rust: Add a Sealed trait

Some traits exposed by the kernel crate may not be intended to be
implemented by downstream modules. Add a Sealed trait to allow avoiding
this using the sealed trait pattern.

Signed-off-by: Asahi Lina <lina@asahilina.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

Author: hoshinolina

rust/kernel/lib.rs

@@ -93,6 +93,11 @@ pub use bindings;
 pub use macros;
 pub use uapi;
 
+pub(crate) mod private {
+    #[allow(unreachable_pub)]
+    pub trait Sealed {}
+}
+
 /// Prefix to appear before log messages printed from within the `kernel` crate.
 const __LOG_PREFIX: &[u8] = b"rust_kernel\0";