diff --git a/src/librustc_allocator/expand.rs b/src/librustc_allocator/expand.rs
index 5c9bf99d15cf..547d5f26cf84 100644
--- a/src/librustc_allocator/expand.rs
+++ b/src/librustc_allocator/expand.rs
@@ -13,7 +13,7 @@ use rustc_errors;
 use syntax::{
     ast::{
         self, Arg, Attribute, Crate, Expr, FnHeader, Generics, Ident, Item, ItemKind,
-        LitKind, Mod, Mutability, StrStyle, Ty, TyKind, Unsafety, VisibilityKind,
+        LitKind, Mac, Mod, Mutability, StrStyle, Ty, TyKind, Unsafety, VisibilityKind,
     },
     attr,
     codemap::{
@@ -167,6 +167,11 @@ impl<'a> Folder for ExpandAllocatorDirectives<'a> {
         info!("exit submodule");
         ret
     }
+
+    // `fold_mac` is disabled by default. Enable it here.
+    fn fold_mac(&mut self, mac: Mac) -> Mac {
+        fold::noop_fold_mac(mac, self)
+    }
 }
 
 struct AllocFnFactory<'a> {