linux-kernel - Re: [PATCH 03/10] rust: xarray: add `contains

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Message-ID: <87o6n3f4uz.fsf@t14s.mail-host-address-is-not-set>
Date: Fri, 09 Jan 2026 11:38:28 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: Tamir Duberstein <tamird@...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 <lossin@...nel.org>, Alice
 Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Danilo
 Krummrich <dakr@...nel.org>, Daniel Gomez <da.gomez@...nel.org>,
 rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/10] rust: xarray: add `contains_index` method

"Tamir Duberstein" <tamird@...il.com> writes:

> On Thu, Jan 8, 2026 at 4:38 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>
>> Tamir Duberstein <tamird@...il.com> writes:
>>
>> > On Wed, Jan 7, 2026 at 1:34 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>> >>
>> >> Tamir Duberstein <tamird@...il.com> writes:
>> >>
>> >> > On Wed, Dec 3, 2025 at 5:27 PM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>> >> >>
>> >> >> Add a convenience method `contains_index` to check whether an element
>> >> >> exists at a given index in the XArray. This method provides a more
>> >> >> ergonomic API compared to calling `get` and checking for `Some`.
>> >> >
>> >> > It isn't clear when you'd want this API, and neither this nor the
>> >> > example are particularly motivating.
>> >>
>> >> I added this when I had a line reading `if xa.get(index).is_none()
>> >> {...}`. I think it reads better as `if !xa.contains_index(index) {...}`.
>> >
>> > What was the code surrounding it?
>> >
>> >> Do you have an idea of how to improve the motivational factor of the
>> >> example? Writing motivating examples is not my top skill.
>> >
>> > IMO writing a better example is not the issue; rather it would be good
>> > to understand why you need it. In my experience `Option::is_none` is a
>> > smell, but hard to say without seeing the surrounding code.
>>
>>     fn get_cache_page(&mut self, sector: u64) -> Result<&mut NullBlockPage> {
>>         let index = Self::to_index(sector);
>>
>>         if self.cache_guard.contains_index(index) {
>>             Ok(self.cache_guard.get_mut(index).expect("Index is present"))
>>         } else {
>>             let page = if self.disk_storage.cache_size_used.load(ordering::Relaxed)
>>                 < self.disk_storage.cache_size
>>             {
>>                 self.hw_data_guard
>>                     .page
>>                     .take()
>>                     .expect("Expected to have a page available")
>>             } else {
>>                 self.extract_cache_page()?
>>             };
>>             Ok(self
>>                 .cache_guard
>>                 .insert_entry(index, page, Some(&mut self.hw_data_guard.preload))
>>                 .expect("Should be able to insert")
>>                 .into_mut())
>>         }
>>     }
>>
>> For lifetime reasons, I cannot borrow `self` in the taken arm.
>
> That's surprising. Couldn't you destructure Self so that all the
> references derive from the single mutable reference &mut self?

I don't think so, because I still need `&mut self` around to do the call
to `extract_cache_page`.


Best regards,
Andreas Hindborg