add a rustc::query_stability lint

compiler/rustc_expand/src/lib.rs

@@ -9,6 +9,7 @@
 #![feature(proc_macro_span)]
 #![feature(try_blocks)]
 #![recursion_limit = "256"]
+#![cfg_attr(not(bootstrap), allow(rustc::potential_query_instability))]
 
 #[macro_use]
 extern crate rustc_macros;