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authorWedson Almeida Filho <wedsonaf@gmail.com>2022-12-28 06:03:42 +0000
committerMiguel Ojeda <ojeda@kernel.org>2023-01-16 22:20:24 +0100
commitf75cb6fce4c91847d3b7cf2c5fc7c8eb4bc2d8f0 (patch)
treea10985c0522079bea9c407d05b00b4a2de3c3a15 /rust
parent53528772fb5a174c8606cdf0047ac4899ce05be7 (diff)
rust: sync: allow coercion from `Arc<T>` to `Arc<U>`
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@garyguo.net> Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> Reviewed-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Gary Guo <gary@garyguo.net> Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com> Acked-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Diffstat (limited to 'rust')
-rw-r--r--rust/kernel/lib.rs2
-rw-r--r--rust/kernel/sync/arc.rs27
2 files changed, 28 insertions, 1 deletions
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