Re: [PATCH 1/5] rust: ptr: add panicking index projection variant

["Alexandre Courbot" <acourbot@nvidia.com>]

On Thu Apr 16, 2026 at 4:57 AM JST, Gary Guo wrote:
> There have been a few cases where the programmer knows that the indices are
> in bounds but compiler cannot deduce that. This is also
> compiler-version-dependent, so using build indexing here can be
> problematic. On the other hand, it is also not ideal to use the fallible
> variant, as it adds error handling path that is never hit.
>
> Add a new panicking index projection for this scenario. Like all panicking
> operations, this should be used carefully only in cases where the user
> knows the index is going to be in bounds, and panicking would indicate
> something is catastrophically wrong.
>
> To signify this, require users to explicitly denote the type of index being
> used. The existing two types of index projections also gain the keyworded
> version, which will be the recommended way going forward.
>
> The keyworded syntax also paves the way of perhaps adding more flavors in
> the future, e.g. `unsafe` index projection. However, unless the code is
> extremely performance sensitive and bounds checking cannot be tolerated,
> panicking variant is safer and should be preferred, so it will be left to
> future when demand arises.

Review nit (no need to respin for this): this patch would probably be
easier to review if the renaming of `index` to `build_index` was split
into its own patch. It would reduce the diff, while also removing the
burden of having to keep in mind that `index` means a different thing
depending on whether the line is removed or added. I've found it a bit
difficult to keep track of this.

>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> ---
>  rust/kernel/dma.rs            |  3 ++
>  rust/kernel/ptr/projection.rs | 98 +++++++++++++++++++++++++++++++++++--------
>  2 files changed, 84 insertions(+), 17 deletions(-)
>
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 4995ee5dc689..3e4d44749aaf 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -1207,6 +1207,9 @@ macro_rules! dma_write {
>      (@parse [$dma:expr] [$($proj:tt)*] [.$field:tt $($rest:tt)*]) => {
>          $crate::dma_write!(@parse [$dma] [$($proj)* .$field] [$($rest)*])
>      };
> +    (@parse [$dma:expr] [$($proj:tt)*] [[$flavor:ident: $index:expr] $($rest:tt)*]) => {
> +        $crate::dma_write!(@parse [$dma] [$($proj)* [$flavor: $index]] [$($rest)*])
> +    };
>      (@parse [$dma:expr] [$($proj:tt)*] [[$index:expr]? $($rest:tt)*]) => {
>          $crate::dma_write!(@parse [$dma] [$($proj)* [$index]?] [$($rest)*])
>      };
> diff --git a/rust/kernel/ptr/projection.rs b/rust/kernel/ptr/projection.rs
> index 140ea8e21617..845811795393 100644
> --- a/rust/kernel/ptr/projection.rs
> +++ b/rust/kernel/ptr/projection.rs
> @@ -26,14 +26,14 @@ fn from(_: OutOfBound) -> Self {
>  ///
>  /// # Safety
>  ///
> -/// The implementation of `index` and `get` (if [`Some`] is returned) must ensure that, if provided
> -/// input pointer `slice` and returned pointer `output`, then:
> +/// The implementation of `index`, `build_index` and `get` (if [`Some`] is returned) must ensure
> +/// that, if provided input pointer `slice` and returned pointer `output`, then:
>  /// - `output` has the same provenance as `slice`;
>  /// - `output.byte_offset_from(slice)` is between 0 to
>  ///   `KnownSize::size(slice) - KnownSize::size(output)`.
>  ///
> -/// This means that if the input pointer is valid, then pointer returned by `get` or `index` is
> -/// also valid.
> +/// This means that if the input pointer is valid, then pointer returned by `get`, `index` or
> +/// `build_index` is also valid.
>  #[diagnostic::on_unimplemented(message = "`{Self}` cannot be used to index `{T}`")]
>  #[doc(hidden)]
>  pub unsafe trait ProjectIndex<T: ?Sized>: Sized {
> @@ -42,9 +42,12 @@ pub unsafe trait ProjectIndex<T: ?Sized>: Sized {
>      /// Returns an index-projected pointer, if in bounds.
>      fn get(self, slice: *mut T) -> Option<*mut Self::Output>;
>  
> +    /// Returns an index-projected pointer; panic if out of bounds.
> +    fn index(self, slice: *mut T) -> *mut Self::Output;

This looks like this could have a default implementation:

    fn index(self, slice: *mut T) -> *mut Self::Output {
        Self::get(self, slice).unwrap()
    }

I'm sure there is a good reason for now having this though, so at least
for my education: why not? :)

In any case,

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>