Every type that implements `AsBytes` should be able to provide its byte
representation. Introduce the `as_bytes` method that returns the
implementer as a stream of bytes, and provide a default implementation
that should be suitable for any type that satisfies `AsBytes`'s safety
requirements.

Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
---
 rust/kernel/transmute.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
index 1c7d43771a37b90150de86699f114a2ffb84db91..d541c9960904d8c7f755351f22d06e4e8dbd546a 100644
--- a/rust/kernel/transmute.rs
+++ b/rust/kernel/transmute.rs
@@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
 ///
 /// Values of this type may not contain any uninitialized bytes. This type must not have interior
 /// mutability.
-pub unsafe trait AsBytes {}
+pub unsafe trait AsBytes {
+    /// Returns `self` as a slice of bytes.
+    fn as_bytes(&self) -> &[u8] {
+        let data = core::ptr::from_ref(self).cast::<u8>();
+        let len = size_of_val(self);
+
+        // SAFETY: `data` is non-null and valid for reads over `len * sizeof::<u8>()` bytes.
+        unsafe { core::slice::from_raw_parts(data, len) }
+    }
+}
 
 macro_rules! impl_asbytes {
     ($($({$($generics:tt)*})? $t:ty, )*) => {

-- 
2.50.1

On Mon Jul 28, 2025 at 2:47 PM CEST, Alexandre Courbot wrote:
> diff --git a/rust/kernel/transmute.rs b/rust/kernel/transmute.rs
> index 1c7d43771a37b90150de86699f114a2ffb84db91..d541c9960904d8c7f755351f22d06e4e8dbd546a 100644
> --- a/rust/kernel/transmute.rs
> +++ b/rust/kernel/transmute.rs
> @@ -47,7 +47,16 @@ macro_rules! impl_frombytes {
>  ///
>  /// Values of this type may not contain any uninitialized bytes. This type must not have interior
>  /// mutability.
> -pub unsafe trait AsBytes {}
> +pub unsafe trait AsBytes {
> +    /// Returns `self` as a slice of bytes.
> +    fn as_bytes(&self) -> &[u8] {
> +        let data = core::ptr::from_ref(self).cast::<u8>();

Let's add a `CAST` comment above this that says:

    // CAST: `Self` implements `AsBytes` thus all bytes of `self` are initialized.

> +        let len = size_of_val(self);
> +
> +        // SAFETY: `data` is non-null and valid for reads over `len * sizeof::<u8>()` bytes.

s/over/of/

---
Cheers,
Benno

> +        unsafe { core::slice::from_raw_parts(data, len) }
> +    }
> +}
>  
>  macro_rules! impl_asbytes {
>      ($($({$($generics:tt)*})? $t:ty, )*) => {