Merge RFC 3695: Boolean literals as cfg predicates

The FCP for RFC 3695 completed on 2024-09-28 with a disposition to
merge.  Let's merge it.

text/3695-cfg-boolean-literals.md

@@ -0,0 +1,80 @@
+- Feature Name: `cfg_boolean_literals`
+- Start Date: 2024-09-16
+- RFC PR: [rust-lang/rfcs#3695](https://github.com/rust-lang/rfcs/pull/3695)
+- Tracking Issue: [rust-lang/rust#131204](https://github.com/rust-lang/rust/issues/131204)
+
+# Summary
+[summary]: #summary
+
+Allow `true` and `false` boolean literals as `cfg` predicates, i.e. `cfg(true)`/`cfg(false)`.
+
+# Motivation
+[motivation]: #motivation
+
+Often, we may want to temporarily disable a block of code while working on a project; this can be useful, for example, to disable functions which have errors while refactoring a codebase.
+
+Currently, the easiest ways for programmers to do this are to comment out the code block (which means syntax highlighting no longer works), or to use `cfg(any())` (which is not explicit in meaning).
+
+By allowing `#[cfg(false)]`, we can provide programmers with an explicit and more intuitive way to disable code, while retaining IDE functionality.
+
+Allowing `cfg(true)` would also make temporarily enabling `cfg`'ed out code easier; a `true` may be added to a `cfg(any(..))` list. Adding a `cfg(all())` is the current equivalent of this.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Boolean literals (i.e. `true` and `false`) may be used as `cfg` predicates, to evaluate as always true/false respectively.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The syntax for configuration predicates should be extended to include boolean literals:
+
+> **<sup>Syntax</sup>**\
+> _ConfigurationPredicate_ :\
+> &nbsp;&nbsp; &nbsp;&nbsp; _ConfigurationOption_\
+> &nbsp;&nbsp; | _ConfigurationAll_\
+> &nbsp;&nbsp; | _ConfigurationAny_\
+> &nbsp;&nbsp; | _ConfigurationNot_ \
+> &nbsp;&nbsp; | `true` | `false`
+
+And the line
+> - `true` or `false` literals, which are always `true`/`false` respectively
+
+should be added to the explanation of the predicates.
+
+`cfg(r#true)` and `cfg(r#false)` should continue to work as they did previously (i.e. enabled when `--cfg true`/`--cfg false` are passed).
+
+`true` and `false` should be expected everywhere Configuration Predicates are used, i.e.
+- the `#[cfg(..)]` attribute
+- the `cfg!(..)` macro
+- the `#[cfg_attr(.., ..)]` attribute
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+By making it more convenient, this may encourage unconditionally disabled blocks of code being committed, which is undesirable.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- This could instead be spelled as `cfg(disabled|enabled)`, or `cfg(none)` for disabling code only. However, giving special meaning to a valid identifier will change the meaning of existing code, requiring a new edition
+- As the existing predicates evaluate to booleans, using boolean literals is the most intuitive way to spell this
+
+# Prior art
+[prior-art]: #prior-art
+
+Many languages with conditional compilation constructs have a way to disable a block entirely.
+
+- C: `#if 0`
+- C#: `#if false`
+- Dlang: `version(none)`
+- Haskell: `#if 0`
+
+Searching for `cfg(false)` on [GitHub](https://github.com/search?q=%23%5Bcfg%28false%29%5D+language%3ARust&type=code) reveals many examples of projects (including Rust itself) using `cfg(FALSE)` as a way to get this behavior - although this raises a `check-cfg` warning.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+A future lint could suggest replacing constructs such as `cfg(any())` with `cfg(false)`, and `cfg(all())` with `cfg(true)`.
+
+The `check-cfg` lint could be with a special case for identifiers such as `FALSE` and suggest `cfg(false)` instead.