Re: [PATCH v9 2/6] rust: enable `clippy::ptr_cast_constness` lint

[Tamir Duberstein]

On Thu, Apr 17, 2025 at 1:12 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> On Wed, Apr 16, 2025 at 01:36:06PM -0400, Tamir Duberstein wrote:
> > 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
> >   `core::ptr::from_ref(self).cast_mut()`.
> >
> > 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@xxxxxxxxx>
> > Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>
> > ---
> >  Makefile                        | 1 +
> >  rust/kernel/block/mq/request.rs | 4 ++--
> >  rust/kernel/dma.rs              | 2 +-
> >  3 files changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/Makefile b/Makefile
> > index 5d2931344490..7b85b2a8d371 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -481,6 +481,7 @@ 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 4a5b7ec914ef..af5c9ac94f36 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.
> > @@ -155,7 +155,7 @@ pub(crate) fn wrapper_ref(&self) -> &RequestDataWrapper {
> >          // 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(core::ptr::from_ref(self).cast_mut()).as_ref() }
> >      }
> >  }
> >
> > diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> > index f395d1a6fe48..43ecf3c2e860 100644
> > --- a/rust/kernel/dma.rs
> > +++ b/rust/kernel/dma.rs
> > @@ -186,7 +186,7 @@ pub fn alloc_attrs(
> >              dev: dev.into(),
> >              dma_handle,
> >              count,
> > -            cpu_addr: ret.cast(),
> > +            cpu_addr: ret.cast::<T>(),
>
> Is this change necessary? The rest looks good to me.

Ah, it belongs in the preceding patch. I'll fix that on the next spin.