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 <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Andreas Hindborg <a.hindborg@samsung.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20230408122429.1103522-6-y86-dev@protonmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: BennoLossin

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`.