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

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Message-Id: <DETC6QG6AT7A.2OPSK3M0G90SG@nvidia.com>
Date: Tue, 09 Dec 2025 11:39:47 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Gary Guo" <gary@...yguo.net>, "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>, <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 Mon Dec 8, 2025 at 10:27 PM JST, Gary Guo wrote:
> On Mon, 08 Dec 2025 18:54:40 +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.
>> 
>> 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.
>> +    (),
>
> I'd avoid use the word "empty" as it has the meaning of uninhabited
> types (never type) in type theory. ZSTs that are inhabited are unit
> types (or singletons).
>
>
> Perhaps better to have a justification that applies to all inhabited
> ZSTs that don't have special semantics. Something like this?
>
> 	// SAFETY: Inhabited ZSTs only have one possible bit pattern.
> 	(),
> 	{<T>} PhantomData<T>,

Sounds good - I'll use your SAFETY statement as-is. Adding `PhantomData`
is also a great idea.

Thanks for the review!