Introduce `-Z` flag to control how proc-macros are run

[mystor]

Proposal

In rust-lang/rust#99123 I am hoping to add a new permanently-unstable -Zproc-macro-execution-strategy={same-thread,cross-thread} flag which can be used to control the way that proc-macros are run when expanding macros in the compiler.

This is being added as after recent changes in the bridge layer, which should have brought the performance of cross-thread execution much closer to same-thread execution than it was previously. Having a flag to change this behaviour without rebuilding rustc should be useful for real-world performance testing or comparison to see how practical a change to cross-thread or otherwise-isolated execution in the future.

Mentors or Reviewers

The patch has already been written by myself, and reviewed by @eddyb. It is a fairly simple change.

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

[eddyb]

@rustbot second

[apiraino]

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