Implement RFC 3323: restrictions

[jhpratt]

This PR implements RFC 3323, which proposed impl and mut restrictions. Both are implemented here.

The current state of the PR is insufficient to merge. mut restrictions are implemented without any known bugs. impl restrictions, however, are not currently enforced on external traits — they are only enforced on same-crate traits. I do not know why this is the case, and would appreciate some help in narrowing down what needs to change to fix this. This is the only known issue with the implementation, and is why this PR is currently marked as a draft.

Once the bug is fixed, I'll also need to clean up the commit history. The current history is due to having it implemented while the RFC was still in progress.

CI will fail for a number of reasons. I still have TODO in the code as opposed to FIXME (because it needs to be done before this gets merged). I haven't added the tracking issue yet. And the bug will also cause a few errors by itself.

I am currently refactoring some code and cleaning up the commit history. After that is done, this PR will be ready to merge.

cc #105077

[rustbot]

r? @eholk

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Error: Label B-rfc-approved can only be set by Rust team members

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[joshtriplett]

Reassigning to T-compiler, as this doesn't need further T-lang review yet.

[bors]

☔ The latest upstream changes (presumably #106256) made this pull request unmergeable. Please resolve the merge conflicts.

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[bors]

☔ The latest upstream changes (presumably #113777) made this pull request unmergeable. Please resolve the merge conflicts.

[estebank]

Maybe

Suggested change

	Visibility or restriction is restricted to a module which isn't an ancestor of
	Visibility or restriction is limited to a module which isn't an ancestor of

to avoid repetition?

[apiraino]

I'll reassign to author(s) to rebase and address a few comments but tbh I'm not sure about who should have the ball right now. Feel free to request a review with @rustbot ready, thanks!

@rustbot author

[petrochenkov]

When it gets to actual landing it will be better to split this PR into separate PRs for individual sub-features, it's too large right now.

[WaffleLapkin]

Agree with @petrochenkov, one of the reasons I'm procrastinating reviewing this (sorry!) is that this PR adds a lot. Maybe an approach closer to what we are doing with the explicit tail calls experiment (a PR per level — one for parsing, one for HIR, ...; maybe a PR "per feature" would make more sense here, I'm not sure) would be a bit easier to manage.

[jhpratt]

No worries on the delay — I know it's a large PR. I can definitely split it up into multiple PRs when I get the chance. That may be a little while, unfortunately, as I've got a few different things going on.

[compiler-errors]

For the record: feel free to follow the new nightly style policy and open a separate formatting PR against rustfmt once this has landed in the compiler.

[Dylan-DPC]

@jhpratt any updates on this?

[jhpratt]

@Dylan-DPC Not at the moment. I have splitting it up on my personal backlog.

[Dylan-DPC]

@jhpratt any updates on this? It's just collecting conflicts at the moment :D

[WaffleLapkin]

r? compiler
(I know this is waiting on author, but I do not plan to review anymore, so rerolling)

[compiler-errors]

I can review this but I'd rather we split it as agreed above (not to seem repetitive, and obviously take your time).

[jhpratt]

The update would be that I'm quite busy, unfortunately. Believe me when I say this is something that I still want to happen 😄

Between work, RustConf prep, and other open source things, this isn't happening in the next month. However, I hope to get around to it within the next few months. It'll likely be a series of PRs, each being an incremental step towards the full implementation that this PR is.