rust: start using the `#[expect(...)]` attribute

In Rust, it is possible to `allow` particular warnings (diagnostics,
lints) locally, making the compiler ignore instances of a given warning
within a given function, module, block, etc.

It is similar to `#pragma GCC diagnostic push` + `ignored` + `pop` in C:

    #pragma GCC diagnostic push
    #pragma GCC diagnostic ignored "-Wunused-function"
    static void f(void) {}
    #pragma GCC diagnostic pop

But way less verbose:

    #[allow(dead_code)]
    fn f() {}

By that virtue, it makes it possible to comfortably enable more
diagnostics by default (i.e. outside `W=` levels) that may have some
false positives but that are otherwise quite useful to keep enabled to
catch potential mistakes.

The `#[expect(...)]` attribute [1] takes this further, and makes the
compiler warn if the diagnostic was _not_ produced. For instance, the
following will ensure that, when `f()` is called somewhere, we will have
to remove the attribute:

    #[expect(dead_code)]
    fn f() {}

If we do not, we get a warning from the compiler:

    warning: this lint expectation is unfulfilled
     --> x.rs:3:10
      |
    3 | #[expect(dead_code)]
      |          ^^^^^^^^^
      |
      = note: `#[warn(unfulfilled_lint_expectations)]` on by default

This means that `expect`s do not get forgotten when they are not needed.

See the next commit for more details, nuances on its usage and
documentation on the feature.

The attribute requires the `lint_reasons` [2] unstable feature, but it
is becoming stable in 1.81.0 (to be released on 2024-09-05) and it has
already been useful to clean things up in this patch series, finding
cases where the `allow`s should not have been there.

Thus, enable `lint_reasons` and convert some of our `allow`s to `expect`s
where possible.

This feature was also an example of the ongoing collaboration between
Rust and the kernel -- we tested it in the kernel early on and found an
issue that was quickly resolved [3].

Cc: Fridtjof Stoldt <xfrednet@gmail.com>
Cc: Urgau <urgau@numericable.fr>
Link: https://rust-lang.github.io/rfcs/2383-lint-reasons.html#expect-lint-attribute [1]
Link: https://github.com/rust-lang/rust/issues/54503 [2]
Link: https://github.com/rust-lang/rust/issues/114557 [3]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Tested-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://lore.kernel.org/r/20240904204347.168520-18-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 11 insertions, 11 deletions

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 10ec90a5f5d8..25057cbed40b 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -35,7 +35,7 @@
 //! that you need to write `<-` instead of `:` for fields that you want to initialize in-place.
 //!
 //! ```rust
-//! # #![allow(clippy::disallowed_names)]
+//! # #![expect(clippy::disallowed_names)]
 //! use kernel::sync::{new_mutex, Mutex};
 //! # use core::pin::Pin;
 //! #[pin_data]
@@ -55,7 +55,7 @@
 //! (or just the stack) to actually initialize a `Foo`:
 //!
 //! ```rust
-//! # #![allow(clippy::disallowed_names)]
+//! # #![expect(clippy::disallowed_names)]
 //! # use kernel::sync::{new_mutex, Mutex};
 //! # use core::pin::Pin;
 //! # #[pin_data]
@@ -120,12 +120,12 @@
 //!   `slot` gets called.
 //!
 //! ```rust
-//! # #![allow(unreachable_pub, clippy::disallowed_names)]
+//! # #![expect(unreachable_pub, clippy::disallowed_names)]
 //! use kernel::{init, types::Opaque};
 //! use core::{ptr::addr_of_mut, marker::PhantomPinned, pin::Pin};
 //! # mod bindings {
-//! #     #![allow(non_camel_case_types)]
-//! #     #![allow(clippy::missing_safety_doc)]
+//! #     #![expect(non_camel_case_types)]
+//! #     #![expect(clippy::missing_safety_doc)]
 //! #     pub struct foo;
 //! #     pub unsafe fn init_foo(_ptr: *mut foo) {}
 //! #     pub unsafe fn destroy_foo(_ptr: *mut foo) {}
@@ -238,7 +238,7 @@ pub mod macros;
 /// # Examples
 ///
 /// ```rust
-/// # #![allow(clippy::disallowed_names)]
+/// # #![expect(clippy::disallowed_names)]
 /// # use kernel::{init, macros::pin_data, pin_init, stack_pin_init, init::*, sync::Mutex, new_mutex};
 /// # use core::pin::Pin;
 /// #[pin_data]
@@ -290,7 +290,7 @@ macro_rules! stack_pin_init {
 /// # Examples
 ///
 /// ```rust,ignore
-/// # #![allow(clippy::disallowed_names)]
+/// # #![expect(clippy::disallowed_names)]
 /// # use kernel::{init, pin_init, stack_try_pin_init, init::*, sync::Mutex, new_mutex};
 /// # use macros::pin_data;
 /// # use core::{alloc::AllocError, pin::Pin};
@@ -316,7 +316,7 @@ macro_rules! stack_pin_init {
 /// ```
 ///
 /// ```rust,ignore
-/// # #![allow(clippy::disallowed_names)]
+/// # #![expect(clippy::disallowed_names)]
 /// # use kernel::{init, pin_init, stack_try_pin_init, init::*, sync::Mutex, new_mutex};
 /// # use macros::pin_data;
 /// # use core::{alloc::AllocError, pin::Pin};
@@ -438,7 +438,7 @@ macro_rules! stack_try_pin_init {
 /// Users of `Foo` can now create it like this:
 ///
 /// ```rust
-/// # #![allow(clippy::disallowed_names)]
+/// # #![expect(clippy::disallowed_names)]
 /// # use kernel::{init, pin_init, macros::pin_data, init::*};
 /// # use core::pin::Pin;
 /// # #[pin_data]
@@ -852,7 +852,7 @@ pub unsafe trait PinInit<T: ?Sized, E = Infallible>: Sized {
     /// # Examples
     ///
     /// ```rust
-    /// # #![allow(clippy::disallowed_names)]
+    /// # #![expect(clippy::disallowed_names)]
     /// use kernel::{types::Opaque, init::pin_init_from_closure};
     /// #[repr(C)]
     /// struct RawFoo([u8; 16]);
@@ -964,7 +964,7 @@ pub unsafe trait Init<T: ?Sized, E = Infallible>: PinInit<T, E> {
     /// # Examples
     ///
     /// ```rust
-    /// # #![allow(clippy::disallowed_names)]
+    /// # #![expect(clippy::disallowed_names)]
     /// use kernel::{types::Opaque, init::{self, init_from_closure}};
     /// struct Foo {
     ///     buf: [u8; 1_000_000],