Lower the unstable feature bound attribute to UnstableFeature predicate

Author: tiif

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -1,6 +1,7 @@
 use std::assert_matches::assert_matches;
 
 use hir::Node;
+use rustc_attr_data_structures::{AttributeKind, find_attr};
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
@@ -333,6 +334,19 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
         predicates.extend(const_evaluatable_predicates_of(tcx, def_id, &predicates));
     }
 
+    let attrs = tcx.hir_attrs(tcx.local_def_id_to_hir_id(def_id));
+    // FIXME(staged_api): We might want to look at the normal stability attributes too but
+    // first we would need a way to let std/core use APIs with unstable feature bounds from
+    // within stable APIs.
+    let allow_unstable_feature_attr =
+        find_attr!(attrs, AttributeKind::UnstableFeatureBound(i) => i)
+            .map(|i| i.as_slice())
+            .unwrap_or_default();
+
+    for (feat_name, span) in allow_unstable_feature_attr {
+        predicates.insert((ty::ClauseKind::UnstableFeature(*feat_name).upcast(tcx), *span));
+    }
+
     let mut predicates: Vec<_> = predicates.into_iter().collect();
 
     // Subtle: before we store the predicates into the tcx, we