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

From: Benno Lossin
Date: Fri Jun 30 2023 - 11:03:44 EST


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