[PATCH v2 28/28] rust: types: add `Opaque` type

[ojeda@kernel.org]

From: Wedson Almeida Filho <wedsonaf@gmail.com>

Add the `Opaque` type, which is meant to be used with FFI objects
that are never interpreted by Rust code, e.g.:

    struct Waiter {
        completion: Opaque<bindings::completion>,
        next: *mut Waiter,
    }

It has the advantage that the objects don't have to be
zero-initialised before calling their init functions, making
the code performance closer to C.

Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
[Reworded, adapted for upstream and applied latest changes]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 rust/kernel/types.rs | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 3b0c44769708..e84e51ec9716 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -2,6 +2,31 @@
 
 //! Kernel types.
 
+use core::{cell::UnsafeCell, mem::MaybeUninit};
+
+/// Stores an opaque value.
+///
+/// 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>>);
+
+impl<T> Opaque<T> {
+    /// Creates a new opaque value.
+    pub const fn new(value: T) -> Self {
+        Self(MaybeUninit::new(UnsafeCell::new(value)))
+    }
+
+    /// Creates an uninitialised value.
+    pub const fn uninit() -> Self {
+        Self(MaybeUninit::uninit())
+    }
+
+    /// Returns a raw pointer to the opaque data.
+    pub fn get(&self) -> *mut T {
+        UnsafeCell::raw_get(self.0.as_ptr())
+    }
+}
+
 /// A sum type that always holds either a value of type `L` or `R`.
 pub enum Either<L, R> {
     /// Constructs an instance of [`Either`] containing a value of type `L`.
-- 
2.38.1