Concrete regions can show up in mir borrowck if the originated from there.

We used to not encounter them here, because we took regions from typeck's opaque type resolution by renumbering them. We don't do that anymore.

compiler/rustc_mir/src/borrow_check/region_infer/opaque_types.rs

@@ -82,15 +82,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
                             .find(|ur_vid| self.eval_equal(vid, **ur_vid))
                             .and_then(|ur_vid| self.definitions[*ur_vid].external_name)
                             .unwrap_or(infcx.tcx.lifetimes.re_root_empty),
-                        ty::ReLateBound(..) => region,
-                        ty::ReStatic => region,
-                        _ => {
-                            infcx.tcx.sess.delay_span_bug(
-                                span,
-                                &format!("unexpected concrete region in borrowck: {:?}", region),
-                            );
-                            region
-                        }
+                        _ => region,
                     });
 
                 debug!(?universal_concrete_type, ?universal_substs);