Improve instrumentation for the bug reported during fn report_borrowed_value_does_not_live_long_enough.

Author: pnkfelix

src/librustc_mir/borrow_check/error_reporting.rs

@@ -471,7 +471,8 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
             | (RegionKind::ReClosureBound(_), _)
             | (RegionKind::ReCanonical(_), _)
             | (RegionKind::ReErased, _) => {
-                span_bug!(drop_span, "region does not make sense in this context");
+                span_bug!(drop_span, "region {:?} does not make sense in this context",
+                          borrow.region);
             }
         }
     }