[PATCH v4 07/16] rust: drm: split Deref for Device context typestates

[Danilo Krummrich <dakr@kernel.org>]

Split the Deref implementation for drm::Device by context:

  - Device<T> (Normal) dereferences to T::Data.
  - Device<T, Registered> dereferences to Device<T> (Normal).

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/drm/device.rs | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/device.rs b/rust/kernel/drm/device.rs
index 6f3af46ff647..86a7fca1d33f 100644
--- a/rust/kernel/drm/device.rs
+++ b/rust/kernel/drm/device.rs
@@ -79,6 +79,9 @@ macro_rules! drm_legacy_fields {
 /// - [`Normal`]: The general-purpose, reference-counted context. A [`Device`] in this context may
 ///   or may not be registered with userspace.
 /// - [`Registered`]: The device has been registered with userspace at some point.
+///
+/// `Device<T, Registered>` dereferences to `Device<T>` ([`Normal`]), so any method available on a
+/// [`Normal`] device is also available on a [`Registered`] one.
 pub trait DeviceContext: Sealed + Send + Sync {}
 
 /// The general-purpose, reference-counted [`DeviceContext`].
@@ -320,7 +323,7 @@ pub(crate) unsafe fn assume_ctx<NewCtx: DeviceContext>(&self) -> &Device<T, NewC
     }
 }
 
-impl<T: drm::Driver, C: DeviceContext> Deref for Device<T, C> {
+impl<T: drm::Driver> Deref for Device<T> {
     type Target = T::Data;
 
     fn deref(&self) -> &Self::Target {
@@ -328,6 +331,17 @@ fn deref(&self) -> &Self::Target {
     }
 }
 
+impl<T: drm::Driver> Deref for Device<T, Registered> {
+    type Target = Device<T>;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The caller holds a `Device<T, Registered>`, which guarantees all invariants
+        // of the weaker `Normal` context.
+        unsafe { self.assume_ctx() }
+    }
+}
+
 // SAFETY: DRM device objects are always reference counted and the get/put functions
 // satisfy the requirements.
 unsafe impl<T: drm::Driver> AlwaysRefCounted for Device<T> {
-- 
2.54.0