bug#70794: 30.0.50; Add Rust compilation regex

[Stefan Kangas]

Mattias Engdegård <mattias.engdegard@gmail.com> writes:

> 3 juni 2024 kl. 18.33 skrev Eli Zaretskii <eliz@gnu.org>:
>
>> Fine by me (I don't use Rust), but is the rule really broken?  What
>> happens if you move it to the end?
>
> It is indeed broken as written because it would match anything
> starting with 'warning:' to something that looks like an arrow further
> down the log which could be an unrelated message a megabyte away.
>
> I could try to slap together a guess at what a better one would be but
> then again, I wouldn't use it myself (rarely use Rust and when I do
> it's one of those external packages which have their own patterns and
> more importantly active maintainers). Furthermore I don't have the
> time to go poking around in the rustc (or cargo?) source code to see
> what the message-emitting parts look like, which is very useful when
> adding patterns.
>
> Even with the obvious fixes, the pattern would still be incomplete and
> not even match parts of the examples given. I'm happy to work with
> Rust users or package maintainers to work out details but I don't want
> to do a rush job now, nor delay Emacs 30.

Randy, would you be interested in working with Mattias to create a good
`compilation-mode` regexp for Rust?