rust: start using the unstable #[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 (currently in beta), 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 <[email protected]>
Cc: Urgau <[email protected]>
Link: https://rust-lang.github.io/rfcs/2383-lint-reasons.html#expect-lint-attribute [1]
Link: rust-lang/rust#54503 [2]
Link: rust-lang/rust#114557 [3]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Diff for: rust/kernel/error.rs

@@ -133,7 +133,7 @@ impl Error {
     }
 
     /// Returns the error encoded as a pointer.
-    #[allow(dead_code)]
+    #[expect(dead_code)]
     pub(crate) fn to_ptr<T>(self) -> *mut T {
         #[cfg_attr(target_pointer_width = "32", allow(clippy::useless_conversion))]
         // SAFETY: `self.0` is a valid error due to its invariant.

Diff for: 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],

Diff for: rust/kernel/init/macros.rs

@@ -162,7 +162,7 @@
 //!     //   not be trivial, e.g. the user has a `#[pin] PhantomPinned` field -- this is
 //!     //   unconditionally `!Unpin` and results in an error. The lifetime tricks the compiler
 //!     //   into accepting these bounds regardless.
-//!     #[allow(dead_code)]
+//!     #[expect(dead_code)]
 //!     struct __Unpin<'__pin, T> {
 //!         __phantom_pin: ::core::marker::PhantomData<fn(&'__pin ()) -> &'__pin ()>,
 //!         __phantom: ::core::marker::PhantomData<fn(Bar<T>) -> Bar<T>>,
@@ -182,13 +182,13 @@
 //!     // Normally `Drop` bounds do not have the correct semantics, but for this purpose they do
 //!     // (normally people want to know if a type has any kind of drop glue at all, here we want
 //!     // to know if it has any kind of custom drop glue, which is exactly what this bound does).
-//!     #[allow(drop_bounds)]
+//!     #[expect(drop_bounds)]
 //!     impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
 //!     impl<T> MustNotImplDrop for Bar<T> {}
 //!     // Here comes a convenience check, if one implemented `PinnedDrop`, but forgot to add it to
 //!     // `#[pin_data]`, then this will error with the same mechanic as above, this is not needed
 //!     // for safety, but a good sanity check, since no normal code calls `PinnedDrop::drop`.
-//!     #[allow(non_camel_case_types)]
+//!     #[expect(non_camel_case_types)]
 //!     trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
 //!     impl<
 //!         T: ::kernel::init::PinnedDrop,
@@ -275,7 +275,7 @@
 //!                     // Additionally we abuse `slot` to automatically infer the correct type
 //!                     // for the struct. This is also another check that every field is
 //!                     // accessible from this scope.
-//!                     #[allow(unreachable_code, clippy::diverging_sub_expression)]
+//!                     #[expect(unreachable_code, clippy::diverging_sub_expression)]
 //!                     let _ = || {
 //!                         unsafe {
 //!                             ::core::ptr::write(
@@ -373,7 +373,7 @@
 //!     unsafe impl ::kernel::init::__internal::PinData for __ThePinData {
 //!         type Datee = Foo;
 //!     }
-//!     #[allow(dead_code)]
+//!     #[expect(dead_code)]
 //!     struct __Unpin<'__pin> {
 //!         __phantom_pin: ::core::marker::PhantomData<fn(&'__pin ()) -> &'__pin ()>,
 //!         __phantom: ::core::marker::PhantomData<fn(Foo) -> Foo>,
@@ -471,7 +471,7 @@
 //!             };
 //!             ::core::mem::forget(__b_guard);
 //!             ::core::mem::forget(__a_guard);
-//!             #[allow(unreachable_code, clippy::diverging_sub_expression)]
+//!             #[expect(unreachable_code, clippy::diverging_sub_expression)]
 //!             let _ = || {
 //!                 unsafe {
 //!                     ::core::ptr::write(
@@ -925,14 +925,14 @@ macro_rules! __pin_data {
         // `Drop`. Additionally we will implement this trait for the struct leading to a conflict,
         // if it also implements `Drop`
         trait MustNotImplDrop {}
-        #[allow(drop_bounds)]
+        #[expect(drop_bounds)]
         impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
         impl<$($impl_generics)*> MustNotImplDrop for $name<$($ty_generics)*>
         where $($whr)* {}
         // We also take care to prevent users from writing a useless `PinnedDrop` implementation.
         // They might implement `PinnedDrop` correctly for the struct, but forget to give
         // `PinnedDrop` as the parameter to `#[pin_data]`.
-        #[allow(non_camel_case_types)]
+        #[expect(non_camel_case_types)]
         trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
         impl<T: $crate::init::PinnedDrop>
             UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}

