Rollup merge of #131034 - Urgau:cfg-true-false, r=nnethercote

Implement RFC3695 Allow boolean literals as cfg predicates

This PR implements https://github.com/rust-lang/rfcs/pull/3695: allow boolean literals as cfg predicates, i.e. `cfg(true)` and `cfg(false)`.

r? `@nnethercote` *(or anyone with parser knowledge)*
cc `@clubby789`

1 files changed, 22 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/language-features/cfg-boolean-literals.md b/src/doc/unstable-book/src/language-features/cfg-boolean-literals.md
new file mode 100644
index 00000000000..ad795ff9d9b
--- /dev/null
+++ b/src/doc/unstable-book/src/language-features/cfg-boolean-literals.md
@@ -0,0 +1,22 @@
+# `cfg_boolean_literals`
+
+The tracking issue for this feature is: [#131204]
+
+[#131204]: https://github.com/rust-lang/rust/issues/131204
+
+------------------------
+
+The `cfg_boolean_literals` feature makes it possible to use the `true`/`false`
+literal as cfg predicate. They always evaluate to true/false respectively.
+
+## Examples
+
+```rust
+#![feature(cfg_boolean_literals)]
+
+#[cfg(true)]
+const A: i32 = 5;
+
+#[cfg(all(false))]
+const A: i32 = 58 * 89;
+```