fix(match_as_ref): suggest as_ref when the reference needs to be cast

[ada4a]

(EDIT: only remembered to add this after the PR was merged already... Fixes #15932)

I first implemented the naive version, which just talks about, and suggests using, Option::as_ref. But the resulting error message sounded a bit misleading -- after all, the manual implementation was factually that of Option::as_mut, and we only suggest as_ref as a means of downcasting. So then I added another help message to mention the need for reference downcasting, which.. still looks awkward.

Honestly it might be the easiest to hide the reference downcasting into the .map after all, so that it encapsulates all the casting needed following the switch to as_ref/as_mut.

changelog: [match_as_ref]: suggest as_ref when the reference needs to be cast

r? @samueltardieu

[ada4a]

Opened another PR for the "two-step cast" approach: #15935, lmk which one you like more

[samueltardieu]

I prefer this version over the other one, as it proposes to use cleaner code even if a bit further from the original code written by the user.

View changes since this review

[samueltardieu]

I would prefer this message.

Suggested change

	diag.note("but the type is coerced to `&_`, and so `as_ref` will need to be used instead");
	diag.note("but the type is coerced to a non-mutable reference, and so `as_ref` can be used instead");

[ada4a]

I would prefer this message.

Yeah this was somewhat of a stand-in.

Though I'm not so sure about "can" here -- after all, it's not like as_mut would work by itself. This could work if we first presented the .as_mut().map() option and only then proposed changing that to .as_ref() as a way to simplify the code, but we currently don't...

That said, there seems to be a general trend in lints to try to make the suggestion type-check by putting extra things on top of it (type ascriptions, casts, etc.), and those rarely -- if ever -- get acknowledged by the help message, not to mention getting the motivation behind them explained.

[samueltardieu]

The idea behind "can be used" is that it "should be used", but without telling the user that they did something wrong. We are showing them the possibility for doing so instead of .as_mut().map().

[ada4a]

@rustbot ready