Handle rustc_const_stable attribute in library feature collector

Author: dtolnay

compiler/rustc_passes/src/lib_features.rs

@@ -29,10 +29,11 @@ impl<'tcx> LibFeatureCollector<'tcx> {
     }
 
     fn extract(&self, attr: &Attribute) -> Option<(Symbol, Option<Symbol>, Span)> {
-        let stab_attrs = [sym::stable, sym::unstable, sym::rustc_const_unstable];
+        let stab_attrs =
+            [sym::stable, sym::unstable, sym::rustc_const_stable, sym::rustc_const_unstable];
 
-        // Find a stability attribute (i.e., `#[stable (..)]`, `#[unstable (..)]`,
-        // `#[rustc_const_unstable (..)]`).
+        // Find a stability attribute: one of #[stable(…)], #[unstable(…)],
+        // #[rustc_const_stable(…)], or #[rustc_const_unstable(…)].
         if let Some(stab_attr) = stab_attrs.iter().find(|stab_attr| attr.has_name(**stab_attr)) {
             let meta_kind = attr.meta_kind();
             if let Some(MetaItemKind::List(ref metas)) = meta_kind {
@@ -52,7 +53,9 @@ impl<'tcx> LibFeatureCollector<'tcx> {
                     // This additional check for stability is to make sure we
                     // don't emit additional, irrelevant errors for malformed
                     // attributes.
-                    if *stab_attr != sym::stable || since.is_some() {
+                    let is_unstable =
+                        matches!(*stab_attr, sym::unstable | sym::rustc_const_unstable);
+                    if since.is_some() || is_unstable {
                         return Some((feature, since, attr.span));
                     }
                 }