diff --git a/src/libsyntax/util/map_in_place.rs b/src/libsyntax/util/map_in_place.rs
index 5dd9fc6e8bc..a237a6e6162 100644
--- a/src/libsyntax/util/map_in_place.rs
+++ b/src/libsyntax/util/map_in_place.rs
@@ -1,3 +1,5 @@
+// FIXME(Centril): Move to rustc_data_structures.
+
 use smallvec::{Array, SmallVec};
 use std::ptr;