linux-kernel - Re: [PATCH v3 03/12] rust: xarray: add `contains

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Message-ID: <87fr77viat.fsf@t14s.mail-host-address-is-not-set>
Date: Wed, 11 Feb 2026 08:41:14 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Liam R. Howlett" <Liam.Howlett@...cle.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 <lossin@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Vlastimil Babka
 <vbabka@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>, Christoph
 Lameter <cl@...two.org>, David Rientjes <rientjes@...gle.com>, Roman
 Gushchin <roman.gushchin@...ux.dev>, Harry Yoo <harry.yoo@...cle.com>,
 Daniel Gomez <da.gomez@...nel.org>, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v3 03/12] rust: xarray: add `contains_index` method

"Liam R. Howlett" <Liam.Howlett@...cle.com> writes:

> * Andreas Hindborg <a.hindborg@...nel.org> [260209 14:38]:
>> 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`.
>
> I think this is going to result in less efficient code for most uses.
>
> Most users use the results returned, not just checking if there is or is
> not a value.  So if you find the value an xarray state and then just
> throw it away and find it again, it'll be less efficient.
>
> If there are users that do use the xarray to just check if something
> exists or not (which there probably are?), then it should be in a
> wrapper for that code and not the generic API.  Otherwise we will have
> users pop up to use this method when they should not.

I agree in that I would prefer matching on the result of a lookup and
using that result. This is not always possible due to a limitation in
the current implementation of the borrow checker. Please see my response
to Tamir [1], it has all the pointers.

Since we cannot use a match statement in certain situations, we have to
fall back to something that does not borrow from the collections, like
`array.get(index).is_some()`. I would argue that

  if array.contains_index(foo) { ... }

is easier to read than

  if array.get(foo).is_some() { ... }

And I don't think `array.contains()` is going to have worse codegen than
`array.get(index).is_some()`.

Best regards,
Andreas Hindborg

[1] https://lore.kernel.org/rust-for-linux/20260209-xarray-entry-send-v3-0-f777c65b8ae2@kernel.org/T/#m95fb90870c511491f4f487dbf852c689cd0733f4