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

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Message-ID: <c30392e2-f0c0-458b-8b14-81661f83c67c@nvidia.com>
Date: Tue, 29 Apr 2025 13:35:20 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Tamir Duberstein <tamird@...il.com>, 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>, 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 4/29/25 1:20 AM, Alice Ryhl wrote:
> On Mon, Apr 28, 2025 at 12:54:19PM -0700, John Hubbard wrote:
...
>>> The `if false` branch is used to trigger a compilation failure when the
>>> macro is used incorrectly. The intent is that the compiler should
>>> optimize it out. I don't think there's anything wrong with that pattern.
>>
>> OK...probably best to either encapsulate that, or at least comment
>> it. I'm accustomed to seeing that pattern in cases where people
>> expected the code to *not* get optimized out, so it triggers me. :)
> 
> Okay ... why exactly would people do that? I can't imagine what purpose
> that would serve.
> 

lol I know, right? Two things, neither of which gives me joy to recall:

a) Configuration games, in which the dead code is available as an
   option that is not configured right now.

b) Binary patching games, sort of the same as above.

And at a slightly higher level, my concern is that when one reads
code like "if false", it sets off "omg, someone is either confused
or doing something questionable...actually, why choose? Probably
both." :)


thanks,
-- 
John Hubbard