Reduce pub exposure in rustc_mir_build

[nnethercote]

r? compiler

[rustbot]

Some changes occurred in match lowering

cc @Nadrieril

[Urgau]

Many errors.rs/lints.rs files have this kind of structure: public diagnostic struct with public fields.
Do you think it would make sense to deny the unreachable_pub lint in those files?

[nnethercote]

This is the first I've heard of this lint! If it's reliable, I think we should use it just about everywhere :)

[Urgau]

As far as I know it is reliable and the only reason it isn't warn-by-default yet is because of pub-fields in non-pub-adt, cf. #120144 (comment).

[compiler-errors]

Would like to see another PR changing (almost?) all diagnostic structs to be pub(crate).

This is good for now @bors r+ rollup

[bors]

📌 Commit 5a5e248 has been approved by compiler-errors

It is now in the queue for this repository.

[nnethercote]

I have started adding #![deny(unreachable_pub)] to every crate in compiler. I've done about a dozen crates so far, it's going well. Diagnostic structs are definitely the most common case, but it's finding other stuff as well. I will file a PR later today for maybe the first 20 crates or so.

[estebank]

@nnethercote on the one hand, I'm glad you're doing that for the sake of the codebase, on the other I have a project that uses the rustc rlibs to build its own rustc_driver and there might be "unused pub items" that I might be using. At least two other tools might be silently affected by that too: miri and klint. That doesn't mean we shouldn't enable that lint, just that you should be aware that people might mildly grumble after the clean up.

[nnethercote]

@estebank: I am familiar with that general problem, and there are now comments on several seemingly unused things in rustc_interface that came about when I (a) tried to remove/hide them, or (b) removed/hid them and then undid that after a complaint :)

But unreachable_pub shouldn't have that problem. It only complains about things marked pub that can't actually be exported because they're within something else (e.g. a struct, a mod) that is itself not pub.