Feature gate boolean lit support in cfg predicates

compiler/rustc_attr/src/builtin.rs

@@ -18,7 +18,7 @@ use rustc_session::parse::feature_err;
 use rustc_session::{RustcVersion, Session};
 use rustc_span::Span;
 use rustc_span::hygiene::Transparency;
-use rustc_span::symbol::{Symbol, sym};
+use rustc_span::symbol::{Symbol, kw, sym};
 
 use crate::fluent_generated;
 use crate::session_diagnostics::{self, IncorrectReprFormatGenericCause};
@@ -603,7 +603,23 @@ pub fn eval_condition(
 
     let cfg = match cfg {
         ast::NestedMetaItem::MetaItem(meta_item) => meta_item,
-        ast::NestedMetaItem::Lit(MetaItemLit { kind: LitKind::Bool(b), .. }) => return *b,
+        ast::NestedMetaItem::Lit(MetaItemLit { kind: LitKind::Bool(b), .. }) => {
+            if let Some(features) = features {
+                // we can't use `try_gate_cfg` as symbols don't differentiate between `r#true`
+                // and `true`, and we want to keep the former working without feature gate
+                gate_cfg(
+                    &((
+                        if *b { kw::True } else { kw::False },
+                        sym::cfg_boolean_literals,
+                        |features: &Features| features.cfg_boolean_literals,
+                    )),
+                    cfg.span(),
+                    sess,
+                    features,
+                );
+            }
+            return *b;
+        }
         _ => {
             dcx.emit_err(session_diagnostics::UnsupportedLiteral {
                 span: cfg.span(),

compiler/rustc_feature/src/unstable.rs

@@ -371,6 +371,8 @@ declare_features! (
     (unstable, async_for_loop, "1.77.0", Some(118898)),
     /// Allows using C-variadics.
     (unstable, c_variadic, "1.34.0", Some(44930)),
+    /// Allows the use of `#[cfg(<true/false>)]`.
+    (unstable, cfg_boolean_literals, "CURRENT_RUSTC_VERSION", Some(131204)),
     /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.
     (unstable, cfg_overflow_checks, "1.71.0", Some(111466)),
     /// Provides the relocation model information as cfg entry

compiler/rustc_span/src/symbol.rs

@@ -543,6 +543,7 @@ symbols! {
         cfg_accessible,
         cfg_attr,
         cfg_attr_multi,
+        cfg_boolean_literals,
         cfg_doctest,
         cfg_eval,
         cfg_fmt_debug,