convert all named regions in opaque types to nll vars

Do it in typeck before entering region inference routines
particularly because we will no longer be able to convert placeholders.

Author: aliemjay

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -229,6 +229,22 @@ pub(crate) fn type_check<'mir, 'tcx>(
                 );
             }
 
+            // Convert all regions to nll vars.
+            let (opaque_type_key, hidden_type) =
+                infcx.tcx.fold_regions((opaque_type_key, hidden_type), |region, _| {
+                    match region.kind() {
+                        ty::ReVar(_) => region,
+                        ty::RePlaceholder(placeholder) => checker
+                            .borrowck_context
+                            .constraints
+                            .placeholder_region(infcx, placeholder),
+                        _ => ty::Region::new_var(
+                            infcx.tcx,
+                            checker.borrowck_context.universal_regions.to_region_vid(region),
+                        ),
+                    }
+                });
+
             (opaque_type_key, hidden_type)
         })
         .collect();