linux-kernel - Re: [PATCH 03/11] rust: macros: convert `#[vtable]` macro to use `syn`

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Message-ID: <CAJ-ks9kFPFjwsgk4ShM1+CLdTSkFLY3WZC6keM=6nKG8Mf7EbQ@mail.gmail.com>
Date: Mon, 5 Jan 2026 10:19:57 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Boqun Feng <boqun.feng@...il.com>, 
	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>, Guilherme Giacomo Simoes <trintaeoitogc@...il.com>, 
	José Expósito <jose.exposito89@...il.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/11] rust: macros: convert `#[vtable]` macro to use `syn`

On Mon, Jan 5, 2026 at 9:27 AM Gary Guo <gary@...yguo.net> wrote:
>
> On Mon, 5 Jan 2026 06:02:35 -0500
> Tamir Duberstein <tamird@...il.com> wrote:
>
> > On Sun, Jan 4, 2026 at 9:18 PM Gary Guo <gary@...yguo.net> wrote:
> > >
> > > On Sun, 4 Jan 2026 18:44:43 -0500
> > > Tamir Duberstein <tamird@...il.com> wrote:
> > >
> > > > On Thu, Dec 11, 2025 at 2:29 PM Gary Guo <gary@...nel.org> wrote:
> > > > >
> > > > > From: Gary Guo <gary@...yguo.net>
> > > > >
> > > > > `#[vtable]` is converted to use syn. This is more robust than the
> > > > > previous heuristic-based searching of defined methods and functions.
> > > > >
> > > > > When doing so, the trait and impl are split into two code paths as the
> > > > > types are distinct when parsed by `syn`.
> > > > >
> > > > > Signed-off-by: Gary Guo <gary@...yguo.net>
> > > >
> > > > Logic looks correct, but the duplication between handle_trait and
> > > > handle_impl is unfortunate. I golfed on this a bit, see if you like
> > > > it: https://github.com/tamird/linux/commit/8354c5a48769f5e1e52963d19ca57c31e5926b08.
> > >
> > > I very much prefer the code to be separate. The trait and impl *should*
> > > be different. It's just that they *look* similar.
> > >
> > > Defining
> > >
> > >         const HAS_FOO: bool = false;
> > >
> > > in trait is defining a new contract while providing *default* value,
> > > while
> > >
> > >         const HAS_FOO: bool = true;
> > >
> > > is implementing such contract with a specific value. They look the
> > > same but I think the way `syn` treats them differently is justified.
> >
> > Yes, the similarity is perhaps superficial, but the duplication in the
> > current patch goes a good bit further because all the surrounding
> > ceremony is also duplicated.
>
> What do you mean? The only code that is really duplicated is the gathering
> of names from items (which have to be duplicate due to type difference
> anyway, and they're still duplicate in your linked commit above.

There's a bunch of boilerplate surrounding this code that is duplicated:
- declaring `functions` and `consts`
- iterating over `functions`
- computing the value of `gen_const_name`
- checking if `gen_const_name` is already in `consts`

These are of course mostly straightforward, but it is duplication
nonetheless, and it is all removed in my commit linked above.

>
> >
> > >
> > > I think the fact that existing code has a boolean and do different
> > > things based on it is a good enough supporting reason to handle
> > > different code path.
> > >
> > > For some new `vtable` features that I am working on would require quite
> > > different impl between the two.
> >
> > It seems unusual to justify current changes with future changes.
>
> I gave both reasons that why I think it should be distinct code path today
> and also why I prefer it to be it for the future. I did not justify the
> current change with future changes.

The today justification (the presence of a boolean) feels weak to me.
It can be made an enum if that changes your position.