Make some region folders a little stricter.

Because certain regions cannot occur in them.

compiler/rustc_middle/src/ty/subst.rs

@@ -829,7 +829,13 @@ impl<'a, 'tcx> TypeFolder<TyCtxt<'tcx>> for SubstFolder<'a, 'tcx> {
                     None => region_param_out_of_range(data, self.substs),
                 }
             }
-            _ => r,
+            ty::ReLateBound(..)
+            | ty::ReFree(_)
+            | ty::ReStatic
+            | ty::RePlaceholder(_)
+            | ty::ReErased
+            | ty::ReError(_) => r,
+            ty::ReVar(_) => bug!("unexpected region: {r:?}"),
         }
     }