Remove unstable `-Zprofile` (gcov-style coverage)

[Zalathar]

Proposal

There is an unstable -Zprofile flag that enables “gcov-style” coverage instrumentation.

• https://github.com/rust-lang/rust/issues/42524

(The profile flag predates and is separate from the stable -Cinstrument-coverage flag.)

Notably, the -Zprofile flag:

• Is largely untested in-tree, having only one run-make test that does not check whether its output is correct or useful.
• Has no known maintainer.
• Has seen no push towards stabilization.
• Has at least one severe regression reported in 2022 that apparently remains unaddressed.
  • https://github.com/rust-lang/rust/issues/100125
• Is confusingly named, since it appears to be more about coverage than performance profiling, and has nothing to do with PGO.
• Is fundamentally limited by relying on counters auto-inserted by LLVM, with no knowledge of Rust beyond debuginfo.

I propose to remove this unstable flag (and its accompanying -Zprofile-emit), implemented at:

• https://github.com/rust-lang/rust/pull/131829.

Mentors or Reviewers

—

Process

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

• [x] 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.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

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

Concerns can be lifted with:

 @rustbot resolve reason-for-concern 

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

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

[jieyouxu]

@rustbot second

[apiraino]

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