Re: [PATCH v4 2/2] rust: transmute: add `as_bytes_mut` method to `AsBytes` trait

[Benno Lossin]

On Mon Jul 28, 2025 at 4:59 PM CEST, Boqun Feng wrote:
> On Mon, Jul 28, 2025 at 04:53:31PM +0200, Benno Lossin wrote:
>> 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 d541c9960904d8c7f755351f22d06e4e8dbd546a..000fda2a78f8e6e8378bbe93cddd8a5008db20cc 100644
>> > --- a/rust/kernel/transmute.rs
>> > +++ b/rust/kernel/transmute.rs
>> > @@ -56,6 +56,19 @@ fn as_bytes(&self) -> &[u8] {
>> >          // SAFETY: `data` is non-null and valid for reads over `len * sizeof::<u8>()` bytes.
>> >          unsafe { core::slice::from_raw_parts(data, len) }
>> >      }
>> > +
>> > +    /// Returns `self` as a mutable slice of bytes.
>> > +    fn as_bytes_mut(&mut self) -> &mut [u8]
>> > +    where
>> > +        Self: FromBytes,
>> > +    {
>> > +        let data = core::ptr::from_mut(self).cast::<u8>();
>> 
>> Also add a `CAST` comment above this line:
>> 
>>     // CAST: `Self` implements both `AsBytes` and `FromBytes` thus making `Self` bi-directionally
>>     // transmutable to `[u8; size_of_val(self)]`.
>> 
>> On that note, `FromBytes` probably entails `Sized`, or does someone have
>> a use-case in mind? (because then we could replace the
>
> Because if `T` is `FromBytes` then we want `[T]` is `FromBytes` too?
> Also it would be weird if `[u8]` itself doesn't implement `FromBytes`.

Oh yeah that's true.

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Cheers,
Benno