linux-kernel - [PATCH v2 3/5] rust: enable `clippy::ptr_cast

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Message-Id: <20250309-ptr-as-ptr-v2-3-25d60ad922b7@gmail.com>
Date: Sun, 09 Mar 2025 12:00:42 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Masahiro Yamada <masahiroy@...nel.org>, 
 Nathan Chancellor <nathan@...nel.org>, Nicolas Schier <nicolas@...sle.eu>, 
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
 Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
 Benno Lossin <benno.lossin@...ton.me>, 
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 "Rafael J. Wysocki" <rafael@...nel.org>, 
 Brendan Higgins <brendan.higgins@...ux.dev>, 
 David Gow <davidgow@...gle.com>, Rae Moar <rmoar@...gle.com>, 
 Bjorn Helgaas <bhelgaas@...gle.com>, Luis Chamberlain <mcgrof@...nel.org>, 
 Russ Weight <russ.weight@...ux.dev>, Rob Herring <robh@...nel.org>, 
 Saravana Kannan <saravanak@...gle.com>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
 rust-for-linux@...r.kernel.org, linux-kselftest@...r.kernel.org, 
 kunit-dev@...glegroups.com, linux-pci@...r.kernel.org, 
 linux-block@...r.kernel.org, devicetree@...r.kernel.org, 
 Tamir Duberstein <tamird@...il.com>
Subject: [PATCH v2 3/5] rust: enable `clippy::ptr_cast_constness` lint

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]
Signed-off-by: Tamir Duberstein <tamird@...il.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