add a rustc::query_stability lint

compiler/rustc_span/src/lib.rs

@@ -21,6 +21,7 @@
 #![feature(negative_impls)]
 #![feature(nll)]
 #![feature(min_specialization)]
+#![cfg_attr(not(bootstrap), allow(rustc::potential_query_instability))]
 
 #[macro_use]
 extern crate rustc_macros;