Use ControlFlow in HIR Visitor

[Jarcho]

Proposal

Currently early returns from visitors are handled by storing a flag or Option type and checking if the result has been found at the start of visit_* methods. The proposal is to have all visit_* functions return ControlFlow<Self::BreakTy> the same way TypeVisitor does.

Most examples of early returns are from clippy, where this is the most use of visitors. There are a not insignificant number of examples in rustc as well.

Related PR: rust-lang/rust#108598

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

[pnkfelix]

@rustbot second

From the associated zulip stream, it is clear that there is support for doing something in this space.

Its possible that the actual implementation that lands won't use ControlFlow, but rather some other trait, or perhaps something else like an associated type, and that is fine, we don't need to flesh out all the details in this MCP ahead of time.

[apiraino]

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