ExitStatusExt unix: Retrospectively seal this trait

As discussed in rust-lang#79982.

I think the "new interfaces", ie the new trait and impl, must be
insta-stable.  This seems OK because we are, in fact, adding a new
restriction to the stable API.

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

Author: ijackson

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

@@ -9,6 +9,14 @@ use crate::process;
 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 {}
+}
+
 /// Unix-specific extensions to the [`process::Command`] builder.
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait CommandExt {
@@ -163,8 +171,11 @@ impl CommandExt for process::Command {
 }
 
 /// Unix-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 = "rust1", since = "1.0.0")]
-pub trait ExitStatusExt {
+pub trait ExitStatusExt: private::Sealed {
     /// Creates a new `ExitStatus` from the raw underlying `i32` return value of
     /// a process.
     #[stable(feature = "exit_status_from", since = "1.12.0")]
@@ -199,6 +210,9 @@ pub trait ExitStatusExt {
     fn into_raw(self) -> i32;
 }
 
+#[stable(feature = "none", since = "1.51.0")]
+impl private::Sealed for process::ExitStatus {}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ExitStatusExt for process::ExitStatus {
     fn from_raw(raw: i32) -> Self {