Add a FIXME to collector

src/librustc_mir/monomorphize/collector.rs

@@ -480,6 +480,8 @@ fn check_type_length_limit<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let type_length_limit = *tcx.sess.type_length_limit.get();
     // We include the const length in the type length, as it's better
     // to be overly conservative.
+    // FIXME(const_generics): we should instead uniformly walk through `substs`,
+    // ignoring lifetimes.
     if type_length + const_length > type_length_limit {
         // The instance name is already known to be too long for rustc.
         // Show only the first and last 32 characters to avoid blasting