linux-kernel - Re: [PATCH v2 1/2] rust: transmute: implement FromBytes and AsBytes for ()

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Message-ID: <fflyalpsifmeusaw2pru6765ry6a3q4ja5x3t52dwjyoqfbihn@krotxfd7vyaj>
Date: Mon, 8 Dec 2025 22:00:45 +1100
From: Alistair Popple <apopple@...dia.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <lossin@...nel.org>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, John Hubbard <jhubbard@...dia.com>, 
	Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi <ttabi@...dia.com>, Edwin Peer <epeer@...dia.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] rust: transmute: implement FromBytes and AsBytes
 for ()

On 2025-12-08 at 20:54 +1100, Alexandre Courbot <acourbot@...dia.com> 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.
> 
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>  rust/kernel/transmute.rs | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index be5dbf3829e2..8d78c81e3749 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -170,6 +170,9 @@ macro_rules! impl_frombytes {
>  }
>  
>  impl_frombytes! {
> +    // SAFETY: This type is empty and thus does not consume any data.
> +    (),
> +
>      // SAFETY: All bit patterns are acceptable values of the types below.
>      u8, u16, u32, u64, usize,
>      i8, i16, i32, i64, isize,
> @@ -230,6 +233,9 @@ macro_rules! impl_asbytes {
>  }
>  
>  impl_asbytes! {
> +    // SAFETY: This type is empty and thus returns an empty slice.

I would have thought the safety comment should have referenced that it cannot
contain any uninitialized data. OTOH that's probably obvious so:

Reviewed-by: Alistair Popple <apopple@...dia.com>

> +    (),
> +
>      // SAFETY: Instances of the following types have no uninitialized portions.
>      u8, u16, u32, u64, usize,
>      i8, i16, i32, i64, isize,
> 
> -- 
> 2.52.0
>