Re: [PATCH v8 6/6] rust: enable `clippy::ref_as_ptr` lint

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Apr 15, 2025 at 04:10:01PM -0400, Tamir Duberstein wrote:
> On Tue, Apr 15, 2025 at 2:18 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> >
> > On Tue, Apr 15, 2025 at 01:58:41PM -0400, Tamir Duberstein wrote:
> > > Hi Boqun, thanks for having a look!
> > >
> > > On Tue, Apr 15, 2025 at 1:37 PM Boqun Feng <boqun.feng@gmail.com> wrote:
> > > >
> > > > On Wed, Apr 09, 2025 at 10:47:23AM -0400, Tamir Duberstein wrote:
> > > > > In Rust 1.78.0, Clippy introduced the `ref_as_ptr` lint [1]:
> > > > >
> > > > > > Using `as` casts may result in silently changing mutability or type.
> > > > >
> > > > > While this doesn't eliminate unchecked `as` conversions, it makes such
> > > > > conversions easier to scrutinize.  It also has the slight benefit of
> > > > > removing a degree of freedom on which to bikeshed. Thus apply the
> > > > > changes and enable the lint -- no functional change intended.
> > > > >
> > > > > Link: https://rust-lang.github.io/rust-clippy/master/index.html#ref_as_ptr [1]
> > > > > Suggested-by: Benno Lossin <benno.lossin@proton.me>
> > > > > Link: https://lore.kernel.org/all/D8PGG7NTWB6U.3SS3A5LN4XWMN@proton.me/
> > > > > Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> > > > > ---
> > > > >  Makefile                 |  1 +
> > > > >  rust/bindings/lib.rs     |  1 +
> > > > >  rust/kernel/device_id.rs |  3 ++-
> > > > >  rust/kernel/fs/file.rs   |  3 ++-
> > > > >  rust/kernel/str.rs       |  6 ++++--
> > > > >  rust/kernel/uaccess.rs   | 10 ++++------
> > > > >  rust/uapi/lib.rs         |  1 +
> > > > >  7 files changed, 15 insertions(+), 10 deletions(-)
> > > > >
> > > > > diff --git a/Makefile b/Makefile
> > > > > index eb5a942241a2..2a16e02f26db 100644
> > > > > --- a/Makefile
> > > > > +++ b/Makefile
> > > > > @@ -485,6 +485,7 @@ export rust_common_flags := --edition=2021 \
> > > > >                           -Wclippy::no_mangle_with_rust_abi \
> > > > >                           -Wclippy::ptr_as_ptr \
> > > > >                           -Wclippy::ptr_cast_constness \
> > > > > +                         -Wclippy::ref_as_ptr \
> > > > >                           -Wclippy::undocumented_unsafe_blocks \
> > > > >                           -Wclippy::unnecessary_safety_comment \
> > > > >                           -Wclippy::unnecessary_safety_doc \
> > > > > diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs
> > > > > index b105a0d899cc..2b69016070c6 100644
> > > > > --- a/rust/bindings/lib.rs
> > > > > +++ b/rust/bindings/lib.rs
> > > > > @@ -27,6 +27,7 @@
> > > > >  #[allow(dead_code)]
> > > > >  #[allow(clippy::cast_lossless)]
> > > > >  #[allow(clippy::ptr_as_ptr)]
> > > > > +#[allow(clippy::ref_as_ptr)]
> > > > >  #[allow(clippy::undocumented_unsafe_blocks)]
> > > > >  mod bindings_raw {
> > > > >      // Manual definition for blocklisted types.
> > > > > diff --git a/rust/kernel/device_id.rs b/rust/kernel/device_id.rs
> > > > > index 4063f09d76d9..37cc03d1df4c 100644
> > > > > --- a/rust/kernel/device_id.rs
> > > > > +++ b/rust/kernel/device_id.rs
> > > > > @@ -136,7 +136,8 @@ impl<T: RawDeviceId, U, const N: usize> IdTable<T, U> for IdArray<T, U, N> {
> > > > >      fn as_ptr(&self) -> *const T::RawType {
> > > > >          // This cannot be `self.ids.as_ptr()`, as the return pointer must have correct provenance
> > > > >          // to access the sentinel.
> > > > > -        (self as *const Self).cast()
> > > > > +        let this: *const Self = self;
> > > >
> > > > Hmm.. so this lint usually just requires to use a let statement instead
> > > > of as expression when casting a reference to a pointer? Not 100%
> > > > convinced this results into better code TBH..
> > >
> > > The rationale is in the lint description and quoted in the commit
> > > message: "Using `as` casts may result in silently changing mutability
> > > or type.".
> > >
> >
> > Could you show me how you can silently change the mutability or type? A
> > simple try like below doesn't compile:
> >
> >         let x = &42;
> >         let ptr = x as *mut i32; // <- error
> >         let another_ptr = x as *const i64; // <- error
> 
> I think the point is that the meaning of an `as` cast can change when
> the type of `x` changes, which can happen at a distance. The example

So my example shows that you can only use `as` to convert a `&T` into a
`*const T`, no matter how far it happens, and..

> shown in the clippy docs uses `as _`, which is where you get into real
> trouble.
> 

... no matter whether `as _` is used or not. Of course once you have a
`*const T`, using `as` can change it to a different type or mutability,
but that's a different problem. Your argument still lacks convincing
evidences or examples showing this is a real trouble. For example, if
you have a `x` of type `&i32`, and do a `x as _` somewhere, you will
have a compiler error once compilers infers a type that is not `*const
i32` for `_`. If your argument being it's better do the
reference-to-pointer conversion explicitly, then that makes some sense,
but I still don't think we need to do it globablly.

> > also from the link document you shared, looks like the suggestion is to
> > use core::ptr::from_{ref,mut}(), was this ever considered?
> 
> I considered it, but I thought it was ugly. We don't have a linter to
> enforce it, so I'd be surprised if people reached for it.
> 

I think avoiding the extra line of `let` is a win, also I don't get why
you feel it's *ugly*: having the extra `let` line is ugly to me ;-)

Regards,
Boqun

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