We should never see unevaluated type-level constants after monomorphization unless errors occurred

compiler/rustc_mir/src/monomorphize/collector.rs

@@ -648,7 +648,13 @@ fn visit_const(&mut self, constant: &&'tcx ty::Const<'tcx>, location: Location)
             ty::ConstKind::Value(val) => collect_const_value(self.tcx, val, self.output),
             ty::ConstKind::Unevaluated(unevaluated) => {
                 match self.tcx.const_eval_resolve(param_env, unevaluated, None) {
-                    Ok(val) => collect_const_value(self.tcx, val, self.output),
+                    // The `monomorphize` call should have evaluated that constant already.
+                    Ok(val) => span_bug!(
+                        self.body.source_info(location).span,
+                        "collection encountered the unevaluated constant {} which evaluated to {:?}",
+                        substituted_constant,
+                        val
+                    ),
                     Err(ErrorHandled::Reported(ErrorReported) | ErrorHandled::Linted) => {}
                     Err(ErrorHandled::TooGeneric) => span_bug!(
                         self.body.source_info(location).span,