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

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Message-Id: <20251215-transmute_unit-v4-1-477d71ec7c23@nvidia.com>
Date: Mon, 15 Dec 2025 14:49:09 +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>, Eliot Courtney <ecourtney@...dia.com>, 
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Alexandre Courbot <acourbot@...dia.com>
Subject: [PATCH v4 1/2] rust: transmute: implement FromBytes and AsBytes
 for inhabited ZSTs

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.
+    (),
+    {<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,

-- 
2.52.0