linux-kernel - Re: [PATCH 0/4] rust: add `FromPrimitive` support

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Message-ID: <CA+tqQ4Ji486i=Gzy5oEsdZLOFDmGHpxT=Wkng2cyZpJ=u14FPA@mail.gmail.com>
Date: Thu, 26 Jun 2025 23:23:01 +0900
From: Jesung Yang <y.j3ms.n@...il.com>
To: Alexandre Courbot <acourbot@...dia.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 <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: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	nouveau@...ts.freedesktop.org
Subject: Re: [PATCH 0/4] rust: add `FromPrimitive` support

On Wed, Jun 25, 2025 at 11:07 PM Alexandre Courbot <acourbot@...dia.com> wrote:
>
> If you add an associated type, I guess this means the derive macro
> should have a helper attribute to specify it?

The current implementation tries to detect the enum's representation
(e.g., `#[repr(u8)]`) and falls back to `isize` if nothing is provided,
since it's the default [1]. However, when `#[repr(C)]` is used, the
internal representation is not controlled on the Rust side. To quote
the reference [2]:

For field-less enums, the C representation has the size and
alignment of the default enum size and alignment for the target
platform's C ABI.

Given this, it would definitely help with determinism if users provided
an explicit attribute. Being explicit also often improves clarity and
makes the intent more obvious.

[1]: https://doc.rust-lang.org/reference/items/enumerations.html#r-items.enum.discriminant.repr-rust
[2]: https://doc.rust-lang.org/reference/type-layout.html?highlight=repr#r-layout.repr.c.enum

Best regards,
Jesung