rust: make `UnsafeCell` the outer type in `Opaque`

When combining `UnsafeCell` with `MaybeUninit`, it is idiomatic to use
`UnsafeCell` as the outer type. Intuitively, this is because a
`MaybeUninit<T>` might not contain a `T`, but we always want the effect
of the `UnsafeCell`, even if the inner value is uninitialized.

Now, strictly speaking, this doesn't really make a difference. The
compiler will always apply the `UnsafeCell` effect even if the inner
value is uninitialized. But I think we should follow the convention
here.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Link: https://lore.kernel.org/r/20230614115328.2825961-1-aliceryhl@google.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/types.rs

@@ -206,17 +206,17 @@ impl<T, F: FnOnce(T)> Drop for ScopeGuard<T, F> {
 ///
 /// This is meant to be used with FFI objects that are never interpreted by Rust code.
 #[repr(transparent)]
-pub struct Opaque<T>(MaybeUninit<UnsafeCell<T>>);
+pub struct Opaque<T>(UnsafeCell<MaybeUninit<T>>);
 
 impl<T> Opaque<T> {
     /// Creates a new opaque value.
     pub const fn new(value: T) -> Self {
-        Self(MaybeUninit::new(UnsafeCell::new(value)))
+        Self(UnsafeCell::new(MaybeUninit::new(value)))
     }
 
     /// Creates an uninitialised value.
     pub const fn uninit() -> Self {
-        Self(MaybeUninit::uninit())
+        Self(UnsafeCell::new(MaybeUninit::uninit()))
     }
 
     /// Creates a pin-initializer from the given initializer closure.
@@ -240,7 +240,7 @@ impl<T> Opaque<T> {
 
     /// Returns a raw pointer to the opaque data.
     pub fn get(&self) -> *mut T {
-        UnsafeCell::raw_get(self.0.as_ptr())
+        UnsafeCell::get(&self.0).cast::<T>()
     }
 
     /// Gets the value behind `this`.
@@ -248,7 +248,7 @@ impl<T> Opaque<T> {
     /// This function is useful to get access to the value without creating intermediate
     /// references.
     pub const fn raw_get(this: *const Self) -> *mut T {
-        UnsafeCell::raw_get(this.cast::<UnsafeCell<T>>())
+        UnsafeCell::raw_get(this.cast::<UnsafeCell<MaybeUninit<T>>>()).cast::<T>()
     }
 }