Delegation: reject C-variadics

compiler/rustc_hir_analysis/src/delegation.rs

@@ -409,6 +409,11 @@ fn check_constraints<'tcx>(
         emit("recursive delegation is not supported yet");
     }
 
+    if tcx.fn_sig(sig_id).skip_binder().skip_binder().c_variadic {
+        // See issue #127443 for explanation.
+        emit("delegation to C-variadic functions is not allowed");
+    }
+
     ret
 }