Seal the BuilderExt trait

library/std/src/os/horizon/thread.rs

@@ -21,10 +21,15 @@
 
 #![unstable(feature = "horizon_thread_ext", issue = "none")]
 
+use crate::sealed::Sealed;
+
 /// Extensions on [`std::thread::Builder`] for the Nintendo 3DS.
 ///
+/// This trait is sealed: it cannot be implemented outside the standard library.
+/// This is so that future additional methods are not breaking changes.
+///
 /// [`std::thread::Builder`]: crate::thread::Builder
-pub trait BuilderExt: Sized {
+pub trait BuilderExt: Sized + Sealed {
     /// Sets the priority level for the new thread.
     ///
     /// Low values gives the thread higher priority. For userland apps, this has

library/std/src/thread/mod.rs

@@ -272,6 +272,10 @@ pub struct Builder {
     pub(crate) native_options: imp::BuilderOptions,
 }
 
+/// Allows extension traits within `std`.
+#[unstable(feature = "sealed", issue = "none")]
+impl crate::sealed::Sealed for Builder {}
+
 impl Builder {
     /// Generates the base configuration for spawning a thread, from which
     /// configuration methods can be chained.