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

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Message-ID: <20260105021756.5df32974.gary@garyguo.net>
Date: Mon, 5 Jan 2026 02:17:56 +0000
From: Gary Guo <gary@...yguo.net>
To: Tamir Duberstein <tamird@...il.com>
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 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.

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.

Best,
Gary