Re: [PATCH v3 03/12] rust: xarray: add `contains_index` method

[Liam R. Howlett]

* Andreas Hindborg <a.hindborg@xxxxxxxxxx> [260212 07:40]:
> "Alice Ryhl" <aliceryhl@xxxxxxxxxx> writes:
> 
> > On Thu, Feb 12, 2026 at 11:52 AM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
> >>
> >> Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:
> >>
> >> > As far as I understand, this is a borrow checker limitation. It is easy
> >> > for us to look at this code and decide that the borrow on line 51 will
> >> > never alias with the borrow on line 49.
> >>
> >> I did a bit of googling, and this seems to be a well known issue with
> >> the current implementation of lifetime analysis in the rust compiler.
> >> Apparently this kind of code used to be OK [1] but the Rust devs decided
> >> to remove the code that allowed this, because it was causing excessive
> >> compilation times [2]. The upside is that this is solved by the new
> >> lifetime analysis implementation called "Polonius" and it is the
> >> intention to replace the existing implementation with Polonius at some
> >> point [3].
> >
> > I believe the standard fix for this issue is to provide an entry api
> > similar to HashMap::entry(). See the rbtree for an example, as it
> > already provides such API.

Alice, can you provide a link to the rbtree code please?

> 
> The example above [1] is using the BTreeMap entry API to produce the
> issue. Are the BTreeMap and HashMap entry APIs significantly different,
> or is there something else I missed?

>From what I can find, the HashMap is different specifically for this
reason.

This is where my question about get_or_insert() came from, the HashSet
has this workaround, maybe?

AFAICT, the hash workaround is done in the Entry code that takes a
different reference(?) based on the variant (enum?) returned [1].

Or maybe it's about the way branches are evaluated by the checker?  Are
these different

But I'm really fumbling around this while I learn what you are all
doing!  Thanks for all the education on this stuff, it's helping me
understand where we are headed.  Hopefully I help along the way this
time.

Thanks,
Liam


[1]. https://doc.rust-lang.org/std/collections/hash_map/enum.Entry.html