Re: [PATCH v3 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

[Benno Lossin]

On Tue Dec 9, 2025 at 3:57 AM CET, Alexandre Courbot wrote:
> This is useful when using types that may or may not be empty in generic
> code relying on these traits. It is also safe because technically a
> no-op.
>
> Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx>
> Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxx>
> ---
>  rust/kernel/transmute.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index be5dbf3829e2..a888a312e7ff 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -170,6 +170,10 @@ macro_rules! impl_frombytes {
>  }
>  
>  impl_frombytes! {
> +    // SAFETY: Inhabited ZSTs only have one possible bit pattern.

Even inhabited ZSTs cannot just be conjured out of thin air, since there
might be safety invariants on the creation of such a type. Now these
two concrete ones do not have any, so this is fine, but should still be
mentioned in the comment IMO.

Cheers,
Benno

> +    (),
> +    {<T>} core::marker::PhantomData<T>,
> +
>      // SAFETY: All bit patterns are acceptable values of the types below.
>      u8, u16, u32, u64, usize,
>      i8, i16, i32, i64, isize,