Re: [PATCH v2 01/16] rust: io: add static `cast()` method for views

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

On Thu Aug 6, 2026 at 1:35 AM JST, Gary Guo wrote:
> Add a compile-time checked variant of `try_cast()` using the minimum size
> and alignment information.
>
> Signed-off-by: Gary Guo <gary@garyguo.net>
> ---
>  rust/kernel/io.rs | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
>
> diff --git a/rust/kernel/io.rs b/rust/kernel/io.rs
> index a38c20ba3d23..adfc555de7d0 100644
> --- a/rust/kernel/io.rs
> +++ b/rust/kernel/io.rs
> @@ -436,6 +436,45 @@ fn is_empty<T>(self) -> bool
>          self.len() == 0
>      }
>  
> +    /// Convert into a different typed I/O view.
> +    ///
> +    /// The target type must be known (statically) to be of the same or smaller size to current
> +    /// type, and the current view is properly aligned for the target type.

grammar nit: s/is/must be.

> +    ///
> +    /// # Examples
> +    ///
> +    /// ```no_run
> +    /// use kernel::io::{
> +    ///     io_project,
> +    ///     Mmio,
> +    ///     Io,
> +    ///     Region,
> +    /// };
> +    /// #[derive(FromBytes, IntoBytes)]
> +    /// #[repr(C)]
> +    /// struct MyStruct { field: u32, }
> +    ///
> +    /// # fn test(mmio: &Mmio<'_, Region<0x1000>>) {
> +    /// // let mmio: Mmio<'_, Region>;

Should this be `Region<0x1000>`? Because as written the `cast` right
below wouldn't build since `Region::MIN_SIZE == 0`.

Also, looking at Sashiko's comment about `KnownSize` we should maybe
bite the bullet and make it unsafe? There was some talk about it [1] but
the reason why we kept it safe back then doesn't seem to protect us
here.

[1] https://lore.kernel.org/rust-for-linux/DGSDGDIVUHO0.P594H9B4LLO5@kernel.org/