linux-kernel - Re: [PATCH v3] rust: check type of `$ptr` in `container

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Message-ID: <CAJ-ks9myad-3MA6-TN_XyE98ZgefOgtoCiXC57KVYFjm3pw2LQ@mail.gmail.com>
Date: Thu, 24 Apr 2025 09:47:54 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Cc: 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 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] rust: check type of `$ptr` in `container_of!`

On Thu, Apr 24, 2025 at 8:48 AM Miguel Ojeda
<miguel.ojeda.sandonis@...il.com> wrote:
>
> On Wed, Apr 23, 2025 at 7:40 PM Tamir Duberstein <tamird@...il.com> wrote:
> >
> > +        if false {
> > +            let container_field_ptr = ::core::ptr::addr_of!((*container_ptr).$($fields)*).cast_mut();
> > +            [field_ptr, container_field_ptr]; // typeof(`$field_ptr`) == typeof(`$Container.$($fields)*`)
>
> If I understand correctly, we keep the `// typeof ...` in the same
> line so that it appears in the error message and thus it is clearer to
> the user, right?
>
> In that case, could we nevertheless please clarify things a bit at the
> top of the `if false` block, i.e. something like:
>
>     // Ensure that both types are equal while avoiding codegen .....
>     // ... i.e. effectively compare `typeof(...) == ...`.
>     if false {
>         ...
>
> etc.?
>
> Or, perhaps even better, we move this into its own macro, so that we
> document it there and why we chose this particular approach, assuming
> the error message still prints the right thing.
>
> Speaking of magic, to be honest, is this approach worth it? I liked v1
> quite more. The error seems concise enough, and the first line that
> the compiler points out is `assert_same_type` which makes it super
> clear, and showed the actual expressions involved without using a
> comment.
>
> With v1, we could also just put `assert_same_type` outside as a
> utility for others to use, i.e. in the `kernel` crate, which
> simplifies things and makes the error a bit shorter. Moving the
> function out makes the error slightly shorter, would also allow us to
> document its usage, including the suggestion to use `if false` in an
> example.
>
> Regarding the `if false`, the kernel is always built with at least
> -O2. Benno mentioned debug performance -- was that related to
> something like debug assertions being enabled or just optimization
> level? Either way, even with the assertions enabled, I don't see it in
> codegen.
>
> Am I missing something?

I have no strong opinions here, I'm just trying to keep everyone
happy. I'm happy to go back to v1 with or without `assert_same_type`
moved out of the macro body.