rustc/ty: mark a comment as FIXME

src/librustc/ty/mod.rs

@@ -1373,7 +1373,7 @@ pub fn walk_tys(&self) -> IntoIter<Ty<'tcx>> {
             }
         };
 
-        // The only reason to collect into a vector here is that I was
+        // FIXME: The only reason to collect into a vector here is that I was
         // too lazy to make the full (somewhat complicated) iterator
         // type that would be needed here. But I wanted this fn to
         // return an iterator conceptually, rather than a `Vec`, so as