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

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Message-ID: <CAJ-ks9neiwxL6WhK1i0qPtZCGjfEFsXA6J-yVHKmZcwZ8RS06A@mail.gmail.com>
Date: Fri, 9 Jan 2026 10:59:06 -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 Fri, Jan 9, 2026 at 5:38 AM Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
> "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`.

Hmm, I must have missed it before, but can't this all be fixed with an
early return?

         if let Some(page) = self.cache_guard.get_mut(index) {
             return Ok(page);
         }

>
>
> Best regards,
> Andreas Hindborg
>
>
>