[PATCH 3/7] rust: sync: allow coercion from `Arc<T>` to `Arc<U>`

From: Wedson Almeida Filho
Date: Wed Dec 28 2022 - 01:05:00 EST


The coercion is only allowed if `U` is a compatible dynamically-sized
type (DST). For example, if we have some type `X` that implements trait
`Y`, then this allows `Arc<X>` to be coerced into `Arc<dyn Y>`.

Suggested-by: Gary Guo <gary@xxxxxxxxxxx>
Signed-off-by: Wedson Almeida Filho <wedsonaf@xxxxxxxxx>
---
rust/kernel/lib.rs | 2 ++
rust/kernel/sync/arc.rs | 27 ++++++++++++++++++++++++++-
2 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 1a10f7c0ddd9..4bde65e7b06b 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -13,8 +13,10 @@

#![no_std]
#![feature(allocator_api)]
+#![feature(coerce_unsized)]
#![feature(core_ffi_c)]
#![feature(receiver_trait)]
+#![feature(unsize)]

// Ensure conditional compilation based on the kernel configuration works;
// otherwise we may silently break things like initcall handling.
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index e2eb0e67d483..dbc7596cc3ce 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -17,7 +17,11 @@

use crate::{bindings, error::Result, types::Opaque};
use alloc::boxed::Box;
-use core::{marker::PhantomData, ops::Deref, ptr::NonNull};
+use core::{
+ marker::{PhantomData, Unsize},
+ ops::Deref,
+ ptr::NonNull,
+};

/// A reference-counted pointer to an instance of `T`.
///
@@ -82,6 +86,23 @@ use core::{marker::PhantomData, ops::Deref, ptr::NonNull};
/// obj.use_reference();
/// obj.take_over();
/// ```
+///
+/// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
+///
+/// ```
+/// use kernel::sync::Arc;
+///
+/// trait MyTrait {}
+///
+/// struct Example;
+/// impl MyTrait for Example {}
+///
+/// // `obj` has type `Arc<Example>`.
+/// let obj: Arc<Example> = Arc::try_new(Example)?;
+///
+/// // `coerced` has type `Arc<dyn MyTrait>`.
+/// let coerced: Arc<dyn MyTrait> = obj;
+/// ```
pub struct Arc<T: ?Sized> {
ptr: NonNull<ArcInner<T>>,
_p: PhantomData<ArcInner<T>>,
@@ -96,6 +117,10 @@ struct ArcInner<T: ?Sized> {
// This is to allow [`Arc`] (and variants) to be used as the type of `self`.
impl<T: ?Sized> core::ops::Receiver for Arc<T> {}

+// This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be converted to the
+// dynamically-sized type (DST) `U`.
+impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> for Arc<T> {}
+
// SAFETY: It is safe to send `Arc<T>` to another thread when the underlying `T` is `Sync` because
// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
// `T` to be `Send` because any thread that has an `Arc<T>` may ultimately access `T` directly, for
--
2.34.1