Allow boolean literals in `--check-cfg`

[clubby789]

Proposal

Currently, the --check-cfg option denies boolean literals, despite --cfg allowing them (they can be accessed in code via cfg!(r#true).) The flag should be amended to allow this.

This is also consistent with keywords such as while, which can be specified under --cfg and --check-cfg then accessed with raw identifiers (#[cfg(r#while)]).

This was noticed in the cfg_boolean_literals stabilization and requested by T-lang.

The change is implemented in rust-lang/rust#138767 and is insta-stable.

Context:
rust-lang/rust#138632 (comment)
rust-lang/rust#138767

Mentors or Reviewers

@Urgau

Process

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

• File an issue describing the proposal.
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• 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.

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<description of the concern>

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cc @rust-lang/compiler

[jieyouxu]

@rustbot second

[Urgau]

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