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: rust-lang/rust#54503 [2]
Link: rust-lang/rust#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>
(cherry picked from commit 1f9ed172545687e5c04c77490a45896be6d2e459)
[pinned_init: sync ui tests with new expanded code. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

src/__internal.rs

@@ -62,7 +62,7 @@ where
 pub unsafe trait HasPinData {
     type PinData: PinData;
 
-    #[allow(clippy::missing_safety_doc)]
+    #[expect(clippy::missing_safety_doc)]
     unsafe fn __pin_data() -> Self::PinData;
 }
 
@@ -92,7 +92,7 @@ pub unsafe trait PinData: Copy {
 pub unsafe trait HasInitData {
     type InitData: InitData;
 
-    #[allow(clippy::missing_safety_doc)]
+    #[expect(clippy::missing_safety_doc)]
     unsafe fn __init_data() -> Self::InitData;
 }
 

src/lib.rs

@@ -62,7 +62,7 @@
 //! 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)]
 //! # #![feature(allocator_api)]
 //! use pinned_init::*;
 //! # use core::pin::Pin;
@@ -85,7 +85,7 @@
 //! (or just the stack) to actually initialize a `Foo`:
 //!
 //! ```rust
-//! # #![allow(clippy::disallowed_names)]
+//! # #![expect(clippy::disallowed_names)]
 //! # #![feature(allocator_api)]
 //! # #[path = "../examples/mutex.rs"] mod mutex; use mutex::*;
 //! # use pinned_init::*;
@@ -277,7 +277,7 @@ pub use pinned_init_macro::{pin_data, pinned_drop, Zeroable};
 /// # Examples
 ///
 /// ```rust
-/// # #![allow(clippy::disallowed_names)]
+/// # #![expect(clippy::disallowed_names)]
 /// # #![feature(allocator_api)]
 /// # #[path = "../examples/mutex.rs"] mod mutex; use mutex::*;
 /// # use pinned_init::*;
@@ -329,7 +329,7 @@ macro_rules! stack_pin_init {
 /// # Examples
 ///
 /// ```rust
-/// # #![allow(clippy::disallowed_names)]
+/// # #![expect(clippy::disallowed_names)]
 /// # #![feature(allocator_api)]
 /// # #[path = "../examples/error.rs"] mod error; use error::Error;
 /// # #[path = "../examples/mutex.rs"] mod mutex; use mutex::*;
@@ -356,7 +356,7 @@ macro_rules! stack_pin_init {
 /// ```
 ///
 /// ```rust
-/// # #![allow(clippy::disallowed_names)]
+/// # #![expect(clippy::disallowed_names)]
 /// # #![feature(allocator_api)]
 /// # #[path = "../examples/error.rs"] mod error; use error::Error;
 /// # #[path = "../examples/mutex.rs"] mod mutex; use mutex::*;
@@ -479,7 +479,7 @@ 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 pinned_init::*;
 /// # use core::pin::Pin;
 /// # #[pin_data]
@@ -953,7 +953,7 @@ pub unsafe trait Init<T: ?Sized, E = Infallible>: PinInit<T, E> {
     /// # Examples
     ///
     /// ```rust
-    /// # #![allow(clippy::disallowed_names)]
+    /// # #![expect(clippy::disallowed_names)]
     /// # use pinned_init::*;
     /// struct Foo {
     ///     buf: [u8; 1_000_000],

src/macros.rs

@@ -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: ::pinned_init::PinnedDrop,
@@ -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: $crate::PinnedDrop>
             UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}
@@ -989,7 +989,7 @@ macro_rules! __pin_data {
         //
         // The functions are `unsafe` to prevent accidentally calling them.
         #[allow(dead_code)]
-        #[allow(clippy::missing_safety_doc)]
+        #[expect(clippy::missing_safety_doc)]
         impl<$($impl_generics)*> $pin_data<$($ty_generics)*>
         where $($whr)*
         {

tests/ui/expand/many_generics.expanded.rs

@@ -35,6 +35,7 @@ const _: () = {
         T: Bar<'a, 1>,
     {}
     #[allow(dead_code)]
+    #[expect(clippy::missing_safety_doc)]
     impl<
         'a,
         'b: 'a,

tests/ui/expand/pin-data.expanded.rs

@@ -15,6 +15,7 @@ const _: () = {
     }
     impl ::core::marker::Copy for __ThePinData {}
     #[allow(dead_code)]
+    #[expect(clippy::missing_safety_doc)]
     impl __ThePinData {
         unsafe fn _pin<E>(
             self,
@@ -54,10 +55,10 @@ const _: () = {
         __Unpin<'__pin>: ::core::marker::Unpin,
     {}
     trait MustNotImplDrop {}
-    #[allow(drop_bounds)]
+    #[expect(drop_bounds)]
     impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
     impl MustNotImplDrop for Foo {}
-    #[allow(non_camel_case_types)]
+    #[expect(non_camel_case_types)]
     trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
     impl<
         T: ::pinned_init::PinnedDrop,

tests/ui/expand/pinned_drop.expanded.rs

@@ -15,6 +15,7 @@ const _: () = {
     }
     impl ::core::marker::Copy for __ThePinData {}
     #[allow(dead_code)]
+    #[expect(clippy::missing_safety_doc)]
     impl __ThePinData {
         unsafe fn _pin<E>(
             self,