[PATCH v2] rust: enable arbitrary_self_types and remove `Receiver`

[Gary Guo]

The term "receiver" means that a type can be used as the type of `self`,
and thus enables method call syntax `foo.bar()` instead of
`Foo::bar(foo)`. Stable Rust as of today (1.81) enables a limited
selection of types (primitives and types in std, e.g. `Box` and `Arc`)
to be used as receivers, while custom types cannot.

We want the kernel `Arc` type to have the same functionality as the Rust
std `Arc`, so we use the `Receiver` trait (gated behind `receiver_trait`
unstable feature) to gain the functionality.

The `arbitrary_self_types` RFC [1] (tracking issue [2]) is accepted and
it will allow all types that implement a new `Receiver` trait (different
from today's unstable trait) to be used as receivers. This trait will be
automatically implemented for all `Deref` types, which include our `Arc`
type, so we no longer have to opt-in to be used as receiver. To prepare
us for the change, remove the `Receiver` implementation and the
associated feature. To still allow `Arc` and others to be used as method
receivers, turn on `arbitrary_self_types` feature instead.

This feature gate is introduced in 1.23.0. It used to enable both
`Deref` types and raw pointer types to be used as receivers, but the
latter is now split into a different feature gate in Rust 1.83 nightly.
We do not need receivers on raw pointers so this change would not affect
us and usage of `arbitrary_self_types` feature would work for all Rust
versions that we support (>=1.78).

Cc: Adrian Taylor <ade@xxxxxxxxxxx>
Link: https://github.com/rust-lang/rfcs/pull/3519 [1]
Link: https://github.com/rust-lang/rust/issues/44874 [2]
Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
---
Changes in v2:
- Reword the commit message to explain term "receiver" and add
  background about the feature gate.
- Link to v1: https://lore.kernel.org/rust-for-linux/20240910120721.107877-1-gary@xxxxxxxxxxx/
---
 rust/kernel/lib.rs      | 2 +-
 rust/kernel/list/arc.rs | 3 ---
 rust/kernel/sync/arc.rs | 6 ------
 scripts/Makefile.build  | 2 +-
 4 files changed, 2 insertions(+), 11 deletions(-)

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index f10b06a78b9d5..35a65a085bce5 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -12,10 +12,10 @@
 //! do so first instead of bypassing this crate.
 
 #![no_std]
+#![feature(arbitrary_self_types)]
 #![feature(coerce_unsized)]
 #![feature(dispatch_from_dyn)]
 #![feature(new_uninit)]
-#![feature(receiver_trait)]
 #![feature(unsize)]
 
 // Ensure conditional compilation based on the kernel configuration works;
diff --git a/rust/kernel/list/arc.rs b/rust/kernel/list/arc.rs
index d801b9dc6291d..3483d8c232c4f 100644
--- a/rust/kernel/list/arc.rs
+++ b/rust/kernel/list/arc.rs
@@ -441,9 +441,6 @@ fn as_ref(&self) -> &Arc<T> {
     }
 }
 
-// This is to allow [`ListArc`] (and variants) to be used as the type of `self`.
-impl<T, const ID: u64> core::ops::Receiver for ListArc<T, ID> where T: ListArcSafe<ID> + ?Sized {}
-
 // This is to allow coercion from `ListArc<T>` to `ListArc<U>` if `T` can be converted to the
 // dynamically-sized type (DST) `U`.
 impl<T, U, const ID: u64> core::ops::CoerceUnsized<ListArc<U, ID>> for ListArc<T, ID>
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
index 3021f30fd822f..28743a7c74a84 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -171,9 +171,6 @@ unsafe fn container_of(ptr: *const T) -> NonNull<ArcInner<T>> {
     }
 }
 
-// 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> {}
@@ -480,9 +477,6 @@ pub struct ArcBorrow<'a, T: ?Sized + 'a> {
     _p: PhantomData<&'a ()>,
 }
 
-// This is to allow [`ArcBorrow`] (and variants) to be used as the type of `self`.
-impl<T: ?Sized> core::ops::Receiver for ArcBorrow<'_, T> {}
-
 // This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
 // `ArcBorrow<U>`.
 impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 72b1232b1f7d9..56b8c72687255 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -263,7 +263,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := new_uninit
+rust_allowed_features := arbitrary_self_types,new_uninit
 
 # `--out-dir` is required to avoid temporaries being created by `rustc` in the
 # current working directory, which may be not accessible in the out-of-tree

base-commit: 93dc3be19450447a3a7090bd1dfb9f3daac3e8d2
-- 
2.44.1