linux-kernel - Re: [PATCH v2 1/2] rust: macros: Add derive Display for enums

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Message-Id: <DFHITWGSEB4E.QV5DTPS816XI@kernel.org>
Date: Tue, 06 Jan 2026 13:56:09 +0100
From: "Benno Lossin" <lossin@...nel.org>
To: "Maurice Hieronymus" <mhi@...lbox.org>, "Danilo Krummrich"
 <dakr@...nel.org>
Cc: "Gary Guo" <gary@...yguo.net>, <aliceryhl@...gle.com>,
 <acourbot@...dia.com>, <airlied@...il.com>, <simona@...ll.ch>,
 <nouveau@...ts.freedesktop.org>, <dri-devel@...ts.freedesktop.org>,
 <linux-kernel@...r.kernel.org>, <ojeda@...nel.org>, <boqun.feng@...il.com>,
 <bjorn3_gh@...tonmail.com>, <a.hindborg@...nel.org>, <tmgross@...ch.edu>,
 <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] rust: macros: Add derive Display for enums

On Tue Jan 6, 2026 at 6:56 AM CET, Maurice Hieronymus wrote:
> On Mon, 2026-01-05 at 23:03 +0100, Danilo Krummrich wrote:
>> On Mon Jan 5, 2026 at 10:11 PM CET, Maurice Hieronymus wrote:
>> > Before I start implementing, I want to reach common ground.
>> > 
>> > In my opinion a derive macro which implements Display would be
>> > perfectly fine, as long as the name suggests what it does. So for
>> > example #[derive(DisplayEnumVariant)]. This would communicate the
>> > intent clearly to the user.
>> > 
>> > Benno, would you be okay with that? If not, Gary and Danilo, are

I'd prefer if we stay a bit more cautious about directly deriving
`Display`. The trait with the variant name sounds like a very sensible
idea.

We can talk about this in the team in our weekly meeting, they might
change my mind :)

>> > you
>> > fine with the proposed trait implementation (e.g. the variant_name
>> > function)?
>> 
>> Actually, it might even be reasonable to have both. In the Nova
>> driver we have
>> the case that we want to print the enum variant exactly as it is
>> defined in the
>> code and a lowercase version of the enum variant.
>> 
>> > Are there any common use-cases where one wants to change the case
>> > of
>> > the enum variants? If not, I would not implement an argument and
>> > rather
>> > name the macro accordingly, so the intent is clear.
>> 
>> As mentioned above, we do have a case in Nova where we also want a
>> lowercase
>> representation to construct a firmware path with.
>
> So there would be the need to have two derive macros:
>
> 1. #[derive(DisplayEnumVariant)]
> Implements Display for all enum variants as they are (original case).
>
> 2. #[derive(ImplementVariantName(Case::Lowercase))]
> Implements the mentioned trait. Case could be an Enum where one could
> choose between Case::Lowercase and Case::Original.

You'll need to use a helper attribute, something like:

    #[derive(VariantName)]
    #[variant_name(case = "lowercase")]

Cheers,
Benno