rust: types: add Opaque::raw_get

This function mirrors `UnsafeCell::raw_get`. It avoids creating a
reference and allows solely using raw pointers.
The `pin-init` API will be using this, since uninitialized memory
requires raw pointers.

Signed-off-by: Benno Lossin <[email protected]>
Reviewed-by: Gary Guo <[email protected]>
Reviewed-by: Andreas Hindborg <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: y86-dev

rust/kernel/types.rs

@@ -238,6 +238,14 @@ impl<T> Opaque<T> {
     pub fn get(&self) -> *mut T {
         UnsafeCell::raw_get(self.0.as_ptr())
     }
+
+    /// Gets the value behind `this`.
+    ///
+    /// 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>>())
+    }
 }
 
 /// A sum type that always holds either a value of type `L` or `R`.