remove special-casing of boxes from match exhaustiveness/usefulness analysis

[dianne]

As a first step in replacing box_patterns with deref_patterns, this treats box patterns as deref patterns in the THIR and exhaustiveness analysis. This allows a bunch of special-casing to be removed. The emitted MIR is unchanged.

Incidentally, this fixes a bug caused by box patterns being treated like structs rather than pointers, where enabling exhaustive_patterns (#51085) could give rise to spurious unreachable_patterns lints on arms required for exhaustiveness. Following the lint's advice to remove the match arm would result in an error. I'm not sure what the current state of exhaustive_patterns is with regard to reference/box opsem, or whether there's any intention to have unreachable_patterns be more granular than the whole arm, but regardless this should hopefully make them easier to handle consistently.

Tracking issue for deref patterns: #87121

r? @Nadrieril

[rustbot]

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[rustbot]

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred in exhaustiveness checking

cc @Nadrieril

[Nadrieril]

I'm very happy we can finally remove this :D

@bors r+

[bors]

📌 Commit 98659a3 has been approved by Nadrieril

It is now in the queue for this repository.

[dianne]

between planning this and actually implementing it, I completely forgot to force it to check that box patterns aren't used in the same column as Box constructors; it'll probably ICE in that case. I think that check is still gated by deref_patterns since I didn't want to affect perf for matches without deref patterns or to make that first PR more complex by adding logic to avoid the perf hit. I'll try fixing that in a separate PR. I'm not too worried about that ICE causing trouble in the mean time (surely no one would actually write Box { .. }.. right? especially not in the same column as box pat).

edit: I also wholly forgot that rust-analyzer has all of this box pattern special casing too (though it sort of looks like there's no logic yet to lower box patterns from their HIR to their match_check IR?). in any case, replacing the special-casing there can probably be a separate PR to the r-a repo.