linux-kernel - [PATCH] rust: transmute: implement FromBytes and AsBytes for ()

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Message-Id: <20251208-transmute_unit-v1-1-680c7386b5d9@nvidia.com>
Date: Mon, 08 Dec 2025 13:15:13 +0900
From: Alexandre Courbot <acourbot@...dia.com>
To: 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>
Cc: 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, Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH] rust: transmute: implement FromBytes and AsBytes for ()

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>
---
This is going to be useful in Nova's GSP message handling, as some
messages are empty and we need to explicitly use an empty structure for
them.

If accepted, I would like to merge it through `drm-rust-next` so Nova
code can start using this feature quickly.
---
 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.
+    (),
+
     // SAFETY: Instances of the following types have no uninitialized portions.
     u8, u16, u32, u64, usize,
     i8, i16, i32, i64, isize,

---
base-commit: ba65a4e7120a616d9c592750d9147f6dcafedffa
change-id: 20251208-transmute_unit-78ab58ba9e6e

Best regards,
-- 
Alexandre Courbot <acourbot@...dia.com>