[PATCH v4 3/6] rust: enable `clippy::ptr_cast_constness` lint

[Tamir Duberstein <tamird@gmail.com>]

In Rust 1.72.0, Clippy introduced the `ptr_cast_constness` lint [1]:

> Though `as` casts between raw pointers are not terrible,
> `pointer::cast_mut` and `pointer::cast_const` are safer because they
> cannot accidentally cast the pointer to another type.

There are only 2 affected sites:
- `*mut T as *const U as *mut U` becomes `(*mut T).cast()`
- `&self as *const Self as *mut Self` becomes a reference-to-pointer
  coercion + `(*const Self).cast()`.

Apply these changes and enable the lint -- no functional change
intended.

Link: https://rust-lang.github.io/rust-clippy/master/index.html#ptr_cast_constness [1]
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
 Makefile                        | 1 +
 rust/kernel/block/mq/request.rs | 5 +++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index ec8efc8e23ba..c62bae2b107b 100644
--- a/Makefile
+++ b/Makefile
@@ -484,6 +484,7 @@ export rust_common_flags := --edition=2021 \
 			    -Aclippy::needless_lifetimes \
 			    -Wclippy::no_mangle_with_rust_abi \
 			    -Wclippy::ptr_as_ptr \
+			    -Wclippy::ptr_cast_constness \
 			    -Wclippy::undocumented_unsafe_blocks \
 			    -Wclippy::unnecessary_safety_comment \
 			    -Wclippy::unnecessary_safety_doc \
diff --git a/rust/kernel/block/mq/request.rs b/rust/kernel/block/mq/request.rs
index 7943f43b9575..10c6d69be7f3 100644
--- a/rust/kernel/block/mq/request.rs
+++ b/rust/kernel/block/mq/request.rs
@@ -69,7 +69,7 @@ pub(crate) unsafe fn aref_from_raw(ptr: *mut bindings::request) -> ARef<Self> {
         // INVARIANT: By the safety requirements of this function, invariants are upheld.
         // SAFETY: By the safety requirement of this function, we own a
         // reference count that we can pass to `ARef`.
-        unsafe { ARef::from_raw(NonNull::new_unchecked(ptr as *const Self as *mut Self)) }
+        unsafe { ARef::from_raw(NonNull::new_unchecked(ptr.cast())) }
     }
 
     /// Notify the block layer that a request is going to be processed now.
@@ -151,11 +151,12 @@ pub(crate) unsafe fn wrapper_ptr(this: *mut Self) -> NonNull<RequestDataWrapper>
     /// Return a reference to the [`RequestDataWrapper`] stored in the private
     /// area of the request structure.
     pub(crate) fn wrapper_ref(&self) -> &RequestDataWrapper {
+        let this: *const _ = self;
         // SAFETY: By type invariant, `self.0` is a valid allocation. Further,
         // the private data associated with this request is initialized and
         // valid. The existence of `&self` guarantees that the private data is
         // valid as a shared reference.
-        unsafe { Self::wrapper_ptr(self as *const Self as *mut Self).as_ref() }
+        unsafe { Self::wrapper_ptr(this.cast_mut()).as_ref() }
     }
 }
 

-- 
2.48.1