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

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Message-ID: <CAJ-ks9=JJ6S6U-Bq0WC1PfJ0P9G3PvugfDdE2q9Q516M-QpsoA@mail.gmail.com>
Date: Thu, 8 Jan 2026 06:05:56 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Andreas Hindborg <a.hindborg@...nel.org>
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

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?

>
>
> Best regards,
> Andreas Hindborg
>
>