[PATCH] rust: types: make `Opaque` be `!Unpin`

[Benno Lossin]

Adds a `PhantomPinned` field to `Opaque<T>`. This removes the last Rust
guarantee: the assumption that the type `T` can be freely moved. This is
not the case for many types from the C side (e.g. if they contain a
`struct list_head`). This change removes the need to add a
`PhantomPinned` field manually to Rust structs that contain C structs
which must not be moved.

Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>
---
This patch depends on the patch that swaps `UnsafeCell` with
`MaybeUninit` inside `Opaque` [1].

[1]: https://lore.kernel.org/rust-for-linux/20230614115328.2825961-1-aliceryhl@xxxxxxxxxx/
---
 rust/kernel/types.rs | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index fb41635f1e1f..e664a2beef30 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -6,7 +6,7 @@
 use alloc::boxed::Box;
 use core::{
     cell::UnsafeCell,
-    marker::PhantomData,
+    marker::{PhantomData, PhantomPinned},
     mem::MaybeUninit,
     ops::{Deref, DerefMut},
     ptr::NonNull,
@@ -224,17 +224,26 @@ fn drop(&mut self) {
 ///
 /// This is meant to be used with FFI objects that are never interpreted by Rust code.
 #[repr(transparent)]
-pub struct Opaque<T>(UnsafeCell<MaybeUninit<T>>);
+pub struct Opaque<T> {
+    value: UnsafeCell<MaybeUninit<T>>,
+    _pin: PhantomPinned,
+}
 
 impl<T> Opaque<T> {
     /// Creates a new opaque value.
     pub const fn new(value: T) -> Self {
-        Self(UnsafeCell::new(MaybeUninit::new(value)))
+        Self {
+            value: UnsafeCell::new(MaybeUninit::new(value)),
+            _pin: PhantomPinned,
+        }
     }
 
     /// Creates an uninitialised value.
     pub const fn uninit() -> Self {
-        Self(UnsafeCell::new(MaybeUninit::uninit()))
+        Self {
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+            _pin: PhantomPinned,
+        }
     }
 
     /// Creates a pin-initializer from the given initializer closure.
@@ -258,7 +267,7 @@ pub fn ffi_init(init_func: impl FnOnce(*mut T)) -> impl PinInit<Self> {
 
     /// Returns a raw pointer to the opaque data.
     pub fn get(&self) -> *mut T {
-        UnsafeCell::get(&self.0).cast::<T>()
+        UnsafeCell::get(&self.value).cast::<T>()
     }
 
     /// Gets the value behind `this`.
-- 
2.41.0