Eagerly check for accidentally registered region constraints instead of waiting until borrowck is done

Author: oli-obk

compiler/rustc_borrowck/src/type_check/canonical.rs

@@ -39,6 +39,12 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
 
         let TypeOpOutput { output, constraints, error_info } =
             op.fully_perform(self.infcx, locations.span(self.body))?;
+        if cfg!(debug_assertions) {
+            let data = self.infcx.take_and_reset_region_constraints();
+            if !data.is_empty() {
+                panic!("leftover region constraints: {data:#?}");
+            }
+        }
 
         debug!(?output, ?constraints);