Implment #[cfg] and #[cfg_attr] in where clauses

compiler/rustc_ast/src/ast.rs

@@ -417,9 +417,11 @@ impl WhereClause {
 /// A single predicate in a where-clause.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct WherePredicate {
+    pub attrs: AttrVec,
     pub kind: WherePredicateKind,
     pub id: NodeId,
     pub span: Span,
+    pub is_placeholder: bool,
 }
 
 /// Predicate kind in where-clause.