linux-kernel - Re: [PATCH v4 1/2] rust: transmute: implement FromBytes and AsBytes for inhabited ZSTs

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Message-ID: <20251215113134.233fdd1e.gary@garyguo.net>
Date: Mon, 15 Dec 2025 11:31:34 +0000
From: Gary Guo <gary@...yguo.net>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 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>, Alistair Popple
 <apopple@...dia.com>, Joel Fernandes <joelagnelf@...dia.com>, Timur Tabi
 <ttabi@...dia.com>, Edwin Peer <epeer@...dia.com>, Eliot Courtney
 <ecourtney@...dia.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/2] rust: transmute: implement FromBytes and AsBytes
 for inhabited ZSTs

On Mon, 15 Dec 2025 14:49:09 +0900
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.
> 
> Reviewed-by: Alistair Popple <apopple@...dia.com>
> Reviewed-by: Gary Guo <gary@...yguo.net>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>
> ---
>  rust/kernel/transmute.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index be5dbf3829e2..5711580c9f9b 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, and these two have no invariant.
> +    (),
> +    {<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,
> @@ -230,6 +234,10 @@ macro_rules! impl_asbytes {
>  }
>  
>  impl_asbytes! {
> +    // SAFETY: Inhabited ZSTs only have one possible bit pattern, and these two have no invariant.

Some nit: technically you can turn any ZST into bytes, even those with
invariants or uninhabited. So while the comment is correct, it's kinda
redundant for the object -> bytes direction :)

No change required for this patch.

Best,
Gary

> +    (),
> +    {<T>} core::marker::PhantomData<T>,
> +
>      // SAFETY: Instances of the following types have no uninitialized portions.
>      u8, u16, u32, u64, usize,
>      i8, i16, i32, i64, isize,
>