linux-kernel - Re: [PATCH v9] rust: transmute: Add methods for FromBytes trait

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Message-ID: <aKMagEO9O6WNhIUK@google.com>
Date: Mon, 18 Aug 2025 12:20:16 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Alexandre Courbot <acourbot@...dia.com>
Cc: Christian <christiansantoslima21@...il.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, 
	"Björn Roy Baron" <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, ~lkcamp/patches@...ts.sr.ht, 
	richard120310@...il.com
Subject: Re: [PATCH v9] rust: transmute: Add methods for FromBytes trait

On Mon, Aug 18, 2025 at 08:28:18PM +0900, Alexandre Courbot wrote:
> On Wed Aug 13, 2025 at 3:00 AM JST, Christian wrote:
> > Hi, Alexandre.
> >
> >> I mentioned it on v8 [1] and v7 [2], but the tests that break due to
> >> this change need to be updated by this patch as well. This includes:
> >>
> >> * The doctests in `rust/kernel/dma.rs`,
> >> * The `samples/rust/rust_dma.rs` sample,
> >> * The example for `FromBytes` introduced by this patch which uses `?` without
> >> defining a function.
> >
> > Sorry for my inattention, I'll fix this in the next version.
> 
> Ah, as it turns out you might not need to.
> 
> We discussed this patch a bit during the DRM sync, and the consensus was
> that it would probably be better to keep things a bit simpler for the
> first version. The `FromBytesSized` trait in particular was not very
> popular; a better long-term way to provide implementations for
> `FromBytes` would be to use a derive macro, but that's out of scope for
> now.
> 
> Instead, we agreed that the following would make a good first version:
> 
> - Make the `FromBytes` trait depend on `Sized`,
> - Provide default implementations for `from_bytes` and `from_bytes_mut`
>   directly in the `FromBytes` trait,

You can put the Sized requirement on the methods, rather than on the
trait.

trait FromBytes {
    fn from_bytes(bytes: &[u8]) -> Option<&Self>
    where
        Self: Sized,
    {
        ...
    }
}

> - No implementation for slices for now,
> - Consequently, no user code will break due to the addition of the
>   methods, which is a big plus.
> 
> The simpler version that would result from this covers all the immediate
> use-cases and would be easier to merge, which will give us some time to
> think about how to handle the non-sized use cases (probably via a derive
> macro).
> 
> Do you think you could write the next version along these lines?
> 
> I feel like I misdirected you with the `FromBytesSized` trait, so please
> accept my apologies for that.