct MockOptions;
+
+impl OpenOptionsExt for MockOptions {
+    fn access_mode(&mut self, _: u32) -> &mut Self { self }
+    fn share_mode(&mut self, _: u32) -> &mut Self { self }
+    fn custom_flags(&mut self, _: u32) -> &mut Self { self }
+    fn attributes(&mut self, _: u32) -> &mut Self { self }
+    fn security_qos_flags(&mut self, _: u32) -> &mut Self { self }
+}
+
+fn main() {}