ExitStatusExt windows: Retrospectively seal this trait

Signed-off-by: NIan Jackson <ijackson@chiark.greenend.org.uk>

library/std/src/sys/windows/ext/process.rs

@@ -7,6 +7,14 @@
 use crate::sys;
 use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 
+mod private {
+    /// This trait being unreachable from outside the crate
+    /// prevents other implementations of the `ExitStatusExt` trait,
+    /// which allows potentially adding more trait methods in the future.
+    #[stable(feature = "none", since = "1.51.0")]
+    pub trait Sealed {}
+}
+
 #[stable(feature = "process_extensions", since = "1.2.0")]
 impl FromRawHandle for process::Stdio {
     unsafe fn from_raw_handle(handle: RawHandle) -> process::Stdio {
@@ -73,8 +81,11 @@ fn into_raw_handle(self) -> RawHandle {
 }
 
 /// Windows-specific extensions to [`process::ExitStatus`].
+///
+/// This trait is saeled (since Rust 1.51): it cannot be implemented outside the standard library.
+/// This is so that future additional methods are not breaking changes.
 #[stable(feature = "exit_status_from", since = "1.12.0")]
-pub trait ExitStatusExt {
+pub trait ExitStatusExt: private::Sealed {
     /// Creates a new `ExitStatus` from the raw underlying `u32` return value of
     /// a process.
     #[stable(feature = "exit_status_from", since = "1.12.0")]
@@ -88,6 +99,9 @@ fn from_raw(raw: u32) -> Self {
     }
 }
 
+#[stable(feature = "none", since = "1.51.0")]
+impl private::Sealed for process::ExitStatus {}
+
 /// Windows-specific extensions to the [`process::Command`] builder.
 #[stable(feature = "windows_process_extensions", since = "1.16.0")]
 pub trait CommandExt {