document RUSTC_BOOTSTRAP in the unstable book

[jyn514]

Proposal

It is a simple fact that people outside the rust project are using RUSTC_BOOTSTRAP. Given that, we should make it more clear what it does.

I propose the following:

• Document the variable in the unstable book so people know what it does, why it exists, and why we would really prefer them not to use it. Right now it spreads by word of mouth, and I do not think people know exactly what breakage they are opting into. document RUSTC_BOOTSTRAP, RUSTC_OVERRIDE_VERSION_STRING, and -Z allow-features in the unstable book rust#139885

• Document that libraries that use nightly features should use an explicit opt-in: Document that nightly features should be opt-in using a --cfg flag api-guidelines#284 EDIT(jieyouxu): not compiler concern

• change the name to RUSTC_ALLOW_UNSTABLE_ON_STABLE (name subject to bikeshedding) that has exactly the current semantics

• give a hard warning (i.e. not silence-able or deny-able) if people use RUSTC_BOOTSTRAP

• simultaneously, change cargo and all related tooling to use the new name. the hard error for build scripts setting RUSTC_BOOTSTRAP will apply to both the old and new name.

Mentors or Reviewers

@onur-ozkan, @bjorn3, @Mark-Simulacrum

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.

[rustbot]

Important

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 can formally be registered here by adding a comment.

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

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@rfcbot resolve reason-for-concern

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

cc @rust-lang/compiler

[Noratrieb]

sounds good, I agree that renaming is needlessly annoying
@rustbot second

[jieyouxu]

This was discussed in a compiler triage meeting, and this doesn't really need a MCP, so we're just going to close this as accepted.

@rustbot label: +major-change-accepted