Policy change around adding new unstable flags

[davidtwco]

Proposal

Our policy is currently to add new flags under -Z or using -Zunstable-options prior to stabilisation. Typically the latter is only used when extending an existing already stable flag (e.g. a new argument for -Csplit-debuginfo), and -Z is used for entirely new flags.

Presumably, the convention of using -Z came about from wanting to avoid users depending on unstable flags, but it ends up causing churn for those projects which need depend on an unstable flags which will soon be stabilised, such as Rust for Linux - once a flag is stabilised, as all of its uses need to be updated from -Z to the new stable flag and failure to do so could cause various CI breakages and other such hassle.

We should change our policy so that new flags are added as they are intended to be stabilised, but require -Zunstable-options to use. For example, instead of adding -Zfixed-x18, instead add -Cfixed-x18 and require -Zunstable-options.-Z would be reserved for never-stable flags such as -Ztreat-err-as-bug.

-Zunstable-options is currently a blanket approval for any unstable feature that is gated behind it, this may or may not make sense as it is used more, but whether or not to require features be explicitly enabled by arguments to -Zunstable-options is left to a follow-up MCP.

Mentors or Reviewers

@oli-obk @nikomatsakis

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