Implement boolean lit support in cfg predicates

compiler/rustc_ast/src/attr/mod.rs

@@ -527,6 +527,16 @@ impl NestedMetaItem {
         }
     }
 
+    /// Returns the `MetaItem` if `self` is a `NestedMetaItem::MetaItem` or if it's
+    /// `NestedMetaItem::Lit(MetaItemLit { kind: LitKind::Bool(_), .. })`.
+    pub fn meta_item_or_bool(&self) -> Option<&NestedMetaItem> {
+        match self {
+            NestedMetaItem::MetaItem(_item) => Some(self),
+            NestedMetaItem::Lit(MetaItemLit { kind: LitKind::Bool(_), .. }) => Some(self),
+            _ => None,
+        }
+    }
+
     /// Returns the `MetaItem` if `self` is a `NestedMetaItem::MetaItem`.
     pub fn meta_item(&self) -> Option<&MetaItem> {
         match self {

compiler/rustc_attr/src/builtin.rs

@@ -603,6 +603,7 @@ 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,
         _ => {
             dcx.emit_err(session_diagnostics::UnsupportedLiteral {
                 span: cfg.span(),
@@ -649,7 +650,7 @@ pub fn eval_condition(
         }
         ast::MetaItemKind::List(mis) => {
             for mi in mis.iter() {
-                if !mi.is_meta_item() {
+                if mi.meta_item_or_bool().is_none() {
                     dcx.emit_err(session_diagnostics::UnsupportedLiteral {
                         span: mi.span(),
                         reason: UnsupportedLiteralReason::Generic,

compiler/rustc_expand/src/config.rs

@@ -466,9 +466,9 @@ pub fn parse_cfg<'a>(meta_item: &'a MetaItem, sess: &Session) -> Option<&'a Nest
             sess.dcx().emit_err(InvalidCfg::MultiplePredicates { span: l.span() });
             None
         }
-        Some([single]) => match single.is_meta_item() {
+        Some([single]) => match single.meta_item_or_bool() {
-            true => Some(single),
+            Some(meta_item) => Some(meta_item),
-            false => {
+            None => {
                 sess.dcx().emit_err(InvalidCfg::PredicateLiteral { span: single.span() });
                 None
             }

compiler/rustc_metadata/src/native_libs.rs

@@ -309,7 +309,7 @@ impl<'tcx> Collector<'tcx> {
                                 .emit_err(errors::LinkCfgSinglePredicate { span: item.span() });
                             continue;
                         };
-                        if !link_cfg.is_meta_item() {
+                        let Some(link_cfg) = link_cfg.meta_item_or_bool() else {
                             sess.dcx()
                                 .emit_err(errors::LinkCfgSinglePredicate { span: item.span() });
                             continue;

compiler/rustc_trait_selection/src/error_reporting/traits/on_unimplemented.rs

@@ -413,7 +413,9 @@ impl<'tcx> OnUnimplementedDirective {
         } else {
             let cond = item_iter
                 .next()
-                .ok_or_else(|| tcx.dcx().emit_err(EmptyOnClauseInOnUnimplemented { span }))?;
+                .ok_or_else(|| tcx.dcx().emit_err(EmptyOnClauseInOnUnimplemented { span }))?
+                .meta_item_or_bool()
+                .ok_or_else(|| tcx.dcx().emit_err(InvalidOnClauseInOnUnimplemented { span }))?;
             attr::eval_condition(cond, &tcx.sess, Some(tcx.features()), &mut |cfg| {
                 if let Some(value) = cfg.value
                     && let Err(guar) = parse_value(value, cfg.span)

tests/ui/cfg/true-false.rs

@@ -0,0 +1,29 @@
+//@ run-pass
+
+#![feature(link_cfg)]
+
+#[cfg(true)]
+fn foo() -> bool {
+    cfg!(true)
+}
+
+#[cfg(false)]
+fn foo() -> bool {
+    cfg!(false)
+}
+
+#[cfg_attr(true, cfg(false))]
+fn foo() {}
+
+#[link(name = "foo", cfg(false))]
+extern "C" {}
+
+fn main() {
+    assert!(foo());
+    assert!(cfg!(true));
+    assert!(!cfg!(false));
+    assert!(cfg!(not(false)));
+    assert!(cfg!(all(true)));
+    assert!(cfg!(any(true)));
+    assert!(!cfg!(not(true)));
+}