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@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.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

On Mon, 15 Dec 2025 14:49:09 +0900
Alexandre Courbot <acourbot@nvidia.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@nvidia.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.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,
>

On Mon Dec 15, 2025 at 6:49 AM CET, Alexandre Courbot 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@nvidia.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Reviewed-by: Benno Lossin <lossin@kernel.org>

Cheers,
Benno

> ---
>  rust/kernel/transmute.rs | 8 ++++++++
>  1 file changed, 8 insertions(+)

On Mon Dec 15, 2025 at 4:45 PM JST, Benno Lossin wrote:
> On Mon Dec 15, 2025 at 6:49 AM CET, Alexandre Courbot 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@nvidia.com>
>> Reviewed-by: Gary Guo <gary@garyguo.net>
>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>
> Reviewed-by: Benno Lossin <lossin@kernel.org>

Miguel, are you ok if we take this one through the drm-rust tree? Nova
is likely to make use of it this cycle.