Remove P<> from visit_s in ast MutVisitor

[maxcabrajac]

On #128974, one divergence I found between versions of the trait were P<> wrappers on some types on the mutable one.
Some are really easy to remove, while others are not.
This PR tries to remove them.

r? @petrochenkov

related to #128974 & #127615

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @petrochenkov (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[petrochenkov]

(The PR was marked as a draft, so I assumed you want to try removing the rest of P<>s.)

[maxcabrajac]

Yup, that was the idea... But I'm starting to think this might not be a good one. From the Vistor's perspective it surely is, but when I looked at how rustc_expand heavily uses substitution on this P<>s it seems not to be great to remove them.

I'll look deeper into it on the weekend and keep you up to date!

If I end up finding a way to remove them from the visits then its best that we do another perf test.

[maxcabrajac]

For now, I removed P<>s from all visits, but this just pushed the problem into rustc_expand. What I see as the problem now is calling .into_inner() on the P<>s we get from .make_*() methods. A better approach is to change them to return T instead of P<T>. This would possibly remove some memory allocations, which I see as a good thing!

[bors]

☔ The latest upstream changes (presumably #131284) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #136564) made this pull request unmergeable. Please resolve the merge conflicts.

[petrochenkov]

The status of this PR was posted on another thread - #128974 (comment).

@maxcabrajac
Do you think this is ready for review?
We can merge this even if not all P<>s are eliminated.

[petrochenkov]

Closing due to inactivity.