[PATCH 6.12.y] rust: init: allow `dead_code` warnings for Rust >= 1.89.0

[Miguel Ojeda <ojeda@kernel.org>]

Starting with Rust 1.89.0 (expected 2025-08-07), the Rust compiler
may warn:

    error: trait `MustNotImplDrop` is never used
       --> rust/kernel/init/macros.rs:927:15
        |
    927 |         trait MustNotImplDrop {}
        |               ^^^^^^^^^^^^^^^
        |
       ::: rust/kernel/sync/arc.rs:133:1
        |
    133 | #[pin_data]
        | ----------- in this procedural macro expansion
        |
        = note: `-D dead-code` implied by `-D warnings`
        = help: to override `-D warnings` add `#[allow(dead_code)]`
        = note: this error originates in the macro `$crate::__pin_data`
                which comes from the expansion of the attribute macro
                `pin_data` (in Nightly builds, run with
                -Z macro-backtrace for more info)

Thus `allow` it to clean it up.

This does not happen in mainline nor 6.15.y, because there the macro was
moved out of the `kernel` crate, and `dead_code` warnings are not
emitted if the macro is foreign to the crate. Thus this patch is
directly sent to stable and intended for 6.12.y only.

Similarly, it is not needed in previous LTSs, because there the Rust
version is pinned.

Cc: Benno Lossin <lossin@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
Greg, Sasha: please note that an equivalent patch is _not_ in mainline.

We could put these `allow`s in mainline (they wouldn't hurt), but it
isn't a good idea to add things in mainline for the only reason of
backporting them, thus I am sending this directly to stable.

The patch is pretty safe -- there is no actual code change.

 rust/kernel/init/macros.rs | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/rust/kernel/init/macros.rs b/rust/kernel/init/macros.rs
index b7213962a6a5..e530028bb9ed 100644
--- a/rust/kernel/init/macros.rs
+++ b/rust/kernel/init/macros.rs
@@ -924,6 +924,7 @@ impl<'__pin, $($impl_generics)*> ::core::marker::Unpin for $name<$($ty_generics)
         // We prevent this by creating a trait that will be implemented for all types implementing
         // `Drop`. Additionally we will implement this trait for the struct leading to a conflict,
         // if it also implements `Drop`
+        #[allow(dead_code)]
         trait MustNotImplDrop {}
         #[expect(drop_bounds)]
         impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
@@ -932,6 +933,7 @@ impl<$($impl_generics)*> MustNotImplDrop for $name<$($ty_generics)*>
         // 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(dead_code)]
         #[expect(non_camel_case_types)]
         trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
         impl<T: $crate::init::PinnedDrop>

base-commit: fbad404f04d758c52bae79ca20d0e7fe5fef91d3
--
2.50.1