Make Iterator a lang item

[oli-obk]

r? @compiler-errors

pulled out of #116447

We're doing this change on its own, because iterator was the one diagnostic item that was load bearing on us correctly emitting errors about diagnostic_item mis-uses. It was used in some diagnostics as an early abort, before the actual checks of the diagnostic, so effectively the compiler was unconditionally checking for the iterator diagnostic item, even if it didn't emit any diagnostics. Changing those uses to use the lang item, caused us not to invoke the all_diagnostic_items query anymore, which then caused us to miss some issues around diagnostic items until they were actually used.

The reason we keep the diagnostic item around is that clippy uses it a lot and having Iterator be a lang item and a diagnostic item at the same time doesn't cost us anything, but makes clippy's internal code simpler

[compiler-errors]

r=me after nit, we probably should remove the diagnostic item (unless other places still rely on it that you didn't want to touch)

[oli-obk]

yea, that change was very painful, and flip1995 (from clippy) asked me to not do it

The full change is 6ea822c

[compiler-errors]

Ok -- is there plans on doing this eventually? Otherwise, perhaps we should revert the changes to is_diagnostic_item in the compiler if we plan on keeping it around forever.

[oli-obk]

Otherwise, perhaps we should revert the changes to is_diagnostic_item in the compiler if we plan on keeping it around forever.

done. Without them we also don't "need" the all_diagnostic_items invocation, but it still seems good to keep around

[compiler-errors]

r=me

[oli-obk]

@bors r=compiler-errors

[bors]

📌 Commit 268ec72 has been approved by compiler-errors

It is now in the queue for this repository.