Eliminate `ast::ptr::P`

[nnethercote]

Proposal

ast::ptr::P was added in rust-lang/rust#13316, in the year 2014 CE. It was created as an alternative to the now-defunct @T. It is a thin wrapper around Box and is used in many parts of the AST. It has a moderate-sized API, but a lot of the operations are unused, and the rest are barely used and easily replaced with Box operations.

At this point P just a useless layer of indirection and obfuscation. Many direct uses of Box have crept into the AST alongside the uses of P. Box is superior in every way except brevity. (Especially box patterns!)

Proposed transition:

1. Remove the few uses of P::map.
2. Remove the one use of P<[T]>.
3. Remove all the P<[T]> operations.
4. Remove unused P<T> operations (most of them).
5. Reduce P<T> to a typedef of Box<T>, plus the P constructor fn.
6. Eliminate P<T>, by mass-converting P<T> to Box<T> and P(x) calls to Box::new(x).

rust-lang/rust#141603 does the first five steps.

Mentors or Reviewers

N/A

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

[rustbot]

Important

This issue is not meant to be used for technical discussion. There is a Zulip stream for that.
Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections can formally be registered here by adding a comment.

@rfcbot concern reason-for-concern
<description of the concern>

Concerns can be lifted with:

@rfcbot resolve reason-for-concern

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler

[ZuseZ4]

@rustbot second

[nnethercote]

Ten days with no objection: approved.

[apiraino]

@rustbot label -final-comment-period +major-change-accepted