Allow #[cold] on match and if arms

[x17jiri]

Proposal

Rust already has likely and unlikely intrinsics, but they don't work.

In this issue rust-lang/rust#26179 it is suggested that they could be replaced with #[likely] and #[unlikely] attributes. This is certainly a possibility, but since rust already has #[cold] attribute and likely on branch A can be replaced with unlikely on branch B, I suggest to reuse the #[cold] attribute.

Use would be as follows:

if cond #[cold] { ... } else { ... } // 'then' branch is cold
if cond { ... } else #[cold] { ... } // 'else' branch is cold

match x {
    #[cold] true => { ... }
    _ => { ... }
}

As a proof of concept, I locally implemented the change for match arms. It generates llvm branch weights. I can provide it to anyone for review.

Mentors or Reviewers

This is my first work on the compiler and I don't know anyone on the team, so I'd appreciate if you could assign someone.

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.

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

[Noratrieb]

A compiler MCP is the wrong way to do this. The compiler team isn't in charge of adding features like this. You should talk to the language team on Zulip in #t-lang instead.
Thank you for trying make progress here anyways!