linux-kernel - Re: [PATCH v4 6/6] rust: bitfield: Use 'as' operator for setter type conversion

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Message-ID: <CANiq72k3kE-6KPkKwiDLgfkGHCQj4a2K7h9c4T13WMa5b4BAnQ@mail.gmail.com>
Date: Mon, 29 Sep 2025 15:59:32 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, dakr@...nel.org, acourbot@...dia.com, 
	Alistair Popple <apopple@...dia.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, 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>, 
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>, 
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard <mripard@...nel.org>, 
	Thomas Zimmermann <tzimmermann@...e.de>, John Hubbard <jhubbard@...dia.com>, Timur Tabi <ttabi@...dia.com>, 
	joel@...lfernandes.org, Elle Rhumsaa <elle@...thered-steel.dev>, 
	Yury Norov <yury.norov@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, 
	nouveau@...ts.freedesktop.org
Subject: Re: [PATCH v4 6/6] rust: bitfield: Use 'as' operator for setter type conversion

On Sat, Sep 20, 2025 at 8:23 PM Joel Fernandes <joelagnelf@...dia.com> wrote:
>
> The bitfield macro's setter currently uses the From trait for type
> conversion, which is overly restrictive and prevents use cases such as
> narrowing conversions (e.g., u32 storage size to u8 field size) which
> aren't supported by From.

Being restrictive is a good thing -- it would be nice to know more
context about this change, like Alexandre points out.

In particular, the line:

    .set_nibble(0x12345678_u32)    // truncated to 0x8

sounds fairly alarming, and not what we usually want. Why cannot the
caller cast on their side, if they really want that?

We avoid `as` for similar reasons and nowadays enable some Clippy
warnings to prevent its use where not needed.

(By the way, please follow our usual coding conventions for comments.)

Thanks!

Cheers,
Miguel