Add notes that data-structures should potentially move to different crate.

Author: pnkfelix

src/librustc_borrowck/bitslice.rs

@@ -8,6 +8,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// FIXME: move this to `rustc_data_structures` and potentially merge
+// with `bitvec` there.
+
 use std::mem;
 
 pub type Word = usize;

src/librustc_borrowck/indexed_set.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// FIXME: move this to `rustc_data_structures`
+
 use std::fmt;
 use std::marker::PhantomData;
 use std::mem;