Messages in this thread Patch in this message |  | | Date | Sat, 31 Dec 2022 19:37:48 +0000 | From | Gary Guo <> | Subject | Re: [PATCH 3/7] rust: sync: allow coercion from `Arc<T>` to `Arc<U>` |
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On Wed, 28 Dec 2022 06:03:42 +0000 Wedson Almeida Filho <wedsonaf@gmail.com> wrote:
> 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: Gary Guo <gary@garyguo.net>
> --- > 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
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