Split TypeVisitable out from TypeFoldable

[eggyal]

Proposal

rustc currently has a single TypeFoldable trait for traversing types, which has methods for both folding and visiting operations; by contrast, chalk currently has distinct Fold and Visit traits (and indeed they have slightly different sets of implementors).

As part of ongoing efforts to integrate chalk into rustc, it is proposed to split the visiting methods out from TypeFoldable so that rustc's approach is more closely aligned with that of chalk. This would entail creating a new TypeVisitable trait, implementing it on all types that currently implement TypeFoldable and making it a supertrait of TypeFoldable. Being a supertrait allows folding operations to perform visiting traversals, e.g. in order to optimise into a no-op where appropriate.

The alternative, also under consideration, is to merge chalk's Fold and Visit traits so that it is better aligned with rustc.

We're conscious that making this change could, at least initially, introduce quite a bit of boilerplate code; it might be possible to refactor some of this into macros, although that would of course add some cognitive overhead. It might also be possible to identify some types that do not need to support folding and remove their TypeFoldable implementations.

Mentors or Reviewers

@jackh726
@compiler-errors

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.

Comments

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.

[rustbot]

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.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[jackh726]

@rustbot second

[apiraino]

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