Tracking issue for RFC 3695: Allow boolean literals as cfg predicates

[clubby789]

This is a tracking issue for the RFC "Allow boolean literals as cfg predicates" (rust-lang/rfcs/#3695).
The feature gate for the issue is #![feature(cfg_boolean_literals)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Accept an RFC.
  • RFC: Allow boolean literals as cfg predicates rfcs#3695
• Implement in nightly.
  • Implement RFC3695 Allow boolean literals as cfg predicates #131034
• Add documentation to the reference.
  • Add cfg(true) and cfg(false) to conditional compilation (RFC 3695) reference#1762
• Stabilize.
  • Stabilize cfg_boolean_literals #138632

Unresolved Questions

None.

Related

• Implement RFC3695 Allow boolean literals as cfg predicates #131034
• Implement RFC3695: Allow boolean literals as cfg predicates cargo#14649

[Urgau]

Call for testing

This is call for testing regarding RFC3695: Allow boolean literals as cfg predicates.

RFC 3695 adds support for true & false in #[cfg], #[cfg_attr] and cfg! predicates.

Testing instructions

Requires: nightly-2024-10-06 or newer with feature(cfg_boolean_literals).

• Use #[cfg(true)], #[cfg(false)] with (or without) any/all/not.

Feedback

• Did the feature work as expected?
• Were there surprising results?

@rustbot label +call-for-testing

[U007D]

Hi. This will appear in This Week in Rust issue #581 under "Call for Testing". You may remove the call-for-testing label, or retain it (in which case it will appear again in the following issue of TWiR).

[clubby789]

With a quick review:

• #[cfg(true)], #[cfg(false)] and variants (all/any/not) work as expected
• Same for cfg_attrs
• cfg!(true) and cfg!(false) work
• r#true and r#false are also unchanged as specified:

[poc.rs:4:5] cfg!(true) = true
[poc.rs:5:5] cfg!(false) = false
[poc.rs:6:5] cfg!(r#true) = false
[poc.rs:7:5] cfg!(r#false) = false

[joshtriplett]

Works well here, behaving exactly as expected.

[joshtriplett]

Reference update at rust-lang/reference#1762 .

[joshtriplett]

@traviscross Good catch, yes, the FCP belongs on the PR.

[clubby789]

#138632