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rust: enable arbitrary_self_types and remove Receiver
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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@hohum.me.uk>
Link: rust-lang/rfcs#3519 [1]
Link: rust-lang/rust#44874 [2]
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240915132734.1653004-1-gary@garyguo.net
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
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nbdd0121 authored and ojeda committed Oct 7, 2024
1 parent 283e628 commit ab48a65
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Showing 4 changed files with 2 additions and 11 deletions.
2 changes: 1 addition & 1 deletion rust/kernel/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -12,11 +12,11 @@
//! do so first instead of bypassing this crate.

#![no_std]
#![feature(arbitrary_self_types)]
#![feature(coerce_unsized)]
#![feature(dispatch_from_dyn)]
#![feature(lint_reasons)]
#![feature(new_uninit)]
#![feature(receiver_trait)]
#![feature(unsize)]

// Ensure conditional compilation based on the kernel configuration works;
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3 changes: 0 additions & 3 deletions rust/kernel/list/arc.rs
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Expand Up @@ -441,9 +441,6 @@ where
}
}

// 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>
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6 changes: 0 additions & 6 deletions rust/kernel/sync/arc.rs
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Expand Up @@ -171,9 +171,6 @@ impl<T: ?Sized> 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> {}
Expand Down Expand Up @@ -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>>
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2 changes: 1 addition & 1 deletion scripts/Makefile.build
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Expand Up @@ -248,7 +248,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
# Compile Rust sources (.rs)
# ---------------------------------------------------------------------------

rust_allowed_features := lint_reasons,new_uninit
rust_allowed_features := arbitrary_self_types,lint_reasons,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
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