pr. adding unstable attr w/ tracking issue

library/std/src/os/windows/fs.rs

@@ -477,7 +477,7 @@ pub trait MetadataExt {
     /// renames, attributes, etc.
     /// 
     /// This will return `None` if the `Metdata` instance was not created using the FILE_BASIC_INFO type,
-    // #[stable(feature = "metadata_ext", since = "1.1.0")] not sure what to do here
+    #[unstable(feature = "windows_change_time", issue = "121478")]
     fn change_time(&self) -> Option<u64>;
 }