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

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Message-ID: <87344gh2pk.fsf@t14s.mail-host-address-is-not-set>
Date: Thu, 08 Jan 2026 10:29:43 +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 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.


Best regards,
Andreas Hindborg