Diff for: rust/kernel/ioctl.rs

@@ -4,7 +4,7 @@
 //!
 //! C header: [`include/asm-generic/ioctl.h`](srctree/include/asm-generic/ioctl.h)
 
-#![allow(non_snake_case)]
+#![expect(non_snake_case)]
 
 use crate::build_assert;
 

Diff for: rust/kernel/lib.rs

@@ -14,6 +14,7 @@
 #![no_std]
 #![feature(coerce_unsized)]
 #![feature(dispatch_from_dyn)]
+#![feature(lint_reasons)]
 #![feature(new_uninit)]
 #![feature(receiver_trait)]
 #![feature(unsize)]

Diff for: rust/kernel/print.rs

@@ -140,7 +140,7 @@ pub fn call_printk_cont(args: fmt::Arguments<'_>) {
 #[doc(hidden)]
 #[cfg(not(testlib))]
 #[macro_export]
-#[allow(clippy::crate_in_macro_def)]
+#[expect(clippy::crate_in_macro_def)]
 macro_rules! print_macro (
     // The non-continuation cases (most of them, e.g. `INFO`).
     ($format_string:path, false, $($arg:tt)+) => (

Diff for: rust/kernel/std_vendor.rs

@@ -14,7 +14,7 @@
 ///
 /// ```rust
 /// let a = 2;
-/// # #[allow(clippy::disallowed_macros)]
+/// # #[expect(clippy::disallowed_macros)]
 /// let b = dbg!(a * 2) + 1;
 /// //      ^-- prints: [src/main.rs:2] a * 2 = 4
 /// assert_eq!(b, 5);
@@ -52,7 +52,7 @@
 /// With a method call:
 ///
 /// ```rust
-/// # #[allow(clippy::disallowed_macros)]
+/// # #[expect(clippy::disallowed_macros)]
 /// fn foo(n: usize) {
 ///     if dbg!(n.checked_sub(4)).is_some() {
 ///         // ...
@@ -71,7 +71,7 @@
 /// Naive factorial implementation:
 ///
 /// ```rust
-/// # #[allow(clippy::disallowed_macros)]
+/// # #[expect(clippy::disallowed_macros)]
 /// # {
 /// fn factorial(n: u32) -> u32 {
 ///     if dbg!(n <= 1) {
@@ -118,7 +118,7 @@
 /// a tuple (and return it, too):
 ///
 /// ```
-/// # #![allow(clippy::disallowed_macros)]
+/// # #![expect(clippy::disallowed_macros)]
 /// assert_eq!(dbg!(1usize, 2u32), (1, 2));
 /// ```
 ///
@@ -127,7 +127,7 @@
 /// invocations. You can use a 1-tuple directly if you need one:
 ///
 /// ```
-/// # #[allow(clippy::disallowed_macros)]
+/// # #[expect(clippy::disallowed_macros)]
 /// # {
 /// assert_eq!(1, dbg!(1u32,)); // trailing comma ignored
 /// assert_eq!((1,), dbg!((1u32,))); // 1-tuple

Diff for: samples/rust/rust_print.rs

@@ -15,7 +15,7 @@ module! {
 
 struct RustPrint;
 
-#[allow(clippy::disallowed_macros)]
+#[expect(clippy::disallowed_macros)]
 fn arc_print() -> Result {
     use kernel::sync::*;
 

Diff for: scripts/Makefile.build

@@ -263,7 +263,7 @@ $(obj)/%.lst: $(obj)/%.c FORCE
 # Compile Rust sources (.rs)
 # ---------------------------------------------------------------------------
 
-rust_allowed_features := new_uninit
+rust_allowed_features := 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