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

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[rustbot]

Important

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cc @rust-lang/compiler

[jieyouxu]

@rustbot second