RFC: Allow boolean literals as cfg predicates

[clubby789]

This RFC proposes allowing boolean literals as cfg predicates, e.g. cfg(true) and cfg(false).

---

Rendered

Tracking:

• Tracking issue for RFC 3695: Allow boolean literals as cfg predicates rust#131204

[clarfonthey]

Also worth noting that, in addition to cfg(any()) meaning cfg(false), cfg(all()) means cfg(true) as well. So, both technically have easy versions, but I agree that true and false care clearer.

[joshtriplett]

This is simple, straightforward, solves an existing problem, and has no compatibility issues.

@rfcbot merge

[rfcbot]

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @pnkfelix
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[kennytm]

the RFC should mention somewhere that the raw identifiers r#true and r#false continue to work:

rustc --cfg false --check-cfg 'cfg(r#false, values(none()))' 1.rs
#     ^~~~~~~~~~~

// 1.rs
#![deny(warnings)]

#[expect(unexpected_cfgs)]
mod a {
  #[cfg(r#true)]
  pub fn foo() {}
}

mod b {
  #[cfg(r#false)]
  pub fn bar() {}
}

fn main() { 
    b::bar()
}

[nikomatsakis]

Yes please. I'm always surprised I can't do cfg(false).

@rfcbot reviewed

[clubby789]

Added a note about r#true/r#false

[clubby789]

Also worth noting that, in addition to cfg(any()) meaning cfg(false), cfg(all()) means cfg(true) as well.

Mentioned this in a couple of places for completeness

[lolbinarycat]

probably should also mention cfg_attr for completeness

[traviscross]

@rfcbot reviewed

In particular, now that we're warning about #[cfg(False)], #[cfg(FALSE)], etc., this is somewhat more pressing.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[clubby789]

Updated the name to cfg_boolean_literals: (rust-lang/rust#131034 (comment))

[traviscross]

The team has accepted this RFC, and it's now been merged.

Thanks to @clubby789 for writing this up and pushing it forward.

For further updates, follow the tracking issue:

• Tracking issue for RFC 3695: Allow boolean literals as cfg predicates rust#131204

[lolbinarycat]

for posterity, here is the IRLO thread that lead to this RFC: https://internals.rust-lang.org/t/cfg-none-for-disabling-code/21520